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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 build | ||||||||||||||||||||||||||||||||||||||||||
