wood trusses fox nov 08

7/27/2019 Wood Trusses Fox Nov 08

1/4STRUCTURE magazine November 2008

discussionand

updatesonstructuralmaterials

Building

Blocks

46

Repair of Wood TrussesBy Steven E. Fox, P.E.

I you are not amiliar with metal plateconnected (MPC) wood trusses, take alook around. They are everywhere thesedays. From humble beginnings 50 yearsago, the use o wood trusses has grownat an amazing rate. They are ound innearly all types o residential construc-

tion, and are highly competitive inagricultural buildings as well as smallcommercial projects. You may even haveseen them used as part o elaborate con-crete orm-work or large construc-tion projects. While the growth o thestructural building component industryhas been excellent over the years, it is not

without some growing pains. As the useo trusses has increased throughout thecountry, so has the incidence o trussdamage and modication.Trusses are generally considered two-

dimensional structural elements. They

are typically manuactured rom dim-ensional lumber 12 inches thick and

joined with 16, 18, or 20 gauge metalplates, with integral teeth stamped intothe steel. While quite strong in theplane o the truss, they are susceptible todamage i allowed to fex out o plane.

This so called out-o-plane bendingmost oten occurs during unloading orerection, and can result in breaks andsplits in the lumber as well as connectorplate damage ranging rom toothpull-out to steel ailure (Figures 1 and

2). Residential construction typicallyincurs less handling damage, due to thesmaller truss spans involved. However,the complex nature o the roo andceiling planes common in todays homeshas resulted in an increase in the modica-tions required to correct or geometric

errors and homeowners preerences.

Truss repairs range rom the simple addition o dimensional lumber to morinvolved reinorcements utilizing gussets, beams, preabricated truss sectionsand even steel plates and angles. Unortunately, there are no code provisionand very limited text book examples t

guide engineers at the design stage. Thentire repair process relies heavily othe individual engineers ability to appltheory, real world judgment, engineering common sense and knowledge oeld conditions to develop a practicaeconomical, and realistic repair or eacgiven situation. Most engineers that design truss repairs learn the skills romanother engineer with past experience

While the theory o connections is documented, the art o truss repair remainan elusive concept that can only b

developed over years o practice.The repair design process varies witthe complexity o the damage. Typicallyor damaged trusses, the original analysiis a valid starting point or determininthe axial, shear and bending orcein the damaged area o the truss

Almost all MPC wood trusses are designed on proprietary sotware romthe connector plate manuacturer. Thisotware is dedicated exclusively to trusdesign, and contains all o the lumbeand connector plate properties require

to design MPC wood trusses. While is possible to use independent structuradesign sotware, the time involved tmatch an existing truss design is oteextensive due to the lack o speciconnector plate values. In cases wherthere is a change required in the trusshape, the truss must be redesigne

with the new prole. Keep in mind, thexisting materials must now be capablo withstanding the new orces due tthe altered prole. Any new materiaadded should be available locally oprovisions should be made through thtruss manuacturer to ship less commomaterials to the job site.Once the orces have been determined

the ocus turns to connections. There ara variety o ways to reconnect memberin wood trusses. The two most commoare dimensional lumber scabs angussets o plywood/Oriented StranBoard (O.S.B.). Plywood and O.S.B. argenerally interchangeable. The AmericanPlywood Associations (APA)Panel Desig

Figure 1: A typical damaged joint. Notice the connector plate teeth have pulled out o the lumberon the right side o the joint, and one web is split along the grain. Also, note the mud on thelumber, a consideration when using adhesives.

Figure 2: Damage to the ends o three trusses resulting when additional trusses were dropped on topo them during delivery.



Specifcation provides design values or bothO.S.B. and plywood. Allowable stresses ordimensional lumber are ound in AmericanForest and Paper Associations (AFPA)National Design Specifcation (NDS) and sup-

plement. In certain instances, plated trusssections are manuactured to solve problems

where gussets and lumber will not suce dueto strength or geometric considerations. Thenew truss sections are attached to the aceo the existing truss and connected in theappropriate aligning members. I available,a portable hydraulic press may be used toinstall new members within the plane o theexisting truss with connector plates. Repairedtrusses utilizing a portable press are otenindistinguishable rom a new truss directlyrom the manuacturer. This can be very ad-

vantageous in instances where appearance isa concern.Fastener selection can oten prove to be a

challenging exercise, due to the wide variety oconnectors available. The metal plates used inthe manuacture o wood trusses have excellentgrip due to the numerous teeth embedded intothe lumber. When calculating the quantitieso other mechanical asteners required torepair a damaged plate, it is oten surprisingto see how much larger the connection areasbecome. Generally, the nail is the most widelyused astener or wood construction. Nails

are commonly reerred to by penny-weight.Unortunately, this designation does nothave clearly dened dimensions. There maybe our or more dierent nails commonlyreerred to by a single penny-weight. Forexample, a 10 penny (10d) nail could reer toa sinker, common, box, cooler or pneumatic(gun) nail, all o which are slightly dierent.To eliminate conusion, it is important tospeciy all nails by length and diameter. Woodscrews are another option. Wood screws can

have higher shear values than nails, but otenrequire pilot holes to prevent splitting. Theengineer must judge when the extra laborinvolved to drill pilot holes is worth the eort.Please note that many general purpose screws,

such as deck screws and drywall screws,may share a common gauge number with aheavier wood screw, but are not consideredstructural due to the lower grade steel used intheir manuacture and should be avoided ortruss repairs. Recently, specialty screws havebecome available with sel-drilling tips, largerdiameters, and high yield strengths, resultingin superior perormance in relation to standardscrews. These screws are manuactured byUnited Steel Product, Simpson Strong-tie,and Fasten-Master, to name a ew. Due tothe proprietary nature o these asteners, it is

best to reer to the manuacturers literatureor design values. Finally, machine bolts aregenerally used where high axial orces areinvolved. Typically, bolts such as ASTM

A307 are specied. Connections in woodtrusses rarely benet rom high strength bolts,making them unnecessary. Carriage bolts arenot recommended; the lack o a washer andsolid bearing on the head o the bolt resultsin poor perormance in relation to machinebolts. Lag screws may be used, but are olimited capacity due to the relatively thin(12-inch) thickness o the truss members.

The lack o penetration limits their lateralstrength signicantly.The use o adhesives or truss repairs raises

considerable debate among engineers. Thequality o some adhesives today allow orbonding wood with greater strength than the

wood itsel, resulting in much smaller connec-tions. However, the conditions under whichmany truss repairs are perormed can com-promise the glue bond. For example, reezingtemperatures, surace dirt, and unsupervised

8.2"

12.4"

12.0"

3600 lbs

5937 lbs

4111 lbs

DAMAGED PLATE

F

P

O

S

Figure 3: Minimum required gussets widths.



have been 2000 pounds. The result othe improper nailing is a connectionthat achieves only 75 percent o thdesign strength. One option is to takinto account the 75 percent eectivstrength and increase the quantity onails at the design stage, assuring aadequate connection.To add perspective to the previou

discussion, consider the ollowin

example. Figure 3 (page 47) depicta typical vaulted truss with a damaged connector plate at the bottomchord apex. The truss was originalldesigned on MiTek Industries 20/2engineering sotware. The orces determined or the original analysis arstill valid because there has not beeany change to the truss geometry oloading. Members F-P, O-P, and P-

are the most critical or this connectionThese members are all in tension undegravity loading, which requires an investigation o the gussets tensile strength. Plywooand O.S.B. have variable tension capacitieat dierent angles relative to the strengthaxis o the gusset. For axial orces paralleto the strength axis, the allowable tensiocapacity, Ft, is taken rom APAs Panel DesigSpecifcation as 467 pounds/inch/pair at degrees and 208 pounds/inch/pair at 90 degrees, assuming2-inch gussets on each aco the truss. Linear interpolation is used tdetermine the eective tension capacity avarious angles. Chord member O-P require12.0 inches o gusset based on an angle o12.9 degrees and a load duration increase o1.15. The required width or the other critical members is calculated in a similar mannerThese widths are illustrated by the dashelines along the members in Figure 3 .It can be seen rom the drawing that th

required widths are signicantly larger thathe widths o the truss members. A rule othumb or limiting the eective width ogussets is twice the member width. In thicase, the members are all 2 x 4s, allowing inches maximum. The required widths exceed this maximum in all o the criticamembers. However, due to the nature o thi

particular joint, it is justiable to assume thathe orces, as they are developed by the nails

will not be ully transerred into the gusseuntil very near joint P. Figure 4 shows thassumed distribution o orces based upothe minimum required gusset widths. Due tthe wide spread o the members pulling thgusset rom dierent angles, the distributioo orces is reasonable or the conditions othis truss. Thereore, the gusset is adequator the orces present. Figure 5shows the ugusset in position on the truss. Due to th

PROJECTED AREA OF 48" x 96" GUSSET

ASSUMED FORCEDISTRIBUTION(HATCHED AREA)

Figure 4: Distribution o tension orces through the gusset.

labor make the capacity o the glued jointsdicult, i not impossible, to judge. Me-chanical asteners can be counted to veriy

conormance with the repair specication.Unortunately, once the glue is applied andcovered, it cannot be seen without destroyingthe repair. Thereore, adhesive use is best letor controlled environments, such as actoriesand jobs where the responsible engineer canobserve the application o the adhesives. I anadhesive is to be used, the engineer must con-sider the curing time. In many instances, atruss must be repaired in place. Once a repairis completed, temporary supports are otenremoved and construction materials areplaced on the truss immediately. This prac-tice does not allow proper time or the adhe-sive to cure. One solution to this problem isto provide mechanical asteners, designed tosupport the construction loads, allowing theadhesive the required time to gain sucientstrength beore the ull design load is appliedto the truss. One other school o thought isto speciy the adhesive, but not to consider itin the calculations. This is most oten donewhen repairing foor trusses. The mechanicalasteners transer the orces and the adhesive

Project M US Patent 7,037,030is Available O-the-Shel Framing or diverse use-groups. Pre-welded / pre-nished members and

resilient insulating panels. Fastening accessories available mechanical

or hinged connections. All parts structurally designed and oered in

Kit ormat or swit assemblage.

Keith McLemore, MBA773-878-3423

[email protected]

provides insurance against squeaks. I the ad-hesive is omitted, the repair is still valid. I itis used, it only adds strength and stiness to

the repair.Shear values or gusset connections usingnails are given in NDS. A common practiceused by many engineers to reduce the con-nection size is to clinch the nails used withgussets, resulting in each nail being placed indouble shear. In order to clinch nails, a gussetis placed on each ace o a single ply truss.The gusset must be clamped or held in placesecurely during the nailing procedure. Nailsare driven through the ront gusset, throughthe truss, and through the gusset on the backace. The nails must then be bent over fatagainst the surace o the back gusset. NDSspecies the conditions required to considerclinching. Please bear in mind that clinchingmust be perormed properly. Unortunately,many repairs speciying clinched nails areperormed incorrectly at the jobsite. The rstgusset is applied and nails are driven throughthe gusset and truss and then bent over againstthe back ace o the truss. The second gusset isthen applied and nails are driven through allthree members and clinched. The small gap

created between the ace o the truss andthe second gusset due to the rst set oclinched nails, while not ideal, is generallynot a concern. However, this processresults in hal the nails in single shearand hal in double shear. For example,consider a repair using twenty nails, tenrom each ace. The rst set o ten nails(assuming 50 pounds per nail in singleshear), has a capacity o 500 pounds.The second ten nails are in double shear(with 100 pounds per nail) yielding1000 pounds. The total capacity is 1500pounds. I all twenty nails were properlyclinched, the resulting capacity would

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wood trusses fox nov 08

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