Chapter 7. Fastenings. Contents. Lawrence A. Soltis

Transcription

1 Fastnings Lawrnc A. Soltis Chaptr 7 Contnts Nails 7 Withdrawal Rsistanc 7 Latral Rsistanc 7 5 Spiks 7 8 Stapls 7 8 Drift Bolts 7 9 Wood Scrws 7 9 Withdrawal Rsistanc 7 9 Latral Rsistanc 7 1 Lag Scrws 7 11 Withdrawal Rsistanc 7 11 Latral Rsistanc 7 1 Bolts 7 14 Baring Strss of Wood Undr Bolts 7 14 Loads at an Angl to th Grain 7 14 Stl Sid Plats 7 15 Bolt Quality 7 15 Effct of Mmbr Thicknss 7 15 Two-Mmbr, Multipl-Mmbr Joints 7 15 Spacing, Edg, and End Distanc 7 16 Effct of Bolt Hols 7 16 Pr-1991 Allowabl Loads 7 17 Post-1991 Yild Modl 7 18 Connctor Joints 7 18 Paralll-to-Grain Loading 7 18 Prpndicular-to-Grain Loading 7 18 Dsign Loads 7 Modifications 7 1 Nt Sction 7 3 End Distanc and Spacing 7 3 Placmnt of Multipl Connctors 7 3 Cross Bolts 7 4 Multipl-Fastnr Joints 7 4 Mtal Plat Connctors 7 5 Fastnr Had Embdmnt 7 6 Rfrncs 7 7 h strngth and stability of any structur dpnd havily on th fastnings that hold its parts togthr. On prim advantag of wood as a structural matrial is th as with which wood structural parts can b joind togthr with a wid varity of fastnings nails, spiks, scrws, bolts, lag scrws, drift pins, stapls, and mtal connctors of various typs. For utmost rigidity, strngth, and srvic, ach typ of fastning rquirs joint dsigns adaptd to th strngth proprtis of wood along and across th grain and to dimnsional changs that may occur with changs in moistur contnt. Maximum latral rsistanc and saf dsign load valus for small-diamtr (nails, spiks, and wood scrws) and largdiamtr dowl-typ fastnrs (bolts, lag scrws, and drift pins) wr basd on an mpirical mthod prior to Rsarch conductd during th 198s rsultd in latral rsistanc valus that ar currntly basd on a yild modl thory. This thortical mthod was adaptd for th 1991 dition of th National Dsign Spcification for Wood Construction (NDS). Bcaus litratur and dsign procdurs xist that ar rlatd to both th mpirical and thortical mthods, w rfr to th mpirical mthod as pr-1991 and th thortical mthod as post-1991 throughout this chaptr. Withdrawal rsistanc mthods hav not changd, so th pr- and post-1991 rfr only to latral rsistanc. Th information in this chaptr rprsnts primarily Forst Products Laboratory rsarch rsults. A mor comprhnsiv discussion of fastnings is givn in th Amrican Socity of Civil Enginrs Manuals and Rports on Enginring Practic No. 84, Mchanical Connctions in Wood Structurs. Th rsarch rsults of this chaptr ar oftn modifid for structural safty, basd on judgmnt or xprinc, and thus information prsntd in dsign documnts may diffr from information prsntd in this chaptr. Additionally, rsarch by othrs srvs as a basis for som currnt dsign critria. Allowabl strss dsign critria ar prsntd in th National Dsign Spcification for Wood Construction publishd by th Amrican Forst and Papr Association; limit stats dsign critria ar prsntd in th Standard for Load and Rsistanc Factor Dsign (LRFD) for Enginrd Wood Construction publishd by th Amrican Socity of Civil Enginrs. 7 1

2 Nails Nails ar th most common mchanical fastnings usd in wood construction. Thr ar many typs, sizs, and forms of nails (Fig. 7 1). Th load quations prsntd in this chaptr apply for bright, smooth, common stl wir nails drivn into wood whn thr is no visibl splitting. For nails othr than common wir nails, th loads can b adjustd by factors givn latr in th chaptr. Nails in us rsist withdrawal loads, latral loads, or a combination of th two. Both withdrawal and latral rsistanc ar affctd by th wood, th nail, and th condition of us. In gnral, howvr, any variation in ths factors has a mor pronouncd ffct on withdrawal rsistanc than on latral rsistanc. Th srvicability of joints with nails latrally loadd dos not dpnd gratly on withdrawal rsistanc unlss larg joint distortion is tolrabl. Th diamtrs of various pnny or gaug sizs of bright common nails ar givn in Tabl 7 1. Th pnny siz dsignation should b usd cautiously. Intrnational nail producrs somtims do not adhr to th dimnsions of Tabl 7 1. Thus pnny sizs, although still widly usd, ar obsolt. Spcifying nail sizs by lngth and diamtr dimnsions is rcommndd. Bright box nails ar gnrally of th sam lngth but slightly smallr diamtr (Tabl 7 ), whil cmnt-coatd nails such as coolrs, sinkrs, and coatd box nails ar slightly shortr (3. mm (1/8 in.)) and of smallr diamtr than common nails of th sam pnny siz. Hlically and annularly thradd nails gnrally hav smallr diamtrs than common nails for th sam pnny siz (Tabl 7 3). Withdrawal Rsistanc Th rsistanc of a nail shank to dirct withdrawal from a pic of wood dpnds on th dnsity of th wood, th diamtr of th nail, and th dpth of pntration. Th surfac condition of th nail at th tim of driving also influncs th initial withdrawal rsistanc. Figur 7 1. Various typs of nails: (lft to right) bright smooth wir nail, cmnt coatd, zinc-coatd, annularly thradd, hlically thradd, hlically thradd and barbd, and barbd. Tabl 7 1. Sizs of bright common wir nails Siz Gaug Lngth (mm (in.)) Diamtr (mm (in.)) 6d 11-1/ 5.8 ().87 (.113) 8d 1-1/ (-1/) 3.33 (.131) 1d (3) 3.76 (.148) 1d (3-1/4) 3.76 (.148) 16d (3-1/) 4.11 (.16) d (4) 4.88 (.19) 3d (4-1/) 5.6 (.7) 4d (5) 5.7 (.5) 5d (5-1/) 6. (.44) 6d 15.4 (6) 6.65 (.6) Tabl 7. Sizs of smooth box nails Siz Gaug Lngth (mm (in.)) Diamtr (mm (in.)) 3d 14-1/ 31.8 (1-1/4) 1.93 (.76) 4d (1-1/).3 (.8) 5d (1-3/4).3 (.8) 6d 1-1/ 5.8 ().49 (.98) 7d 1-1/ 57. (-1/4).49 (.98) 8d 11-1/ 63.5 (-1/).87 (.113) 1d 1-1/ 76. (3) 3.5 (.18) 16d (3-1/) 3.43 (.135) d (4) 3.76 (.148) Tabl 7 3. Sizs of hlically and annularly thradd nails Siz Lngth (mm (in.)) Diamtr (mm (in.)) 6d 5.8 () 3.5 (.1) 8d 63.5 (-1/) 3.5 (.1) 1d 76. (3) 3.43 (.135) 1d 8.6 (3-1/4) 3.43 (.135) 16d 88.9 (3-1/) 3.76 (.148) d 11.6 (4) 4.5 (.177) 3d (4-1/) 4.5 (.177) 4d 17. (5) 4.5 (.177) 5d (5-1/) 4.5 (.177) 6d 15.4 (6) 4.5 (.177) 7d (7) 5.6 (.7) 8d 3. (8) 5.6 (.7) 9d 8.6 (9) 5.6 (.7) 7

3 For bright common wir nails drivn into th sid grain of sasond wood or unsasond wood that rmains wt, th rsults of many tsts hav shown that th maximum withdrawal load is givn by th mpirical quation p=54 1G 5 /. DL p=7 85G 5 /, DL (mtric) (inch pound) (7 1a) (7 1b) whr p is maximum load (N, lb), L dpth (mm, in.) of pntration of th nail in th mmbr holding th nail point, G spcific gravity of th wood basd on ovndry wight and volum at 1% moistur contnt (s Ch. 4, Tabls 4 to 4 5), and D diamtr of th nail (mm, in.). (Th NDS and LRFD us ovndry wight and volum as a basis.) Th loads xprssd by Equation (7 1) rprsnt avrag data. Crtain wood spcis giv tst valus that ar somwhat gratr or lss than th quation valus. A typical load displacmnt curv for nail withdrawal (Fig. 7 ) shows that maximum load occurs at rlativly small valus of displacmnt. Although th quation for nail-withdrawal rsistanc indicats that th dns, havy woods offr gratr rsistanc to nail withdrawal than do th lowr dnsity ons, lightr spcis should not b disqualifid for uss rquiring high rsistanc to withdrawal. As a rul, th lss dns spcis do not split as radily as th dnsr ons, thus offring an opportunity for incrasing th diamtr, lngth, and numbr of th nails to compnsat for th wood s lowr rsistanc to nail withdrawal. Th withdrawal rsistanc of nail shanks is gratly affctd by such factors as typ of nail point, typ of shank, tim th nail rmains in th wood, surfac coatings, and moistur contnt changs in th wood. Withdrawal load (N) Displacmnt (in.) 6d smooth box nail.49-mm (.98-in.) diamtr 31.8 mm (1-1/4 in.) pntration dpth Douglas-fir 1% moistur contnt.54 spcific gravity Displacmnt (mm) Figur 7. Typical load displacmnt curv for dirct withdrawal of a nail Withdrawal load (lb) Effct of Sasoning With practically all spcis, nails drivn into grn wood and pulld bfor any sasoning taks plac offr about th sam withdrawal rsistanc as nails drivn into sasond wood and pulld soon aftr driving. Howvr, if common smooth-shank nails ar drivn into grn wood that is allowd to sason, or into sasond wood that is subjctd to cycls of wtting and drying bfor th nails ar pulld, thy los a major part of thir initial withdrawal rsistanc. Th withdrawal rsistanc for nails drivn into wood that is subjctd to changs in moistur contnt may b as low as 5% of th valus for nails tstd soon aftr driving. On th othr hand, if th wood fibrs dtriorat or th nail corrods undr som conditions of moistur variation and tim, withdrawal rsistanc is rratic; rsistanc may b rgaind or vn incrasd ovr th immdiat withdrawal rsistanc. Howvr, such sustaind prformanc should not b rlid on in th dsign of a naild joint. In sasond wood that is not subjctd to apprciabl moistur contnt changs, th withdrawal rsistanc of nails may also diminish du to rlaxation of th wood fibrs with tim. Undr all ths conditions of us, th withdrawal rsistanc of nails diffrs among spcis and shows variation within individual spcis. Effct of Nail Form Th surfac condition of nails is frquntly modifid during th manufacturing procss to improv withdrawal rsistanc. Such modification is usually don by surfac coating, surfac roughning, or mchanical dformation of th shank. Othr factors that affct th surfac condition of th nail ar th oil film rmaining on th shank aftr manufactur or corrosion rsulting from storag undr advrs conditions; but ths factors ar so variabl that thir influnc on withdrawal rsistanc cannot b adquatly valuatd. Surfac Modifications A common surfac tratmnt for nails is th so-calld cmnt coating. Cmnt coatings, contrary to what th nam implis, do not includ cmnt as an ingrdint; thy gnrally ar a composition of rsin applid to th nail to incras th rsistanc to withdrawal by incrasing th friction btwn th nail and th wood. If proprly applid, thy incras th rsistanc of nails to withdrawal immdiatly aftr th nails ar drivn into th softr woods. Howvr, in th dnsr woods (such as hard mapl, birch, or oak), cmnt-coatd nails hav practically no advantag ovr plain nails, bcaus most of th coating is rmovd in driving. Som of th coating may also b rmovd in th sid mmbr bfor th nail pntrats th main mmbr. Good-quality cmnt coatings ar uniform, not sticky to th touch, and cannot b rubbd off asily. Diffrnt tchniqus of applying th cmnt coating and variations in its ingrdints may caus larg diffrncs in th rlativ rsistanc to withdrawal of diffrnt lots of cmnt-coatd nails. Som nails may show only a slight initial advantag ovr plain nails. In th softr woods, th incras in withdrawal rsistanc of 7 3

4 cmnt-coatd nails is not prmannt but drops off significantly aftr a month or so. Cmnt-coatd nails ar usd primarily in construction of boxs, crats, and othr containrs usually built for rough handling and rlativly short srvic. Nails that hav galvanizd coatings, such as zinc, ar intndd primarily for uss whr corrosion and staining rsistanc ar important factors in prmannc and apparanc. If th zinc coating is vnly applid, withdrawal rsistanc may b incrasd, but xtrm irrgularitis of th coating may actually rduc it. Th advantag that uniformly coatd galvanizd nails may hav ovr nongalvanizd nails in rsistanc to initial withdrawal is usually rducd by rpatd cycls of wtting and drying. Nails hav also bn mad with plastic coatings. Th usfulnss and charactristics of ths coatings ar influncd by th quality and typ of coating, th ffctivnss of th bond btwn th coating and bas fastnr, and th ffctivnss of th bond btwn th coating and wood fibrs. Som plastic coatings appar to rsist corrosion or improv rsistanc to withdrawal, whil othrs offr littl improvmnt. Fastnrs with proprly applid nylon coating tnd to rtain thir initial rsistanc to withdrawal compard with othr coatings, which xhibit a markd dcras in withdrawal rsistanc within th first month aftr driving. A chmically tchd nail has somwhat gratr withdrawal rsistanc than som coatd nails, as th minutly pittd surfac is an intgral part of th nail shank. Undr impact loading, howvr, th withdrawal rsistanc of tchd nails is littl diffrnt from that of plain or cmnt-coatd nails undr various moistur conditions. Sand-blastd nails prform in much th sam mannr as chmically tchd nails. Shap Modifications Nail shanks may b varid from a smooth, circular form to giv an incras in surfac ara without an incras in nail wight. Spcial nails with barbd, hlically or annularly thradd, and othr irrgular shanks (Fig. 7 1) ar commrcially availabl. Th form and magnitud of th dformations along th shank influnc th prformanc of th nails in various wood spcis. In wood rmaining at a uniform moistur contnt, th withdrawal rsistanc of ths nails is gnrally somwhat gratr than that of common wir nails of th sam diamtr. For instanc, annular-shank nails hav about 4% gratr rsistanc to withdrawal than common nails. Howvr, undr conditions involving changs in moistur contnt of th wood, som spcial nail forms provid considrably gratr withdrawal rsistanc than th common wir nail about four tims gratr for annularly and hlically thradd nails of th sam diamtr. This is spcially tru of nails drivn into grn wood that subsquntly dris. In gnral, annularly thradd nails sustain largr withdrawal loads, and hlically thradd nails sustain gratr impact withdrawal work valus than do th othr nail forms. Nails with dformd shanks ar somtims hardnd by hat tratmnts for us whr driving conditions ar difficult or to obtain improvd prformanc, such as in pallt assmbly. Hardnd nails ar brittl and car should b xrcisd to avoid injuris from fragmnts of nails brokn during driving. Nail Point A smooth, round shank nail with a long, sharp point will usually hav a gratr withdrawal rsistanc, particularly in th softr woods, than th common wir nail (which usually has a diamond point). Howvr, sharp points accntuat splitting in crtain spcis, which may rduc withdrawal rsistanc. A blunt or flat point without tapr rducs splitting, but its dstruction of th wood fibrs whn drivn rducs withdrawal rsistanc to lss than that of th common wir nail. A nail taprd at th nd and trminating in a blunt point will caus lss splitting. In havir woods, such a taprd, blunt-pointd nail will provid about th sam withdrawal rsistanc, but in lss dns woods, its rsistanc to withdrawal is lss than that of th common nail. Nail Had Nail had classifications includ flat, oval, countrsunk, dp-countrsunk, and brad. Nails with all typs of hads, xcpt th dp-countrsunk, brad, and som of th thin flathad nails, ar sufficintly strong to withstand th forc rquird to pull thm from most woods in dirct withdrawal. Th dp-countrsunk and brad nails ar usually drivn blow th wood surfac and ar not intndd to carry larg withdrawal loads. In gnral, th thicknss and diamtr of th hads of th common wir nails incras as th siz of th nail incrass. Th dvlopmnt of som pnumatically opratd portabl nailrs has introducd nails with spcially configurd hads, such as T-nails and nails with a sgmnt of th had cut off. Corrosion and Staining In th prsnc of moistur, mtals usd for nails may corrod whn in contact with wood tratd with crtain prsrvativ or fir-rtardant salts (Chs. 14 and 17). Us of crtain mtals or mtal alloys will rduc th amount of corrosion. Nails of coppr, silicon bronz, and 34 and 316 stainlss stl hav prformd wll in wood tratd with ammoniacal coppr arsnat and chromatd coppr arsnat. Th choic of mtals for us with fir-rtardant-tratd woods dpnds upon th particular fir-rtardant chmical. Staining causd by th raction of crtain wood xtractivs (Ch. 3) and stl in th prsnc of moistur is a problm if apparanc is important, such as with naturally finishd siding. Us of stainlss stl, aluminum, or hot-dippd galvanizd nails can allviat staining. In gnral, th withdrawal rsistanc of coppr and othr alloy nails is comparabl with that of common stl wir nails whn pulld soon aftr driving. Driving Th rsistanc of nails to withdrawal is gnrally gratst whn thy ar drivn prpndicular to th grain of th wood. Whn th nail is drivn paralll to th wood fibrs (that is, 7 4

5 into th nd of th pic) withdrawal rsistanc in th softr woods drops to 75% or vn 5% of th rsistanc obtaind whn th nail is drivn prpndicular to th grain. Th diffrnc btwn sid- and nd-grain withdrawal loads is lss for dns woods than for softr woods. With most spcis, th ratio btwn th nd- and sid-grain withdrawal loads of nails pulld aftr a tim intrval, or aftr moistur contnt changs hav occurrd, is usually somwhat gratr than that of nails pulld immdiatly aftr driving. To nailing, a common mthod of joining wood framwork, involvs slant driving a nail or group of nails through th nd or dg of an attachd mmbr and into a main mmbr. To nailing rquirs gratr skill in assmbly than dos ordinary nd nailing but provids joints of gratr strngth and stability. Tsts show that th maximum strngth of tonaild joints undr latral and uplift loads is obtaind by (a) using th largst nail that will not caus xcssiv splitting, (b) allowing an nd distanc (distanc from th nd of th attachd mmbr to th point of initial nail ntry) of approximatly on-third th lngth of th nail, (c) driving th nail at a slop of 3 with th attachd mmbr, and (d) burying th full shank of th nail but avoiding xcssiv mutilation of th wood from hammr blows. Th rsults of withdrawal tsts with multipl nail joints in which th pic attachd is pulld dirctly away from th main mmbr show that slant driving is usually suprior to straight driving whn nails ar drivn into drywood and pulld immdiatly, and dciddly suprior whn nails ar drivn into grn or partially dry wood that is allowd to sason for a month or mor. Howvr, th loss in dpth of pntration du to slant driving may, in som typs of joints, offst th advantags of slant nailing. Cross slant driving of groups of nails through th sid grain is usually somwhat mor ffctiv than paralll slant driving through th nd grain. Nails drivn into lad hols with a diamtr slightly smallr (approximatly 9%) than th nail shank hav somwhat gratr withdrawal rsistanc than nails drivn without lad hols. Lad hols also prvnt or rduc splitting of th wood, particularly for dns spcis. Clinching Th withdrawal rsistanc of smooth-shank, clinchd nails is considrably gratr than that of unclinchd nails. Th point of a clinchd nail is bnt ovr whr th nail protruds through th sid mmbr. Th ratio btwn th loads for clinchd and unclinchd nails varis normously, dpnding upon th moistur contnt of th wood whn th nail is drivn and withdrawn, th spcis of wood, th siz of nail, and th dirction of clinch with rspct to th grain of th wood. In dry or grn wood, a clinchd nail provids 45% to 17% mor withdrawal rsistanc than an unclinchd nail whn withdrawn soon aftr driving. In grn wood that sasons aftr a nail is drivn, a clinchd nail givs 5% to 46% gratr withdrawal rsistanc than an unclinchd nail. Howvr, this improvd strngth of a clinchd-nail joint dos not justify th us of grn lumbr, bcaus th joints may loosn as th lumbr sasons. Furthrmor, laboratory tsts wr mad with singl nails, and th ffcts of drying, such as warping, twisting, and splitting, may rduc th fficincy of a joint that has mor than on nail. Clinching of nails is gnrally confind to such construction as boxs and crats and othr containr applications. Nails clinchd across th grain hav approximatly % mor rsistanc to withdrawal than nails clinchd along th grain. Fastning of Plywood Th nailing charactristics of plywood ar not gratly diffrnt from thos of solid wood xcpt for plywood s gratr rsistanc to splitting whn nails ar drivn nar an dg. Th nail withdrawal rsistanc of plywood is 15% to 3% lss than that of solid wood of th sam thicknss. Th rason is that fibr distortion is lss uniform in plywood than in solid wood. For plywood lss than 1.5 mm (1/-in.) thick, th gratr splitting rsistanc tnds to offst th lowr withdrawal rsistanc compard with solid wood. Th withdrawal rsistanc pr unit lngth of pntration dcrass as th numbr of plis pr unit lngth incrass. Th dirction of th grain of th fac ply has littl influnc on th withdrawal rsistanc from th fac nar th nd or dg of a pic of plywood. Th dirction of th grain of th fac ply may influnc th pull-through rsistanc of stapls or nails with svrly modifid hads, such as T-hads. Fastnr dsign information for plywood is availabl from APA Th Enginrd Wood Association. Allowabl Loads Th prcding discussion dalt with maximum withdrawal loads obtaind in short-tim tst conditions. For dsign, ths loads must b rducd to account for variability, duration-of-load ffcts, and safty. A valu of on-sixth th avrag maximum load has usually bn accptd as th allowabl load for long-tim loading conditions. For normal duration of load, this valu may b incrasd by 1%. Normal duration of load is dfind as a load of 1-yar duration. Latral Rsistanc Pr-1991 Tst loads at joint slips of.38 mm (.15 in.) (approximat proportional limit load) for bright common wir nails in latral rsistanc drivn into th sid grain (prpndicular to th wood fibrs) of sasond wood ar xprssd by th mpirical quation p= KD 3 / (7 ) whr p is latral load pr nail, K a cofficint, and D diamtr of th nail. Valus of cofficint K ar listd in Tabl 7 4 for rangs of spcific gravity of hardwoods and softwoods. Th loads givn by th quation apply only whr th sid mmbr and th mmbr holding th nail point ar of 7 5

6 Tabl 7 4. Cofficints for computing tst loads for fastnrs in sasond wood a (pr-1991) Spcific Latral load cofficint K (mtric (inch pound)) gravity rang b Nails c Scrws Lag scrws Hardwoods (1,44) 3.17 (3,36) 6.34 (3,8) (,) (4,64) 9.51 (4,8) (,7) (6,4) (4,95) Softwoods (1,44) 3.17 (3,36) 3.3 (3,38) (1,8) 9.79 (4,3) 6.34 (3,8) (,) 36.4 (5,8) 9.51 (4,8) a Wood with a moistur contnt of 15%. b Spcific gravity basd on ovndry wight and volum at 1% moistur contnt. c Cofficints basd on load at joint slip of.38 mm (.15 in.) approximatly th sam dnsity. Th thicknss of th sid mmbr should b about on-half th dpth of pntration of th nail in th mmbr holding th point. Th ultimat latral nail loads for softwoods may approach 3.5 tims th loads xprssd by th quation, and for hardwoods thy may b 7 tims as grat. Th joint slip at maximum load, howvr, is mor than tims.38 mm (.15 in.). This is dmonstratd by th typical load slip curv shown in Figur 7 3. To maintain a sufficint ratio btwn ultimat load and th load at.38 mm (.15 in.), th nail should pntrat into th mmbr holding th point by not lss than 1 tims th nail diamtr for dns woods (spcific gravity gratr than.61) and 14 tims th diamtr for low dnsity woods (spcific gravity lss than.4). For spcis having dnsitis btwn ths two rangs, th pntration may b found by straight lin intrpolation. Post-1991 Th yild modl thory slcts th worst cas of yild mods basd on diffrnt possibilitis of wood baring and nail bnding. It dos not account for nail had ffcts. A dscription of th various combinations is givn in Figur 7 4. Mod I is a wood baring failur in ithr th main or sid mmbr; mod II is a rotation of th fastnr in th joint without bnding; mods III and IV ar a combination of wood baring failur and on or mor plastic hing yild formations in th fastnr. Mods I m and II hav not bn obsrvd in nail and spik connctions. Th yild modl thory is applicabl to all typs of dowl fastnrs (nails, scrws, bolts, lag scrws), and thus th wood baring capacity is dscribd by a matrial proprty calld th dowl baring strngth. Load (kn) Slip in joint (in.) Th yild mod quations (Tabl 7 5) ar ntrd with th dowl baring strngth and dimnsions of th wood mmbrs and th bnding yild strngth and diamtr of th fastnr. Th dowl baring strngth of th wood is xprimntally dtrmind by comprssing a dowl into a wood mmbr. Th strngth basis is th load rprsnting a 5% diamtr offst on th load dformation curv (Fig. 7 3). Dowl baring strngth F (Pa, lb/in ) is mpirically rlatd to spcific gravity G by F F x fastnr Slip in joint (mm) diamtr Yild strngth 184. =114. 5G 184. =16, 6G 8d common nail Figur 7 3. Typical rlation btwn latral load and slip in th joint and 5% offst dfinition. (mtric) (inch pound) whr spcific gravity is basd on ovndry wight and volum Load (lb) (7 3a) (7 3b) Spacing End distanc, dg distanc, and spacing of nails should b such as to prvnt unusual splitting. As a gnral rul, nails should b drivn no closr to th dg of th sid mmbr than on-half its thicknss and no closr to th nd than th thicknss of th pic. Smallr nails can b drivn closr to th dgs or nds than largr ons bcaus thy ar lss likly to split th wood. Grain Dirction Effcts Th latral load for sid-grain nailing applis whthr th load is in a dirction paralll to th grain of th pics joind or at right angls to it. Whn nails ar drivn into th nd grain (paralll with th wood fibrs), limitd data on softwood spcis indicat that thir maximum rsistanc to latral displacmnt is about two-thirds that for nails drivn into th sid grain. Although th avrag proportional limit loads appar to b about th sam for nd- and sid-grain nailing, th individual rsults ar mor rratic for nd-grain nailing, and th minimum loads approach only 75% of corrsponding valus for sid-grain nailing

7 (a) (b) (Not applicabl) Mod I m Mod I s Mod II Mod I m Mod I s Mod II (Not applicabl) Mod III m Mod III s Mod IV Mod III m Mod III s Mod IV Figur 7 4. Various combinations of wood-baring and fastnr-bnding yilds for (a) two-mmbr connctions and (b) thr-mmbr connctions. Moistur Contnt Effcts Nails drivn into th sid grain of unsasond wood giv maximum latral rsistanc loads approximatly qual to thos obtaind in sasond wood, but th latral rsistanc loads at.38 mm (.15 in.) joint slip ar somwhat lss. To prvnt xcssiv dformation, latral loads obtaind for sasond wood should b rducd by 5% for unsasond wood that will rmain wt or b loadd bfor sasoning taks plac. Whn nails ar drivn into grn wood, thir latral proportional limit loads aftr th wood has sasond ar also lss than whn thy ar drivn into sasond wood and loadd. Th rratic bhavior of a naild joint that has undrgon on or mor moistur contnt changs maks it difficult to stablish a latral load for a naild joint undr ths conditions. Structural joints should b inspctd at intrvals, and if it is apparnt that th joint has loosnd during drying, th joint should b rinforcd with additional nails. Dformd-Shank Nails Dformd-shank nails carry somwhat highr maximum latral loads than do th sam pnnywight common wir nails, but both prform similarly at small distortions in th joint. It should b notd that th sam pnnywight dformd-shank nail has a diffrnt diamtr than that of th common wir nail. Ths nails oftn hav highr bnding yild strngth than common wir nails, rsulting in highr latral strngth in mods III and IV. Latral Load Slip Modls A considrabl amount of work has bn don to dscrib, by mathmatical modls, th latral load slip curv of nails. Ths modls hav bcom important bcaus of thir nd as input paramtrs for advancd mthods of structural analysis. On thortical modl, which considrs th nail to b a bam supportd on an lastic foundation (th wood), dscribs th initial slop of th curv: δ= P + ( J J ) ( L L ) 1 1 ( K1+ K) (7 4) whr P is th latral load and δ is th joint slip. Th factors L 1, L, J 1, J, K 1, and K (Tabl 7 6) ar combinations of hyprbolic and trigonomtric functions of th quantitis λ 1 a and λ b in which a and b ar th dpth of pntration of th nail in mmbrs 1 and, rspctivly. For smooth round nails, 4 k λ = (7 5) 3 πed whr k is lastic baring constant, D nail diamtr, and E modulus of lasticity of th nail. For sasond wood, th lastic baring constant k (N/mm 3, lb/in 3 ) has bn shown to b rlatd to avrag spcis spcific gravity G if no lad hol is usd by k = 58G (mtric) (7 6a) k =, 144, G (inch pound (7 6b) If a prbord lad hol qual to 9% of th nail diamtr is usd, k = 869G (mtric) (7 7a) k = 3,, G (inch pound) (7 7b) Othr mpirically drivd modls attmpt to dscrib th ntir load slip curv. On such xprssion is P = A log 1 ( 1+ Bδ ) (7 8) whr th paramtrs A and B ar mpirically fittd. 7 7

8 Tabl 7 5. Th 5% offst latral yild strngth (Z) for nails and scrws for a two-mmbr joint Mod Z valu for nails Z valu for scrws I s III m III s IV Dfinitions D Spiks Dt F s s Dt F s s k1dpf 1+ R kdtf s + R D m m FmFyb 31 ( + R ) Common wir spiks ar manufacturd in th sam mannr as common wir nails. Thy hav ithr a chisl point or a diamond point and ar mad in lngths of 76 to 35 mm (3 to 1 in.). For corrsponding lngths in th rang of 76 to 15 (3 to 6 in.), thy hav largr diamtrs (Tabl 7 7) than common wir nails, and byond th 6d siz thy ar usually dsignatd by diamtr. kdtf 3 s + R m 175. FmF D 31 ( + R ) nail, spik, or scrw diamtr, mm (in.) (for annularly thradd nails, D is thrad-root diamtr; for scrws, D is ithr th shank diamtr or th root diamtr if th thradd portion of th scrw is in th shar plan) F m dowl baring strss of main mmbr (mmbr holding point), kpa (lb/in ) F s dowl baring strss of sid mmbr, kpa (lb/in ) F yb bnding yild strss of nail, spik, or scrw, kpa (lb/in ) p pntration of nail or spik in main mmbr, mm (in.) t s thicknss of sid mmbr, mm (in.) Z offst latral yild strngth R = F m /F s k k k 1 3 Fyb( 1+ R ) D = 1+ ( 1+ R) + 3F p = 1+ = 1+ m 1 ( + R ) F ( + R ) D yb + R 3F t m s 1 ( + R ) F ( + R ) D yb + R F t m s yb Tabl 7 6. Exprssions for factors in Equation (7 4) Factor Th withdrawal and latral rsistanc quations and limitations givn for common wir nails ar also applicabl to spiks, xcpt that in calculating th withdrawal load for spiks, th dpth of pntration is takn as th lngth of th spik in th mmbr rciving th point, minus two-thirds th lngth of th point. Stapls Exprssion a L1 λ1 sinhλ1a cosh λ1a sinλ1a cos λ1a k 1 sinh λ1a sin λ1a L λ sinhλb cosh λb sinλb cos λb k sinh λb sin λb J1 λ1 sinh λ1a + sin λ1a k 1 sinh λ1a sin λ1a J λ sinh λb + sin λb k sinh λb sin λb K1 3 λ1 sinhλ1a cosh λ1a + sinλ1a cos λ1a k 1 sinh λ1a sin λ1a K 3 λ sinhλb cosh λb + sinλb cos λb k sinh λb sin λb a k 1 = k 1 d and k = k d, whr k 1 and k ar th foundation moduli of mmbrs 1 and, rspctivly. Diffrnt typs of stapls hav bn dvlopd with various modifications in points, shank tratmnt and coatings, gaug, crown width, and lngth. Ths fastnrs ar availabl in clips or magazins for us in pnumatically opratd portabl staplrs. Most factors that affct th withdrawal and latral loads of nails similarly affct th loads on stapls. Th withdrawal rsistanc, for xampl, varis almost dirctly with th circumfrnc and dpth of pntration whn th typ of point and shank ar similar to nails. Thus, Equation (7 1) has bn usd to prdict th withdrawal load for on lg of a stapl, but no vrification tsts hav bn don. Th load in latral rsistanc varis approximatly as th 3/ powr of th diamtr whn othr factors, such as quality of mtal, typ of shank, and dpth of pntration, ar similar to nails. Th diamtr of ach lg of a two-lggd stapl must thrfor b about two-thirds th diamtr of a nail to provid a comparabl load. Equation (7 ) has bn usd to prdict th latral rsistanc of stapls. Howvr, yild modl thory quations hav not yt bn xprimntally vrifid for stapls. 7 8

9 Tabl 7 7. Sizs of common wir spiks Lngth Diamtr Siz (mm (in.)) (mm (in.)) 1d 76. (3) 4.88 (.19) 1d 8.6 (3-1/4) 4.88 (.19) 16d 88.9 (3-1/) 5.6 (.7) d 11.6 (4) 5.7 (.5) 3d (4-1/) 6. (.44) 4d 17. (5) 6.68 (.63) 5d (5-1/) 7.19 (.83) 6d 15.4 (6) 7.19 (.83) 5/16 in (7) 7.9 (.31) 3/8 in (8-1/) 9.53 (.375) In addition to th immdiat prformanc capability of stapls and nails as dtrmind by tst, factors such as corrosion, sustaind prformanc undr srvic conditions, and durability in various uss should b considrd in valuating th rlativ usfulnss of a stapld connction. Drift Bolts A drift bolt (or drift pin) is a long pin of iron or stl, with or without had or point. It is drivn into a bord hol through on timbr and into an adjacnt on, to prvnt th sparation of th timbrs connctd and to transmit latral load. Th hol in th scond mmbr is drilld sufficintly dp to prvnt th pin from hitting th bottom. Th ultimat withdrawal load of a round drift bolt or pin from th sid grain of sasond wood is givn by p= G DL (mtric) (7 9a) p= 6, 6G DL (inch pound) (7 9b) whr p is th ultimat withdrawal load (N, lb), G spcific gravity basd on th ovndry wight and volum at 1% moistur contnt of th wood, D diamtr of th drift bolt (mm, in.), and L lngth of pntration of th bolt (mm, in.). (Th NDS and LRFD us ovndry wight and volum as a basis.) This quation provids an avrag rlationship for all spcis, and th withdrawal load for som spcis may b abov or blow th quation valus. It also prsums that th bolts ar drivn into prbord hols having a diamtr 3. mm (1/8 in.) lss than th bolt diamtr. Data ar not availabl on latral rsistanc of drift bolts. Th yild modl should provid latral strngth prdiction, but th modl has not bn xprimntally vrifid for drift bolts. Dsignrs hav usd bolt data and dsign mthods basd on xprinc. This suggsts that th load for a drift bolt drivn into th sid grain of wood should not xcd, and ordinarily Gag or diamtr of shank /3 lngth should b takn as lss than, that for a bolt of th sam diamtr. Bolt dsign valus ar basd on th thicknss of th main mmbr in a joint. Thus th dpth of pntration of th drift bolt must b gratr than or qual to th mainmmbr thicknss on which th bolt dsign valu is basd. Howvr, th drift bolt should not fully pntrat its joint. Wood Scrws Th common typs of wood scrws hav flat, oval, or round hads. Th flathad scrw is most commonly usd if a flush surfac is dsird. Ovalhad and roundhad scrws ar usd for apparanc, and roundhad scrws ar usd whn countrsinking is objctionabl. Th principal parts of a scrw ar th had, shank, thrad, and cor (Fig. 7 5). Th root diamtr for most sizs of scrws avrags about two-thirds th shank diamtr. Wood scrws ar usually mad of stl, brass, othr mtals, or alloys, and may hav spcific finishs such as nickl, blud, chromium, or cadmium. Thy ar classifid according to matrial, typ, finish, shap of had, and diamtr or gaug of th shank. Currnt trnds in fastnings for wood also includ tapping scrws. Tapping scrws hav thrads th full lngth of th shank and may hav som advantag for crtain spcific uss. Withdrawal Rsistanc Cor or root diamtr Exprimntal Loads Th rsistanc of wood scrw shanks to withdrawal from th sid grain of sasond wood varis dirctly with th squar of th spcific gravity of th wood. Within limits, th withdrawal load varis dirctly with th dpth of pntration of th thradd portion and th diamtr of th scrw, providd th scrw dos not fail in tnsion. Th scrw will fail in tnsion whn its strngth is xcdd by th withdrawal strngth from th wood. Th limiting lngth to caus a tnsion failur dcrass as th dnsity of th wood incrass sinc th withdrawal strngth of th wood incrass with dnsity. Th longr lngths of standard scrws ar thrfor suprfluous in dns hardwoods. Lngth A B C Figur 7 5. Common typs of wood scrws: A, flathad; B, roundhad; and C, ovalhad. 7 9

10 Th withdrawal rsistanc of typ A tapping scrws, commonly calld sht mtal scrws, is in gnral about 1% gratr than that for wood scrws of comparabl diamtr and lngth of thradd portion. Th ratio btwn th withdrawal rsistanc of tapping scrws and wood scrws varis from 1.16 in dnsr woods, such as oak, to 1.5 in lightr woods, such as rdwood. Ultimat tst valus for withdrawal loads of wood scrws insrtd into th sid grain of sasond wood may b xprssd as p=18. 5G DL (mtric) (7 1a) p=15, 7G DL (inch pound) (7 1b) whr p is maximum withdrawal load (N, lb), G spcific gravity basd on ovndry wight and volum at 1% moistur contnt, D shank diamtr of th scrw (mm, in.), and L lngth of pntration of th thradd part of th scrw (mm, in.). (Th NDS and LRFD us ovndry wight and volum as a basis.) Ths valus ar basd on raching ultimat load in 5- to 1-min. This quation is applicabl whn scrw lad hols hav a diamtr of about 7% of th root diamtr of th thrads in softwoods, and about 9% in hardwoods. Th quation valus ar applicabl to th scrw sizs listd in Tabl 7 8. (Shank diamtrs ar rlatd to scrw gaugs.) For lngths and gaugs outsid ths limits, th actual valus ar likly to b lss than th quation valus. Th withdrawal loads of scrws insrtd in th nd grain of wood ar somwhat rratic, but whn splitting is avoidd, thy should avrag 75% of th load sustaind by scrws insrtd in th sid grain. Lubricating th surfac of a scrw with soap or similar lubricant is rcommndd to facilitat insrtion, spcially in dns woods, and it will hav littl ffct on ultimat withdrawal rsistanc. Tabl 7 8. Scrw sizs appropriat for Equation (7 1) Scrw lngth (mm (in.)) Gaug limits 1.7 (1/) 1 to (3/4) to (1) 3 to (1-1/) 5 to () 7 to (-1/) 9 to (3) 1 to Fastning of Particlboard Tapping scrws ar commonly usd in particlboard whr withdrawal strngth is important. Car must b takn whn tightning scrws in particlboard to avoid stripping th thrads. Th maximum amount of torqu that can b applid to a scrw bfor th thrads in th particlboard ar strippd is givn by T = X (mtric) (7 11a) T = X (inch pound) (7 11b) whr T is torqu (N m, in lb) and X is dnsity of th particlboard (kg/m 3, lb/ft 3 ). Equation (7 11) is for 8-gaug scrws with a dpth of pntration of 15.9 mm (5/8 in.). Th maximum torqu is fairly constant for lad hols of to 9% of th root diamtr of th scrw. Ultimat withdrawal loads P (N, lb) of scrws from particlboard can b prdictd by 1 / 54 / P = KD ( L D 3) G (7 1) whr D is shank diamtr of th scrw (mm, in.), L dpth of mbdmnt of th thradd portion of th scrw (mm, in.), and G spcific gravity of th board basd on ovndry wight and volum at currnt moistur contnt. For mtric masurmnts, K = 41.1 for withdrawal from th fac of th board and K = 31.8 for withdrawal from th dg; for inch pound masurmnts, K =,655 for withdrawal from th fac and K =,55 for withdrawal from th dg. Equation (7 1) applis whn th stting torqu is btwn 6% to 9% of T (Eq. ( 7 11)). Withdrawal rsistanc of scrws from particlboard is not significantly diffrnt for lad hols of 5% to 9% of th root diamtr. A highr stting torqu will produc a somwhat highr withdrawal load, but thr is only a slight diffrnc (3%) in valus btwn 6% to 9% stting torqus (Eq. (7 11)). A modst tightning of scrws in many cass provids an ffctiv compromis btwn optimizing withdrawal rsistanc and stripping thrads. Equation (7 1) can also prdict th withdrawal of scrws from fibrboard with K = 57.3 (mtric) or 3,7 (inch pound) for th fac and K = 44.3 (mtric) or,86 (inch pound) for th dg of th board. Latral Rsistanc Pr-1991 Th proportional limit loads obtaind in tsts of latral rsistanc for wood scrws in th sid grain of sasond wood ar givn by th mpirical quation p= KD (7 13) whr p is latral load, D diamtr of th scrw shank, and K a cofficint dpnding on th inhrnt charactristics of th wood spcis. Valus of scrw shank diamtrs for various scrw gaugs ar listd in Tabl

11 Tabl 7 9. Scrw shank diamtrs for various scrw gaugs Scrw numbr Diamtr or gaug (mm (in.)) 4.84 (.11) (.15) (.138) (.151) (.164) (.177) (.19) (.3) (.16) (.4) (.68) (.94) 8.13 (.3) (.37) Valus of K ar basd on rangs of spcific gravity of hardwoods and softwoods and ar givn in Tabl 7 4. Thy apply to wood at about 15% moistur contnt. Loads computd by substituting ths constants in th quation ar xpctd to hav a slip of.18 to.5 mm (.7 to.1 in.), dpnding somwhat on th spcis and dnsity of th wood. Equation (7 13) applis whn th dpth of pntration of th scrw into th block rciving th point is not lss than svn tims th shank diamtr and whn th sid mmbr and th main mmbr ar approximatly of th sam dnsity. Th thicknss of th sid mmbr should b about on-half th dpth of pntration of th scrw in th mmbr holding th point. Th nd distanc should b no lss than th sid mmbr thicknss, and th dg distancs no lss than onhalf th sid mmbr thicknss. This dpth of pntration (svn tims shank diamtr) givs an ultimat load of about four tims th load obtaind by th quation. For a dpth of pntration of lss than svn tims th shank diamtr, th ultimat load is rducd about in proportion to th rduction in pntration, and th load at th proportional limit is rducd somwhat lss rapidly. Whn th dpth of pntration of th scrw in th holding block is four tims th shank diamtr, th maximum load will b lss than thr tims th load xprssd by th quation, and th proportional limit load will b approximatly qual to that givn by th quation. Whn th scrw holds mtal to wood, th load can b incrasd by about 5%. For ths latral loads, th part of th lad hol rciving th shank should b th sam diamtr as th shank or slightly smallr; that part rciving th thradd portion should b th sam diamtr as th root of th thrad in dns spcis or slightly smallr than th root in low-dnsity spcis. Scrws should always b turnd in. Thy should nvr b startd or drivn with a hammr bcaus this practic tars th wood fibrs and injurs th scrw thrads, sriously rducing th load carrying capacity of th scrw. Post-1991 Scrw latral strngth is dtrmind by th yild modl thory (Tabl 7 5). Mods I, III, and IV failurs may occur (Fig. 7 4). Th dowl baring strngth valus ar basd on th sam spcific gravity quation usd to stablish valus for nails (Eq. (7 3)). Furthr discussion of scrw latral strngth is found in ASCE Manual No. 84, Mchanical Connctions in Wood Structurs. Lag Scrws Lag scrws ar commonly usd bcaus of thir convninc, particularly whr it would b difficult to fastn a bolt or whr a nut on th surfac would b objctionabl. Commonly availabl lag scrws rang from about 5.1 to 5.4 mm (. to 1 in.) in diamtr and from 5.4 to 46 mm (1 to 16 in.) in lngth. Th lngth of th thradd part varis with th lngth of th scrw and rangs from 19. mm (3/4 in.) with th 5.4- and 31.8-mm (1- and 1-1/4-in.) scrws to half th lngth for all lngths gratr than 54 mm (1 in.). Lag scrws hav a hxagonal-shapd had and ar tightnd by a wrnch (as opposd to wood scrws, which hav a slottd had and ar tightnd by a scrw drivr). Th following quations for withdrawal and latral loads ar basd on lag scrws having a bas mtal avrag tnsil yild strngth of about 31.3 MPa (45, lb/in ) and an avrag ultimat tnsil strngth of 53.9 MPa (77, lb/in ). Withdrawal Rsistanc Th rsults of withdrawal tsts hav shown that th maximum dirct withdrawal load of lag scrws from th sid grain of sasond wood may b computd as 3 / 34 / p=15. 4G D L 3 / 34 / p=8, 1G D L (mtric) (inch pound) (7 14a) (7 14b) whr p is maximum withdrawal load (N, lb), D shank diamtr (mm, in.), G spcific gravity of th wood basd on ovndry wight and volum at 1% moistur contnt, and L lngth (mm, in.) of pntration of th thradd part. (Th NDS and LRFD us ovndry wight and volum as a basis.) Equation (7 14) was dvlopd indpndntly of Equation (7 1) but givs approximatly th sam rsults. Lag scrws, lik wood scrws, rquir prbord hols of th propr siz (Fig. 7 6). Th lad hol for th shank should b th sam diamtr as th shank. Th diamtr of th lad hol for th thradd part varis with th dnsity of th wood: For low-dnsity softwoods, such as th cdars and whit pins, 4% to 7% of th shank diamtr; for Douglas-fir and Southrn Pin, 6% to 75%; and for dns hardwoods, such as oaks, 65% to 85%. Th smallr prcntag in ach rang applis to lag scrws of th smallr diamtrs and th largr 7 11

12 Tabl 7 1. Multiplication factors for loads computd from Equation (7 15) Figur 7 6. A, Clan-cut, dp pntration of thrad mad by lag scrw turnd into a lad hol of propr siz, and B, rough, shallow pntration of thrad mad by lag scrw turnd into ovrsizd lad hol. prcntag to lag scrws of largr diamtrs. Soap or similar lubricants should b usd on th scrw to facilitat turning, and lad hols slightly largr than thos rcommndd for maximum fficincy should b usd with long scrws. In dtrmining th withdrawal rsistanc, th allowabl tnsil strngth of th lag scrw at th nt (root) sction should not b xcdd. Pntration of th thradd part to a distanc about svn tims th shank diamtr in th dnsr spcis (spcific gravity gratr than.61) and 1 to 1 tims th shank diamtr in th lss dns spcis (spcific gravity lss than.4) will dvlop approximatly th ultimat tnsil strngth of th lag scrw. Pntrations at intrmdiat dnsitis may b found by straight-lin intrpolation. Th rsistanc to withdrawal of a lag scrw from th ndgrain surfac of a pic of wood is about thr-fourths as grat as its rsistanc to withdrawal from th sid-grain surfac of th sam pic. Latral Rsistanc Pr-1991 Th xprimntally dtrmind latral loads for lag scrws insrtd in th sid grain and loadd paralll to th grain of a pic of sasond wood can b computd as p= KD (7 15) whr p is proportional limit latral load (N, lb) paralll to th grain, K a cofficint dpnding on th spcis spcific gravity, and D shank diamtr of th lag scrw (mm, in.). Valus of K for a numbr of spcific gravity rangs can b found in Tabl 7 4. Ths cofficints ar basd on avrag rsults for svral rangs of spcific gravity for hardwoods and softwoods. Th loads givn by this quation apply whn th thicknss of th sid mmbr is 3.5 tims th shank diamtr of th lag scrw, and th dpth of pntration in th main Ratio of thicknss of sid mmbr to shank diamtr of lag scrw Factor Tabl Multiplication factors for loads applid prpndicular to grain computd from Equation (7 15) with lag scrw in sid grain of wood Shank diamtr of lag scrw (mm (in.)) Factor 4.8 (3/16) (1/4) (5/16) (3/8) (7/16) (1/) (5/8) (3/4).55. (7/8) (1).5 mmbr is svn tims th diamtr in th hardr woods and 11 tims th diamtr in th softr woods. For othr thicknsss, th computd loads should b multiplid by th factors listd in Tabl 7 1. Th thicknss of a solid wood sid mmbr should b about on-half th dpth of pntration in th main mmbr. Whn th lag scrw is insrtd in th sid grain of wood and th load is applid prpndicular to th grain, th load givn by th latral rsistanc quation should b multiplid by th factors listd in Tabl For othr angls of loading, th loads may b computd from th paralll and prpndicular valus by th us of th Scholtn nomograph for dtrmining th baring strngth of wood at various angls to th grain (Fig. 7 7). 7 1

13 Y P QN Load or strss paralll to grain Load or strss prpndicular to grain Load of strss at inclination θ with dirction of grain 45 b 3 15 X a Allowabl load or strss for P, Q, and N in units, tns, hundrds, or thousands Figur 7 7. Scholtn nomograph for dtrmining th baring strss of wood at various angls to th grain. Th dashd lin ab rfrs to th xampl givn in th txt. Th nomograph provids valus as givn by th Hankinson quation, PQ N = (7 16) Psin θ+ Qcos θ whr P is load or strss paralll to th grain, Q load or strss prpndicular to th grain, and N load or strss at an inclination θ with th dirction of th grain. Exampl: P, th load paralll to grain, is 6, lb, and Q, th load prpndicular to th grain, is, lb. Th load at an angl of 4 to grain, N, is found as follows: Connct with a straight lin 6, lb (a) on lin OX of th nomograph with th intrsction (b) on lin OY of a vrtical lin through, lb. Th point whr lin ab intrscts th lin rprsnting th givn angl 4 is dirctly abov th load, 3,3 lb. Valus for latral rsistanc as computd by th prcding mthods ar basd on complt pntration of th unthradd shank into th sid mmbr but not into th main mmbr. Whn th shank pntrats th main mmbr, th prmittd incrass in loads ar givn in Tabl 7 1. Whn lag scrws ar usd with mtal plats, th latral loads paralll to th grain may b incrasd 5%, providd th plat thicknss is sufficint so that th baring capacity of th stl is not xcdd. No incras should b mad whn th applid load is prpndicular to th grain. Lag scrws should not b usd in nd grain, bcaus splitting may dvlop undr latral load. If lag scrws ar so usd, howvr, th loads should b takn as two-thirds thos for latral rsistanc whn lag scrws ar insrtd into sid grain and th loads act prpndicular to th grain. Th spacings, nd and dg distancs, and nt sction for lag scrw joints should b th sam as thos for joints with bolts (discussd latr) of a diamtr qual to th shank diamtr of th lag scrw. Lag scrws should always b insrtd by turning with a wrnch, not by driving with a hammr. Soap, bswax, or othr lubricants applid to th scrw, particularly with th dnsr wood spcis, will facilitat insrtion and prvnt damag to th thrads but will not affct prformanc of th lag scrw. Post-1991 Lag scrw latral strngth is dtrmind by th yild modl thory tabl similar to th procdur for bolts. Mods I, III, and IV yild may occur (Fig. 7 4). Th dowl baring Tabl 7 1. Prmittd incrass in loads whn lag scrw unthradd shank pntrats foundation mmbr Ratio of pntration of shank into foundation mmbr to shank diamtr Incras in load (%)

14 strngth valus ar basd on th sam paralll- and prpndicular-to-grain spcific gravity quations usd to stablish valus for bolts. Bolts Baring Strss of Wood Undr Bolts Th baring strss undr a bolt is computd by dividing th load on a bolt by th product LD, whr L is th lngth of a bolt in th main mmbr and D is th bolt diamtr. Basic paralll-to-grain and prpndicular-to-grain baring strsss hav bn obtaind from tsts of thr-mmbr wood joints whr ach sid mmbr is half th thicknss of th main mmbr. Th sid mmbrs wr loadd paralll to grain for both paralll- and prpndicular-to-grain tsts. Prior to 1991, baring strss was basd on tst rsults at th proportional limit; sinc 1991, baring strss is basd on tst rsults at a yild limit stat, which is dfind as th 5% diamtr offst on th load dformation curv (similar to Fig. 7 3). Th baring strss at proportional limit load is largst whn th bolt dos not bnd, that is, for joints with small L/D valus. Th curvs of Figurs 7 8 and 7 9 show th rduction in proportional limit bolt-baring strss as L/D incrass. Th baring strss at maximum load dos not dcras as L/D incrass, but rmains fairly constant, which mans that th ratio of maximum load to proportional limit load incrass as L/D incrass. To maintain a fairly constant ratio btwn maximum load and dsign load for bolts, th rlations btwn baring strss and L/D ratio hav bn adjustd as indicatd in Figurs 7 8 and 7 9. Th proportional limit bolt-baring strss paralll to grain for small L/D ratios is approximatly 5% of th small clar crushing strngth for softwoods and approximatly 6% for hardwoods. For baring strss prpndicular to th grain, th ratio btwn baring strss at proportional limit load and th small clar proportional limit strss in comprssion prpndicular to grain dpnds upon bolt diamtr (Fig. 7 1) for small L/D ratios. Spcis comprssiv strngth also affcts th L/D ratio rlationship, as indicatd in Figur 7 9. Rlativly highr bolt proportional-limit strss prpndicular to grain is obtaind with wood low in strngth (proportional limit strss of 3,93 kpa (57 lb/in ) than with matrial of high strngth (proportional limit strss of 7,86 kpa (1,14 lb/in )). This ffct also occurs for bolt-baring strss paralll to grain, but not to th sam xtnt as for prpndicular-to-grain loading. Th proportional limit bolt load for a thr-mmbr joint with sid mmbrs half th thicknss of th main mmbr may b stimatd by th following procdurs. For paralll-to-grain loading, (a) multiply th spcis small clar comprssiv paralll strngth (Tabls 4 3, 4 4, or 4 5) by.5 for softwoods or.6 for hardwoods, (b) multiply this product by th appropriat factor from Figur 7 8 for th L/D ratio of th bolt, and (c) multiply this product by LD. Ratio of long- to short-bolt baring strss (%) For prpndicular-to-grain loading, (a) multiply th spcis comprssion prpndicular-to-grain proportional limit strss (Tabls 4 3, 4 4, or 4 5) by th appropriat factor from Figur 7 1, (b) multiply this product by th appropriat factor from Figur 7 9, and (c) multiply this product by LD. Loads at an Angl to th Grain A L/D ratio Figur 7 8. Variation in bolt-baring strss at th proportional limit paralll to grain with L/D ratio. Curv A, rlation obtaind from xprimntal valuation; curv B, modifid rlation usd for stablishing dsign loads. Ratio of long- to short-bolt baring strss (%) A-1 A- B-1 B L/D ratio Figur 7 9. Variation in bolt-baring strss at th proportional limit prpndicular to grain with L/D ratio. Rlations obtaind from xprimntal valuation for matrials with avrag comprssion prpndicular strss of 7,86 kpa (1,14 lb/in ) (curv A 1) and 3,93 kpa (57 lb/in ) (curv A ). Curvs B 1 and B, modifid rlations usd for stablishing dsign loads. For loads applid at an angl intrmdiat btwn thos paralll to th grain and prpndicular to th grain, th boltbaring strss may b obtaind from th nomograph in Figur 7 7. B 7 14