Structures Congress 2012 © ASCE 20121970 Failure of Wood Connections and the Evolution of Connection Design Jaret Lynch, P.E. 1 & John Cocca, P.E. 2 Wiss, Janney, Elstner Associates, Inc., 2 Trap Falls Road, Suite 502, Shelton, CT 06484; PH (203) 944-9424; FAX (203) 944-6997; email:
[email protected] 2 Wiss, Janney, Elstner Associates, Inc., 2 Trap Falls Road, Suite 502, Shelton, CT 06484; PH (203) 944-9424; FAX (203) 944-6997; email:
[email protected] Downloaded from ascelibrary.org by Paulo Sepulveda on 12/19/12. Copyright ASCE. For personal use only; all rights reserved. 1 ABSTRACT Wood has been used as a construction material long before concrete and steel were available as structural materials. Wood is easy to work with and can be used for many different types of framing. Wood trusses have been used for long span roof framing in the United States since the 1800’s. Over time the material, wood, has remained largely unchanged; however, the connections have evolved as well as the standards to which they are designed. Before the 1850’s, truss connections typically consisted of mortise and tenon joinery. These connections make good use of the inherent strength of the wood members with little or no reliance on fasteners or engineering analysis. During the late 1800’s and early 1900’s these connections evolved into side lapped joints with steel bolts which bear on the wood in order to transfer shear through the joint. During the mid 1900’s the invention of shear plates and split rings allowed joint forces to be transferred with fewer bolts. During the late 1900’s pressed plate connections were developed which permited wood trusses to be readily pre-fabricated. In addition to the evolution of the connections, advancements in the testing of materials and creation of design standards have evolved into what they are today. This paper provides examples of actual woods connection failures from the different time periods mentioned above and discusses the cause of the failures. Also provided is a comparison of historic references versus modern design methods for the evaluation of the failed connections. INTRODUCTION Below are examples of four wood connections dating from 1843 to 1955 that do not have adequate capacity to resist the anticipated loading. These examples demonstrate the deficiencies typically found in these types of joinery. Comparisons are made between historical references and modern day design standards for each of these connections. 1843 TIMBER FRAME In January of 2011, WJE was hired to assess the condition of a 2-1/2 story rectory in northwestern Connecticut. This timber-framed structure dates from 1843. The wood Structures Congress 2012 Figure F 1: Tim mber Frame C Connection The e struts are connected c to the wall post with tradi itonal mortis se and tenon n joinery. Although the proportions p of this con nnection are e simialar to o those foun nd in other tim mber frames of o the period d. WJE’s W asses ssment found d that both s struts on the center bent hav ve failed causing the kne ee walls to le ean outward at their cent ters (Figure 1). app pears to be oak. this type of o mortise an nd tenon is id deally reserv ved for knee braces transmit tting compre ession forces s through dir rect bearing on the wood d. A lapped dov vetail or a la arger mortise and tenon with more shear pegs would have been more app propriate for this tension n brace conne ection. all rights reserved. There is a short kneewal ll formed by y the posts ex xtending abo ove the level l of the attic floor. A sing gle oak peg is used to secure each connection. For personal use only. Closer e examination of the t joint reve ealed that th he oak peg ha ad torn out th the end of th he tenon due to the large tensile force de eveloped in the t strut (Fig gure 2). Copyright ASCE. o The roof structur re consists o of hand hew wn common rafters and pur rlins which connect c to th hree timber-framed bent ts. The connecting c g girts (wall t top plate) are fashioned from m one leng gth of timbe er suggestin ng that the roof framin ng was asse embled one mem mber at a tim me instead of o being rais sed as comp leted bents. c A gap had developed b between the sho oulder of the tenon and the t mortise at a the top of f the wall po ost. Structures Congress 2012 .Structures Congress 2012 © ASCE 2012 1971 Downloaded from ascelibrary. Diag gonal timber stru uts are used to resolve th he thrust cre eated by the rafters by ty ying the top of the post to the t attic floo or framing. This s condition is often avoid ded entirely by connecting the t rafter tails directly to t the framin ng at the att tic floor leve el. although the useable space in the atti ic is reduced d. Thi is type of blo ock shear ru upture of the mat terial behind d the peg ho ole is known n as a relish h failure (M Miller & Schm midt.org by Paulo Sepulveda on 12/19/12. The ben nts are 24-fee et wide and spa aced at fiftee en feet on ce enter. 2004) and d is caused by insufficien nt end distan nce on the ten non. WJE was s hired to in nvestigate th he cause of two outwar rdly leaning mas sonry walls at a church in New Jersey. Copyright ASCE. b The e trusses are e comprised d of machin ne sawn lum mber that is typically side lapped and bolted with h 5/8-inch d diameter squ quare headed d bolts. WJ JE’s assessm ment found d that wide espread fail lure of the e bolted sc cissor truss con nnections was causing the roof to o sag and t thrust the m masonry bea aring walls outward.org by Paulo Sepulveda on 12/19/12. The co ondition has s reportedly wor rsened in recent years. A shallow ba arrel vaulted d ceiling (wo ood lathe an nd plaster) sp prings from nar rrow soffits running r the length of the e north and south walls. all rights reserved. The T trusses are variant ts of a scis ssor truss w which incor rporate two hor rizontal tie beams. The walls w were found fo to be leaning l a ma aximum of a approximate ely 5-inches Structures Congress 2012 . For personal use only. e roof of the e church is supported s by y nine wood d trusses spa anning 46-fe eet between The bea aring walls. Figure 2: Failed Conne ection 185 52 SCISSOR R TRUSS In June J of 2010. Samples taken t from the truss m members wer re identified d as Spruce (Pic cea sp.Structures Congress 2012 © ASCE 2012 1972 Downloaded from ascelibrary. A notable excepti ion is the lo ower horizon ntal tie beam m which eng gages a bird’s mouth in the sloping top chord and is drawn up tight t with sq quare headed d bolts and h hand forged iron n fixtures. The ow wner reported d that the his story of this con ndition dates s back to at t least 1902 2 when thre e metal tie rods were installed to rest train the spr reading of the t north an nd south wa alls. The walls are 16-inch thic ck unreinforc ced brick masonry and rise r to a heig ght of appro oximately 38 8-feet above the level of the sidewalk. including i loc calized failu ure of the ma asonry and p plaster wall at one o of the tie e rod anchora ages. .). all rights reserved. Each scissor chord member is fashioned f fr rom one con ntinuous leng gth of sawn lum mber measuri ing approxim mately 2-1/4 4 inches wide e by 9 inche es deep. Downloaded from ascelibrary. The ro oof was shor red until rep pairs could b be designed and d implemente ed.Structures Congress 2012 © ASCE 2012 1973 eac ch with much h of the tilt occurring o ab bove the leve el of the san nctuary floor r diaphragm loca ated approxi imately 28 ft. The sc cissor conne ection to the top chord has failed f in num merous locati ions. Eac ch top chord mem mber is fashioned fro om one con ntinuous len ngth of saw wn lumber measuring app proximately 3-7/8 inches s wide by 9-5/8 inches d deep. Failed c connections were found at all a nine of th he roof trusses. Figure e 4 shows a splitting fail lure passing thro ough two fas steners in a row. Copyright ASCE. f below the e truss bearin ng. r Figure 4: : Splitting Fa ailure Structures Congress 2012 .org by Paulo Sepulveda on 12/19/12. For personal use only. Although dama age was foun nd throughou ut the trusses s. These e connection ns are located d at Joints 2 & 4 as show wn in Figure 3. Figure 3: : Truss Geom metry The e two slopin ng bottom chord c memb bers of each h truss exem mplify the scissor truss form m. our analys sis indicated d that failure of the scissor chord conne ection to th he top chord d was likely y the root ca ause of the pro oblem. Not t only were there t not eno ough bolts. b but many of the bolts we ere installed with insufficien nt distance from f the edge and/or en nd of the m member. For personal use only. Copyright ASCE. It is important to realize th hat the seve erely overstr ressed bolted d connection ns were the cau use of the ro oof truss fail lure. The bu uilder’s relian ance on throu ugh bolted c connections inst tead of trad ditional timb ber joinery is i noteworth hy because the level of f overstress fou und in the tru uss members s would not have been e enough to pr recipitate the failure by itse elf. WJE E was asked d to analyze t the roof of a an existing t train station in upstate New w York to verify v its ab bility to car rry loads sp pecified in the current building code. the e bolted conn nections wer re shown to be carrying thr ree to four times what would be p permitted by y modern w wood design stan ndards. The main building b was originally constructed d in 1918 w with a gable styl le roof that is supported d by six timb ber trusses t that span in the east/we est direction betw ween unrein nforced brick k masonry bearing b wall ls. all rights reserved. Downloaded from ascelibrary. Wood pu urlins span b between the Structures Congress 2012 .Structures Congress 2012 © ASCE 2012 1974 Fig gure 5 shows s one square e headed bol lt which has s split free f from the me ember while the remaining two bolts are a tilted and d crushing t the wood fi fibers beneat th the plate was shers. More imp portantly. Figure 5: 5 Splitting Failure F with h Crushing B Bolts Our r analysis in ndicated that the truss me embers (in th heir undama aged state) w were slightly ove erstressed for dead load alone and si ignificantly overstressed d for the exp pected wind and d snow loads s on the roof f.org by Paulo Sepulveda on 12/19/12. Sev veral of the bolts were let in i to the thic ckness of th he side memb ber by chise eling away t the wood to acc commodate bolts b which were too short to pass s through th he full thick kness of the trus ss members. 191 18 King Pos st Truss In the t winter of 2009. In this type of connection c the t thrust fro om the top chord is res sisted by the e cantilevere ed plate and notch in the bottom b chord. dia ameter bolts that t pass throug gh the top an nd bottom ch hords near th heir bearings s (Figure 7). all rights reserved. Downloaded from ascelibrary. T The purpose e of the bolt ts is not to tran nsfer force th hrough the connection. The plate wraps w the en nd of the top p chord and d is notched into o the top of the bottom chord. Des sign referen nces from th his time fra ame (Kidder r 1916) pro ovide allowa able design valu ues for this type t of conn nection however in the r roofs current t configurati ion. x 9 in. an nd the center r vertical me ember is a s steel rod enc cased by no on-structural woo od members s (Figure 6). Structures Congress 2012 . there is no excess capac city for wind d or snow loads. . h Stru uctural analy ysis of the entire truss an nd its conne ections was p performed to o determine the adequacy of o the truss to carry both h uniform an nd unbalance ed loads as specified in ASCE-7-02. the con nnection of the top and bottom chords was found to o be signific cantly overst tressed when n subjected t to wind and sno ow loads inc cluded in the e present da ay design co odes. This distress is consistent w with the fa ailure mode predicted by ou ur analysis of f the connec ction. the ere are four r. Copyright ASCE. The wood was identifi fied as dense e grained so outhern pine and d its member rs were grade ed using visu ual observat tions of the w wood’s natur ral defects. Th he top chord d of the trus ss is fashion ned of two m members wi ith nominal dim mensions of 4 in. The diagonal web mem mbers have nominal dim mensions of f 7 in n. The bottom chord of o the truss is s fashioned of two mem mbers with no ominal dime ensions of 4 in. For personal use only. c they t are mer rely there to o keep the pa arts aligned. The T truss members m and d connection ns were fou und to hav ve adequate cap pacity to resi ist the dead load of the roof. Add ditionally. howe ever.5 ft. f and bear on blueston ne pads at th he top of the e wall. x 14 in n.org by Paulo Sepulveda on 12/19/12.Structures Congress 2012 © ASCE 2012 1975 trus sses in the north/south n direction d sup pporting the e common ra afters. Close up inspectio on revealed larg ge splits in th he wood spa anning from the tip of th he notched c connection to o the end of f the bottom ch hord (Figure e 8). 7/8 in. Shear an nd bearing o of the wood d parallel to o the grain con ntrols the de esign of this s type of co onnection. x 12 in. At the t time of our o inspectio on. thick. the bolts were found to be loose and the nuts s removable by hand. The r roof trusses spa an approximately 40. Fi igure 6: Typic cal Truss and d Geometry The e top chord d to bottom m chord co onnection c consists of an end plate that is app proximately 1 in. Structures Congress 2012 © ASCE 2012 1976 Downloaded from ascelibrary. Copyright ASCE. all rights reserved. Figu ure 7: Truss Connection C C Configuration Figur re 8: Location n of Split in F Field Inspectio on Structures Congress 2012 . For personal use only.org by Paulo Sepulveda on 12/19/12. Copyright ASCE. (i. Stru uctural analy ysis verified d our observ vations that the failure o occurred in the bottom cho ord of the tru uss. Fi igure 9: Bow wstring Truss s Collapse The e roof was constructed c of wood bo owstring trus sses spannin ng 71 feet in n the northsou uth direction n and spaced d at 8 ft. For personal use only. . The e top and bot ttom chords wer re fashioned d of glue laminated lu umber with the top ch hord curved to form a par rabolic arch. The ends of the e trusses bea ar on 12 in. These bow string trusses typically t pro ovide adequ uate capacity ty under dea ad load alone. Prior to 1 1980. Web memb bers were fa ashioned fro m solid saw wn dimensional lumber. all rights reserved. A second s type of failure co ommon to th hese bowstrin ng trusses th hat was not observed in this s particular example e is failure f of the e web memb ber to chord connections s as a result of unbalanced u load from drifting d snow w. The cod des typically y only provi ided guidan nce for the i inclusion of f a uniform sno ow load acr ross the ent tire roof sp pan.Structures Congress 2012 © ASCE 2012 1977 195 55: BOW ST TRING TRU USS WJ JE was called d to investig gate the caus se of a bows string truss r roof collapse in upstate New w York tha at occurred during a major m snow storm in th he winter of f 2008 that resu ulted in 39 in n. most building codes did not pro ovide provisi ions for estim mating the unbalanced u lo oads on roof fs as a result t of drifting sno ow. sn now and win nd) can result in member or connect tion failure. The splice con nsists of steel side plates on each side e of the mem mber and bol lted shear pl lates. of snow in n less than a 48 hour per riod (Figure 9). The e bottom ch hord is splice ed 3 ft. from m the center of the truss. These unbalanced loads can cause web Structures Congress 2012 .org by Paulo Sepulveda on 12/19/12. but the add dition of live e load. on center.e. This type of failure under u heavy y snow load is i not uncom mmon in our experience. Con nnections be etween the web w members and chords s consist of b bolted shear r plates. The failu ure was loca ated at the sp plice connec ction where the demand sign nificantly ex xceeded the capacity of f the shear p plate connec ctors (Figur re 10). unr reinforced ho ollow concre ete masonry unit (CMU) ) walls. The top chord to bo ottom chord connection c consists c of a steel bearin ng shoe with h shear plate con nnectors. Downloaded from ascelibrary. Res search and advancement a t of wood tec chnology bro ought about the develop pment of the firs st National Design D Spec cification (NDS) for woo od in 1944. The sa afe working join stre esses publish hed in these references are a surprisin ngly close to o the current t guidelines due e to the large e factors of safety s which were histori ically applie ed. m th he allowable ress for woo od members was s determined d using a cor rrelation to the t bending stress. Structures Congress 2012 . design d refere ences for wo ood construc ction and its nery became e available. first in Europe. The earlie est connectio ons. hen evaluatin ng existing wood w structu ures. then la ater in Amer rica. the allowab ble tensile st tresses were red duced by ap pproximately y 40% com mpared to current stan ndards (Kri istie 1996).Structures Congress 2012 © ASCE 2012 1978 mem mbers to ca arry loads much m greater than those required by y the uniform m load case resu ulting in fail lure of the members m and/ /or connectio ons. These j joints were con nstructed by skilled craf ftsmen using g knowledge e gained thr rough trial a and error as wel ll as knowl ledge passed d down thro ough the ge enerations. One reason e tensile str for this is that until the mid-1960’s. particu ular attention n should be g given to the Wh con nnections and d their abilit ty to resist th he anticipate d loads. Current gui idelines for ana alyzing and designing timber t framed connecti ions have been develop ped by The Tim mber Frame Engineering E g Council (TF FEC 1-07). The allowa able stresses pub blished in th his NDS are significantl ly higher tha an currently permitted. For personal use only. Copyright ASCE. Once e full scale tension tests of wood w memb bers were dev veloped and d performed. Figu ure 10: Failed Bottom C Chord Splice e CO ONCLUSIONS Des sign of wood d connection ns has evolv ved over time e. connection c capacities c were w also red duced comp pared to cur rrent design stan ndards. Fur rthermore. such as mortise and tenon t joiner ry were ge enerally not t engineere ed.org by Paulo Sepulveda on 12/19/12. At the beginnin ng of the 19t th Century. Downloaded from ascelibrary. all rights reserved. R. Timber Frame Engineering Council. Minimum design loads for buildings and other structures. 25-30. New York.” Research Report. For personal use only. Miller J. Wyoming. University of Wyoming. R. and Johnson.. 1(1). Struct. (2007). Period.org by Paulo Sepulveda on 12/19/12. Standard for design of timber frame structures and commentary. F. E. “Capacity of pegged mortise and tenon joinery. of Civil & Arch. Structures Congress 2012 .” Pract. (2005). Kristie. Washington. American Society of Civil Engineers (ASCE). (1996). J. Copyright ASCE. (2004). Becket. “Investigating and repairing wood bowstring trusses. Laramie. and Schmidt. F. Architects’ and builders’ pocket-book. Downloaded from ascelibrary.Structures Congress 2012 © ASCE 2012 1979 REFERENCES Kidder. ASCE.. (1916). Engineering. Wiley. J. A. D. Constr. (2002). P. New York. Mass. Dept. American Forest & Paper Association (AFPA). all rights reserved. National design specification for wood construction. Des.C.