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Welcome to the Frequently Asked Questions PageClick one of the below topic headings to find answers to questions regarding that topic. If you cannot find the answer to a question you have, click on the button below to submit a question of your own! Lumber Specialties Topic Headings:Bracing Cutting Dimensioning Load Bearing Nailing Setting Trusses We Stock Other WTCA Topic Headings:**Below FAQs provided courtesy of WTCA - Wood Truss Council of AmericaConnectionsDesign Loads Design Responsibilities Fire Handling, Installing and Bracing Heavy Timber Introduction Lumber Metal Connector Plates Misc. Truss Information Partition Separation Site-Built Trusses Testing Truss Design and Specifications Truss Repairs Truss Repairs and Alterations Lumber Specialties FAQsBracingTopLATERAL AND T BRACING Question: What is the difference between Lateral and T Bracing? Answer: Lateral bracing is used when you have three or more consecutive trusses with the same web configuration. Lumber Specialties uses T bracing when we have a single style truss. STRONG BACKS Question: I just installed a set of floor trusses for my house. Do I need to install the strong back bracing you call out for every 10'-0" on center? Answer: No, the strong backs are not required for residential construction, however, they are strongly recommended to minimize floor vibration. CuttingTopI-JOIST Question: Why do we cut the top flange of an I-Joist? Answer: When you have a continuous span joist and one span is much longer than the other. This may cause an up lift situation on the shorter span. If you cut the top flange of the I-joist over the bearing point, it will alleviate the situation. DimensioningTopDIMENSION 12-8-4 Question: What does the dimension 12-8-4 equate to? Answer: The dimensions on Lumber Specialties layouts and truss design drawings are in feet, inches, sixteenths. 12-8-4 would be 12' 8 4/16", or 12' 8 1/4". Load BearingTopPROVIDE UPLIFT CONNECTION PER SCHEDULE Question: On your truss design drawing it shows: PROVIDE UPLIFT CONNECTION PER SCHEDULE Support 1 124# Support 2 127# What does this actually mean? Answer: Some trusses may have a tendency to "fly off of the building" under certain load case (mainly wind loads). In order to prevent the trusses from being sucked off of the building, they need to be connected for the uplift reaction shown in the provided schedule. In other words, Support-One needs to be connected to the top plate for 124 pounds and Support-Two should be connected for 127 lbs. L/480 DEFLECTION Question: What does L/480 deflection equate to? Answer: L/ is a ratio of deflection versus the length of the member in inches. For example: a 10'-0, or 120", structural member that is L/480 can deflect a maximum of 1/4". The length in inches is simply divided by the deflection factor, 120"/480 = .25". Deflection factors are typically referred to in terms of live load deflection. Code requires a minimum of L/360 live load deflection for floor systems. However, Lumber Specialties typically designs floor systems at L/480 to improve floor performance. Remember, the higher the L/ factor, the lower the deflection. g NailingTopNAILING TRUSSES Question: Should I nail the truss down to the interior walls? Answer: Nailing the truss to a non bearing wall is not recommended. If this is somethin you want to do, you should use a roof truss clip. Roof truss clips are used for alignment control between a roof truss and non-bearing wall. The clip permits vertical truss chord movement when loads are applied. SettingTopHIP FLAP INSERT TRUSSES Question: Should the hip flat insert trusses be set with the beveled side up or down? Answer: Hip flat trusses (HF01) should be set with the bevel down. Trusses We StockTopSTOCKED TRUSSES Question: What trusses do you stock? Answer: We stock trusses 20' to 30' in two foot increments, 4/12 pitch, 2' on centers, 2' overhangs, normal heels and use 2/3 webs. OtherTopSEALED DRAWINGS Question: Can I get a set of sealed truss design drawings? Answer: Yes; however, they may not be required. Most residential and agricultural type structures may not require any structural design documents because they are exempt structures. One should check local, county or state requirements as applicable and let Lumber Specialties know in advance if sealed drawings are required. Obtaining a seal may add extra expense and unnecessary delays to the project. WTCA FAQsConnectionsTopATTACHMENT OF SCISSORS TRUSSES Question: What is the correct method of attaching scissors trusses to the top plate? I read recently in a trade magazine that this type of truss should be toe-nailed on one end and attached with slotted clips on the other end. According to the article, this is to allow for movement of the truss. We require PE stamped spec sheets from the truss manufacturer to verify trusses meet wind and snow loads. These sheets give bracing requirements but never give recommended attachment requirements. Answer: The reason this information is never on the stamped truss design drawing is because the P.E.'s design responsibility does not cover truss-to-bearing connections. The truss design drawing will only list the expected horizontal deflection. It is then up to the building designer (P.E. or architect of record) to determine how to design the structure to accommodate this deflection. BLOCKING FOR LATERAL DISPLACEMENT AND ROTATION Question: I am a Building Inspector and have some questions regarding the second sentence of section 2326.12.8 of the 94 UBC that states, "Roof trusses shall be supported laterally at points of bearing by solid blocking to prevent rotation and lateral displacement". Is this requirement true for all end bearing conditions regardless of truss heel height? What are the proper methods to achieve solid blocking? Is the combination of roof sheathing, gypboard on the ceiling and a hurricane clip on one side of a truss an acceptable substitution for solid blocking? How would one solid block a truss at intermediate bearing points? Do truss engineers account for rotation and lateral displacement in their truss design or is this the responsibility of the structural engineer or building designer? Answer: Do truss engineers account for rotation and lateral displacement in their truss design or is this the responsibility of the structural engineer or building designer? The truss designer assumes (per ANSI/TPI 1-1995) that the truss will be installed in-plumb and in-plane and will carry only in-plane loads. The Building Designer is responsible for designing the system to resist forces from rotation and lateral displacement. Is this requirement true for all end bearing conditions regardless of truss heel height? Yes, but the block doesn't need to go the full height of the heel to effectively block it and keep it from rotating. What are the proper methods to achieve solid blocking? Blocks, blocking panels, structural panels (plywood, OSB). BRACKETLESS CONNECTIONS Question: Are there any trusses that are supported strictly by the wood itself without any mechanical connections such as brackets? Answer: Most trusses do not require metal connectors to attach to bearing. Instead, they bear directly on the wall, beam, header, etc. that is supporting them. In this case, nails driven in at an angle (toenails) are sufficient to keep the trusses in place. If there are any uplift forces at the bearing point then the truss must be held down in place. This is where metal connectors are usually specified. HANGER NAILS ABOVE THE NEUTRAL AXIS Question: I recall seeing a design recommendation several years ago regarding installation of hanger nails above the neutral axis of the bottom chord of a plated girder truss. The intent was to avoid dumping large loads into the bottom chord, below the neutral axis. Can you tell me where I can find this information? Answer: The recommendation is actually a design provision (section 10.3.4.2) in the Truss Plate Institute's ANSI/TPI 1-1995 National Design Specification for Metal Plate Connected Wood Trusses. NOTCHED TOP CHORDS ON GABLE ENDS Question: The industry suggests notching the gable end truss to support the overhang. Is this wise? What about a structural gable, a gable designed with drag loads, or one with only partial bearing? How safe is it for a framer working with a truss that has the top chord cut repeatedly? Answer: You are correct the notched top chord should not be used for structural gable ends, drag struts or partial bearing trusses. The only exceptions would be trusses that have been analyzed and sealed by a truss engineer that take the notched top chord into account. A better solution is to use a drop top chord detail, which allows the out-lookers to be installed on edge and cantilevered back to the top chord of the second last truss. This produces a much stronger rake overhang and is a much easier installation than notching. ROOF TRUSS NAILING WITH HURRICANE CLIPS Question: If you use hurricane clips to secure roof trusses from uplift, are you allowed to use less than the typical three nails into the bearing heel of the roof truss? I am concerned because we want to do the right fastening schedule, but 3nails in addition to the hurricane clip splits the wood. What is the standard recommendation? Answer: When using hurricane clips, install per the clip manufacturer's instructions. No other nails are needed nor should be used. TRUSS ATTACHMENT INFORMATION Question: Would you please inform me of the specified requirements of the size and the amount of nail attachments from the truss to the top plate? Answer: WTCA addresses this topic in one of our Truss Technology in Building brochures titled Toe-Nailing For Uplift Reactions. This brochure provides guidance on the correct usage of toenailing as a means of uplift connection. In a simple yet effective format, the following concerns are addressed:
UPLIFT REACTION NUMBERS Question: As building inspectors in a 100mph exposure C, UBC'97 design area, we have been failing framing inspections due to inadequate truss to top plate connectors for the uplift reactions indicated on the wet sealed truss drawings required to be furnished by the various truss fabricators at time of frame inspection. Typically double & triple girder truss bearing points that may only have a standard truss clip rated in the 400 to 500 lb. range with an uplift reaction of 2000 to 3000 lb. range. Because of various suppliers, all having different software, and therefore different information on the truss sheets, we are getting conflicting information as to what the uplift reaction number actually represents. Some are very specific and state "to provide for mechanical connection of the truss to the top plate with a connector capable of withstanding a specific load". Others simply list the uplift reaction with no further information. These are the ones that have caused a debate as to what the number actually represents. Some say the uplift is a net number to size a connector to, and others say it is a gross # that can be reduced and a lower rated truss connector used. Unless the engineer can provide calculations and be willing to "stamp" the calculations, we have stood by the uplift load listed, as the load to size the connector to. Any insight on this issue would be greatly appreciated. Answer: WTCA contacted several engineers representing the truss design software companies that calculate these uplift reactions. We asked them what their listed uplift force represents. The overall answer to the question, "Does the listed uplift force represent the resistance for which the connection needs to be designed?" is YES. The reaction data is the worst case at each support considering all the load cases for which the truss is designed. The uplift reaction should not be reduced. You should definitely call the professional engineer sealing the design if you have any questions or concerns. You should continue to stand by the uplift load listed unless a professional engineer is willing to take the responsibility of recalculating it. Design LoadsTopADDING TILE TO EXISTING FLOOR TRUSSES Question: We are planning to add 1/2" cement board and 3/8" quarry tile to a kitchen floor. We need to know if the floor trusses will handle the additional weight. The floor trusses are 19.2" o.c. and the loading numbers are 40-10-0-5. What do these numbers mean? Answer: The numbers 40-10-0-5 refer to the design loads for the truss in pounds per square foot (psf). There are two sets of numbers - one set for the top chord of the truss and one set for bottom chord of the truss. The top chord live load is 40 psf; this is standard loading for residential floors. Live loads account for loads from people, furniture, moveable objects, etc. The top chord dead load is 10 psf, which accounts for things like the weight of flooring materials and part of the weight of the truss. The bottom chord live load is usually zero, as it is in this case. The bottom chord dead load is 5 psf which accounts for dead loads applied to the bottom chord like gypsum board, insulation, ductwork, lighting and part of the weight of the truss. You will be increasing the top chord dead load by adding cement board and tile to the floor. Tile loads are usually higher than 10 psf so chances are your trusses were not designed to account for this. A couple other considerations are the on center spacing of the trusses. Most sources say that sub floor for tile should not span more than 16" oc to reduce deflection of the sub floor and minimize cracking of the tile. The fact is there are many things to consider here. What is the span of the truss? How far is this extra load from bearing? What deflection criteria was the truss designed for - L/360, L/480 or higher? You may install the tile and find that it is problem free, on the other hand it may not be. It's difficult to make recommendations without having seen the assembly. You should determine the load from the tile and the cement board (and all the other materials on the floor). The manufacturers of each product should have that information. Then you should call the truss manufacturer and ask them to do an analysis of the truss with the additional loading. They may be able to suggest some options for this installation if you choose to proceed. ADDITIONAL LOADS Question: I am a structural engineer designing pool cage structures that often attach to the house at the fascia board. Sometimes, the structure is attached where trusses are behind the fascia board and other times when there is a framed gable end overhang. Do you know of any information concerning this additional load on the trusses or overhang under design wind loads. Is there a limiting distance on the amount of overhang? I know trusses are designed for certain uplift and the pool cage will add to this uplift at design load, but what about the gable end overhangs? Are there any calculations regarding the amount of uplift they can take? Answer: Assumptions cannot be made regarding loading and uplift without knowing the original design and the assumptions with which it was based. Most design software does account for wind uplift at overhangs and cantilevers per specifications - only when wind loading is specified by the building designer. We contacted an engineer of one of the plate manufacturers and he stated that he does not know of any standardized limits governing overhang or cantilever length beyond those limits resulting from standard structural design procedures (strength and deflection limitations). Regardless of the overhang or cantilever distance, the truss or the element of the truss that the fascia board is attached to, the uplift connectors fastening the truss to its supports, or anything else in the load path could be at its maximum allowable limit prior to any further application of load from a pool cage structure. The only way to be certain that you are not over-stressing the truss is to find the original truss and building design, or to repeat those design calculations. CONSTRUCTION LOADS ON FLOOR TRUSS ASSEMBLIES (From the April 2001 issue of WOODWORDS) Question: How much OSB can be stacked onto a floor deck without damaging the trusses? Answer: During the construction process, it is common practice for builders to store stacks of building materials on top of floor truss assemblies. Unfortunately, a check to see if this "construction loading" causes excessive stresses in truss members is far less common. Trusses that are overstressed due to construction loading may exhibit loosened connector plates, hairline compression fractures, and excessive sagging. These, in turn, may lead to problems ranging from finishing difficulties to structural collapse. It is important to realize that in most cases, truss manufacturers design their products to support loads under the following conditions:
DECKING ON BOTTOM CHORD Question: Can I put a wood deck (1/2 inch plywood) on top of the bottom chord of a 26 foot span truss 24 inches on center? Answer: Whether or not you can add wood decking onto the bottom chord of your roof trusses depends on what load your trusses were designed for in the first place. What are you planning to use the wood decking for...storage? 1/2" plywood itself only weighs 1.5 pounds per square foot (psf), which probably would not affect the integrity of the truss design. If you plan on adding dead weight from storing in the attic space, this may lead to problems. Do you have the original sealed truss design drawings from the Truss Manufacturer? These drawings will provide the bottom chord load for which your trusses were designed for. This will help you determine whether or not you can adequately add additional weight in the roof system. According to the National Design Standard for Metal Plate Connected Wood Trusses, ANSI/TPI 1-1995, bottom chord dead load shall be the actual weight of the material but not less than 10 psf for residential use. Is your bottom chord seeing a larger dead load than 10 psf? (In lieu of talking with the original truss manufacturer, contact a local truss manufacturer to discuss your load situation. A list of manufacturers located in your state in our Members Section. DRAG LOADS Question: Can you explain drag loads and how to calculate a drag load pertaining to roof trusses? Answer: Drag loads are due to lateral (horizontal) loads generated in high-wind or seismic events. These loads are generated within the structure and transferred into load carrying elements (like drag strut trusses, shear walls or roof diaphragms) which then transfer the loads to the foundation and then safely into the ground. Therefore the loads must be calculated by the building designer (usually required to be a structural engineer in seismic areas on the West coast) based on the expected environmental loads and the configuration of the structural elements. WTCA does not have any information on calculating drag loads, but the location, magnitude, and direction of these loads should be provided by the building designer. The truss technician or the truss designer is neither qualified nor responsible for calculating drag loads in the structure. Some truss design software programs analyze lateral loads so the design of a drag truss should be straightforward once the loading information is provided. If not, check with the truss designer to make sure all information is provided before the truss is designed and manufactured. Another point to consider, load transfer from the structure to the drag truss is only as good as the connector used. Proper connector detailing and installation is required for the drag truss to perform as expected. MAIN WIND FORCE RESISTING SYSTEMS VS. COMPONENTS & CLADDING Question: As an engineer, I have noticed truss designers in two high wind states routinely using "Primary Frame" wind pressure coefficients (of the UBC) as opposed to "Elements and Components" coefficients to design for wind uplift. A truss is not a primary frame and an Element or Component would have to have a tributary area of more than 1000 sq. ft before qualifying for the lower Primary Frame coefficients. In my experience this practice is routine. I have to wonder why the design software would even allow the selection of "Primary Frames". I resolve the problem in my truss specs but apparently few other designers are even aware of the problem. Any comments would be appreciated. Answer: : Charles Hoover of Alpine Engineered Products, Inc. wrote a paper in 1996 which presented a method for applying wind loads in the design of wood trusses based on the criteria provided in the 1993 edition of the ASCE standard. This document can be found in the Metal Plate Connected Wood Truss Handbook which can be purchased through WTCA. I hope that this information can provide some insight. SAFETY ISSUES WITH DESIGN DEAD LOADS Question: Can I safely install 3/4" TG, OSB on 2x4 trusses that are 24" O.C.? My roof was installed over 5/8" plywood without clips that have caused a lot of sagging and the shingles need replacing. I want to "fix" it one time and install architecture type shingles, but the garage is 24' wide and 28' long without any load bearing walls. My concern is the weight on the trusses. 5/8" plywood weighs 52 lbs and the OSB weighs 78 lbs for each 4'x 8' sheet. The roof will require about 84 4' x 8' sheets to cover, which equals about 2,184 lbs additional weight plus the small increase for the different shingles. The roofers here indicate it is not a problem, but I wanted to hear it from someone "outside the box" that is not trying to sell me something. The shingles I'm talking about are asphalt that resemble shake and are a little heavier than the normal three tab type. Answer: You are correct in thinking there may be safety concerns. If your safety concern stems from installing 3/4" OSB on your roof instead of the originally designed 5/8" plywood, you should not worry. The design dead loads for top chords of trusses are generally high enough to compensate for any minor differences in sheathing thickness. If you want specifics, we suggest that you contact APA-The Engineered Wood Association (253/620-7400, www.apawood.org). Our concern is regarding the type of shingles that you are planning to install. What is the difference in the existing shingles and the new "architecture type" shingles that you want to install? If you are replacing asphalt or composite shingles with ceramic or concrete tiles then you are increasing the dead load drastically. Trusses that are designed for asphalt or composite type shingles can have dead loads of around 7-10 pounds per square foot (psf). Trusses that are designed for concrete tile can have top chord dead loads of 15 psf or higher. If you have the original design information on the trusses you should be able to determine the top chord design dead load. If not, you should contact the truss manufacturer to request help on tracking down this information. By replacing the 5/8" plywood with the OSB, you are only increasing the design dead load on the top chord of the truss by 0.8125 psf. This is solved by taking the difference of 78 lbs and 52 lbs and dividing it by 4 x 8 feet {[(78-52)lbs/(32) sq. feet]= 0.8125 psf}. A small increase like this is negligible and can be ignored due to the fact, as was stated before, design dead loads for top chords of trusses are generally high enough to compensate for any minor differences in sheathing thickness. TOP CHORD DEAD LOAD Question: I have built a 30 ft. x 40 ft. pole barn with nine 30 ft. 2x4 7/12 pitch trusses that are 5 ft. o.c. I am planning to finish out the interior and will attach 7/16 x 4 x 8 osb sheets to the trusses for my ceiling. Along with this, I will have to add several 2x4 nailers across the 30 ft. span between the trusses to attach the sheeting to. My question is will these trusses have any problem supporting this ceiling? I am not planning on anything being placed in the section above the ceiling and there will be no walls or supports erected between the ceiling and the floor. The trusses came with data sheets that have a great deal of information but none of it directly answers my question. It does state that the truss ID is R01 and the loading is 20-4-10 60.00" O.C. Answer: If there is ever a question regarding a specific truss design, you should contact the original Truss Designer that issued the Truss Design Drawings for truss R01. The Truss Design Drawing contains valuable information for anyone building with or inspecting trusses. 60.00" O.C. means that the trusses are designed to be spaced 60 inches on center (5 ft.). Every Truss Design Drawing must specify the loads that have been accounted for in the design. In other words, was the truss designed for the additional loading on the purlins and sheathing? 20-4-10 means that the truss was designed for 20 psf top chord live load (TCLL), 4 psf top chord dead load (TCDL), 0 psf bottom chord live load (BCLL), and 10 psf bottom chord dead load (BCDL). Now, the trusses themselves are going to be part of the TCDL and BCDL. So, just because the trusses were designed for some dead load, does not necessarily mean that the trusses can handle the additional load. Again, we suggest that you contact the original Truss Designer. TRUSS DEFLECTION ONTO NON-LOAD BEARING WALLS Question: I am installing a 40-foot scissor truss that is designed to deflect about ½-inch. I am concerned that the deflection will cause an interior partition wall to pick up some load from the truss and transfer it to the floor system. Should I double up the I-joists under this partition to pick up the extra load? Answer: There are a lot of variables that affect how much load the truss will transmit at the "unintentional" bearing wall. If it's not near a truss panel point, chances are that it should not be a huge concern. If you would like to get an idea of how much load will be transferred, ask the truss manufacturer to analyze the truss with an additional bearing point at that location. Keep in mind that the actual load transferred may be even less because the floor is a lot less stiff than the solid bearing that the truss design assumes. It goes back to that old engineering adage "stiffness attracts load", so the less stiff the bearing location (i.e. truss mid-panel) and the supporting floor below the partition wall, the less load it will pick up. If you are very concerned, then yes, you may want to double up the joists at that location. The question now becomes, based on the analysis of the additional bearing location, is that enough? Design ResponsibilitiesTopDESIGN RESPONSIBILITIES Question: My company supplied roof trusses for a hotel project. The building inspector shut the project down because the trusses were not designed to account for additional snowdrift loading. The construction plans did not contain any snowdrift loading information. The architect is claiming it is our responsibility to determine drift loading; therefore we must fix the problem. Do you have any documentation to help us dispute the architect's claim? Answer: Yes, Standard Design Responsibilities in the Design Process WTCA 1-1995 was developed to avoid situations just like this. This document outlines what is expected from the five parties involved in the design process-building owner, building designer, contractor, truss manufacturer and truss designer. This document is helpful in clearing up expensive misunderstandings like this one and could very well be included in your customer contracts. In your case, the building designer may not be persuaded by this information after the fact. That is why it is smart to include this document with every bid/proposal/purchase order you submit so that your scope of work is well defined from the outset. For example, Section 3.2.5 states that one of the building designer's responsibilities is to provide "The location, direction and magnitude of all dead and live loads attributable to: roof, floor, partition, mechanical, fire sprinkler, attic, storage, wind, snow drift and seismic." At this point, WTCA 1-1995 is completely voluntary and is not enforced by any agency or building code. By the end of the year, however, TPI should have a new standard in place called ANSI/WTCA/TPI 4-2000 that is based directly on WTCA 1-1995. The fact that it will become an ANSI document does not make it enforceable but it gives the document more credibility and clout for having passed through the public consensus process. When you combine this with ensuring that all the language in the specifications and/or contracts/purchase order complies with WTCA 1-1995, you have provided a foundation that will keep you from assuming more responsibility than is yours to take. And if you desire to take on this "extra" work, you can more easily justify getting paid "extra" for it. INDUSTRY STANDARD RESPONSIBILITIES Question: What is the industry standard for ordering residential roof truss systems: Should the general contractor/builder field measure before ordering trusses or should he rely on the blueprint? Who is responsible for their accuracy - the plan service, the truss manufacturer, the builder/general contractor or the framing contractor? Answer: WTCA's Engineering Review Committee in cooperation with the Truss Plate Institute developed an industry standard outlining the responsibilities in the design process. Titled, WTCA 1-1995, this document outlines the owner, building designer, contractor, truss manufacturer, truss designer, and other responsibilities. As far as your specific question, the builder or general contractor is responsible for giving the truss manufacturer the right information. The truss manufacturer can only work from what is given them by the builder. If the initial measurements were wrong, the truss manufacturer cannot be blamed. It is important that you as the builder or general contractor provide accurate information to the truss manufacturer. You can request shop drawings from the truss manufacturer before the trusses are built to insure that they are accurate and will fit the given job. RESPONSIBILITY FOR MULTI-PLY GIRDER TRUSS ASSEMBLY Question: I am reviewing a truss package that includes multi-ply trusses. Where do I find the requirements for the attachment of the individual trusses to each other (nails and/or bolts)? Is this a requirement that the structural engineer of record needs to supply or is it the responsibility of the truss manufacturer to design? Answer: According to WTCA 1-1995 Standard Responsibilities in the Design Process Involving Metal Plate Connected Wood Trusses and the Commentary & Appendices to ANSI/TPI 1-1995, the responsibility for specifying the connection requirements of truss ply-to-ply belongs to the Truss Designer. RESPONSIBILITY OF SNOW DRIFT LOAD CALCULATIONS Question: Is the Truss Designer or the Building Designer responsible to calculate snow drift loads on a roof system? Answer: It is the Building Designer's responsibility to supply snow drift loading information. WTCA's Standard Design Responsibilities in the Design Process WTCA 1-1995, outlines what is expected from the five parties involved in the design process - building owner, building designer, contractor, truss manufacturer and truss designer. For example, Section 3.2.5 states that one of the building designer's responsibilities is to provide "The location, direction and magnitude of all dead and live loads attributable to: roof, floor, partition, mechanical, fire sprinkler, attic, storage, wind, snow drift and seismic." At this point, WTCA 1-1995 is completely voluntary and is not enforced by any agency or building code. By the end of the year [2001], however, TPI should have a new standard in place called ANSI/WTCA/TPI 4-2001 that is based on WTCA 1-1995. The fact that it will become an ANSI document does not make it enforceable but it gives the document more credibility and clout for having passed through the public consensus process. As such, it may be a good idea for the truss manufacturer to point out to the building designer the missing snow drift loading information before the trusses are designed and manufactured. This will avoid the loading from being overlooked entirely. FireTopDUCT AND LIGHT PENETRATIONS OF FIRE RATED ASSEMBLIES Question: In a small scale multifamily residential project, I'd like to use a wood truss floor ceiling assembly to achieve a one hour separation between units. I'd like to directly attach the drywall to the underside of the trusses & use the truss space for ducts & lighting (the floor above will be lightweight concrete on plywood sub-floor). UL assemblies do not seem to address the duct/light penetrations in such an assembly. Can I achieve a one-hour rating in such an assembly and how are penetrations addressed? Can the ducts in the truss space serve both units above and below? Answer: There is one UL assembly that addresses ducts and lighting - No. L529. This assembly however is for suspended ceilings with metal furring channel. You have a good question though with the upper and lower duct penetrations. We do not have any information on that. You may try contacting US Gypsum, they have been very helpful in the fire rated assembly area. Their web site is www.usg.com. FIRE ASSEMBLY FOR TRUSSES BUILT WITH 2x3 LUMBER Question: Are there any published studies or guidelines on the fire rating of floor trusses built with 2x3 lumber? Answer: Jager Industries of Canada has tested some 45-minute and one-hour rated assemblies for trusses with 2x3 material using Jager/Truswal connector plates. Alpine Engineered Products' assemblies also include the use of 2x3 members. Contact us if you would like a copy of these assemblies. The Canadian Wood Council is currently in the process of conducting fire endurance tests for assemblies constructed with various wood products. They are planning to test wood truss assemblies in the near future. WTCA has asked them to test at least one assembly built with 2x3 trusses. We will keep the industry informed as to the results of this testing. One option that will provide a one-hour assembly is to use a directly applied ceiling of two layers of 5/8-in. type X gypsum board (GA File No. FC 5406). This assembly is listed in the Gypsum Association's Fire Resistance Manual. The assembly is drawn using 2x10 joists, however, the note at the bottom of the assembly description indicates that the two layer gypsum wallboard ceiling will provide one hour protection, and therefore wood trusses can be used in place of the wood joists. FIRE ASSEMBLY RATING FOR SLOPED ROOF TRUSSES Question: A question has come up concerning sloped roof trusses and fire assembly ratings. Some are reluctant to rely on test results from flat (parallel chord) trusses applied to sloped roof trusses. Do you have any information regarding the suitability of the fire rated ceiling assemblies for sloped roof trusses? Does the "minimum depth" requirement of the parallel chord assembly apply to the minimum depth of a sloped roof truss? (I.e. heel height?) Answer: The reluctance to rely on approved test results is unwarranted as explained within Section 17 of the Metal Plate Connected Wood Truss Handbook. You will find a good description of why sloped assemblies can be used and how that should address your questions. As you know, we have all our current listings for several one-hour assemblies and a two-hour assembly. The Metal Plate Connected Wood Truss Handbook is an extremely useful tool and we hope you have a copy of it. Let us know if this information does not get you the precise information you need in a form that works for you. FIRE ASSEMBLY RATING FOR SLOPED ROOF TRUSSES Question: I'm looking at the Section 17 report on fire-rated assemblies with trusses. A number of assemblies specify "parallel chord" trusses, but the discussion on page 17-16 seems to imply that this specification is only meant to insure a minimum truss depth - deeper trusses that don't necessarily have parallel chords would meet the assembly requirements as long as the minimum depth is maintained. Furthermore, figure 17.6.1 (p. 17-29) shows "typical 1-hr. roof-ceiling assemblies" with sloping trusses. However, several assemblies don't specify a minimum truss depth at all: UBC Table 7-C Item 21-1.1, UL L542, GA FC 5515, and GA 5516. Does this mean that these assemblies, even though "parallel chord" might be specified, could actually be used for sloping trusses (i.e., typical hipped or gable roof applications)? Answer: You are correct in stating that deeper trusses that do not necessarily have parallel chords and that they would meet the assembly requirements as long as the minimum depth is maintained. Section 17 states on page 17-16, "These tested assemblies are available for specification by architects or building designers, and for use by all truss manufactures where one-hour rated assembly is required, and can generally be applied to both floor and roof applications." It further states according to the UL directory's Design Information Section, "Thus, larger and deeper trusses can be used under the auspices of the same design number. This approach has often been applied to roof truss applications since roof trusses are usually much deeper than the tested assemblies." This does mean that these assemblies listed in Section 17 can be used for sloping trusses as long as the minimum depth is maintained. You are correct, some of the assemblies do not specify a minimum truss depth, for the most part a 12" minimum is standard. FIRE DAMAGE ASSESSMENT Question: How do you evaluate if a metal plate connected truss is still usable after exposure to fire? Are there any recommended tests? Answer: There is no one way to evaluate trusses after exposure to fire. All steel connections conduct heat into the wood at some point in a fire. Truss plate connections, being steel, do as well. However, they also reflect heat for a period of time during a fire, which protects the wood below these connections. Once the plate gets hot enough, it conducts heat, and contributes to the charring of the wood below the plate and, presumably, around the truss plate teeth. Eventually, charring becomes significant enough that the truss plate loses its holding power and fails. When the char becomes great enough, the load on the truss plate connection causes the wood member to pull away from the truss plate. The fire will not cause the plate to pull or curl away from the joint. It is the load on the wood members that would cause this action. Under ASTM E119 fire test exposures, wood ignites in approximately two minutes. Charring then proceeds at a rate of approximately 1/30-in. per minute for the next eight minutes. Thereafter, the char layer has an insulating effect and the rate decreases to 1/40 in. per minute. With this information, you can often calculate the approximate time that the truss will fail under standard ASTM E119 fire exposures. Mass and surface area are the most significant factors in determining the fire endurance performance of steel. Heavy, thicker plates will have greater resistance than lighter, thinner ones. The first steps in assessing the truss damage caused by a fire are to inspect the plates and the wood. How long were the trusses in contact with the fire? Is there significant char of the wood members? Are the plates discolored? Are the plates well embedded into the wood members? FIRE DAMAGED TRUSSES Question: I have been hired by an insurance company to determine the extent of damage to roof trusses exposed to fire. How much fire damage compromises the structural integrity of the truss? Answer: As far as we know, there is no specific information available to determine the strength degradation of plates and lumber after trusses have survived a fire. We contacted some engineers in the industry to get an informal description of their methods for dealing with this situation. Some will specify repair or removal of any charred material. Some will specify repair or removal of lumber that has lost over 10 percent of its cross section due to charring. Some will allow up to 1/16" char depth on the assumption that it will not reduce the strength markedly. Lumber that is discolored by smoke damage but not charred is usually considered acceptable after it has been cleaned. If there is damage to the plate area, the plate is discolored or there is charring under the plate, the plate should be considered ineffective. Truss chords and webs can be repaired using properly sized and attached lumber scabs over the damaged areas. Joints are often repaired using plywood or OSB gussets that are properly sized and attached to transfer 100 percent of the forces in that joint. In some cases, the entire truss is replaced. All of the engineers we spoke to stressed the fact that these were not one-size-fits-all solutions and that one should consider the specific circumstances before choosing a repair strategy. Each situation requires a separate professional engineering assessment. The most conservative solution is to replace all charred or smoke damaged trusses. GA RC 2601 Question: In RC 2601, is RC-1 Channel used? Answer : As you can see from the description below, RC 2601 does NOT include resilient channel. GA FILE NO. RC 2601 1 HOUR FIRE Approx. Ceiling Weight: 5 psf Fire Test: FM FC 172, 2-25-72 GYPSUM BOARD, WOOD JOISTS, ROOF COVERING Base layer 5/8" type X gypsum wallboard applied at right angles to 2 x 10 wood joists 24" o.c. with 1¼" Type W or S drywall screws 24" o.c. Face layer 5/8" type X gypsum wallboard or gypsum veneer base applied at right angles to joists with 17/8" Type S drywall screws 12" o.c. at joints and intermediate joists and 1½" Type G drywall screws 12" o.c. placed 2" back on either side of end joints. Joints offset 24" from base layer joints. Wood joists supporting ½" plywood with exterior glue applied at right angles to joists with 8d nails. Appropriate roof covering. Ceiling provides one hour fire resistance protection for wood framing, including trusses. As far as hanging two layers of 5/8" gyp to a single resilient channel, there are fire rated assemblies that do have more than one layer of gypsum applied with resilient channel. See UL L-556 for example. INFORMATION ON TRUSS FIRE ISSUES Question: I am trying to develop a guideline for my firefighters regarding initial fire attack in buildings with light weight trusses. My concern is truss failure especially when exposed to fire. Is there any information on failure time related to flame impingement? Any information about truss failure - especially in a fire condition would be helpful. Answer: The best report on this subject is by the National Fire Protection Research Foundation entitled the "National Engineered Lightweight Fire Research Project." This is a 320-page analysis of exactly your issue. For a summary of this report please contact Lumber Specialties. PENETRATION OF FIRE WALLS Question: Can a roof truss penetrate a one-hr. tenant separation wall without having a one-hr. rated ceiling? The building official insists that the 2x4 chords are combustible and nullify the integrity of the one-hr. rated partition that is constructed in an attic above an 8" C.M.U. bearing wall. The partition consists of gypsum board attached to 2 x 4 stud framing. The building official insists that a ledger must be attached through the gypsum board to support the trusses each side of the wall. This is almost saying that any rated assembly must have bearing only on other rated assemblies. Would you want gypsum board between your truss bearing and the double top plate of a wood stud wall? Answer: You pose some very good questions. Structurally, what the building official is suggesting seems inadequate. You cannot have a gypsum board separation between a ledger and the bearing wall to which it is being attached. One solution that comes to mind is UL one-hr assembly no. U338. This is a non-structural separation of 2x4s on flat covered with 5/8" gypsum board on each side. Gable end frames (truss shapes with vertical studs) are often used with this assembly. Since the assembly is only 2.75" thick on the center of an 8" CMU wall there is 2.625" of bearing left on either side for the roof trusses and penetration of the vertical fire assembly is not required. Check with the truss manufacturer to see if this is sufficient bearing for the roof trusses. If not, they should be able to provide you with some method to achieve sufficient bearing under this configuration. SPRINKLER LOADING Question: : Does the NFPA book address sprinkler loading? If so, how and where do I buy it? What other information do you have on sprinkler loading? How do other truss companies price jobs for sprinkler loading when on the plans they do not include a sprinkler layout or even say what size/type of sprinkler system will be used? Answer: The NFPA standard will provide you with everything you need to know in order to design a sprinkler system. You can buy this from NFPA at www.nfpa.org. Unless you intend to do the sprinkler design for the building, this will not help you with the specific loads that should be applied to the trusses. Regarding your question on bidding trusses with sprinklers, ask the customer how they want the bid done since it does not include the specific sprinkler plan. Let them know that these loads can change the design of the trusses significantly depending on where the concentrated loads are placed. If this is not done, it becomes a complete guess and that can only lead to trouble. SPRINKLERS ABOVE GYPBOARD CEILING Question: I have a 29 x 72 mobile office with a 2-foot wooden truss above the ceiling that a client is required to sprinkler. Is there any way to avoid sprinklering above the gypboard ceiling? Answer: From Rodney A. McPhee, CET, CIP, SFPE- Manager, Building Codes, Canadian Wood Council: "In North America, NFPA 13 allows for such spaces in sprinklered buildings to not have to be sprinklered if the space is filled with noncombustible insulation or to have the wood materials coated with a fire retardant paint or pressure impregnated fire retardant chemical (Flame Spread Rating 25 or less)". Since this property is presumably already constructed, you're likely to have to remove the ceiling, giving you three choices: sprinkler the space, fill it with noncombustible insulation, or paint the trusses and wood deck with FR paint. TWO-HOUR ASSEMBLY UL NUMBER Question: We have a local builder wishing to use floor trusses with a 2-hour assembly. I have shown him the 2-hour design beginning on page 17-27 of the second edition of the Metal Plate Connected Wood Truss Handbook. Is this assembly UL approved? If so, what is the design number? Answer: This two-hour assembly does not have a UL number. However, it is identical to UL No L538 with the exception of I-joists being used instead of wood trusses. Both of these assemblies were developed using the Component Additive Method (CAM) which adds up the known fire resistance of each of the assembly elements to determine the whole assembly resistance. The CAM is allowed by the major model building codes (UBC, SBC, BOCA) so it should be acceptable to local building inspectors. The L538 report does not explicitly state that it is a calculated assembly but generally when Finish Ratings are given in broad terms (e.g. more than 90 min.) that indicates a calculated assembly rather than a tested one. Tested assemblies usually list a specific Finish Rating (e.g., L537 lists 21 minutes). For your information, there is another two-hour assembly available (FR-SYSTEM 1TM, FR-SYSTEM 2TM, AND FR SYSTEM 3 TM, FIRE-RESISTANCE-RATED FLOOR-CEILING AND ROOF-CEILING ASSEMBLIES WITH WOOD TRUSSES.) It was donated to WTCA by Alpine and it uses the FR Quik metal channel for blocking at all drywall edges. Handling, Installing and BracingTop60-FT. TRUSS SPAN INSTALLATION DANGERS Question: How do I, as a truss manufacturer, adequately advise my customer against the dangers of 60' and over truss span installations (i.e., exceeding the scope of WTCA'S Warning Poster)? Answer: The recommendations in WTCA's Warning Poster are based upon actual field experience but are only presented as a guide for the qualified building designer, builder, or erection contractor. It is not the responsibility of you, the truss manufacturer, to determine the exact handling, installing, and bracing of the trusses you manufacture. You offer the poster as recommendations. As you know, the warning poster notes that for trusses over 60 feet, an engineer should be involved. This is because of the increased risk involved with trusses with large spans. When you achieve large spans like 60+ ft., temporary bracing becomes an extremely important issue! As far as hoisting 60+ ft. trusses, warning poster still provide recommendations (strongback/spreader should be used). As far as temporary bracing, the recommendations still hold true. You should take extra caution on long span trusses. Know who is going to erect them and go over the warning poster with them. What you might expect when using poor temporary erection bracing methods:
You should also be aware of WTCA's Truss Technology in Building brochure on Temporary Bracing. This 4-page document is based more on actual field experience than just WTCA's Warning Poster). This document may help you explain the dangers involved in 60 foot or larger truss erection. BRACING SCHEMATICS Question: Are there any schematics available on how to horizontally brace a 7/12 pitch roof? Answer: The best review a schematic of the bracing required, view the following documents from WTCA's Truss Technology in Building Series: Web Member Permanent Bracing: Brace it for Stability, Always Diagonally Brace for Safety and WTCA's Job Site Warning Poster. CONTINUOUS LATERAL BRACING (From the December 1999 issue of WOODWORDS) Question: How important is continuous lateral bracing in roof trusses? Is it detrimental to the roof integrity if they are missing? Answer: To answer this question directly, continuous lateral bracing is VERY important. Certain truss members require this bracing in order to carry the full design load. Without it, these members may buckle at loads far less than what they were designed to support. The truss design drawings will specify which members require this bracing and the approximate location that this bracing is to be applied. It is the responsibility of the building designer to determine the size, type and method of attachment of this bracing to the trusses, as well as how this bracing is to be tied into the rest of the building. It may be possible to brace the truss members with other types of bracing, such as "T," "L," or "scabs." Contact your truss manufacturer for acceptable alternatives for any specific application. FALL PROTECTION Question: Are wood trusses designed to be fall protection anchors that would support a worker should he fall? Answer: : WTCA has a document called "Fall Protection" (part of our Truss Technology in Building Series) that explains graphically how trusses are NOT designed to be fall protection anchors. It then presents several tips for framing crews to safely and efficiently install trusses while meeting OSHA's Fall Protection Guidelines. GABLE END BRACING (From June/July '03 SBC Magazine) Manufactured gable ends are actually frames even though they are often referred to as trusses. The webs are “studs” oriented vertically and usually spaced at 12, 16 or 24 inches on center. The gable end frame is designed to transfer vertical loads from the roof to the continuous bearing wall below. Another way gable end frames are different from trusses placed in the interior of the structure is that frames experience perpendicular wind loads. The sheathed frame transfers the wind loads to the roof and ceiling diaphragms and vice versa. The roof and ceiling diaphragms transfer shear loads to the gable end frame, which transfers these loads to the end wall below and into the foundation. In order to do this, the frame relies upon well designed connections to the bearing wall and diaphragms. If the wind load is high enough and the vertical studs are long enough, the frame may also require a brace to prevent it from rotating the frame and/or buckling the verticals. This is essentially the same concept as permanent web bracing; therefore, the Truss Designer is responsible for indicating the location of the bracing for the vertical studs. The Building Designer is responsible for designing the size and attachment of the brace and how it transfers all the forces into the structure. Question: For gable ends, what is the maximum length a vertical member can be before a lateral brace is required? Answer: The maximum length of the vertical member depends on a number of factors. The first factor is the wind force which in turn depends on the code-defined wind speed, mean roof height, building category, and wind exposure due to topographical conditions. Other factors are roof loads; and size, species and on center spacing of the vertical studs. Clearly it is not a straightforward matter. Luckily most Truss Designers at truss software companies have simplified matters by producing standard tables and details based on these factors. The best way to find the answer to your specific situation is to call your Truss Designer to discuss the gable end bracing details for your project. The Truss Designer may suggest using L-braces along the length of the individual verticals, or horizontal braces that must be stabilized with a diagonal brace that extends into the diaphragms (See Figure 2 attachment is now available). These standard tables do some of the work of the Building Designer with respect to incorporating the gable end frame into the overall structural design but they do not take the place of a full analysis by the Building Designer. Other factors the Building Designer must consider are:
References:
GABLE END BRACING Question: Is there a detailed table for gable studs that gives the maximum length a vertical member can be before a lateral brace is required? Answer: Most truss plate companies have technical brochures on Gable End Bracing. You should contact your plate supplier or sealing engineer to receive this information. INSTALLATION GUIDELINES Question: Are there any associations that have recommendations for the installation of wood trusses? Answer: WTCA has a document called "Always Diagonally Brace for Safety" and "Commentary for Always Diagonally Brace for Safety" (part of our Truss Technology in Building Series) that explains graphically how trusses are NOT designed to be fall protection anchors. It then presents several tips for framing crews to safely and efficiently install trusses while meeting OSHA's Fall Protection Guidelines. INSTALLATION GUIDELINES Question: What are the requirements on the permanent bracing of bottom chords? Can gypsum board diaphragms be used? Answer: The National Design Standard for Metal Plate Wood Truss Construction (ANSI/TPI 1-1995) discusses bottom chord bracing in section 5.3.2. It is referenced in all the building codes as THE standard for MPCWT construction. So the answer is yes, gypsum board diaphragms can be used to brace the bottom chord of the truss. In high wind or seismic areas, the gypsum board may not be sufficient and properly restrained; therefore, continuous lateral reinforcement may also be required. Either way, this is something that the Building Designer needs to verify and design. INSTALLING VALLEY SETS Question: What are the requirements for installing "valley set" overlay roof trusses. I am interested in nailing and support conditions. Some Engineer's ask for the bottom chord of the valley truss to be ripped to match the roof pitch of the underlying trusses. Is this necessary? Answer: Valley sets are non-structural, but they do transfer roof dead loads and live loads to the underlying trusses. Each bearing point would have a tributary area of 4 sq. ft. assuming a 2-foot spacing for both the valley set and the underlying trusses. The typical roof total load at the high end is 60 psf which results in 240 lbs of load applied to the top chord of the carrier truss from the truss above. If we take the 240 and divide it by the compression perp value for SPF, which is the worst case, typically you get 240 lbs./425 psi. or 0.567 sq. in. and if you divide this by 1.5 you get 0.37 inches of width. Depending on the slope of the roof will determine if there is less or more than 0.37 inches across the 1.5 inch thickness to support this weight. If one also considers that the live load is rarely on the roof and the factor of safety prior to crushing for compression perpendicular to grain is 1.9. Then the 0.37 can be reduced to a very small number. For example if the dead load is 20 psf then the width is 0.066 in. ([(20psf times 4)/425] divided by 1.5 divided by 1.9). Even with all of this, we suspect that for the amount of load that is typically going to be seen there will not be crushing that will be of any significance in terms of visibility and certainly this will never become a structural failure problem. The key issue is probably getting the proper nailing done to hold the truss on the top in the proper place. Again, given the load, all the nailing has to do is to ensure that there is contact of wood to wood and keep the truss in plane. If possible it would be good to nail through the face of the top truss down into the top of the top chord in the bottom truss as perpendicular as possible and again depending on the slope the angles should allow for a pretty good connection using nails through the fact of the top truss. PERMANENT WEB BRACING (From the December 1999 issue of WOODWORDS) Question: I inspect truss installations and sometimes I see a lateral brace attached to a web on the opposite side that is shown on the truss drawing. Is this a problem? Answer: Permanent web bracing is required to provide lateral support and to reduce the buckling length of specific web members subjected to axial compressive forces due to the load conditions applied to the truss. These web members can be thought of as simple 2x_ columns that may buckle about their narrow edge (weak axis) unless they are braced. It makes no difference as to which narrow edge the bracing is attached. What is critical, however, is that the bracing is applied. PLYWOOD SHEATHING ON A CURVED SURFACE Question: We are working on the structural drawings for a building that has outside walls curved at about a 400 foot radius. Our roof trusses are on radial lines and are not parallel. We are proposing to use 3/4" plywood roof sheathing. Since the trusses are not parallel the 4'X 8' plywood panels will need to be cut so that they will splice over a roof truss. Also, the roof is not a plane surface so the plywood will have to be warped to fit the single curved surface. Does the Wood Truss Council have any experience with roof trusses and plywood sheathing as described above? Answer: We do not have any direct experience with this specific issue, although we do have experience regarding plywood. As long as the plywood sheathing is properly attached to the trusses you should not incur any problems. It is important to follow the plywood recommendations set forth by your engineer. For example, when the span of the plywood gets too long you may have to back block so that the load on the plywood will not cause the plywood to deform greater than is expected. Following the APA recommendations for plywood spans should guide you on when back blocking will be needed. You may also want to discuss your situation with the contractors and make sure that it is feasible. Can 3/4" plywood bend that much? TEMPORARY BRACING DOCUMENTS Question: Is it common practice for the supplier/distributor of a truss to provide a publication regarding temporary bracing with the delivery of the material? Answer: Yes, a large number of truss manufacturers send a copy of our WTCA's Warning Poster. This document outline the "dos and don'ts" of wood truss erection and provide temporary bracing recommendations. TRUSS BLOW DOWN Question: If trusses blew down from insufficient temporary bracing and the contractor put them back up without the knowledge of the Truss Manufacturer and gave the Truss Manufacturer a letter stating that the trusses were okay, is that sufficient? Do you know of any Truss Manufacturer who would accept this? Answer: WTCA's position: 1) If there is any question at all replace the trusses. 2) If a PE inspects the trusses, says they are okay, and signs off on it- he/she is taking on the responsibility, not the Truss Manufacturer. Other than the above points, this is dangerous and the liability is much greater than the reward. We do not know of any Truss Manufacturer or Designer that would knowingly do this. TRUSS BRACING AT SUPPORTS Question: I almost always see wood trusses erected with no stability bracing at points of support. It seems to me that common sense and NDS 3.3.3.4 require that lateral support be provided at points of bearing. Plywood decking doesn't provide any more restraint for a wood truss than it does for a roof joist. I doubt if it was a concern with short span trusses having 4" high heels, but I see lots of longer span trusses with energy heels that are not braced. At times, it is hard to draw the line between an energy truss and a bottom chord bearing truss. When a truss can clearly be identified as bottom chord bearing, there is not as much resistance to bracing it. When I tell someone that the trusses should be braced, I cannot point to a code requirement that specifically addresses this facet of bracing. Is there any published guidance on a requirement for this type of bracing? Answer: The type of permanent bracing that you are describing is part of the building design that needs to be calculated and details provided by the Building Designer. This is bracing which is ultimately part of the diaphragm design. This is not something to be left to the Truss Designer. If the Building Designer does not specify or address this situation, it would ultimately go unnoticed - which is definitely not a good thing. One can use blocking panels, OSB, plywood that runs the side of the building for bracing to counter act the trusses urge to rotate on the bearing. To answer your question, there is no specific document that covers guidelines for this specific type of bracing. It should be addressed on a case by case basis, but it should certainly be addressed. TRUSSES EXPOSED TO THE ELEMENTS Question: Occasionally, I will get a truss load from the factory that is more "weathered" than normal or I might have a load of trusses sit on the ground for three to four weeks. When should I be worried about the effect of the elements on the trusses? How much dampness can they safely be exposed to? Answer: The wood used in trusses is kiln dried (moisture content equal to or less than 19%). As far as how long can trusses go "exposed" to the elements - a ballpark number is about a month. During long-term storage, trusses need to be protected from the environment in a manner that provides for adequate ventilation of the trusses. If tarpaulins or other materials are used, the ends need to be left open for ventilation. Plastic is not recommended, since it can trap moisture. When storing the trusses horizontally, blocking needs to be used on eight to ten foot centers, or as required, to minimize lateral bending and moisture gain. Things you can look for to determine if the trusses are over-exposed to moisture: the teeth in the metal plates will actually start to pull out of the lumber and fungus and mold will grow on the trusses. Teeth back out of the lumber because the lumber swells when it is wet and shrinks when it dries down. When this happens, a few times the teeth get pushed out of the wood by this "shrink-swell" process. The plates will actually rust over time. If there is white rust it is probably not a problem but the galvanizing has begun to do its job of protecting the plate. If it is red rust there is trouble as the plate strength has begun to degrade and there is no way to stop the continuation of rusting once red rust has started. However, this process is not fast and is dependent on the amount of moisture present as to how fast it will continue. TRUSSES EXPOSED TO THE ELEMENTS Question: We are having a house built and on a recent visit we brought along a friend who builds houses for a living. He noticed that the trusses were wet and a bit moldy. The wood seemed warped, brown, and had white splotches on it. We are worried that it would later make the roof uneven. How could we tell how long the trusses have been exposed to the elements? What options do we have from here? Do we request new trusses, or can these be repaired? Answer: During long-term storage, trusses need to be protected from the environment and moisture gain. Coverings like tarpaulins should be left loose to allow for ventilation. When storing the trusses horizontally, blocking needs to be used on eight to ten foot centers, or as required, to minimize lateral bending and moisture gain. As far as how long trusses can be exposed to the elements - a ballpark number is about a month. The moisture content of the wood members does have an impact on the overall integrity of the truss. If the wood used in trusses is kiln-dried the moisture content is equal to or less than 19%. Once the trusses are installed, the moisture content of the wood will drop to its in-service moisture content, which ranges from 8-12% depending on where you live. Things you can look for to determine if the trusses are over-exposed to moisture: the metal plates will work their way out of the lumber from the repeated wetting and drying cycles. Fungus and mold will grow on the trusses if the lumber moisture content exceeds and remains above 20%. This growth will cease once the moisture content drops to 19% or less. Unequal shrinkage during the drying process and inherent lumber characteristics produce warped lumber. The trusses should not become warped due to being left out on the ground for a week. If the metal plates are still embedded in the truss, you should not worry about the look of the lumber (i.e. brown with white spots). As far as uneven rooflines are concerned, this is most often caused by mis-installation of trusses, not the moisture content. LumberTopAGE RESTRICTIONS ON WOOD TRUSSES Question: I would like to know if there is an age restriction on wood trusses? How long after production are they safe to use? If they are at least 3 to 4 months old, are they still safe to use? They have not been covered the entire time and show signs of age. Answer: There are many factors that affect the strength (which in turn affects the life expectancy) of wood. One of the main factors regarding strength is the moisture content. In most cases, the wood used in trusses is kiln dried (moisture content equal to or less than 19%). As far as how long can trusses go "exposed" to the elements - a ballpark number is about a month. During long-term storage, trusses need to be protected from the environment in a manner that provides for adequate ventilation of the trusses. If tarpaulins or other materials are used, the ends need to be left open for ventilation. Plastic is not recommended, since it can trap moisture. When storing the trusses horizontally, blocking needs to be used on eight to ten foot centers, or as required, to minimize lateral bending and moisture gain. ALLOWABLE MOLD LEVELS FOR INDOOR AIR QUALITY Question: Some questions have been raised regarding the levels of mold in the lumber used in one of our truss projects. This issue relating to 'Indoor Air Quality' is up and coming. Do you have any information regarding standard allowable values? Answer: The following information on mold and Indoor Air Quality were found at this web site: www.montana.edu/wwwcxair/bugsmold.htm "How can we test for these contaminants? It is not practical for a non-professional to test for the presence of biological contaminants. But if contaminants are suspected in the home, an investigation should be conducted to remove and control them because of the health consequences. Left unchecked, mold can continue to grow and cause health problems for sensitive people. Because there are no standards for "normal" levels of mold, tests are not usually conducted. When tests are done, however, they compare types and levels of molds in the house with molds in the outside air." An important thing to remember is that moisture content levels in lumber need to be over 19% to support mold and fungi growth and cause decay. Normal installation conditions for wood framing usually fall well below this (8 to 11% yearly average moisture content in the US). For information on mold remediation, check www.epa.gov/iaq/mold HEALTH HAZARDS OF MOLD ON LUMBER Question: What are the health hazards of black fungus on Douglas-Fir lumber? Answer: There is a PDF of a brochure from the Western Wood Products Association (www.wwpa.org) on moisture in lumber. They mention surface mold and mildew. You might also want to consult some web sites that further discuss indoor air quality. LIFE EXPECTANCY OF WOOD Question: What is the life expectancy of wood that was used in an attic truss? Does fire retardant change the life expectancy? Answer: There are many factors that affect the strength (which in turn affects the life expectancy) of wood. The strength of wood depends heavily on the direction of the loads with respect to the grain. The moisture content (MC) of the wood also will have an impact on the strength. All other things being equal, the drier the wood, the stronger it will be. Changes in strength properties occur between 0-30% MC. The MC is dependent on relative humidity and temperature. If your attic is well ventilated, this should not be a factor. As far as the direct affect of temperature, strength properties of wood generally decrease when temperatures increase and increase when temperatures decrease (inverse relationship). At temperatures greater than 200 degrees Fahrenheit strength loss is irreversible. Normally, this also will not be a factor in an attic. Even though chemical solutions reduce wood's strength, chemicals are used as preservative and fire-retardant treatments (FRT). The reduced capacity of using FRT lumber is taken into account when originally designing the attic truss. Therefore, whether or not your attic truss was built using FRT lumber, your attic trusses should not decrease in life expectancy. All things considered, if your attic is well ventilated and your trusses were designed properly taking all factors into consideration, your attic trusses should last indefinitely. There are wood structures still standing that were built centuries ago. The Gol Church at the Norwegian Folk Museum in Olso is over 800 years old. LUMBER CONCERNS IN HIGH TEMPERATURE, LOW HUMIDITY CLIMATES Question: I have been told that high temperatures and low humidity present in attics can cause deterioration of wood leading to truss failure. Is this true? And, if so, what length of time are we looking at and can we see the deterioration in the wood by visual inspection? Answer: No, these conditions will not cause truss failure. First of all, trusses are made of dimensional lumber as are conventionally framed rafters. If low humidity and high temperatures caused lumber deterioration then failures would occur in ALL wood framed roofs, not just in trussed roofs. The fact is that low humidity does not cause lumber deterioration - high humidity does. Once the moisture content of lumber exceeds 19%, decay can begin. The average yearly moisture content of standard use in-service lumber in the Southwest is only about 8%. High temperatures (over 150 degrees Fahrenheit) can cause lumber to weaken but not decay. Fortunately, that kind of temperature cannot be reached and maintained (even in an attic space located in a desert climate) long enough to cause any damage. LUMBER FATIGUE AND DECAY Question: What is fatigue in lumber and how does long term fatigue affect lumber? Does long term fatigue cause cracks in lumber? What is the difference between long term fatigue and decay? How do you determine whether lumber has been subject to long term fatigue or decay? Answer: One of the following lumber associations may be able to give you a more case specific American Forest & Paper Association - www.awc.org Canadian Wood Council - www.cwc.ca Southern Forest Products Association - www.sfpa.org Southern Pine Council - www.southernpine.com Western Wood Products Association - www.wwpa.org In addition, according to "Wood Engineering and Construction Handbook, Second Edition", McGraw Hill, fatigue is defined as the progressive damage and failure that can occur when a member or structure is subjected to cyclic, repeated loading at levels less than the static strength. It is directly related to the duration of the load. Fatigue properties are generally of little concern in many applications of wood but can become important in applications where there are many repetitions of stress. Long term fatigue can cause damage to lumber if it is not considered in the truss design when repetitions of design stress or near-design stress are expected to be more than 100,000 cycles during the normal life of the structure. The Forest Products Laboratory (www.fpl.fs.fed.us) should be contacted to obtain data on the fatigue properties of wood. Decay is completely different hazard on wood than fatigue. It results from the action of certain types of fungi, which use the wood substance as a food source. Wood-destroying fungi require favorable conditions of moisture, temperature, and access to air as well as access to wood. Lack of any of these will inhibit the growth of fungi. Usually, trusses are designed with wood members with moisture content of 19% or lower. Keeping wood below 20% moisture content is the most effective way of preventing decay. LUMBER GRADES IN TRUSS DESIGN Question: In truss manufacturing, can hemlock, fir and spruce be used instead of Douglas Fir/Larch? What grades can be used: Structural Light Framing (Select Structural through #3) only, or Light Framing (Construction through Utility) as well? Answer: Yes, species combinations like Hem-Fir and Spruce-Pine-Fir are used regularly in truss manufacturing as well as Southern Yellow Pine, and as mentioned, Doug Fir-Larch. It generally depends on the part of the country and what is in most plentiful supply. Structural framing grades like Select Structural, No. 1, No. 2, No. 3, MSR, and MEL are used. It is important to make sure that the proper lumber size, species and grade chosen in the truss design program are being used because that information is used to pull lumber and plate strength values from the programs "lumber file" in order to design the truss. If the species and grade used does not included in the lumber file, it may mean that the plate manufacturing company does not have data on plate strength values in that particular type of lumber. At that point, contact the plate manufacturer for more information. LUMBER GRADES IN WOOD TRUSSES Question: The grading rules usually permit some percentage of visually graded lumber to be below grade - I believe it is 5%. ANSI/TPI 1-1995 requires that all lumber used in trusses be of the grade specified by truss design. A strict interpretation of ANSI/TPI 1 would require that the fabricator cull the 5% that is off grade. What are your thoughts on this? Answer: The 5% tolerance is to account for variations in lumber grader judgement and for human error. It is not there to intentionally assign inaccurate grades to lumber. Truss manufacturers buy graded lumber - it is not a reasonable expectation to have them cull through lumber or essentially "re-grade" the lumber they buy. They rely on the grade to be accurate. The fact that the lumber is marked with the grade stamp specified on the truss design is enough to meet the intent of the ANSI/TPI 1-1995 provision 4.3.1: "Truss lumber shall be of the minimum grade, size, moisture content and species combination specified by the truss design." However, any grade-allowed lumber defects such as wane or knots that occur in plated areas must fall within the quality tolerances outlined in section 4.5 of ANSI TPI 1-1995. LUMBER SIZES AND PROPERTIES Question: Where can I find tolerances and measurements of green and dry lumber? Answer: The Western Wood Products lumber guide can be downloaded from their web site at www.wwpa.org. You can find sizes of dry and green (unseasoned) lumber on page 13 of the guide. There is also quite a bit of very good information in the guide. Keep in mind that this is only one species group (see the last page of the guide for species included under the "Western Woods" category). You will have to contact other wood marketing agencies or any grading rules if you require information for any other species not listed there. LUMBER SPECIES Question: What type of wood is usually used for wood trusses? I live in Louisiana and I would guess pine, but I want to make sure. Answer: It depends on what type of wood is available in your market. You are right in that Southern Pine is predominantly used in the southern states such as Louisiana. Truss manufacturers in the western states may have better access to Douglas-Fir and Hemlock-Fir species and use those. Truss manufacturers further north may have better supplies of Canadian Spruce-Pine-Fir. Many different species can be used in truss construction as long as the lumber strength properties and plate grip values used in the software design program reflect the materials used in the truss. The National Design Specification for Wood Construction (NDS) lists different species or species combinations and their corresponding design values which are used in the design of metal plate connected wood trusses. This document is available through the American Forest & Paper Association - American Wood Council (www.awc.org). MOISTURE CONTENT EQUILIBRIUM AND TRUSS MOVEMENT Question: How long does it take the moisture content to stabilize in a wood truss? I live in Southern California. Also, how much deflection should be expected in a scissors truss over a partition wall? Answer: Moisture content of lumber is dependent on the temperature and relative humidity of the surrounding environment. Most lumber used in California is S-GRN D-Fir. That means that the trusses are designed with lumber values corresponding to a moisture content greater than 19%. Most truss manufacturers use kiln dried lumber, which has 19% or less moisture content. In most parts of the US including Southern California, you can expect that to drop to a yearly annual average of 8% after the building is closed in and finished. We have no information on how long it takes lumber's moisture content to reach equilibrium with its environment but have heard that new homeowners can expect movement and settling in wood structures for up to two years. Vertical deflection limits for trusses are outlined by the Uniform Building Code as L/240 where L is the span of the truss in inches. However, flat bottom chord roof trusses generally exhibit much less vertical deflection than this. Horizontal deflection is usually more pronounced in scissors trusses. According to ANSI/TPI 1-1995, The National Design Specification for Metal Plate Connected Wood Truss Construction, the amount of horizontal deflection for a truss is generally limited to 1.25" for total load (which in Southern California the truss will rarely see). Maximum dead load deflection is limited to 0.75". MOLD Question: I am looking for some information regarding black surface mold on lumber. Does it alter the integrity of the lumber stress values? Answer: No, mold or stain generally does not have an effect on lumber values. It may be a warning flag, however, for high moisture levels, which CAN cause problems. If the moisture content of the lumber is over 19% and stays at this level, two things will happen: lumber strength decreases and decay can begin. If trusses are being designed and manufactured under wet service conditions (over 19% moisture content), then you should check with your plate/software vendor to see if the trusses should be designed as such; even if you expect their moisture levels to drop to below 19% after they are in service. The Western Wood Product Association (www.wwpa.org) publishes a pamphlet called "Unseasoned (Green) Lumber" that discusses moisture and mold in more detail. It's available as free download in the Technical Publications section of their web site. TREATED LUMBER IN WOOD TRUSSES Question: Can one use CCA treated lumber in metal plate connected truss construction? If so, are special plates or coatings required? Answer: If the lumber is kiln dried after treatment, the CCA is inert and normal plates can be used. The key is drying the lumber and keeping it dry in use. VISUAL GRADING CRITERIA Question: Where can I find a copy of the criteria used to visually grade lumber? Answer: There are standard grading rules for each lumber category all of which have been approved by the American Lumber Standards Board of Review. The standard grading rules for lumber stamped by the Northeastern Lumber Manufacturers' Association represents: Eastern White Pine, Red Pine (Norway Pine), Eastern Spruces, Eastern Hemlock, Tamarack, Northern White Cedar, Balsam Fir, Aspen, Jack Pine, Pitch Pine, Eastern Softwoods, Spruce-Pine-Fir south, Red Maple, Mixed Maple, Beech-Birch-Hickory, Mixed Oaks, Red Oak, Northern Red Oak, White Oak, Yellow Poplar This standard can be purchased from: 272 Tuttle Road; PO Box 87A; Cumberland Center, ME 04021 207/829-6901 207/829-4293 (fax) Douglas Fir, Hem Fir, Western Red Cedar, Spruce-Pine-Fir South This standard can be purchased from: PO Box 23145, Portland, Oregon 97281 503/639-0651 503/684-8928 (fax) 522 SW 5th Ave, Suite 500; Portland, Oregon 97204-2122 503/224-3930 503/224-3934 (fax) 4709 Scenic Highway; Pensacola, Florida 32504 904/434-2611 960 Quayside Drive Suite 406, New Westminster, BC Canada V3M 6G2 604/524-2393 WET LUMBER (From the March 2001 issue of WOODWORDS) Question: I am looking for some information regarding black surface mold on lumber. Does it alter the integrity of the lumber stress values? I have a client who wants to use "all dry" lumber for his construction but I do not know if this is really necessary. Answer: Generally, mold or stain does not have an effect on the lumber values. However, it may be a warning sign of high moisture levels that definitely can cause problems. If the moisture content of the lumber is over 19 percent, two things will happen: lumber strength will decrease and decay can begin. If trusses were being designed and manufactured with green or wet lumber (over 19 percent moisture content), check with your plate/software vendor to see if the truss design should be modified. Do this even if you expect the moisture levels to drop to below 19 percent after they are in service. WET LUMBER (From the March 2001 issue of WOODWORDS) Question: I would like to know if there is an age restriction on wood trusses? How long after production are they safe to use? The trusses in question are at least three to four months old. Are they still safe to use? They have not been covered the entire time and are showing signs of age. Answer: There are many factors that affect the strength, which in turn affects the life expectancy, of wood. One of the main factors regarding strength is the moisture content. The wood used in trusses is kiln dried (moisture content equal to or less than 19 percent). In the event of long-term storage, trusses need to be protected from the environment in a manner that provides for adequate ventilation. If tarpaulins or other materials are used, the ends need to be left open for ventilation. Plastic is not recommended, since it can trap moisture. When storing the trusses horizontally, blocking needs to be used on eight to ten foot centers, or as required, to minimize lateral bending and moisture gain. Things you can look for to determine if the trusses are over-exposed to moisture: the teeth in the metal plates will actually start to pull out of the lumber, fungus and mold will grow on the trusses, and the truss plates may begin to show signs of rusting starting with white rust that will eventually migrate to red rust. Anytime you see red rust, it is time to take a much closer look at the trusses. Metal Connector PlatesTopAVERAGE PLATE VALUES Question: I am a Structural Engineer trying to locate average plate values such that I can recreate truss designs for a roof structure that is over two decades old. The plates are not marked with any type of identification, therefore I need to take a conservative approach in the design and determine worst case scenarios. Answer: The ICBO link below is to all the plate's evaluation reports. Select the "All reports with an ER prefix, listed by number." link and pick the following numbers to get the report. ICBO ES Evaluation Reports Alpine ER-5352 (WAVE plate) Cherokee's SBCCI ER-9765 Computrus ER-4211 Eagle Metal ER-4420 Jager ER-4805 (Canadian) MiTek ER-4922 Robbins ER-4994 Tee-Lok ER-5243 TrusWal ER-1607 DETERMINING TRUSS MANUFACTURER Question: I am trying to determine the manufacturer of some wood trusses installed in the early 1980's in New Jersey. The stamp mark on the connector plates reads, to the best of my ability, TPL-153A (it could be PPL or TPI, but I am unsure). I need to get in contact with this truss manufacturer in order to determine design loads for the truss, as I am remodeling the store under the roof. Answer: We contacted the Truss Plate Institute (TPI) to gain their perspective. From the information you provided, they were unable to determine the truss plate manufacturer. One thing to keep in mind, the stamp marks on the metal connector plates can only at best provide the information to determine the metal plate connector manufacturer. This information is not going to help to determine the truss manufacturer. Many different truss manufacturers use the same metal connector plates. We realize that the trusses in question are about twenty years old, but you should try to locate some type of inspection stamp on the truss itself. This stamp, possibly on one of the chords, would contain the plant number which in turn would allow you to backtrack the original manufacturer of the truss. Misc. Truss InformationTopATTIC VENTILATION Question: I need to put a new roof on my single-family dwelling. The house originally had gable and soffit/eave vents. The Roofing Contractor suggested that we install a ridge vent when new roof is installed. Should the existing gable vents be blocked off or does it not matter? I have read some debates about whether or not the combination of gable and ridge vents substantially reduces the effect of the soffit vents by having the intake now at the gable vents and exhaust at the ridge. Would this type of ventilation affect the truss warranty? Answer: WTCA is a trade association that represents metal plate connected wood truss manufacturers all over the country. Any truss warranty that you may have will be with a specific manufacturer, so we really cannot comment on the terms of the warranty. We will say, however, that the design assumption for all wood framing in structures is that the moisture content will be below 19%. Most structures reach average moisture contents of 8-11% after construction. As long as the ventilation does not allow moisture build-up in the attic, the trusses should be fine. This is really a question of whether or not the new ventilation configuration will allow moisture build-up in the attic. We really do not have any expertise in that area so your question was forwarded to an engineer at the National Association of Homebuilders (NAHB) Research Center. Their comments are below: “General wisdom on attic ventilation is that you can't have too much attic ventilation. I suspect that the combination of the gable ventilation and soffit ventilation is enough without the addition of the ridge ventilation, but the addition of the ridge ventilation shouldn't hurt the ventilation. I would leave the existing ventilation open”. BELOW GROUND STRUCTURES Question: Our scout troop has been helping in the construction of a new youth camp. The basic idea is to make the place as traditional as possible, but still accommodate contemporary needs. We need to construct a shelter that is safe from storms, yet as unobtrusive as possible. Since the area is hilly, we thought a below ground structure built into the top of a hill would make sense. Kind of a dugout if you can envision it. It would not be visible from the main part of camp, but could be readily accessed as needed. It would need space for 20 to 30 people in a pinch, have a washroom, kitchen, and serving area. We thought something about 16 ft. by 32 ft. would work. We have been getting some information on wooden foundation structures from the Southern Pine Marketing Council, but haven't found any information on trusses for roofs that are covered by soils. We would greatly appreciate any suggestions that you can give on finding sources of design information or materials. Answer: As you know, this is an unusual application for metal plate connected wood roof trusses. Your local building code will have specifications that you will need to follow for below grade structures. This means that you will probably need to use treated plywood and trusses. The loads may be very high depending on the soil depth. You will also need to tend to the water and drainage. There will also be long term creep that you will have to account for. The best advice that we can give you is to hire an engineer (or find a student engineering group like ASCE, Chi Epsilon, or Tau Beta Pi to see if they would like to tackle a project like this) to do this work, as this is quite a bit more complex than it looks on the “surface”. CHANGES IN ROOF PITCH Question: We are changing plans from a two story home to a one story home. The original roof pitch is 12/12 with 3 dormer windows, one large in center and two smaller on either side. Can you tell us what pitch would be advisable under this new plan still using the dormers? Answer: Since we're not sure whether or not your home will be conventionally framed or framed using components. Using metal plate connected wood trusses, you can design to almost any pitch you would like. We have seen truss pitches range anywhere from 0.25/12 to 24/12. Practically any type of roof layout you can imagine can be designed using trusses. Are you using attic trusses in your design? Attic trusses are built such that the opening in the center can be useful for storage or additional living space applications. If you are using this type of design, the rule of thumb is the steeper the slope- the larger the room. You need to consult with the building designer to determine what pitch would be advisable with your changed plans. Your pitch is dependent upon the dimensions of your roof and what type of roof profile you are trying to achieve. COMPARISON OF STICK BUILT VS. COMPONENTS Question: Does the WTCA produce any products detailing how to market and sell roof and floor trusses to builders whom stick framing has been the standard way of building? Products that detail the cost savings when compared to traditional stick framing? Answer: WTCA sponsored a demonstration project called Framing the American Dream® at the 1996 NAHB Homebuilders Show. This project consisted of two identical 2,600 sq. ft. houses built side by side - one was completely stick built and the other was component built from floor and roof trusses and wall panels. What We Learned by Framing the American Dream is a four-color brochure that outlines the time and cost savings on the component built house. Time-lapsed videos of the project are also available. DECK LEDGER BOARDS Question: Is it possible to attach deck ledger boards to the ends of floor trusses? If not, are there any modifications that would make this possible? Answer: The best person to answer this question is the truss manufacturer's engineer. Any extra attachments (like deck ledger boards) or extra loading should be supplied to the manufacturer up front so that the trusses can be designed to accommodate these details. DRILLING TRUSSES Question: Can you drill small holes in trusses for electrical wires? Answer: Trusses should never be modified in any way without first receiving approval and an engineered "repair detail" from the truss manufacturer reflecting the modification. You should contact your truss manufacturer to get this information. DRYWALL CRACKING UNDER SCISSORS TRUSSES Question: We keep having problems with dry wall joints rising on the vaulted bottom chords in our track homes that we are building. The joints run from front to back of the house and the trusses bear on the side walls. The trusses are mostly 2-point bearing (a few in the front and back of the building are tri-bearing). The trusses span 40 ft. and have a 4/12 pitch with a tile roof. Is it common to have dry wall problems as the trusses deflect? How long should the roof be loaded before hanging sheetrock? Should the heel of the truss be allowed to slide out on the top plate? Answer: There are a couple things that could be going on in this situation. The first is partition separation, which describes the gap between partition walls and truss bottom chords. You can read more about this subject under the FAQ - Partition Separation section. The second thing is horizontal deflection in scissors trusses. This is an outward thrust that occurs in all vaulted framing. This seems less likely as it is not clear how this would manifest itself as raised drywall joints. A good article on horizontal deflection is posted on the Alpine web site: www.alpeng.com. Make sure that the truss manufacturer knows that there is a tile roof on these houses as the loads for them are quite a bit higher than for asphalt shingles. Make sure that all necessary permanent bracing is installed. Keep in mind that you can always request a higher deflection criterion on the truss. This means that you will get a stronger truss that will deflect less under load, but it will increase production costs. EFFECTS OF WOOD TRUSSES EXPOSED TO IMPROPERLY VENTILATED ATTICS Question: I am a structural engineer currently assessing the structural condition of a 2 story wood framed home. The home has approximately 50% on the available attic ventilation as outlined in the applicable building code. The home utilizes wood trusses with metal connector plates to support the roof. Wood floor trusses (2x4, 14" deep) support the 2nd floor. What does the lack of attic heat dissipation do to the structural capacity/longevity of the home? The home is 4 years old. Answer: According to the National Design Specification for Wood Construction (NDS), lumber in roof systems needs to reach temperatures in excess of 150 degrees Fahrenheit for a sustained period of time for permanent lumber damage to occur. NDS also states that this seldom happens even in the severest of attic conditions. What may be more damaging in the roof system you mention is moisture. Insufficient ventilation in attic spaces may lead to abnormally high moisture conditions that can cause problems for lumber and metal connectors (e.g. nails, joist hangers, truss plates). High moisture contents lead to reduced lumber strength properties and prolonged high moisture can lead to decay of lumber and corrosion of metal. If you are located in a heating climate then you have to consider what's happening to moisture in the under-ventilated attic in the winter. Excessive moisture build-up can cause movement in wood structures and water leakage into the structure. ENERGY HEELS Question: What is the definition of an energy heel? Answer: There is no one definition of an energy heel. It is simply higher than a standard heel truss. Energy or raised heel trusses are used for the purpose of better insulation and ventilation. Energy heel trusses help with ventilation, which can reduce moisture and heat build-up in the attic. The high heel also helps the insulation retain its R-value by not compressing it in to a smaller space. This reduces thermal bridging and "cold corners" at the joint between the wall and roof. In winter climates, energy heels are a good way to go. FLOOR TRUSSES WITH MASONRY BEARING WALLS Question: We would like to know if there is a problem with using wood floor trusses with masonry bearing walls without a ledger. Our local supplier has told me that encasing the wood truss in the masonry wall could be detrimental to the truss due to moisture. What if any suggestions do you have for this condition? If a bottom chord bearing is used, do the trusses need fire cuts? Please provide me with any details you might have that pertain to this condition. The two-story building contains masonry bearing walls with wood trusses on the second floor and wood trusses at the roof. Answer: You can use wood floor trusses with masonry bearing walls without a ledger. We suggest that you use a bucket hanger that can be encased in the masonry wall and used as a bottom chord bearing for the floor truss. A detail of this can be found in the Metal Plate Connected Wood Truss Handbook in Section 16 Standard Detail HS page 16-34 or on our web site. If you still wish for the bearing of the truss to be on masonry, the truss should be fire cut to allow it to rotate out of the wall if it burns through. The truss must also be anchored against pulling away from the wall, either with lag screws or a lug on the iron bearing wall. The connectors that you would need are available through any connectors catalog. FRAMING GABLE END OVERHANGS Question: What is the recommended way to frame gable end overhangs? Answer: 3. 2 x 6 ladders with two dropped trusses should be used for overhangs up to 3'. 4. In all instances, it is a good idea to have the ladder extend into the roof a distance at least equal to the overhang. IMPACT SOUND TRANSMISSION THROUGH FLOOR ASSEMBLY Question: A condominium project I am involved with is experiencing a sound transfer problem through the floor/ceiling assembly between the first and second floor. You can hear every footstep from an adult to a child. The floor assembly consists of a carpet and pad, ¾-in. OSB sub-flooring, 15-in. deep wood trusses at 16-in. on center, 9-in. fiberglass batt insulation and a 5/8-in. gypsum board ceiling. The trusses span 20 ft with air/heat ducts between the trusses. Will an additional layer of 5/8-in. gypsum board attached to 7/8-in. furring strips oriented perpendicular to the trusses, minimize the sound of the footsteps? If not what would be the way to go with this problem? Any suggestions would be appreciated. Answer: Based on the background information you provided, it appears your problem has more to do with impact sound transmission rather than airborne sound transmission. Impact sound transmission occurs when a structural element is set into vibration by direct impact (i.e. footfall). The vibrating surface generates sound waves on both sides of the element. The Impact Insulation Class (IIC) is a method of rating the impact sound transmission performance of an assembly. The higher the IIC rating, the better the impact noise control of the element and assembly. IIC values for various construction materials may be found in several publications, including WTCA's Metal Plate Connected Wood Truss Handbook. The impact insulation characteristics of a floor/ceiling assembly can be calculated by adding up the IIC values of the individual components. The IIC value of the assembly described above should be approximately 53-58, depending on the thickness of the carpet and pad. The addition of resilient and another layer of 5/8-in. gypsum board will increase the IIC rating of your assembly by 10 to 12 points. Whether this increase is enough to eliminate the impact sound problem is hard to say, but it certainly will help reduce it. You might also consider adding a sound deadening board to the ceiling, as this can significantly improve the impact sound performance of the assembly depending on the type you choose. The National Research Council of Canada is currently doing quite a few sound transmission tests. Information is available from Canadian Wood Council at 800/463-5091. American Forest and Paper Association may have information at their help desk number at 202/463-4713. The Forest Product Laboratory has several publications on sound transmission (608/231-9200). The Gypsum Association has assembly information in its Fire Resistance and Sound Control Design Manual (202/289-5440). The Truss Plate Institute has assembly information in its ANSI/TPI Appendix E (608/833-5900). Finally, WTCA's Metal Plate Connected Wood Truss Handbook has calculation and assembly information. IN-FLOOR RADIANT HEAT Question: Do you have any information on the use of in-floor radiant heat with 4x2 open web floor trusses? Are there any adverse effects to the lumber as a result of moisture build up? Answer: We do not have anything that addresses radiant heat with 4 x 2 open web floor trusses. There would be no more adverse affects with 4 x 2 open web floor truss than with a 2 x 10. Lumber is lumber and as long as the pipes are installed properly (i.e., no leaking), there should not be a problem with using 4 x 2 open web floor trusses. INFORMATION ON FLOOR TRUSSES Question: I am researching building products for a new home and I wanted to use metal plate connected floor trusses. When I took my plans to the builder, they cautioned me against using these floor trusses. They stated that they were not engineered products, they do not carry any kind of warranty, and they do not know how long they last over time. My basement span is 30 ft., they said trusses could not span 30 ft. I really would like to use floor trusses, but now I am unsure. Could you offer any information? Answer: Metal plate connected wood floor trusses ARE engineered products, have been around for over 30 years with a proven design and testing to back it up, and can span 30 ft. Some background: Introduced by Truswal Systems Corp. in 1970, engineered parallel chord floor and roof trusses are manufactured throughout the United States. They offer strength and rigidity over long spans. Like pitched chord trusses, they offer design flexibility. With the use of parallel chord trusses, room size is not limited to wood joist spans. The open web member system permits running plumbing, duct work and wiring inside the trusses. For the designer, parallel chord trusses provide long, stiff, clear spans in floors and ceilings, engineered to an optimum depth for the project. The advantages of using parallel chord floor and roof trusses are as plentiful as the advantages of using pitched chord trusses:
PCT trusses are used as both roof and floor trusses. 4x2 PCT trusses are used as floor trusses more commonly than PCT trusses that use 2x4 lumber on edge. ANSI/TPI 1-1995 discusses criteria for the determination of secondary stresses assuming frictionless pin joints with primary (axial) forces determined by static equations or graphical analysis. More exact methods of analysis, such as PPSA4, are recommended for designing more complicated trusses. Design aids that assume an equivalent beam for approximating PCT member forces are shown in ANSI/TPI 1. Web member forces are determined by the method of shears that involves use of the beam shear diagram. Unknown member forces are determined by using the ordinate of the shear diagram, the web member angle and web member vertical component. Maximum allowable reactions for PCT trusses have been recommended based on specific bearing details. Truss bearing clearances and connector plate unsupported contact area are provisions that have been considered. The guidelines are included in ANSI/TPI 1. One of our products that you may find useful is the Floor Truss Fact Sheet. In addition, any of our members would be more than willing to assist you with your floor truss design. INFORMATION ON TRUSS HISTORY Question: For one of my classes, I need to write a report on the history of trusses. Your web site has been a great help to me and appreciate that very much. Do you have any additional information? Answer: Glad to hear you found our web site useful. The invention of the first metal truss connector plate in Florida in 1952 by A. Carroll Sanford (founder of Sanford Industries in Pompano Beach, Florida) marked the beginning of the industry that in just four decades has vastly changed home, apartment and commercial building construction. Section 2 of the Metal Plate Connected Wood Truss Handbook addresses the Evolution of the Industry. The plate suppliers also have a wealth of information. After all, as you will find from reading Section 2, it was the invention of the metal connector plate that spurred the industry. Presently there are eight plate suppliers. You should contact them to obtain additional information. The Truss Plate Institute (608/ 833-5900) also would be a great source for information on our history. JOBSITE INSPECTIONS (From January/February '03 SBC Magazine) Consider this statement from a set of construction contract documents: “The Truss Manufacturer is responsible for inspecting the truss installation, bracing, anchorage and bearings and preparing a letter of verification stating that trusses are installed and braced properly.” At first glance, it may seem that the Truss Manufacturer is the most logical party to perform such an inspection. They are the truss experts, are they not? But on second thought, why is the Truss Manufacturer singled out to perform installation inspections of their products when they have no control over the installation process? Is the same service required from the window supplier, the concrete supplier or the plumbing contractor? There is no good reason for a Truss Manufacturer to supply framing inspections. If the justification for the request is the lack of knowledge on the part of the end user or building inspector, then education is in order instead of more risk exposure for the Truss Manufacturer. Consider the following question and answer the next time you are asked to provide a jobsite inspection. Question: By sending an independent engineer to inspect trusses, beams, and hardware that we, the Truss Manufacturers, supplied, could we be held responsible for other components of the building or the entire building itself? Also, where would the responsibility of our inspecting engineer end if he or she noticed, for example, a column supporting a girder was not adequate? Answer: Aside from a truss collapse or claim relating to your trusses, from a risk management and liability standpoint, it is not recommended that the Truss Manufacturer undertake jobsite inspections, through a professional engineer or otherwise. If the requirement to inspect is already part of your contractual obligation and cannot be avoided, then the inspector must do the job thoroughly and comply with the obligation stated. Otherwise, if you choose to inspect, it is important to clearly define the scope of your investigation and any limitations. For example, the report should clearly define what was inspected and what was not. If you are named in any possible future construction defect litigation, a well defined scope of inspection may be the only thing that will protect you against assertions of a failure on your part. If the inspector is an engineer, he is likely to be obligated to report any condition that may involve an issue of life safety. Another problem with supplying an inspector is there may be no contractual obligation on the part of the inspector or framer as to who will do the work to comply with the inspection recommendations. A better solution is for the builder or framer to hire the inspector who then reports to them. Ultimately, the local building official is responsible for the framing inspection and ensuring it is in compliance with the plans and the code. LONG-TERM PERFORMANCE OF METAL PLATE CONNECTED WOOD TRUSSES (From the September/October 1999 issue of WOODWORDS) Question: How do trussed roofs perform in long-term service? Are there any reports of rusting in the nail plates, rot/insect attack, do-it-yourself modifications, or condensation and ventilation problems in the attic? Answer: Metal plate connected wood trusses have been available as a building component in the United States since the early 1950’s. The long-term performance for properly designed and installed trusses is outstanding. Today, wood trusses are used in the roof systems of over 60% of all buildings built in the U.S. Metal plate connected wood trusses are susceptible to any in-service or environmental condition(s) that adversely affect the wood and/or metal connector plates. Elevated moisture, insects, decay, condensation and ventilation issues must be considered when using trusses, just as these issues must be considered when using other building materials made from wood and/or steel. Fortunately, these issues can be handled easily and cost effectively during the design stage for both the building and trusses. Trusses are a manufactured component product and should not be field modified without prior approval from the truss manufacturer. WTCA developed a comprehensive reference manual called the Metal Plate Connected Wood Truss Handbook that contains considerable information about wood trusses and how to properly use them. PURLIN DESIGN Question: Do you have information on purlin grades and species to meet 50 psf top chord live load using 2x4s on edge 24" o.c. over trusses at 6' o.c.? Answer: WTCA does not have any information on purlin sizing. Purlins are part of the permanent bracing system, and as such, a Building Designer responsibility. Contact the National Frame Builders Association (a trade association of post-frame building professionals) for more information on purlin requirements. Their contact information is: National Frame Builder’s Association (NFBA); 4840 W. 15th St., Ste. 1000; Lawrence, Kansas 66049; 785/843-2444; 785/843-7555 fax; www.postframe.org REMEDY FOR BOUNCY FLOOR TRUSSES Question: We are experiencing problems with bouncy wood floor trusses. I'm wondering what the industry standard is on deflections (live and total load). Also, do you have any ideas on how to decrease the deflection without affecting the profit margin significantly? Any ideas would be extremely helpful. Answer: Floor trusses are built to have a certain amount of deflection/ stiffness. Structural designers describe stiffness in terms of deflection criteria. You may have heard of the terms L/360, L/480, L/600, etc. This is the deflection level the designer or customer chooses and the individual floor or roof component is designed to meet or exceed that level. The building code generally requires at least L/360 for floor components regardless if they are sold sawn joists, I-joists or trusses. The L in this equation represents span, so if you divide the span by 360, that is the expected amount of deflection the truss will experience under initial full live loading. For example, a 20' truss is 240", 240/360 = 0.67", therefore, a 20' truss designed to L/360 deflection criteria would see 0.67" of deflection. The same span truss designed to an L/480 deflection criteria would see 0.5" of deflection. THIRD PARTY INSPECTIONS Question: Are third party inspections required by the state or in accordance with building codes or both? I know that certain states require some but I do not know if it is due to the state law or the building code. Answer: There is not a state that we are aware of that has a law regarding third party inspection -- the requirement comes form the building code that is adopted at the time and this has the force of state law we suspect. The final controlling authority in this case is the local building code official who can really require anything he wants as long as it fits into his/her interpretation of the code language. In this case, the model codes say that the building official can ask for special inspection if a third party inspection process is not in place in the plant. TRUSS UPLIFT Question: What is truss uplift? Answer: Truss uplift describes two completely unrelated conditions. The first truss uplift, as in truss arching, is one of the causes of Partition Separation. WTCA has a publication in our TTB series on Partition Separation that describes this occurrence and ways to prevent it. What the brochure does not emphasize is that when truss arching occurs it is because of a moisture content difference between the top chord and bottom chord of the truss. One easy way to avoid this is to ensure plenty of ventilation of the attic space, do not discharge household vents into the attic, and ensure that ventilation areas are not blocked by insulation. The second uplift describes the tendency of framing (trusses, rafters, joists, etc.) to lift off their supports due to wind forcing them upward, or due to multiple bearings or cantilevers. These uplift forces are generally listed on truss design drawings. In some cases, toe nailing the framing to the top plate does not provide enough resistance to the uplift force and a mechanical connector like a metal hurricane tie, anchor or clip must be used. WTCA also has a publication our TTB series on Toe-Nailing which describes this more completely. WEIGHTS AND TERMINOLOGY OF TRUSSES Question: How do I go about estimating the weight of a roof truss framing system? Is there a table of span vs. pitch vs. weight of trusses? Is there a guide available that shows how the components of various roof styles (hip, gable, etc.) are commonly framed and the associated terminology? Answer: Depending on the size of the members (top chords, bottom chords, webs) there is a corresponding weight per linear foot. Multiplying this number by your total span should give you a rough estimate of the weight of the trusses themselves. (VIEW CHART that can be used to determine the weights of both floor and roof trusses.) Another resource is WTCA's Truss Technology in Building: Truss Configurations brochure which addresses differenct truss configurations. WOOD FRAMING (From the December 2000 issue of WOODWORDS) Question: I am conducting an investigation on the costs associated with building a 15,000 sq. ft. addition to an existing school building. I need to determine if wood trusses, steel bar joists or light gauge steel trusses would be the most economical material for the building system. The truss spans range from 42 ft. to 56 ft. Please respond if you have any suggestions on resources or helpful advice. Answer: All other factors being equal, wood is usually the least expensive of the three building systems that you list. To get an accurate cost estimate for this project, we recommend that you contact a truss manufacturer in your local area. Go to the Members' Section to search for members in your area.) One point that you may need to explore further is if your building code permits wood as a construction material for that specific type of construction. Under some codes, there are restrictions for building heights and areas based on construction type (in this case wood framing), occupancy and use (educational), and level of fire protection (are fire rated assemblies or sprinklers specified?). To determine if wood is an allowable construction material for a 15,000 sq. ft. school, you can use a nifty little program called Code Conforming Wood Design from the American Wood Council. It is available as a free download at www.awc.org. You can enter the building parameters like height, area, number of stories, occupancy and use. Then, based on your model building code, the program will list all viable wood framing options. Partition SeparationTopLAG BOLTING Question: Is lag bolting the bottom chord of a truss to the top plate of a wall a proper fix to correct truss arching assuming the attic is properly vented? Answer: We, as an industry, do not suggest attaching the bottom chord of a truss to the partition wall as a proper preventative measure against truss arching. This will not prevent the truss from arching in response to the moisture content differential between the top and bottom chords. In some cases, the attachment of bottom chord to the partition wall has lifted the entire wall assembly as the truss arches. PARTITION SEPARATION Question: Question: I have read a lot of information on partition separation. Nothing so far has discussed the subject as it applies to nonbearing walls with scissor trusses. I have scissor trusses in my bedroom and I would like to add non-bearing walls for a closet in the room. Do the same techniques apply to the scissor truss as a common truss? I'm thinking of the deflection of the scissor trust and how it will affect my drywall. Answer: You are probably well aware of the mechanisms of partition separation. If you are not experiencing it in the rest of your house, it would be safe to assume that it will not occur in the portion with scissor trusses. If it is a big concern, you may want to consider employing the floating drywall corner details from WTCA's Partition Separation Brochure. What may be of more concern is the scissor truss movement you mention or horizontal deflection. This is the tendency of all vaulted framing to thrust outwards. On shorter spans (<40 feet) it is usually not a noticeable problem but you may want to account for that when you are framing up the partition wall. Many framing connector companies manufacture a product like this. The trick here is you have to decide which is more of a concern the vertical or the horizontal movement? You can't allow for both unless the top of the wall remains completely unattached which would not affect the truss but would leave the top of the wall laterally unstable. PARTITION SEPARATION (From the December 1999 issue of WOODWORDS) Question: I installed roof trusses in my house and dry walled the ceiling to the bottom of the trusses. Now I seem to have a problem with the change of the seasons, where the ceiling meets the interior wall a gap opens and closes-in winter it opens, in summer it closes. What did I do wrong, how can I correct it, and how should the drywall be installed in the first place? Answer: It sounds as if you are experiencing a partition separation problem. This can be caused by several factors working either independently or in combination with one another. The possible causes include building settlement, inconsistent framing, moisture effects, deflection and truss movement. The fact the problem appears to be seasonal leads me to believe that moisture is involved. Do you have enough ventilation in the roof plenum? If you do not, the trusses could be arching due to a substantial difference in moisture content between the top and bottom chords. The easiest way to check if this is part of the problem is to measure the moisture content of several truss chords with a hand-held moisture meter. You should also consider using a level to check for settlement of bearing walls, headers, beams and floor members. WTCA has a brochure on partition separation that discusses this phenomenon in greater detail (contact us for a sample copy). This brochure contains some wall corner details that allow the gypsum to "float" with any differential movement between the wall and the ceiling. It is important that you determine the cause(s) of the problem. Otherwise you will just be treating the symptom and it will never be permanently fixed. TRUSS UPLIFT Question: I want to get a good definition on what causes "truss lift", when the trusses will actually raise off the top plates of interior walls (even when nailed) causing the drywall to crack. Is it from drastically different temperatures in the attic and living area? Answer: Truss arching, sometimes called truss uplift, is caused by wood's natural response to changes in moisture levels. The problem usually shows up in winter climates, when the bottom chords of trusses are warm and dry, buried under ceiling insulation. The top chords are usually exposed to cool, damp outside air. The result is high moisture top chords wanting to expand and low moisture bottom chords wanting to contract. You can't stop Mother Nature and when they both try and go their own way the bottom chords arches or lifts up. Once moisture levels in the lumber start to equalize the problem goes away. Truss arching can be a problem if the truss is over interior partition walls. The arching may be severe enough to develop an opening between the top of the partition wall and the truss - called partition separation. Many contractors and homeowners are aware of truss arching and any time there is a partition separation problem, they assume it's the truss. There can be many factors at work to produce partition separation. In fact, truss arching is the cause of only about 20% of reported cases of partition separation. WTCA offers a one-page Partition Separation brochure that addresses the truths of truss uplift with easy-to-read definitions and diagrams illustrating drywall details. Truss Design and SpecificationsTopADEQUATE TRUSSES Question: Where can I find specs for what would be adequate trusses. I have a room with a 24 foot span and my contractor is using trusses where the bottom plate is composed of 2x4s butted together and fastened with a metal plate. Is this ok? Answer: If you are questioning the integrity of the trusses that your contractor is using, you may want to ask for the copy of the sealed truss design drawings for your 24 foot long room. The truss design drawing is a graphic depiction of an individual truss prepared by the truss designer that prescribes truss geometry, materials and load conditions. This contains the "proof" that the trusses will work. ALLOWABLE DEFLECTION IN FLOOR TRUSSES Question: Is there a chart available on the allowable amount of deflection on floor trusses? Answer: For code compliance, the allowable amount of deflection is l/360, where l is in inches. For a stiffer floor it is common to use l/480. You can also use a length to depth ratio, which is typically 18 for good performance and that is a good number to monitor. The lower the number the stiffer the floor. Most building codes have something similar, it's best to check with local codes to be sure. DEFLECTION MINIMUMS Question: What deflection criterion is customary for top or bottom chord members subjected to uniform or concentrated loads between panel points? Would the traditional deflections of l/360 for live load and l/240 for total load be applied? I have specified this requirement and have received a tremendous amount of resistance from the truss fabricator. Answer: ANSI/TPI 1-1995 lists provisions for deflection. ANSI/TPI 1 is the National Design Specification for Metal Plate Connected Wood Truss Construction and is referenced in all three model building codes and the new I-codes. As a customer, it is not unreasonable to require a deflection criterion that is higher than the code based minimums. You must realize, however, that this increase will result in a stiffer and therefore more costly truss. DEFLECTION OF LONG SPAN SCISSORS TRUSSES (From the May 1999 issue of WOODWORDS) Question: I was wondering if you have any information or anecdotes relating to the deflection performance of long span scissors trusses? We recently sold a job with 70 ft long scissors trusses. The customer is in the process of installing the trusses and doesn't like the deflections he is seeing. To make matters worse the scissors trusses are framing into a valley set (which we provided) that is erected on 35' Howe trusses. The Howe trusses are exhibiting very little deflection. Any ideas? Answer: Since the trusses are currently being installed, check the installation bracing. With 70-ft scissors truss it is suggested that the contractor enlist the services of a registered professional engineer to design the temporary bracing system for this application. If the trusses are buckling due to inadequate bracing, this will certainly have an adverse effect on their deflection performance. If the trusses are sheathed and plumb, the contractor should check to see how much lateral movement is occurring at the top of the bearing wall. The vertical loads applied to the scissors truss create a horizontal thrust at the bearing. This thrust may push out the wall if the truss is not allowed to slip or the wall is not stiff enough to offer resistance. This horizontal movement translates into a vertical deflection, which is most visible at the ridgeline. If the outside wall is a long clear run with no perpendicular walls to take out load and eliminate the movement, both the horizontal and vertical deflection could be noticeable. ANSI/TPI 1-1995 limits this movement to 1-1/4 in. due to total load or ¾-in. due to live load, unless the building designer specifies more restrictive criteria. The building designer is responsible for designing the supporting structure and truss to wall connection accordingly. We would also check the vertical deflection criteria used to design the scissors and Howe trusses. An L/240 deflection for a 70-ft scissors truss will be twice that of a 35ft truss. Theoretically, if the top of the wall is restrained laterally, and the trusses are rigidly fixed to it (i.e. against slip), the vertical deflection due to horizontal movement will be minimized. The trusses will need to be re-designed as pinned-pinned and checked for stress changes due to the horizontal thrust. If this can be accomplished (much easier said than done) the deflection performance will be enhanced. This example helps illustrate the importance of having the building designer specify deflection limits in the contract documents. Addressing this issue beforehand would certainly help reduce the chance of future problems. Also, if the building designer had specified the handling, erection and permanent bracing requirements in the contract documents, this situation may have been avoided. DUCT CHASE SIZES Question: Are there charts illustrating allowable HVAC duct sizes that can fit between truss chords for various truss depths? Answer: Per Stuart Lewis from Alpine Engineered Products: For a standard 4x2 floor truss (meaning Warren-style webs with 30-inch panels) with double-cut webs, the maximum round duct in the end panel is 5.625, 6.5, and 7.25 inches for 12, 14 and 16 inch truss depths, respectively. By end panel, I mean the 15-inch long panel formed by the outermost diagonal and the end vertical, where the duct would be against the end vertical of the truss. The outermost 30-inch panel (formed by the two outermost diagonals) will permit round ducts up to 7.375, 8.875 and 10 inches for 12, 14 and 16 inch truss depths IF the truss plate joining the two webs together is narrow enough that it does not extend into that opening - at ends of trusses, it is not unusual for the truss plate to extend into that opening. Since the plates usually get smaller as you move away from the bearing, these sizes should be possible in subsequent panels further from the bearing. For 4x2 trusses with Fan-style webbing, a.k.a. modified Warren webbing, which is like the Warren-style but the horizontal run per diagonal is twice the distance and vertical webs are added at web-to-bottom chord joints in order to reduce the top chord panel lengths to 30 inches, the maximum round duct sizes are 7, 8.375, and 9.625 inches between vertical and diagonal webs, and 8, 9.875 and 11.625 inches between two diagonals, for 12, 14 and 16 inch depths, respectively. For the Alpine Tension Web style of truss, which has metal webs with Pratt style webbing (meaning all diagonals are in tension under gravity loads) with 24 inch panels (c.c. of verticals), the maximum round duct size is 6, 7 and 8 inches for 12, 14 and 16 inch truss depths, respectively. FLOOR PERFORMANCE (From the September/October 2001 issue of WOODWORDS) Question: How do you avoid floor performance problems? Answer: A multitude of factors may cause floor performance problems. These problems are not limited to wood framing either; steel bar joists and pre-stressed concrete floors can also experience performance problems. Here are some things to keep in mind when designing and installing wood floor systems regardless if they are composed of trusses, I-joists or solid sawn lumber:
Floor vibration, bounce, movement, springiness-call it anything you like, but for most new homeowners, it is an unwelcome guest. It is not something that can be easily designed out of a floor system by following the building codes. In fact, most experts in this area agree that designing to higher deflection criteria (using L/480 versus L/360) does not guarantee better floor performance. The problem with defining good floor performance is that it is highly subjective. What may be fine to one homeowner may be unacceptable to another. Research into floor performance from Trus Joist, A Weyerhaeuser Business, has found that homeowners moving from a slab-on-grade home to a wood floor framed home were very sensitive to floor movements. Once homeowners have been sensitized to the problem, it must be virtually eliminated before they will be satisfied. The trick is to build a floor that, from the outset, avoids the commonly understood causes to floor performance problems. FLOOR VIBRATION Question: We have designed a custom home that needs a good solid floor, with low (very slightly perceptible) vibration perceptibility. The system we have specified consists of 2" of gypcrete over 1 1/8" thick plywood or OSB. The floor trusses are 20" deep at 16" o.c. spanning 31'-6". The preliminary truss design shows a double top and bottom chord with a total load deflection of 1.30" (L/290) and a live load deflection of 0.65" (L/581). The basement ceiling will be 1/2" gypboard nailed to bottom of trusses. We are looking for a way to analyze the system to determine the vibration characteristics. We are familiar with analysis methods for steel joists and beam systems. Are there any programs for wood truss floor systems? Answer: The following articles are useful in understanding floor vibration in floor trusses. The first is an article written by Frank Woeste and Dan Dolan titled "Beyond Code: Preventing Floor Vibration" www.jlconline.com/jlc/archive/framing/floor_vibration/index.html Second is an article written by Dolan called "Designing to Reduce Floor Vibrations in Wood Floors". It is available on Alpine Engineered Products web site www.alpeng.com. TrussJoist (800/628-3997) also has a proprietary program that calculates floor performance levels based on construction materials. Granted they are for I-joists vs. floor trusses but it should give a ballpark idea. HORIZONTAL DEFLECTION OF SCISSOR TRUSSES Question: With scissor trusses that support a 40 psf total load, can the lower chord pitch be 2/3 of the upper chord pitch without causing drywall cracking or other problems? Answer: The general rule of thumb for scissor trusses is that the bottom chord pitch is half of the top chord pitch. The bottom chord can be more steeply pitched than that but that usually means that the truss needs to be made deeper - usually by increasing the depth at the heel (where the truss sits on the wall). This is referred to as a high heel condition. I suspect that the drywall cracking you are concerned with may be due to the horizontal deflection of the trusses. Vaulted framing naturally responds to downward gravity loads by thrusting outwards. ANSI/TPI 1-1995 The National Design Specification for Metal Plate Connected Wood Truss Construction recommends limits to horizontal deflection. It states, "In lieu of specific provisions for lateral movement of trusses and supports, total horizontal deflection at the reactions for the design of trusses shall be limited to 1.25" due to total load or 0.75" due to live load, whichever controls." If this seems excessive to you, you can request a stiffer truss that exhibits less horizontal deflection but realize that this will most likely increase the truss cost. LUMBER SUBSTITUTION (From the January/February 1999 issue of WOODWORDS) Question: If the truss design drawing specifies using 2x4 1650F SPF, can 2x4 No. 2 southern pine be used instead? What are some things that need to be checked? Answer: General provisions for lumber substitution during truss manufacturing are provided in Section 4.3.1 of ANSI/TPI 1-1995. This section states, "Truss Lumber shall be of the minimum grade, size, moisture content and species combination specified by the truss design. Truss lumber of a higher stress grade of the same size and species combination is not prohibited from being substituted for the stress grade, size and species combination as specified. Changes in size and/or species combination or conversion to structural composite lumber shall require additional analysis by the truss designer and review by the building designer to prove equivalency". ANSI/TPI 1 permits direct substitution of a higher stress grade of the same size and species combination since the allowable properties of the substitute lumber will be equal to or greater than the material it is replacing. If the lumber substitution includes a change in grade, species combination (both true for this case) or size, additional analysis is required to ensure the substitution is acceptable. Unless you re-run the design in your computer, you will need to compare the allowable design properties of the substitute lumber with those of the lumber you want to replace. If the allowable properties of the substitute lumber are equal to or greater than the specified lumber, then substitution should be ok. Keep in mind, however, that all the allowable properties need to be checked. In this example the allowable design values are:
As you can quickly tell, a direct substitution is not going to work for this case based strictly on a comparison between allowable properties. A second factor that is important to remember is that plate design values are lower for SPF than they are for southern pine, so in this case substituting southern pine for SPF would be fine if lateral resistance (gripping) controls the plating requirements. This is due to the density differences between southern pine and SPF. Lumber companies are now testing for density with MSR lumber and placing it on the grade stamp. This may help if the density of SPF is the same as HF or DF the truss plate design values improve. SYP however typically has the best truss plate design values so substituting SYP for the other species is easier. It gets more complex going the other way. The quickest, most precise and probably the best way to check the adequacy of this proposed substitution is to re-run the design and see what effects the substitution has on the original design. Depending upon where the substituted lumber is to be used, it is possible that it may work fine. Even if some of the allowable values are less than those for the SPF. WTCA's quality assurance computer program provides a database that can be used as an additional check on lumber substitution. For more information contact WTCA. SLOPING FLOORS Question: In our one-year old home, as you approach the hallway, the floor slopes downward. Our home inspector said that this was most likely due to the fact that the walls were resting on the floor as opposed to being load-bearing walls. What has been your experience with sloping floors in a new home? The floors are very flat elsewhere until the walls start. Do you think this sloping could turn into a structural problem eventually? Answer: It is very difficult to say whether or not this is or could be a problem. Floor systems may exhibit differential deflection (one area may deflect more than an adjacent area) which could cause this floor unevenness. We recommend that you ask the engineer-of-record to come out and take a look at the project because he/she is responsible for the overall performance of the structure. They may be able to provide you with more information after looking at this particular assembly. SOUND RATING INFORMATION Question: Do you have sound rating information for floor truss assemblies (i.e., STC and IIC ratings). We are looking specifically for ratings for 12" floor trusses with 1 1/2" concrete topping and without the topping. We are looking to meet the minimum code rating of a STC of 45 and IIC of 45. Answer: The intensity of sound is measured in decibels (dB). The greater the intensity of sound, the higher the dB. Sound striking a wall or ceiling surface is transmitted through the air in the wall or ceiling cavity. It then strikes the opposite wall surface. Causing it to vibrate and transmit the sound into the adjoining room. Sound also is transmitted through any openings into the room (i.e., air ducts, electrical outlets, window openings, doors). This is called airborne sound transmission. Owens-Corning Fiberglass Corporation in Toledo, OH, developed the Noise Control Guide, which discusses design techniques to reduce noise levels. The following excerpts are from this document. There are three methods of controlling airborne sound transmission: MASS, BREAKING VIBRATION PATH, or CAVITY ABSORPTION. 1 ) Mass: Heavy walls of cement block or other masonry can reduce sound transmission. If surfaces are sealed and joints are tight, each doubling of weight can increase transmission loss 3-6 dB. Weight is not always the cheapest or prettiest answer to good acoustical design. 2) Breaking vibration path: Discontinuous construction with a minimum of direct mechanical connection between the surfaces reduces transmission by breaking the sound path (e.g., Double stud or staggered stud walls, resiliently-mounted wall and ceiling surfaces). Discontinuous construction will improve sound transmission performance by 6-10 dB. 3) Cavity absorption: With discontinuous constructions, blanket-type insulation materials further improve performance by absorbing sound energy in the cavity before the sound can set the opposite wall surface in motion. Fiberglas insulation can improve performance in discontinuous construction 5-15 dB at minimum cost. The Sound Transmission Class (STC) method of rating airborne sounds evaluates the comfort ability of a particular living space. The higher the STC, the better the airborne noise control performance of the structure. An STC of 50 or above is generally considered a good airborne noise control rating. Impact Sound Transmission is produced when a structural element is set into vibration by direct impact. For example, someone walking. The vibrating surface generates sound waves on both sides of the element. The Impact Insulation Class (IIC) is a method of rating the impact sound transmission performance of an assembly. The higher the IIC, the better the impact noise control of the element. An IIC of 55 is generally considered a good impact noise control rating. How do you estimate wood floor sound performance? Sound transmission and impact insulation characteristics of a floor assembly can be calculated by adding up the value of the individual components. The contributions of various products to an STC and IIC rating are shown in WTCA's Metal Plate Connected Wood Truss Handbook. SPACING REQUIREMENTS ON SHEATHING Question: I am looking for help on the spacing required to screw down a BHP B-36 20 gage roof deck to wooden trusses spaced at 5'4" oc and 24" oc. Do you know of any published codes or specifications on the above? Answer: We only require that trusses have continuous top chord bracing (like sheathing) for 24" oc. Once the on center spacing exceeds the capacity of the sheathing (like your 5'4" oc), then purlins are installed on the top chord for support. The on center spacing of the purlins along the truss top chord are part of the information that is input at the design stage. How the sheathing is attached is beyond the scope of any truss design work. Truss designers do not perform structural roof diaphragm designs. We suggest that you contact your roofing manufacturer or supplier with your concerns. STANDARD ON-CENTER SPACING Question: What is the recommended standard spacing for open web wood joists? I have seen them placed 24 inches OC. Is this acceptable for a customer who will have an exercise room with 500 pounds of free weights? I understand there are problems with bouncy floors with 24 inch spacing. Is this true? Answer: Spacing is chosen such that the sheathing edges will fall at the centerline of the trusses. As a result, truss spacing usually divides evenly into a common plywood sheet (8 ft.). Example: Common Truss Spacing (8ft. x 12in./ft.)/5 = 19.2 in. on center (8ft. x 12in./ft.)/6 16 in. on center STRUCTURAL COMPOSITE LUMBER IN TRUSSES (From December '02 SBC Magazine) The lumber used in most metal plate connected wood trusses is either visually or mechanically graded solid sawn dimensional lumber. However, ANSI/TPI 1-1995 and the newly released ANSI/TPI 1-2002 National Design Standard for Metal Plate Connected Wood Truss Construction both allow the option of using structural composite lumber (SCL) products such as laminated veneer lumber (LVL), laminated strand lumber (LSL), and parallel strand lumber (PSL). These engineered wood products can conceivably compete with sawn lumber and complement it in truss designs. The qualification in TPI 1 is that the truss designs incorporating these products must be reviewed and approved by the Truss Designer. Even though the option is available, some structural composite lumber may not be practical in metal plate connected truss applications. Question: As a truss manufacturer, how can I incorporate structural composite lumber (SCL) into my truss designs? Answer: In theory, any type of lumber product can be used in wood trusses as long as the lumber and plate design values for that product are available. In reality, you should check first with your software provider to see if they have values for the product you are considering and talk with your engineer to see if this is something that can be readily incorporated into the design process. We are aware of several products that have been tested and evaluated by software suppliers. Check with your software provider to find out which products have been incorporated into their truss design process and then they can be specified in the truss design and plated just like any other standard lumber species and grade. Because these engineered products can have significantly higher strength properties than standard lumber grades, it makes them a good choice for girders, attic frame bottom chords or tail-bearing top chords. Some other advantages to higher strength structural lumber products are that longer lengths reduce the number of chord splices, greater strength to size ratio allowing smaller member sizing or custom member sizing, no knots and less shrinkage and splitting. These advantages come at a cost, SCL is generally more expensive per board foot than sawn lumber. If a larger attic room can be created or girder ply can be eliminated, there is additional value for the cost differential. The overall value must be calculated as part of the entire cost equation: inventory, design, manufacture, installation, site performance and customer satisfaction. TRUSS DESIGN FACTORS OF SAFETY? (From March '03 SBC Magazine) When I was asked to prepare this month's column, I could not think of a better opportunity to answer a regularly posed question on truss design safety factors than in an issue focused on safety. It is inevitable that during a Truss Technician Training (TTT) course or a seminar, one of the questions asked is "How much safety is built into a truss?" We also continue to receive similar questions by email. One such question follows: Question: I am currently working on a project where a wood truss system was loaded with a heavy spring snow. I do not believe the load was beyond the truss's design capacity. But how do you know? Is there safety built into wood trusses? If the trusses were loaded beyond their design capacity, it would not have been for a long duration. Any assistance would be appreciated. Answer: Structures and structural members must always be designed to carry some reserve load beyond what is expected under normal use. Under allowable stress design, safety is provided by using an allowable stress that is low enough to protect against (1) variation in material properties, (2) errors in design theory, and (3) uncertainties as to the exact load. The ratio of the load that would cause failure to the load for which the structure is designed is called the factor of safety.1 Metal plate connected wood trusses are designed with factors of safety. Metal connector plates are designed with a factor of safety of 3.2 for withdrawal, 1.44 for steel shear yield strength and 1.67 for steel tension yield strength. Both these factors result in a steel factor of safety of 2.0 on overall steel strength. Often it is a tooth withdrawal resistance failure that is seen due to overloading; but steel and lumber failures occur as well.1 Because wood is a heterogeneous product, the factor of safety varies depending on the direction of the applied load to the direction of lumber grain. According to ASTM D245 Standard Practice for Establishing Structural Grades and Related Allowable Properties for Visually Graded Lumber and ASTM D1990 Standard Practice for Establishing Allowable Properties for Visually Graded Dimension Lumber from In-Grade Tests of Full-Size Specimens, the following factors of safety should be applied to the lower fifth percentile exclusion limit on clear softwood properties: 2.1 in bending and tensile strength parallel to grain, 1.9 in compressive strength parallel to grain, and 1.67 in compressive strength perpendicular to grain. Overall, the factor of safety of the composite truss should be in the conservative range of 2 to 2.5, depending on the failure mode, and can easily be as high as 3 to 3.5. When overloaded, plates may exhibit "peeling." The gap be-tween plate and lumber will be at a maximum at the outside of the plate and taper off towards the inside. Lumber may exhibit hairline fractures that are not visible with the naked eye when trusses are overloaded. According to Wood Engineering and Construction Handbook, fatigue is defined as the progressive damage and failure than can occur when a member or structure is subjected to cyclic, repeated loading at levels less than the static strength. It is directly related to the duration of the load. Fatigue properties are generally of little concern in many applications of wood but can become important in applications where there are many repetitions of stress. Long term fatigue can cause damage to lumber if it is not considered in the truss design when repetitions of design stress or near-design stress are expected to be more than 100,000 cycles during the normal life of the structure.3 Contact the Forest Products Laboratory (www.fpl.fs.fed.us) to obtain data on the fatigue properties of wood. We suggest that you contact a local engineer experienced in wood design to look at the trusses and determine if there has in fact been any damage due to overloading. 1 Stalnaker J & Harris E. Structural Design in Wood. Van Nostrand Reinbhold, New York, 1989. 2 Truss Plate Institute ANSI/TPI 2-1995 Truss Plate Institute ANSI/TPI 2-1995 Standard for Testing Performance of Metal Plate Connected Wood Trusses. 3 Faherty K & Williamson T. Wood Engineering and Construction Handbook: 2nd Edition, McGraw-Hill, Inc, New York, 1989. Truss Repairs AlterationsTopCUTTING TRUSSES Question: Our home has 2x4 roof trusses spanning 25 ft. spaced 16 in. o.c. Currently there is limited attic storage on a plywood subfloor on the bottom chords, accessed via a small hatch in a closet. We would like to install a fold-down attic stair with a rough opening size of 25.5 in. x 54 in., with the long dimension perpendicular to the trusses. This would mean cutting the bottom chords of three trusses to frame the opening. Is this possible? If so, would double framing to the bottom chords of the adjacent trusses in the first and fourth bays be adequate to header off the three cut bottom chords? Answer: NEVER cut any structural member of a truss! Any field modification that involves the cutting, drilling, or relocation of any structural truss member or connector plate shall not be done without the approval of the truss manufacturer or a licensed design professional. One question that should also be asked regards what you are planning on using the access for? Be aware that the trusses most likely were not designed to carry additional loads on the bottom chord, if you are planning to use the attic space for storage. If this is what you are planning, you would be well advised to contact the truss manufacturer as to the amount of load the trusses were designed to carry. DAMAGED TRUSSES Question: I have just moved into a new home and during the construction I noticed that one of the trusses in the garage was damaged. The web plate had come off in two different locations. I notified the builder and they said they would take care of it. Approximately two weeks after I moved in, I noticed the ceiling in the garage started to sag, so I went into the space above the garage and found the trusses was never repaired. I again notified the builder and they sent someone to fix it. I went and looked at the fix and this is where my question comes in. Their fix was to take 4 pieces of plywood (I'm not sure if it was 1/2 or 3/4 in) approximately 18 inch square and nail them on either side of the joints where the webbing was. Is this a correct type of fix for a truss joint? If not can you recommend the correct way to fix them? Answer: The type of plywood gusset "fix" that you are describing is a common repair on trusses. To determine if it is the proper repair on your trusses, you will need to obtain the repair detail drawings that most likely came from the truss manufacturer. When you notified the builder of the damaged trusses in your garage, the builder should have notified the truss manufacturer immediately such that the manufacturer's engineer could prepare the repair drawings. Without proper repair detail drawings, repairs should not be attempted. DRILLED HOLE IN TRUSS Question: I need to know how to repair a hole that has been drilled through a truss. In installing the wiring we improperly put the wires through the truss and now need to know the proper way to repair it. Answer: Contact the truss manufacturer if you are concerned about the serviceability of your truss. They will be better suited to discuss whether or not a repair is necessary because they know the loads for which the truss was designed. The truss chord with the drill holes may have been designed near capacity and any reductions from drilled holes could affect the structural performance of the truss. When you notify the truss manufacturer, they in turn will notify their engineer who will prepare the repair drawings if necessary. Without proper repair detail drawings, repairs should not be attempted. REPAIRS TO FIELD MODIFIED/DAMAGED TRUSSES Question: As a home inspector, I have recently inspected an existing home with an attic truss system installed. Several of the truss web members have been cut away to allow access to an attic-mounted heating system. Can you recommend repairs to a truss with cut web members? Answer: Unfortunately there is no "standard" repair detail available for field modified/damaged trusses, as design conditions and the extent of the damage vary considerably from job to job. It is very possible that the damaged trusses you have described can be repaired. The repair will most likely require reinforcement of the bottom and/or top chords with additional lumber as well as reinforcement of the remaining web joints with a field-applied steel plate or structural sheathing. The repair should be designed by a qualified individual. Your first choice should be the original truss manufacturer/truss designer, as they are most familiar with the product. The truss manufacturer/truss designer will be able to compare the "in-use" conditions with those of the original design and, for a fee, develop an acceptable repair. In the event you are unable to determine or contact the truss manufacturer/truss designer, a registered design professional experienced with wood truss design can be hired to design a repair. (From the June/July 1999 issue of WOODWORDS) ROOF/CEILING ALTERATION Question: Is it possible to retrofit a standard rafter/joist style roof to a scissors truss configuration without replacing the existing construction? Answer: It sounds like what you are proposing is not necessarily to make a scissors truss from conventional framing but to vault the ceiling joists. This is certainly an option but we recommend consulting a structural engineer for this project. The only practical way (and that's debatable) to use scissors trusses would be to replace the roof. STATISTICS ON TRUSS REPAIRS Question: How often are trusses repaired? Answer: Trusses have successfully been used for over 50 years and are seldom in need of repairs. The following statistics were provided by a WTCA member: Out of all trusses shippied, they have a maximum repair rate of 0.32%. Out of this rate, the breakdown of causes: 60% of the repairs are for breakage by the framer while loading and spreading of them. 25% are from developer changing HVAC conditions and people cutting webs. The balance is due to changes at the site and /or errors by the developer or manufacturer. TRUSS FAILURE AND REPAIR Question: I am a Professional Engineer looking for information concerning failure of metal plate connected wood trusses and methods of repair. I am looking for this information with regard to developing recommendations for repair of truss systems in floor structure of two story multi-family dwellings. I see frequent failure of truss systems of this type in both in bending and due to catastrophic failure at the gussets. Answer: Most truss repairs that are generated during the construction phase involve lumber scabs over broken webs and chords or plywood gussets over damaged plates or joints. These are generated following the design principles outlined in AF&PA's National Design Specification (NDS) for Wood Construction. One option may be to contact the truss manufacturer and have their engineering support supply the repair details for you. What is of more concern, though, are these frequent catastrophic failures that you describe. What is causing this? Are the trusses being overloaded? Oftentimes, trusses are overloaded with stacks of construction materials (drywall, plywood, etc.) shortly after installation and they can be permanently damaged. Could that be a cause? This problem needs to be addressed in more detail. TRUSS REPAIRABILITY Question: Occasionally we deal with truss failures due to impact and or crushing forces due to trees falling onto roof structures. At what point can the truss no longer be repaired? Also, what should an adjuster look for when determining repairability? Or should a storm adjuster basically punt and call a consultant for every truss failure? Answer: When dealing with truss failures you are almost "stuck" with punting to a professional to assess the damage at the onset. If there is any structural damage to the truss, an engineer must design the repairs. Some examples of structural damage are broken or cracked chords or webs and damaged, loose or missing plates. |
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