repair of wood trusses

8/21/2019 Repair of Wood Trusses

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d i s c u s s i o n a n d

u p d a t e s o n s t r u c t u r a l m a t e r i a l s

B U I L D I N G

B L O C K S

Reprinted and distributed with permission by STRUCTURE® magazine November 2008 www.STRUCTUREmag.or46

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

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

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

without some growing pains. As the useof trusses has increased throughout thecountry, so has the incidence of trussdamage and modification.Trusses are generally considered two-

dimensional structural elements. They

are typically manufactured from 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 of the truss, they are susceptible todamage if allowed to flex out of plane.

This so called “out-of-plane” bendingmost often occurs during unloading orerection, and can result in breaks andsplits in the lumber as well as connectorplate damage ranging from toothpull-out to steel failure (Figures 1 and

2 ). Residential construction typicallyincurs less handling damage, due to thesmaller truss spans involved. However,the complex nature of the roof andceiling planes common in today’s homeshas resulted in an increase in the modifica-tions required to correct for geometric

errors and homeowner’s preferences.

Truss repairs range from the simple addition of dimensional lumber to morinvolved reinforcements utilizing gussets, beams, prefabricated truss sectionsand even steel plates and angles. Unfortunately, there are no code provisionand very limited text book examples t

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

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

developed over years of practice.The repair design process varies witthe complexity of the damage. Typicallyfor damaged trusses, the original analysiis a valid starting point for determininthe axial, shear and bending forcein the damaged area of the truss

Almost all MPC wood trusses are designed on proprietary software fromthe connector plate manufacturer. Thisoftware is dedicated exclusively to trusdesign, and contains all of the lumbeand connector plate properties require

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

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

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

Figure 1: A typical damaged joint. Notice the connector plate teeth have pulled out of the lumberon the right side of 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 of three trusses resulting when additional trusses were dropped on topof them during delivery.


http://slidepdf.com/reader/full/repair-of-wood-trusses 2/4Reprinted and distributed with permission by STRUCTURE® magazine November 2008 www.STRUCTUREmag.org47

Specification provides design values for bothO.S.B. and plywood. Allowable stresses fordimensional lumber are found in AmericanForest and Paper Association’s (AFPA)National Design Specification (NDS) and sup-

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

where gussets and lumber will not suffice dueto strength or geometric considerations. Thenew truss sections are attached to the faceof the existing truss and connected in theappropriate aligning members. If available,a portable hydraulic press may be used toinstall new members within the plane of theexisting truss with connector plates. Repairedtrusses utilizing a portable press are oftenindistinguishable from a new truss directlyfrom the manufacturer. This can be very ad-

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

challenging exercise, due to the wide variety ofconnectors available. The metal plates used inthe manufacture of wood trusses have excellentgrip due to the numerous teeth embedded intothe lumber. When calculating the quantitiesof other mechanical fasteners required torepair a damaged plate, it is often surprisingto see how much larger the connection areasbecome. Generally, the nail is the most widelyused fastener for wood construction. Nails

are commonly referred to by penny-weight.Unfortunately, this designation does nothave clearly defined dimensions. There maybe four or more different nails commonlyreferred to by a single penny-weight. Forexample, a 10 penny (10d) nail could refer toa sinker, common, box, cooler or pneumatic(gun) nail, all of which are slightly different.To eliminate confusion, it is important tospecify all nails by length and diameter. Woodscrews are another option. Wood screws can

have higher shear values than nails, but oftenrequire pilot holes to prevent splitting. Theengineer must judge when the extra laborinvolved to drill pilot holes is worth the effort.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 manufacture and should be avoided fortruss repairs. Recently, specialty screws havebecome available with self-drilling tips, largerdiameters, and high yield strengths, resultingin superior performance in relation to standardscrews. These screws are manufactured byUnited Steel Product, Simpson Strong-tie,and Fasten-Master, to name a few. Due tothe proprietary nature of these fasteners, it is

best to refer to the manufacturer’s literaturefor design values. Finally, machine bolts aregenerally used where high axial forces areinvolved. Typically, bolts such as ASTM

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

The lack of penetration limits their lateralstrength significantly.The use of adhesives for truss repairs raises

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

wood itself, resulting in much smaller connec-tions. However, the conditions under whichmany truss repairs are performed can com-promise the glue bond. For example, freezingtemperatures, surface 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 connectiothat achieves only 75 percent of thdesign strength. One option is to takinto account the 75 percent effectivstrength and increase the quantity onails at the design stage, assuring aadequate connection.To add perspective to the previou

discussion, consider the followin

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 software. The forces determined for 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 for this connectionThese members are all in tension undegravity loading, which requires an investigation of the gussets’ tensile strength. Plywooand O.S.B. have variable tension capacitieat different angles relative to the strengthaxis of the gusset. For axial forces paralleto the strength axis, the allowable tensiocapacity, Ft, is taken from APA’s Panel DesigSpecification as 467 pounds/inch/pair at degrees and 208 pounds/inch/pair at 90 degrees, assuming 2-inch gussets on each facof the truss. Linear interpolation is used tdetermine the effective tension capacity avarious angles. Chord member O-P require12.0 inches of gusset based on an angle o12.9 degrees and a load duration increase o1.15. The required width for 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 from the drawing that th

required widths are significantly larger thathe widths of the truss members. A rule othumb for limiting the effective width ogussets is twice the member width. In thicase, the members are all 2 x 4’s, allowing 7inches maximum. The required widths exceed this maximum in all of the criticamembers. However, due to the nature of thi

particular joint, it is justifiable to assume thathe forces, as they are developed by the nails

will not be fully transferred into the gusseuntil very near joint P. Figure 4 shows thassumed distribution of forces based upothe minimum required gusset widths. Due tthe wide spread of the members pulling thgusset from different angles, the distributioof forces is reasonable for the conditions othis truss. Therefore, the gusset is adequatfor the forces present. Figure 5 shows the fugusset in position on the truss. Due to th

PROJECTED AREA OF 48" x 96" GUSSET

ASSUMED FORCEDISTRIBUTION(HATCHED AREA)

Figure 4: Distribution of tension forces through the gusset.

labor make the capacity of the glued jointsdifficult, if not impossible, to judge. Me-chanical fasteners can be counted to verify

conformance with the repair specification.Unfortunately, once the glue is applied andcovered, it cannot be seen without destroyingthe repair. Therefore, adhesive use is best leftfor controlled environments, such as factoriesand jobs where the responsible engineer canobserve the application of the adhesives. If 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 oftenremoved and construction materials areplaced on the truss immediately. This prac-tice does not allow proper time for the adhe-sive to cure. One solution to this problem isto provide mechanical fasteners, designed tosupport the construction loads, allowing theadhesive the required time to gain sufficientstrength before the full design load is appliedto the truss. One other school of thought isto specify the adhesive, but not to consider itin the calculations. This is most often donewhen repairing floor trusses. The mechanicalfasteners transfer the forces and the adhesive

Project M US Patent 7,037,030is Available

• Off-the-Shelf Framing for diverse use-groups.• Pre-welded / pre-finished members and

resilient insulating panels.• Fastening accessories available mechanical

or hinged connections.• All parts structurally designed and offered in

Kit format for swift assemblage.

Keith McLemore, MBA 773-878-3423

[email protected]

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

the repair.Shear values for 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 face of a single ply truss.The gusset must be clamped or held in placesecurely during the nailing procedure. Nailsare driven through the front gusset, throughthe truss, and through the gusset on the backface. The nails must then be bent over flatagainst the surface of the back gusset. NDSspecifies the conditions required to considerclinching. Please bear in mind that clinchingmust be performed properly. Unfortunately,many repairs specifying clinched nails areperformed incorrectly at the jobsite. The firstgusset is applied and nails are driven throughthe gusset and truss and then bent over againstthe back face of the truss. The second gusset isthen applied and nails are driven through allthree members and clinched. The small gap

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

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Figure 5: Finalized repair.

ATTACH 1/2" PLYWOOD OR OSB GUSSET (15/32" APA RATED SHEATHING 32/16 EXP 1)

TO EACH FACE OF TRUSS WITH 10d (3" X .131") NAILS DRIVEN THROUGH BOTH SHEETS

OF PLYWOOD AND CLINCHED PER THE FOLLOWING NAIL SCHEDULE:

2 ROWS: SPACED @ 0-4-0 O.C.

NAILS TO BE DRIVEN FROM BOTH FACES. STAGGER SPACING FROM FRONT TO BACK FACE FOR A

NET 0-2-0 O.C. SPACING IN THE MAIN MEMBER. USE A M IN. 0-3-0 MEMBER END DISTANCE.

13"

48"

DAMAGED PLATE

48" X 96"

Steven E. Fox, P.E. is currently a Senior Design Engineer for MiTek Industries, Inc. He has specialized in the repair of metal plate connected woodcomponents for the past 18 years. Steven can be reached via email at [email protected].

nature of gusset repairs, there isoften excess material in some areasas is noticeable on the drawing. Itis acceptable to adjust the gussetto a slightly smaller overall sizeor shape, although the simplerthe gusset shape, the easier it isto cut and install. Appearanceis generally not an issue, as therepair will not be visible once theceiling has been installed.

Due to the lack of code provisionsgoverning truss repairs, engineering

judgment becomes critical in thedetermination of what is accept-able. While the design capacitiesof various materials are availableto aid an engineer’s calculations,there are times when repairs tendto bring the art of engineering to alevel almost equal to the science .▪

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repair of wood trusses

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