Truss-Repair.Pdf

B uilding B locks S T R U C T U R E discussion and updates on structural materials Reprinted and distributed with permission by STRUCTURE Reprinted anddistributed withpermission of themduringdelivery. Figure 2:Damage totheendsofthree resulting trusses were whenadditional trusses dropped ontop lumber, aconsideration whenusingadhesives. on therightsideofjoint,andoneweb issplitalongthegrain. Also, notethemudon Figure 1:Atypicaldamagedjoint.Notice theconnectorplateteethhave pulledoutofthelumber They elements. structural dimensional truss modification. and damage of incidence the has so country, the throughout increased has trusses of use the As pains. growing without some has been excellent over the years, it is not industry component building structural the of construc growth the While large projects. tion for form-work crete con- elaborate of part as used them seen small projects.commercial You as haveeven in may well as competitive buildings highly agricultural are and tion, construc residential of types all nearly in found are They rate. amazing an at grown has trusses wood of use the ago, years 50 beginnings humble From days. these everywhere are They around. look a take trusses, wood (MPC) connected By Steven E.Fox, P.E. Repair ofWood Trusses rse ae eeal cniee two- considered generally are Trusses plate metal with familiar not are you If

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ninl ubr 1 lumber ensional dim- from manufactured typically are errors and homeowner’s geometric preferences. for correct to required tions has resulted and in an increase roof in the modifica- the in today’s common planes ceiling homes of nature complex the However, involved. spans truss smaller the to due damage, handling less incurs Copyright tooth 2 from ( failure steel ranging to pull-out damage plate connector as well as and lumber splitsthe in breaks in result can and erection, or unloading during occurs often most bending “out-of-plane” called so This plane. of out flex to allowed if damage the in strong plane they of arethe truss, susceptible to quite While steel. the into stamped teeth integral with plates, metal gauge 20 or 18, 16, with joined ). Residential construction typically typically construction Residential ). 46 ® magazine 2 inches thick and and thick inches iue 1 Figures  November 2008 and and an elusive concept that can only be be only can developed over years ofpractice. that concept elusive an remains repair truss of art the mented, While the theory of connections is docu experience. past with engineer from another skills the learn repairs truss sign de that engineers Most situation. given each for repair realistic and economical, apractical, to develop conditions of field knowledge and sense common ing engineer judgment, world real theory, on apply engineer’sto individual heavily ability the relies process repair entire The stage. design the at engineers guide to examples book text limited very provisions and code no are there tunately, Unfor angles. and plates steel even and sections, truss prefabricated beams, sets, gus utilizing reinforcements involved more to lumber dimensional of dition Plywood Association’s (APA) generally interchangeable. The American Strand and plywood/OrientedBoard (O.S.B.). Plywood and O.S.B. are scabs of lumber gussets dimensional are trusses. common wood most in two The members reconnect to ways of variety a the focus turns to connections. There are materials tothejobsite. common less ship to manufacturer truss or the locally through made be should available provisions be should material new added Any profile. altered the to due forces new the withstanding of capable be now must materials existing redesigned be the mind, in Keep profile. new must the with truss the truss shape, the in required change a is there where specific cases In of values. lack plate connector the to due extensive often is design truss existing to an match involved time the software, design structural independent use to possible is it While trusses. wood MPC design to required properties plate connector and lumber the of all contains and design, truss to exclusively dedicated is software from This manufacturer. plate connector software the proprietary truss. on the de- signed are of trusses wood MPC area all forces Almost damaged bending the and in shear axial, the determining for point starting valid a is for damaged trusses, the original analysis damage. the of Typically,complexity the h rpi dsg poes ais with varies process design repair The Trussad simple the from range repairs Once the forces have been determined, been have forces the Once 

www.STRUCTUREmag.org Panel Design

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F 4111 lbs

12.4" ® 12.0" P 5937 lbs 8.2" DAMAGED PLATE O 3600 lbs S

Figure 3: Minimum required gussets widths. Figure 3 Copyright

Specification provides design values for both have higher shear values than nails, but often O.S.B. and plywood. Allowable stresses for require pilot holes to prevent splitting. The dimensional lumber are found in American engineer must judge when the extra labor Forest and Paper Association’s (AFPA) involved to drill pilot holes is worth the effort. National Design Specification (NDS) and sup- Please note that many general purpose screws, plement. In certain instances, plated truss such as deck screws and drywall screws, visit ADVERTISEMENT Information, - For Advertiser sections are manufactured to solve problems may share a common gauge number with a where gussets and lumber will not suffice due heavier wood screw, but are not considered to strength or geometric considerations. The structural due to the lower grade steel used in new truss sections are attached to the face their manufacture and should be avoided for of the existing truss and connected in the truss repairs. Recently, specialty screws have appropriate aligning members. If available, become available with self-drilling tips, larger a portable hydraulic press may be used to diameters, and high yield strengths, resulting install new members within the plane of the in superior performance in relation to standard existing truss with connector plates. Repaired screws. These screws are manufactured by www. trusses utilizing a portable press are oftenmagazine United Steel Product, Simpson Strong-tie, STRUCTUREmag indistinguishable from a new truss directly and Fasten-Master, to name a few. Due to from the manufacturer. This can be very ad- the proprietary nature of these fasteners, it is

S T R U C T U R E.org vantageous in instances where appearance is best to refer to the manufacturer’s literature a concern. for design values. Finally, machine bolts are Fastener selection can often prove to be a generally used where high axial forces are challenging exercise, due to the wide variety of involved. Typically, bolts such as ASTM connectors available. The metal plates used in A307 are specified. Connections in wood the manufacture of wood trusses have excellent trusses rarely benefit from high strength bolts, grip due to the numerous teeth embedded into making them unnecessary. Carriage bolts are the lumber. When calculating the quantities not recommended; the lack of a washer and of other mechanical fasteners required to solid bearing on the head of the bolt results repair a damaged plate, it is often surprising in poor performance in relation to machine to see how much larger the connection areas bolts. Lag screws may be used, but are of become. Generally, the nail is the most widely limited capacity due to the relatively thin used fastener for wood construction. Nails (12-inch) thickness of the truss members. are commonly referred to by penny-weight. The lack of penetration limits their lateral Unfortunately, this designation does not strength significantly. have clearly defined dimensions. There may The use of adhesives for truss repairs raises Thank you for reviewing this ad proof for the upcoming issue of STRUCTURE® Magazine. be four or more different nails commonly considerable debate among engineers. The To ensure that the proper advertisement for your company is run, please print out this referred to by a single penny-weight. For quality of some adhesives today allow for document, fi ll out the information below and fax it to us at: 608-524-4432. example, a 10 penny (10d) nail could refer to bonding wood with greater strength than the a sinker, common, box, cooler or pneumatic wood itself, resulting in much smaller connec- Yes, the ad looks fi ne. (gun) nail, all of which are slightly different. tions. However, the conditions under which To eliminate confusion, it is important to many truss repairs are performed can com- No, we require the following changes: specify all nails by length and diameter. Wood promise the glue bond. For example, freezing screws are another option. Wood screws can temperatures, surface dirt, and unsupervised

If we recieve no fax within 48 hours of this email, we will assume that there is no change

® necessary and will run the ad as presented here. Thank you for your assistance. Reprinted and distributed with permission by STRUCTURE 47magazine  November 2008  www.STRUCTUREmag.org

Inside_Cover_Outside_Cover_Perfe1 1 6/4/2008 3:14:06 PM have been 2000 pounds. The result of PROJECTED AREA OF 48" x 96" GUSSET the improper nailing is a connection that achieves only 75 percent of the design strength. One option is to take into account the 75 percent effective strength and increase the quantity of nails at the design stage, assuring an adequate connection. ® To add perspective to the previous discussion, consider the following example. Figure 3 (page 47) depicts a typical vaulted truss with a dam- aged connector plate at the bottom ASSUMED FORCE chord apex. The truss was originally DISTRIBUTION designed on MiTek Industries’ 20/20 (HATCHED AREA) engineering software. The forces de- termined for the original analysis are Copyright still valid because there has not been Figure 4: Distribution of tension forces through the gusset. Figure 4 any change to the truss geometry or loading. Members F-P, O-P, and P-S labor make the capacity of the glued joints provides insurance against squeaks. If the ad- are the most critical for this connection. difficult, if not impossible, to judge. Me- hesive is omitted, the repair is still valid. If it These members are all in tension under chanical fasteners can be counted to verify is used, it only adds strength and stiffness to gravity loading, which requires an investiga- conformance with the repair specification. the repair. tion of the gussets’ tensile strength. Plywood Unfortunately, once the glue is applied and Shear values for gusset connections using and O.S.B. have variable tension capacities covered, it cannot be seen without destroying nails are given in NDS. A common practice at different angles relative to the strength the repair. Therefore, adhesive use is best left used by many engineers to reduce the con- axis of the gusset. For axial forces parallel for controlled environments, such as factories nection size is to “clinch” the nails used with to the strength axis, the allowable tension and jobs where the responsible engineer can gussets, resulting in each nail being placed in capacity, Ft, is taken from APA’s Panel Design observe the application of the adhesives. If an double shear. In order to clinch nails, a gusset Specification as 467 pounds/inch/pair at 0 adhesive is to be used, the engineer must con- is placed on each face of a single ply truss. degrees and 208 pounds/inch/pair at 90 de- sider the curing time. In many instances, a The gusset must be clamped or held in place grees, assuming 2-inch gussets on each face truss must be repaired in place. Once a repair securely during the nailing procedure. Nails of the truss. Linear interpolation is used to is completed, temporary supports are often are driven through the front gusset, through determine the effective tension capacity at removed and construction materials aremagazine the truss, and through the gusset on the back various angles. Chord member O-P requires placed on the truss immediately. This prac- face. The nails must then be bent over flat 12.0 inches of gusset based on an angle of tice doesS not allow properT time for theR adhe- againstU the surface of Cthe back gusset. T NDS 12.9U degrees and a loadR duration increaseE of sive to cure. One solution to this problem is specifies the conditions required to consider 1.15. The required width for the other criti- to provide mechanical fasteners, designed to clinching. Please bear in mind that clinching cal members is calculated in a similar manner. support the construction loads, allowing the must be performed properly. Unfortunately, These widths are illustrated by the dashed adhesive the required time to gain sufficient many repairs specifying clinched nails are lines along the members in Figure 3 . strength before the full design load is applied performed incorrectly at the jobsite. The first It can be seen from the drawing that the to the truss. One other school of thought is gusset is applied and nails are driven through required widths are significantly larger than to specify the adhesive, but not to consider it the gusset and truss and then bent over against the widths of the truss members. A rule of in the calculations. This is most often done the back face of the truss. The second gusset is thumb for limiting the effective width of when repairing floor trusses. The mechanical then applied and nails are driven through all gussets is twice the member width. In this fasteners transfer the forces and the adhesive three members and clinched. The small gap case, the members are all 2 x 4’s, allowing 7 created between the face of the truss and inches maximum. The required widths ex- ADVERTISEMENT ceed this maximum in all of the critical For Advertiser Information, visit www.STRUCTUREmag.org the second gusset due to the first set of clinched nails, while not ideal, is generally members. However, due to the nature of this Project M US Patent 7,037,030 not a concern. However, this process particular joint, it is justifiable to assume that is Available results in half the nails in single shear the forces, as they are developed by the nails, • Off-the-Shelf Framing for diverse use-groups. and half in double shear. For example, will not be fully transferred into the gusset • Pre-welded / pre-finished members and consider a repair using twenty nails, ten until very near joint P. Figure 4 shows the resilient insulating panels. from each face. The first set of ten nails assumed distribution of forces based upon • Fastening accessories available mechanical the minimum required gusset widths. Due to or hinged connections. (assuming 50 pounds per nail in single the wide spread of the members pulling the • All parts structurally designed and offered in shear), has a capacity of 500 pounds. gusset from different angles, the distribution Kit format for swift assemblage. The second ten nails are in double shear (with 100 pounds per nail) yielding of forces is reasonable for the conditions on Keith McLemore, MBA 1000 pounds. The total capacity is 1500 this truss. Therefore, the gusset is adequate 773-878-3423 pounds. If all twenty nails were properly for the forces present. Figure 5 shows the full [email protected] clinched, the resulting capacity would gusset in position on the truss. Due to the

® Reprinted and distributed with permission by STRUCTURE magazine48  November 2008  www.STRUCTUREmag.org nature of gusset repairs, there is often excess material in some areas DAMAGED PLATE as is noticeable on the drawing. It is acceptable to adjust the gusset to a slightly smaller overall size or shape, although the simpler 48" X 96" the gusset shape, the easier it is to cut and install. Appearance ® is generally not an issue, as the repair will not be visible once the ceiling has been installed. Due to the lack of code provisions 13" governing truss repairs, engineering judgment becomes critical in the determination of what is accept- 48" able. While the design capacities of various materials are available to aid an engineer’s calculations, Copyright there are times when repairs tend ATTACH 1/2" PLYWOOD OR OSB GUSSET (15/32" APA RATED SHEATHING 32/16 EXP 1) to bring the art of engineering to a TO EACH FACE OF TRUSS WITH 10d (3" X .131") NAILS DRIVEN THROUGH BOTH SHEETS level almost equal to the science.▪ 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. Figure 5: Finalized repair.

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 wood components for the past 18 years. Steven can be reached via email at [email protected].

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® Reprinted and distributed with permission by STRUCTURE 49magazine  November 2008  www.STRUCTUREmag.org

Thank you for reviewing this ad proof for the upcoming issue of STRUCTURE® Magazine. To ensure that the proper advertisement for your company is run, please print out this document, fi ll out the information below and fax it to us at: 608-524-4432.

Yes, the ad looks fi ne.

No, we require the following changes:

If we recieve no fax within 48 hours of this email, we will assume that there is no change necessary and will run the ad as presented here. Thank you for your assistance.

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