Roof Construction Techniques: Pro - PDF document

Roof Construction Techniques: ProÕs and ConÕs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . usses: Special Benefits for Architects and Engineers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Special Benefits for Contractors and Builders. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Special Benefits for the Owner. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .How Does A Truss Work?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . dditional Truss Framing Options alley Sets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 ypical Bearing / Heel Conditions xterior Bearing Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Crushing at the Heel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Optional End Cosmetics Level Return. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Nailer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .arapet. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ansard. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ypical Bearing / Heel Conditions for Floor Trusses op Chord, Bottom Chord, and Mid-Height Bearing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Interior Bearing Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ribbon Boards, Strongbacks and Fire Cut Ends. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Steel Trusses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 TTable of Contentsable of Contents 1 raditional Stick Framing Ð arpenters take 2x6,2x8, 2x10 and 2x12 sticks of lumber to the jobsite. They hand cut and fit this lumber togetherinto a roof or floor system. mber Frame Ð joinery and braces together to form a self-supporting skeleton structure. Engineered Wood Products Ð I-Joists, Glulam,and LVL (laminated veneer lumber) wood productsare shipped to the jobsite, cut to fit, and made intobeams for the floor system, or into ridge beamsand sloping joists for the roof system. russ Systems Ð in two primary forms:Metal Plate Connected Wood Trusses Ð Engineered trusses are designed anddelivered to the jobsite with a truss placementplan. Trusses are quickly erected and braced Steel Trusses Ð Light gauge steel trusses aredesigned and delivered to the jobsite with atruss placement plan. Trusses are lightweight,able to be quickly erected and braced onsite. Builders, ArBuilders, Architects and Home Owners today have manychitects and Home Owners today have manychoices about what to use in rchoices about what to use in roof and floor systemsoof and floor systems 2 Each method has advantages and disadvantages. In this book we will look at those pros and cons, and assistyou in identifying which method is best for your situation. We hope to be able to offer you strengths for eachsystem that can help you sell the value-added difference in the product you choose, as well as help you gaina better understanding of the roof systems themselves, with Òreal worldÓ advice on framing techniques and ______________ Pros os ____________Stick FramingStick Framing TTimber Frameimber Frame ______________ Cons ____________ Lowest cost for materials. skilled labor required.If trusses cannot be deliveredSlow to install and requiresskilled labor.ay not be accepted by thebuilding department, as it isnot an engineered product.equires scarce large lumberSmaller span capability.High jobsite waste. Creates handsome exposedtimber-frame structures.equires skilled labor.Difficult to run ductwork,wiring, pipes through.Slow to install and frame in.Hard to insulate 3 EngineerEngineeredWood Prood Productsoducts WWood Tood Trrusses Steel TSteel Trrusses ______________ Pros os ____________ ______________ Cons ____________ I-Beams are suitable for steeprim-able ends on floors, greatfor angled or rounded walls.Engineered product withuniform quality.Glulam and Laminated VeneerLumber make strong beamDifficult to run ductwork,wiring, pipes through.Limited use in complicated rooflines, coffered ceilings, etc.ay require special connections. Lowest overall cost.to install. equires ordinary tools anddoesnÕt require skilled labor toinstall.ually any roof and ceiling isEngineered product.No job site waste.ou will want to plan things inadvance. YouÕll need a set ofplans for the structure, andwhere theyÕre supposed to be. Lightweight and stiff, soinstalling is easy and fast.ay be preferred in lightcommercial / industrialsettings for fire rating.Straight ceilings for ceilingattachment.ually any roof and ceiling is possible.Engineered product.More expensive than metalIf not analyzed/designed by aleading industry softwareprogram, the joint connections russes bring virtually unlimited versatility , providing simple solutions to complex building design freedom. Using trusses, you have complete flexibility in interior roomarrangements, too. Using AlpineÕs proprietaryVIEW software, truss designers can produceengineered shapes that satisfy virtually anyaesthetic and functional specification by thebuilding design professional.russes are an engineered and tested product.here are nationally recognized standards for trussdesign and manufacturing of metal plateconnected wood trusses. These standards haveis ensures a quality product. Alpine ProfessionalEngineers are committed to providing the highestquality, cost efficient structural products for more open space to work with in the interior.uss manufacturers using Alpine software areavailable for consultation when special framingood trusses connected with metal plates enjoyan outstanding record of more than 35 years ofproven performance and durability. State of the art truss design software allowsmanufacturers to design them to be: ¥ Hurricane-resistant, and/or ¥ ithstand heavy snow loads, and/or ¥ Support storage areas above the ceiling.ou can get a one- or two-hour fire rating trusses using the FR-Quik System. usses: Special Benefits for Arusses: Special Benefits for Ar 4 russes go up faster and easier, with less skilledlabor requirements, no matter how complicatedthe roof or ceiling is.russes put you under roof faster, which helps indrawing construction loans.he use of preassembled components generatesless waste at the jobsite. This improves safety and reduces cleanup costs. On-site losses from mis-cutting, theft and damage are virtually eliminated.russes are built in a computer-aidedmanufacturing environment to assure accuracyand quality.Industry standards for manufacturing and handlingassure code-compliance. Building departmentsecognize these standards and respect thesoftware used to design trusses. While manybuilding departments are wary about inspectingconventional framing, sealed truss designs arerusses are lightweight and easy to install,equiring only normal construction tools.he wide nailing surface of 4x2 floor trusses safelyspeeds deck and flooring installation.xpenses are accurately controlled because trusscosts can be predetermined.Open web design allows easy installation ofplumbing, electrical wiring and heating/coolingrusses are available locally for fast delivery. Morethan 550 truss manufacturers throughout theUnited States and Canada are backed by theexpertise of Alpine Engineered Products, Inc.he owner can enjoy peace of mind, knowing thatthe trusses have been professionally engineeredand quality manufactured for the specific job.he resiliency of wood provides a floor system thatis comfortable.ood is a natural insulator because it is composedof thousands of individual cells, making it a poorconductor of heat and cold.Roof truss details such as tray, vaulted or studioceilings improve the appearance and comfort ofhomes, offices, churches and commercialFloor trusses can conceal mechanical services,leaving a clear plane for ceiling installations. This isideal for finished rooms in a lower level.usses provide clear spans so interior walls canbe moved easily during remodeling or whenmaking additions. It is very economical to remodel Special Benefits for Contractors and BuildersSpecial Benefits for Contractors and BuildersSpecial Benefits for the OwnerSpecial Benefits for the Owner 5 A truss is a frame that supports loads by efficientlytransferring its forces to end supports. While stickframing might use larger 2x8, 2x10, etc. members(which are expensive and hard to find withoutgoing into old growth forests), and might requirewalls, trusses can span a longer distance withoutadditional supports, while using less expensiveand more plentiful 2x4 members, usually arrangedrusses can span up to approximately 90Õ,although very long truss spans are morechallenging to deliver, erect, brace and installproperly. While longer trusses may be ÒwobblyÓ asthey are lifted off of the ground and onto thebearing walls, once they are properly braced, thetruss system is extremely strong. How Does a THow Does a Trruss Wuss Work?ork? 6 Most truss manufacturers utilizesophisticated truss design software that isapable of designing the truss system as from a set of plans, firstcreating a truss placementplan, and then designingeach truss in the system. 7 he job is manufactured and delivered to the jobsite, where the contractor / builder erects them and Cutting the lumberManufacturing the trusseslivering a truckload of trussesErecting the trusses on the jobsiteemporary bracing systemPermanent bracing system russ framing systems, and the names associatedwith them, vary all over the country andthroughout the world. No matter how they areframed or what they are called, though, trusssystems easily provide tremendous flexibility in thelook of the roof system.he illustrations below are designed to help youvisualize typical framing systems, looking at: ¥ A truss placement plan, ¥ he overall 3D look of the roof planes in thatoof system, and ¥ 3D view of the framing system of trusses. TTypical Framing Systemsypical Framing Systems 8 e most basic (and least expensive) of roofs, agable roof rises vertically on the shorter ends ofwhich meet in the middle. In our example youÕllfind a gable frame on either end, each supportedby the continuous wall underneath it, and commontrusses in between, each of which spans from onewall to the other. __________________________________________________________________ Gable _________________________________________________ (* See also Ð Gable framing variations) is roof system could have a sloping ceiling or tray ceiling, if desired. 9 __________________________________________________________________ Hip Set Hip Set _______________________________________________________________________________________ ( * See also Ð Hip set framing variations ) ______________________________________________________ Dutch (Boston) Hip Set Dutch (Boston) Hip Set _______________________________________________________ Notice the vertical rise in the middle of the hip set end plane. 10 _____________________________________________________________ Tudor Hip udor Hip ______________________________________________________________ A tudor hip provides some interesting sloping planes at either end, and is generally less expensive to build than a full hip set. Floor System ( * See also Ð Floor Truss Systems ) Gable Framing VGable Framing Variationsariations 11 If the roof eave extends beyond the end wallenough to require support for the roof sheathing,then a dropped gable is specified. The top chordof the gable end is dropped down enough so thatthe builder can run outlookers from the fascia backto the first common truss. This provides enoughadditional support for the sheathing. ____________________________________________________________ Dropped Gable opped Gable __________________________________________________________ __________________________________________________________ Clear Span Gable Clear Span Gable ______________________________________________________ ile most gable ends have continuous supportunder their bottom chords, a clear span gablemust span from one wall to another. It has to havediagonal webs to help distribute the load out to thewalls, but it also needs to have vertical gable studsto help the gable sheathing resist lateral windforces. Alternative methods for framing a structural gable include the truss manufacturer providing a full gable truss(with just gable studs) and a full common truss, which would be fastened together with the gable end facingthe wind, or providing studs along the outside face of a common truss. 12 ________________________________________________________ Transition Gable ransition Gable ______________________________________________________ A transition gable end truss occurs when a run ofspan trusses. The first longer span truss will facelateral wind loads in the area where it is notshielded by the smaller span trusses. In thesmaller span area of that same gable, it doesnÕthave continuous support under its bottom chord.A transition gable, therefore, has diagonals to helptransfer loads in the smaller span area, and gableuds in the part that faces the wind. Under the planes of a hip set roof, there are manydifferent methods used to frame the trusses.Preferences for any given system hinges uponbuilder preferences, manufacturing efficiencies,cost, and ceiling requirements. Some of the mostcommon hip systems are outlined below. Hip Set Framing VHip Set Framing Variationsariations 13 ______________________________________________________________ Stepdown _______________________________________________ ________________________________________________________________ Midwest ________________________________________________ Notice that a long rafter is provided for the lengthof the hip ridge so no cats or field support of theridge is necessary. All the bottom chords of the end jacks runto the hip girder, allowing for better attachmentof the drywall on the ceiling, and the top chords ofthe end jacks run all the way up to the hip ridge.Proper support of extended top chords is critical. _______________________________________________________________ Californiania_______________________________________________ A stepdown hip set provides a girder truss, with ahip jack running from the corners up the ridge untilthey meet the hip girder. he builder must provide support for the hip ridge, usually in the form of blocking, orhip cats, in between each hip truss up to he Midwest hip set also provides a girder truss,with hip trusses that step up to the peak. However in a Midwest hip set, you run a rafter upfrom the corner of the front wall to the hip girder. All the bottom chords of the end jacks run to thehip girder, allowing for better attachment of thedrywall on the ceiling.Hip cats must be field cut and installed between the hip trusses if a dropped-in gable is not provided (for more info see Dropped-In Purlin Frame). 14 ______________________________________________________________ Northeast ______________________________________________ In a northeast hip set, you will find a short jackarting at the corner and running up the hip ridge.is hip jack stops at a sub-girder. All end jackbottom chords run to the hip girder, allowing forbetter attachment of the drywall on the ceiling.Hip cats must be field cut and installedbetween the hip trusses. Works well withdual-pitched hip systems. _______________________________________________________________ Terminal _______________________________________________ erminal hip sets are used in spans up to about 30'to 32'. A ridge rafter runs from the corner up tothe peak of the hip, and a sub-girder, locatedabout half way to the peak, carries the side jackbottom chords while their top chords run up to theridge rafter. _________________________________________________________ Standard Td Tererminal ________________________________________ he standard terminal hip set is used for very smallspans, i.e. up to about 18Õ to 20Õ. The two hipjacks running up the corners and the center endjack all connect at the peak, which can make thepeak connection difficult on longer spans. _______________________________________________________ Drop-in Purlin Frame op-in Purlin Frame _____________________________________________________ An alternative method for providing support up theridges of a hip set is for the truss manufacturer todrop the flat top chord of the hip trusses by 1 and provide a special purlin frame (in red) for theframing crew to drop in above the hip girder. he frame fits in the plane of the front face of thehip set, and provides additional rigidity to the roofsystem. Nailing sheathing into the 2x4 flat studs is Additional TAdditional Trruss Framing Optionsuss Framing Options lley Set Ð A valley set is a group of trusses whichsit on top of other trusses in order to change theway the roof looks. In the example below, withoutvalleys, we would have an empty ÒholeÓ in the roofbehind the girder. This would need either stickframing, or a valley set. 15 16 Here is what the roof looks like after the valley set (in red) is added. he top chords of the trusses which the valley set sits on must be braced, either by plywood under the valleys,or by alternate bracing. Images courtesy of Simpson Strong-Tie¨ 17 _________________________________________________________ Piggyback TPiggyback Trrusses _________________________________________ hen roof trusses are too tall to be manufacturedand/or delivered, truss manufacturers will ÒcapÓthe trusses and provide piggy back trusses. Thepiggybacks below are shown above the roof sothat the purlin and bracing system can be seen.Special diagonal bracing on the underside of thetop chords (in red) helps brace the purlinsthemselves, and keeps them from shifting out ofplane. Piggy back trusses can be designed usingeither vertical studs (as shown) or diagonal webs. tail of the bracing system, with purlins (blue) and diagonal braces (red). TTypical Typical Trruss Configurationsuss Configurations 18 19 TTypical Typical Trruss Shapesuss Shapes 20 he outside profile of your truss can take on just about any shape you can imagine. Some of the more commontruss shapes appear below. TTypical Bearing / Heel Conditionsypical Bearing / Heel Conditions 21Exterior bearing conditions - he illustrations below show typical support conditions.picalÓ Bottom Chord end conditionpical conditions include a girder heel (one where the top chord stops when it meets the bottom chord), atruss heel without an overhang, and/or with a raised heel, and/or with a sloped ceiling, for example: Bearing support types for each can be any of the following: Image from AlpineÕs VIEW¨ software 22 Additional conditions and options include: ¥Crushing at the heel - Sometimes the forcestransferred through the wood into the bearingwall are enough to cause the wood to crush overtime, unless either a bearing block is used, or, ona raised heel condition, the end vertical is runthrough to the bearing wall (so the wood shear value), or unless the bearing itself is madewider so the forces are transferred through moresurface contact area between the truss heel and ¥Trusses sitting on concrete walls - hen the truss sits on a concrete wall, a seat plate may be required asa moisture barrier: Images courtesy of Simpson Strong-Tie¨ 23 Other Bottom Chord Bearing conditions:russ in a hanger:Truss nailed to a girder/ledger (Right end): ¥Mid-Height bearing Ð sits between the top and bottom chord of thetruss, and a mid-height bearing block (the 2x6vertical shown here) will be used. eg-Thru to the bearing Ð below the truss bottom chord, the end vertical ofthe truss can extend down to sit on the bearing.Special bracing may be required. ¥Top Chord bearing Ð russes can be designed to have support right under the top chord. You will want toshim the bearing solid under the top chord of the truss. 24 ail bearing tray Ð If the room calls for a tray orcoffered ceiling very close to the edge of theoom, the truss can look like these tail bearingtray trusses below. The example on the rightalls for a scab to be attached (an additionalpiece of wood nailed to the face, or both faces,of the top chord at the heel, as shown by the Images courtesy of Simpson Strong-Tie¨ Special Connection Notes: It is important to properly tie down the truss toyour bearing walls, and not just in areas prone tohurricanes. There are many products that aredesigned for this purpose. These productsshould not only anchor the truss into the wall,but also tie the truss down to resist both upliftand lateral forces. Interior bearing conditions wo trusses sharing a wall Interior bearing Built-in kneewall (see 3D image} 25 he top chord that extends beyond the bottomchord and bearing is called an overhang. he outside cut of your overhang can look like anyof the images below. ____________________________________________________Typical Heel Conditions ypical Heel Conditions _____________________________________________________ evel Return: A level return is a horizontalmember creating a soffit under the overhang.he level return on the left side builds up theÓ, and uses a vertical to tiffen it. The level return on the right sidecreates a box soffit, and is plated to the raised Optional End Cosmetics Ð In addition, the truss overhang can have optional cosmetic framing applied. ¥N top chord, and some have a 2x4 top chord, youan keep a consistent fascia edge by applying aripped piece of lumber to the under side of theoverhang. This piece is called a nailer. Horizontal Cut 26 On trusses with raised heels, you canbuild in a parapet condition to your truss. Thisparapet can be used to either hide A/C units (ona flat truss, when the A/C unit is located on top of the flat roof), or to hang signage. Additionalbracing may be required for parapet conditions,especially in high wind areas.Flat roof trusses should always have a sloperunning from the center of the truss to theoutside. This prevents water from accumulatingon the roof, and is especially important withparapets. Adequate scuppers must be providedto ensure proper drainage. Cantilevers and Stubs Ð conditions for trusses are cantilevers truss bottom chord extends beyond the outside stubs (when there is a jog in the wall orsome other reason why the truss would be heldback from its original span). ¥Mansard ends: r storefront signs or just foranother look, special mansard jack trusses canbe manufactured. ____________________________________________________________ Web Bracing eb Bracing __________________________________________________________Bracing ExamplesBracing Examples 27 CLB: Continuous lateral bracing (CLB) is 1x4 or2x4 material nailed to the narrow side of a web. B braces must be fastened across a minimum of3 trusses. If you donÕt have a run of at least 3trusses, you must use another type of brace. he CLB brace here is shown in blue, with diagonalbracing to Òbrace the braceÓ shown in red.he truss drawing will show a brace on the web,and will also have a note specifying the brace, asshown to the left here. T A T-Brace is 1x or 2xmaterial fastened to the narrow faceof an individual web so as to form a(At right) Truss drawing depicting a T-Brace, and bracing note (below) 28 -Brace - Braces are pieces of1x or 2x lumber attached toindividual webs to form an ÒLÓshape. These braces are usually Scab Brace Ð A scab brace isapplied to the wide face of theweb member, using the same Web Block¨ & Other methods for reducing fieldapplied bracing Ð If you are open to reducing theamount of time your framers are in the roof systemjust a little bit extra for the truss package, thesoftware used by truss fabricators allows them touse manufactured solutions like the Web Block(shown, at right), or to increase web grades andsizes to considerably reduce the need for fieldapplied bracing. Talk with your local trussmanufacturer about these alternatives! -Brace note from a truss drawing (below) ErErection of Tection of Trrusses 29 russes may be installed manually, by crane, or byforklift, depending on truss size, wall height andarried vertically to avoid lateral strain andusses installed manually are slid into positionover the sidewall and rotated into place usingpoles. The longer the span, the more workersneeded to avoid excessive lateral strain on thetrusses. Trusses should be supported at joints andLarge trusses should be installed by a crane orforklift employing chokers, slings, spreader barsand strongbacks to prevent lateral bending.russes may be lifted singly, in banded groups, orpreassembled in groups. ag lines should always be used to controlfer to the Building Component SafetyInformation BCSI 1-03 Booklet and/or the BCSI-B1Summary Sheet, both by the Truss Plate Institute(TPI) and the Wood Truss Council of America(WTCA), for proper methods of unloading,toring, lifting, erecting, installing and Installation procedures are theresponsibility of the installer. Jobconditions and proceduresvary considerably. Theseare only guidelines andmay not be proper TTemporary Bracing y Bracing 30 All trusses must be securely braced, both duringerection and after permanent installation.Individual trusses are designed only as structuralcomponents. Responsibility for proper bracingcontractor for they are familiar with local and job-trusses should be installed straight, plumb andaligned at the specified spacing. Trusses shouldalso be inspected for structural damage.here are two types of bracing. Temporary bracingis used during erection to hold the trusses untilpermanent bracing, sheathing and ceilings are inplace. Permanent bracing makes the trusscomponent an integral part of the roof andbuilding structure. Temporary and permanentbracing includes diagonal bracing, cross bracing rmanent lateral bracing, as may be required bytruss design to reduce the buckling length ofindividual truss members, is part of the trussdesign drawing. This bracing must be sufficientlyanchored or restrained by diagonal bracing toprevent its movement. Most truss designs assumecontinuous top and bottom chord lateral supportfrom sheathing and ceilings. Extra lateral anddiagonal bracing is required if this is not the case.Bracing members are typically 2x4s nailed with two16d nails at each cross member unless otherwisespecified on the design drawing. Cross and diagonalbraces should run on an approximate 45 degreeIt is important to temporarily brace the first truss atthe end of the building. One method calls for thetop chord to be braced by ground braces that aresecured by stakes driven in the ground, preferablyoutside and inside. The bottom chord is to besecurely anchored to the end wall. Additionaltrusses are now set and connected together withcontinuous rows of lateral bracing on the topchord. These are typically spaced at 4', 6', 8', or10 feet on centers along the length of the truss.fer to BCSI 1-03 for diagonal spacing. This topchord bracing will be removed as the sheathing isapplied after the other bracing is completed,unless specifically designed to be left in place. 31 emporary bracing should be 2x4 dimensionlumber or larger and should be 8 feet minimum inlength. Continuous lateral bracing maintainsspacing, but without cross bracing, permits trussesto move laterally. See BCSI 1-03. prevent dominoing, diagonal bracing should beinstalled in the plane of the webs as the trusses areinstalled. See BCSI 1-03.the trusses. They should be limited to 8 sheets toa pair of trusses. Likewise, other heavyconcentrated loads should be evenly distributed.Inadequate bracing is the reason for most trussinstallation failures. Proper installation is a vitaltep for a safe and quality roof structure. hese recommendations are offered only as aguide. Refer to Recommended DesignSpecifications for Temporary Bracing of Metal PlateConnected Wood Trusses (DSB-89) by the TrussPlate Institute (TPI), or Building Component SafetyInformation BCSI 1-03 Booklet by TPI and WTCA.or cold formed steel truss systems, refer toÕs two publications, Field Installation Guidefor CFS Trusses, and Design Guide forConstruction Bracing of CFS Trusses. Bracing is extremely IMPORTANT!! Every roofsystem needs adequate bracing. The purpose ofmost bracing is to ensure that the trusses and trussmembers remain straight and do not bow out oftheir plane. Inadequate, improper or incorrectlyinstalled bracing can lead to collapses, failures andserious accidents. An engineered bracing systemwill avoid these pitfalls and ensure the structuralintegrity of the truss system.russes need to be braced during installation,which is called mporary Bracing, and they need rmanent Bracing which will remain installed forthe life of the roof system. mporary Bracing Guidelines: or metal plateconnected wood truss systems, refer to BCSI 1-03 for proper installation bracing guidelines. For coldformed steel truss systems, refer to LGSEAÕs twopublications, Field Installation Guide for C F S T and Design Guide for Construction Bracing of C F S T russes. CHECKLIST FOR TRUSS BRACINGCHECKLIST FOR TRUSS BRACING 32 ____________________________________________ Permanent Bracing System Checklistmanent Bracing System Checklist____________________________________________ 1. Top Chord Planes
Do top chord planes have structural sheathing (plywood, OSB, metal deck)?
If not, do you have a purlin system, with both purlins (perpendicular to the trusses) and diagonal bracing?Purlin systems can be used for standing seam roofs, or with structural sheathing applied on top of thepurlins. Either way, a diagonal brace system must be engineered. Refer to sealed engineered truss 2. Web Bracing Ð be sure to reference sealed engineered truss designs for proper web bracing callouts.
B Bracing crosses a minimum of 3 trusses, including diagonal bracing to Òbrace the bracingÓ?
Properly installed T-Braces, L-Braces (especially on gable ends), Scab Braces, and other web bracingsystems such as the Web Block? 3. Bottom Chord Planes
Do bottom chord planes have structural sheathing directly attached? In many cases drywall isconsidered by the building designer to be lateral bracing, but in some cases it is not.
If not, then you will need a purlin system, which can be attached to the top of your bottom chords, and
If you have any suspended ceilings, do you have a purlin system (including diagonal bracing) on the topor bottom of those bottom chords? 4. Additional Bracing Concerns
Piggyback Systems Ð If you have piggyback systems, do you have a purlin system installed to supportthe bottom chord of the piggyback, as well as purlins and diagonal braces to ensure that the flat topchords of the hip trusses stay in plane?
Vall Under the valley trusses, do you have structural sheathing, or other engineered bracingsystem for the top chords of the trusses underneath? Are the valley bottom chords adequately fastened
High Heel Heights at a Wall Ð for trusses with heel heights greater than a nominal 2x6, is special heelblocking required and installed?
Blocking For the Ridge in Hip Systems Ð Have you added blocks on the ridge between each hip truss(where a rafter or extended hip jack top chord doesnÕt extend to the peak of the hip system) to support Floor Systems Floor Systems 33 Another popular application for truss systems is infloor systems. Floor systems can be trussed,conventionally framed, or built with engineeredwood products such as I-Joists. Both trusses andengineered wood products are engineered, andhave wider nailing surfaces for the floor decking.russes are built with open chases for ductworkand have natural open spaces for plumbing andelectrical wiring. Some engineered woodproducts have specified or marked notches thatan be removed to allow for the same.Floor truss systems are sometimes called System42Õs, because to build them manufacturers turnthe 2x4Õs on their side. This allows for shallowÓ nailing surface. Somefloors are built from 3x2Õs, others from 2x4Õs.Floor trusses can be manufactured with manydifferent possible end conditions to accommodatedifferent installation needs; around raised walls,pocketed beams, headers around stairways, etc.In addition, some manufacturers are takingadvantage of adding an I-Joist to the end of a trussto make it a trim-able end. Then the truss can bemanufactured just a bit long, and easily trimmedback as needed in the field. Two of the mostcommon web patterns for floor trusses appear Is it OK to move a floor truss? Typical floor trussesare engineered to be spaced evenly, and the trussdesign drawing will tell you how far apart thetrusses are designed to be. Occasionally the needwill arise to shift one of the floor trusses fromwhere it was designed to be. When this happens,please contact the truss manufacturer to be sure itmore load on the truss youÕre moving it away from,as shown in the drawing below. Fan configuration web style Warren configuration web style II F YOU SHIFT IT : T HEN THE OVERSTRESSED TRUSS CARRIES : 24Ó 3Ó6.2% more load than it was designed for27Óon center6Ó12.5% Ò30Ótrusses9Ó18.7% Ò33Ó 16Ó3Ó9.3% more load than it was designed for19Óon center6Ó18.7% Ò22Ótrusses9Ó28.1% Ò 25Ó Check with Check with the trthe truss uss manufacturmanufacturerbefore e shifting a trshifting a truss ! uss ! ________________________________________ Exterior bearing conditions Exterior bearing conditions __________________________________________________ ¥Bottom chord bearing Ð trusses can sit on a wall, or in a floor truss hanger. ¥Top Chord bearing Ð hen the bearing is raised under the top chord, the end can be built with or withoutan end vertical, and with or without an additional slider for extra strength. ¥Mid-Height bearing Ð hen the bearing is raised between the top chord height and the bottom chordelevation, the end will use either a solid 4x block of wood, or multiple 4x2 verticals at the heel. It can alsobe built with or without an end vertical, and with or without an additional slider for extra strength. TTypical Bearing / Heel Conditions for Floor Typical Bearing / Heel Conditions for Floor Trrusses 34 35 ________________________________________ Interior bearing conditions Interior bearing conditions __________________________________________________ his truss is supported by an interior load bearing wall. ¥Cut chord condition Ð bearing wall, a truss can also be built with a cut chord condition. This truss is designed to be cutinto two separate trusses in the field. Beam Pocket Ð is truss has a ÒpocketÓ built into it so the support can be recessed up into the truss. ¥Threaded Beam Ð designed to bear on a beam, which will be designed and then threaded into the truss to helpsupport it. 36 ongbacks and Fir ¥Ribbon Notches Ð Floor truss ends can have anotch built into the top of the truss end, thebottom of the truss end, or both. The purposeof these notches is to help the framer line up the trusses. By putting a 2x4 board at the end of thefirst few trusses, the remaining trusses can ribbon board. ¥Strongbacks Ð A strongback helps to distributethe loads on a floor truss, thereby helpingeduce the ÒbounceÓ the floor system mightotherwise have. Strongbacks are typicallyspecified every 10Õ to 11Õ across a floor truss. Asyou can see in the image above, a strongbackhas been attached to the verticals of the longer ¥Fire Cut Ends Ð In some cases, floor systemswill be required to keep the top chord of the truss back away from the endwall. In such circumstances, a firecut end can be provided, as shown here. hile there are many steel truss solutions in themarketplace, AlpineÕs TrusSteel products are thebest, providing:ightweight trusses: One worker alone cantypically lift a 35Õ truss by himself. TrusSteeltrusses are easy to deliver, handle, and install.Meets fire code non-combustible materialsequirements.mmune to insect damage, material deteriorationand shrinkage, as well as from dry-wall nail pops.ItÕs an engineered product, so you can build withhe material is stiff, so installing drywall or otherceiling materials is easier. Steel TSteel Trrusses 37 Ask Charlie VAsk Charlie V.. 38 Strength-wise, what is the difference betweenconventional framing and trusses? IsnÕt V: It should be, but, how do you know? Trussesare designed with 2 to 3 times the designload. They are calculated and tested toperform at that level. The conventionalarpenter in charge of the framing is. The only thing he knows about loading is what's been done in the past from a skilled carpenter. Most don't have formaltraining today. Why use trusses? russes offer virtually unlimited architecturalversatility - complete flexibility of interiorpartitioning and room arrangement -uniformity and accuracy from one truss tothe next - faster and easier erection time -lightweight (generally 20 to 40 percent lessthan most other structural systems) - openweb design - durable. They have a provenperformance record - inspectable and "totalin-place cost" savings.Esthetically, I can take one basic floor planand leave it exactly the same. Yet, from theoutside, I can make it look like four differentfloor plans, just by changing the trusses. The onlycost difference is the trusses, not the floor plan. Iam only showing you four; but, with imagination, Ian do a few more elevations.If I have the resources, I can virtually work allthe house in fair weather, and work on the insideduring inclement weather and get a good day'swork done. If I was conventionally framing, mydays would be more subject to the elements. Most people with construction loans don't get totheir first draw until the roof is dried in. Trusses willget you there weeks ahead of conventionalframing. ith trusses, if the square foot of the floor plan isthe same, no matter what I do to the floor plan, mycost difference would be the cost of adding ordeleting a wall. Since trusses can span longerdistances than conventional framing, I donÕt oftenely on interior load bearing walls, so you canmake changes to the interior without majorproblems. Plus, I can create heavy storage areasabove the ceiling if needed, without changing the Is it OK to cut a truss in the field? Structural members of a truss should neverbe field cut without proper field repair or nearly 46 years, Charlie Vaccarohas worked in virtually every phaseof the truss industry. Originally anaeronautical engineer, he hasand trusses, started truss plants,consultant since 1970. Today, heserves as a speaker, author andfor Alpine Engineered Products, Inc. 39 engineering from the truss manufacturer.Non-structural members, such as overhangsor filler bottom chords may be cut as long asthey do not interfere with the structuralintegrity of the truss. What is temporary bracing? emporary bracing is described as bracingin WTCA and TPI's BCSI 1-03 as bracingwhich is installed to hold the trusses true toline, dimensions, and plumb. In addition,temporary bracing holds the trusses in asuch as roof or floor sheathing, joists, orrigidity of the roof or floor are in place. What is permanent bracing? ermanent bracing is bracing that will beinstalled in the roof system as a permanentpart of the roof system. The most commonis continuous lateral bracing which is aangles to a chord or web to prevent bucklingunder loads less than design loads. Otherexamples are T-braces, L-braces, etc. What are energy heels? An energy heel is a truss with a raised heelto allow for proper ventilation with increasedinsulation at the wall line.A simple energy truss is one where the topof the bottom chord just touches the bottomof the top chord, with a wedge plated on.is provides a raised heel of about 7-8inches, depending on the pitch. This is thecheapest way to go.A standard energy truss will typically have aset heel height, such as 8 or 10 inches.pically, the end condition is made with aA 12" energy heel provides for the mostinsulation and air flow. This is a good choicefor scissors trusses so that a good amount ofinsulation can be applied; and, at the sametime, plenty of room is available for air flow. Themost important issue, of course, is providing theproper air flow (ventilation) so that condensation isminimized. When the temperature differencebetween the insulation space and the attic is large,the potential for condensation is great. Wetinsulation loses its "R" value. It is imperative thatpically, a baffle of cardboard or plastic is stapledto the sheathing between the trusses to allow aclear path for air to flow. The baffle extends a littlebeyond the wall line past the point where theinsulation stops. On the inside, it extends alongthe pitch so that the outlet is above the insulationlayer.One thing that helps the condensation problem isa moisture barrier on the ceiling. This prevents thewarm, moist air from escaping into the attic. IMPLESTANDARD12Ó ENERGY HEELS 40 IÕve got to provide the truss manufacturer withtrusses. What information will they need? our local truss manufacturer may alreadyhave a form to help you with this, but theone below is a good starter. 1) hat is the Span of the truss? ___________________________ 2) Does the bottom chord of the truss run past the wall?YN(circle one)a. If so, how far?_______________________________________ 3) hat is the Height of the truss?__________________________ 4) hat size is the top chord? 2x4 2x6 2x8 Not Sure (circle one) 5) hat is the Heel Height? _______________________________ 6) a. Put an overhang on One Side Both Sides None (circle one)b. Overhang length (measure horizontally)______________________________c. Overhang type Square Cut Plumb Cut(circle one)(Note: Truss shown has a Square Cut overhang on the left, and a Plumb Cut on the right) 7) tical rise for every 24Ó horizontal run, measured using a level?_________________(Please measure to within 1/8Ó accuracy)our Name: _______________________________Phone:___________________________Best Time to Call: __________________________TodayÕs Date:______________________Any additional comments about this truss? ________________________________________ 41 CharlieÕs advice on situations to watch out for inNo purlins, or no lateral bracing under field system to keep the flat top chords of the hiptrusses from bending out of plane (usuallybracing for those purlins (in red). Engineeringfirms familiar with wood and/or steel trusssystems, such as the Alpine StructuralConsultants, can assist in designing the rightsystem. ¥Missing web bracing Ð shows a web brace (CLB, T-Brace, L-Brace, Scabbrace) and hasnÕt been applied, it needs to beapplied! Refer to the truss drawing for brace No decking under a valley set Ð that it is important to brace the top chords of thetrusses under a valley set. Apply sheathingunder the valley set, and attach the valleys wellto that sheathing. eftover girders or other trusses on the If you have set all the trusses on theoof but there are still trusses on the ground,double check the truss placement plan, and callthe truss manufacturer. You may have missedan extra girder ply, or they may have made amistake. Either way, itÕs important to beconfident that you have the roof system in place properly! Deflecting girders, or incorrect fastening of Multiple ply girders andbeams MUST be fastened together BEFORE youput any loads on them!!! The truss drawing willprovide instructions for the nail type you shoulduse and the proper nail spacing for eachmember (Top chords, bottom chords, and webs).Also, if a bearing block is specified on any truss,be sure to install it! russes installed backwards or upside down Ð r any truss that could possibly be installedupside down, or backwards, take time to look atthe truss drawings and find out how it wasdesigned to be installed. If you have installed atruss backwards or upside down, either re-installit properly, or call the truss manufacturer ASAPto work together to resolve the problem. ¥Lumber grade markings Ð russes are built withspecial engineered lumber grades, and thewood is stamped with that grade marking. If youdonÕt see those stamps on the wood, doublecheck to see that it was built using the lumbergrades and sizes specified on your engineered Missing plates, broken or cracked webmembers Ð contractor can cut through a web to installelectrical, etc. You should never cut a trusswithout consulting with the truss manufacturer. Ifyou see any of these situations, contact yourtruss manufacturer for assistance in resolvingthese problems. Trusses are engineered toperform under designed loads, but they assumeall the webs and chords are in good shape.Damage to a web or chord can require a repair(many times a simple repair for such conditions),in order for the roof to be structurally sound. GlossarGlossary of Ty of Tererms IAL FORCE - A push (compression) or pull (tension) force acting along the length of a member [usuallymeasured in pounds (lbs)]. AXIAL STRESS - he axial force acting along the length of a member, divided by the cross-sectional area ofthe member [usually measured in pounds per square inch (psi)]. BEARIN Anything which supports a truss; usually walls, hangers, beams or posts (shown in blue below). BENDING MO A measure of the bending effect on a member due to forces acting perpendicular tothe length of the member. The bending moment at the given point along a member equals the sum of allperpendicular forces, either to the left or right of the point, times their corresponding distances from the point.Usually measured in inch-pounds. BENDING STRESS - he force per square inch acting at a point along the length of a member, resulting fromthe bending moment applied at that point. Usually measured in pounds per square inch (psi). BOTTOM CHORD - Horizontal or inclined members that establish the lower edge of a truss, usually carryingcombined tension and bending stresses. 43 BRACIN See Lateral Bracing, Temporary Bracing, Permanent Bracing BUILT-UP A single unit composed of two or more wood members having the same thickness but notnecessarily the same depth, which provides a greater load carrying capacity as well as greater resistance to BUTT Slight vertical cut at outside end of truss bottom chord made to ensure uniform nominal spanand tight joints. Usually 1/4-inch. CAMBER - An upward vertical displacement built into a truss, usually to offset deflection due to dead load. CANTILEVER - he part of a truss that extends beyond its support. The truss below has a cantilever on CLEAR SPAN - Horizontal distance between interior edges of supports (see image above). COMBINED STRESS - he combination of axial and bending stresses acting on a member simultaneously,such as occurs in the top chord (compression + bending) or bottom chord (tension + bending) of a truss. CONCENTRATED LOAD - An additional load centered at a given point. An example is a crane or hoisthanging from the bottom chord at a panel point or mechanical equipment supported by the top chord. AD LOAD - rmanent loads that are constantly on the truss, i.e.: the weight of the truss itself, purlins,sheathing, roofing, ceiling, etc. DEFLECTION - Downward or horizontal displacement of a truss due to loads. DIAPHRAGM A large, thin structural element that acts as a horizontal beam to resist lateral forces on a DRAG ST pically a horizontal member, such as a truss or beam, which transfers shear from adiaphragm to a shear wall. DROPPED GABLE Ð A gable truss that has its top chord lowered vertically to allow outlookers or a gableladder to form an overhang. DURATION OF LOAD FACTOR - An adjustment in the allowable stress in a wood member, based on theduration of the load causing the stress. The shorter the time duration of the load, the higher the percentageincrease in allowable stress. ENERGY HEELSÐ See page 39. EEL Ð he vertical height of the truss at the end of the span, measured from the top of the top chord to thebottom of the bottom chord. HIP SE A trussed system where three planes come in on a slope, so the outside planes look GABLE END Ð A truss with vertical studs, usually spaced 24Ó on center (or closer). The gable usually sits onan end wall and the studs help provide support for the sheathing and resistance to wind. LATERAL BRACING - A member installed and connected at right angles to a chord or web member of a trussto resist lateral movement. LEVEL RETURN - Lumber filler placed horizontally from the end of an overhang to the outside wall to form LIVE LOAD - Any load which is not of permanent nature, such as snow, wind, seismic, movable concentratedloads, furniture, etc. Live loads are generally of short duration. MID-HEIGHT BEARING Ð A Mid-Height bearing condition is when the elevation of the bearing that supportsthe truss is above the bottom chord, and below the top chord enough to require a short vertical, which willrun from the top chord to the bearing. OVERHANG - he extension of the top chord (usually) or bottom chord of a truss beyond the support. PANEL - he chord segment defined by two successive joints. PANEL LENGTH - he centerline distance between joints measured along the chord. PANEL PO he centerline of the point of intersection in a joint where a web(s) meets a chord. PEAK - int on a truss where the sloped top chords meet. PERMANENT BRACING Ð Bracing put on a roof or floor system that is intended to remain permanently onthe roof to reinforce the structure. PIGGYBACK Ð A cap truss provided which will sit on top of the trusses below (with purlins and blocking),usually when the trusses are too tall to build, or too tall to deliver. PITC he slope of the roof, usually expressed as the vertical rise measured over a run of 12Ó (so if the roofrises 6Ó vertically for every 12Ó inches horizontally). 45 PLUMB CUT - op chord cut that is plumb to the building floor line provided for vertical installation of a fascia. PURLIN - A horizontal member in a roof perpendicular to the truss top chord used to support the decking. REACTION - ces acting on a truss through its supports that are equal but opposite to the sum of the dead RIBBON NOTCHÐ See page 36.. RIDGE Ð A ridge is the line formed when two planes meet. SHEAALL - A wall element that acts as a large vertical beam, cantilevered from the foundation to resistlateral forces on the building. SLE (Pitch) - he inches of vertical rise in 12 inches of horizontal run for inclined members, generallyexpressed as 3/12, 4/12 etc. SPAN Ð he length of the truss, measured from outside bearing to outside bearing, except in the case ofantilever conditions. If the truss is cantilevered beyond an outside bearing, the Span length would includethe length of the bottom chord beyond the outside wall. (Top or Bottom Chord Splice) - he point at which two chord members are joined together to forma single member. UARE CUT Overhang - A cut perpendicular to the slope of the member at its end. STUB Ð hen the truss is held back from its original span. SYSTEM A truss (usually a flat truss) where the 2x4 members have been rotated 90 degrees onto theirsides, resulting in a truss that is 3 _Ó wide instead of 1 _Ó wide. These trusses are usually used floor systems,but can be provided as roof trusses because you can achieve shallow depth trusses when you utilize System42s. These trusses can sometimes be manufactured using 3x2 lumber instead of 4x2 lumber. TEMPORARY BRACIN Bracing added to the roof or floor system to brace it during erection andinstallation. TOP CHORD - An inclined or horizontal member that establishes the upper edge of a truss, usually carryingcombined compression and bending stresses. TOP CHORD BEARING Ð Any time the bearing rests directly under the top chord of the truss. The image onthe right is also often referred to a Òtail bearingÓ truss. USS - A manufactured component that functions as a structural support member. A truss employs one ormore triangles in its construction. ALLEY SET - usses built to change the look of the roof system, which sit in a perpendicular direction ontop of other trusses. VIBRATION - he term associated with the serviceability of a floor. If the occupant feels the floor respond towalking or other input, it may be referred to as vibration or response to load. WEBS - Members that join the top and bottom chords to form the triangular patterns that give truss action,usually carrying tension or compression stresses (no bending). American Forest & Paper Association (AFPA) 202/463-2700 1 19th St. NW, # 700, Washington, DC 20036National Design Specification for Wood Construction(NDS)ood Frame Construction Manual American National Standards Institute (ANSI) 212/642-4900 web.ansi.or 1 West 47th Street, New York, NY 10036See TPI APA - The Engineered Wood Association 206/565-6600 .apa.wood.or 19 A Street, Tacoma, WA 98401Use of Rated Sheathing in Roofs & FloorsFire Rated Systems American Society of Civil Engineers (ASCE) www .asce.or 1801 Alexander Bell Dr., Reston, VA 20191-4400Structures, ASCE7 American Society for Testing and Materials (ASTM) www .as tm.or 1916 Rice Street, Philadelphia, PA 19103est Methods for Fire Tests for Building Construction and Building Officials and Code Administrators International, Inc.(BOCA) 08/799-2300 .boc ar esear 4051 W. Flossmoor Road, Country Club Hills, IL 60478 he BOCA National Building Code Council of AmericanBuilding Officials (CABO) 03/931-4533 .c abo.or 5203 Leesburg Pike, # 798, Falls Church, VA 22041 One and Two Family Dwelling Code orest Products Laboratory www U.S. Department of AgricultureOne Gifford Pinchot Drive, Madison, WI 53705ood Handbook: Wood as an Engineered Material Gypsum Association 202/289-5440 .gypsum.or 10 First St. NE, # 510, Washington, DC 20002Fire Resistance Design Manual, GA-600 International Code Council (ICC) 03-931-4533 .intlcode.or 5203 Leesburg Pike, #600, Falls Church, VA 22041 International Residential Code International Conference of Building Officials (ICBO) 3/699-0541 .icbo.com 5360 S. Workman Mill Rd, Whittier, CA 90601Uniform Building CodeUniform Fire Code NAHB Research Center 301-249-4000 .nahbr c.or 400 Prince Georges Blvd., Upper Marlboro, MD 20774 National Frame Builders Association (NFBA) 913/843-211 .pos 4980 W. 15th St., # 1000, Lawrence, KS 66049ost Frame Building Designost Frame Comes of Ageecommended Practices-Post Frame Construction Southern Forest Products Association (SFPA) 504/443-4464 .southernpine.com P. O. Box 64100, Kenner, LA 70064Southern Pine Maximum Spans for Joists and Rafters BCCI) 205/591-1853 .sbcci.or 900 Montclair Road, Birmingham, AL 35213-1206andard Building Codeind Design Standard, SSTD 10-93 russ Plate Institute (TPI) 608/833-5900 www .tpins t.or 583 DÕOnofrio Drive, Suite 200, Madison, WI 53719National Design Standard for Metal Plate Connectedood Truss Construction, ANSI/TPI 1-2002andard for Testing Metal Plate Connected Woodrusses, ANSI/TPI 2-1995ecommended Design Specification for TemporaryBracing of MPC Wood Trusses, DSB-89 estern Wood Products Association (WWPA) 503/224-3930 .wwpa.or 533 SW Fifth Ave., Portland, OR 97204estern Lumber Product Use Manual ood Truss Council of America 608/274-4849 .woodtruss.com One WTCA Center6300 Enterprise Ln., Madison, WI 53719Metal Plate Connected Wood Truss HandbookJob-Site Bracing Poster - TTB SeriesBuilding Component Safety Information BCSI 1-03 ____________ _______ Canadian References ences ___________ Alpine Systems Corporation 905/879-0700 .alpeng.com Moyal Court, Concord, ON L4K 4R8russ Design Procedures and Specifications for LightMetal Plate Connected Wood Trusses (Limit StatesDesign), published by TPIC Canadian Wood Truss Association - L'AssociationCanadienne des Fabricants de Fermes de Bois 3/747-5544 400 Blair Place, Suite 210, Ottawa, ON K1J 9B8ood Design Manual Canadian Standards Association 16/747-4044 .cs exdale Boulevard ¥ Rexdale, ON M9W 1R3SA 086.194 "Engineering Design in Wood (Limit States Design)"SA S347-M1980 "Method of Test for Evaluation of Truss National Research Council of Canada 613/993-2463 www .nr a/ir Institute for Research in Construction500 Montreal Road ¥ Ottawa, ON K1A 9Z9National Building Code of Canada (NBCC)National Farm Building Code of Canada (NFBCC) light Gauge Steel Refer Light Gauge Steel Engineers Association 866/465-4732 .lgsea.com 5th Street, N.W., Suite 320, Washington, DC 20005Field Installation Guide for CFS Roof Trussesesign Guide: Permanent Bracing of CFS Steel Trusses Appendix B Ð ReferAppendix B Ð Referencesences 48