Creep of LVL and Its Effect on the Structures

Transcription

1 Crp of LVL ad Is Effc o h Srucurs H. Z. Zhou PhD Sud Harb Isu of Tchology Harb, Cha E. C. Zhu Profssor Harb Isu of Tchology Harb, Cha S. W. Wag Egr Cha Souhws Archcural Dsg ad Rsarch Isu Chgdu, Cha Summary Sx ss of s apparauss wr dsgd ad maufacurd, of whch, ach hr wr for h so ad comprsso ss, rspcvly. Th yar-log so ad comprsso ss of LVL wr coducd udr hr srss lvls. O h bass of h s rsuls, h cosuv modl of LVL was dvlopd. Th modl was corporad o h commrcal FE sofwar ABAQUS by dvlopg a usr dfd subrou-uma. FE modls o prdc h log-rm prformac of LVL srucurs, bucklg parcular, wr dvlopd. Bucklg aalyss of a rculad Kw-6 LVL shll was coducd. A arly lar rlaoshp bw h bucklg load ad h logarhmc bucklg m of h shll was rvald hrough h aalyss. Th safy load agas bucklg durg srvc lf of h shll was dfd. Th rmag load poal of h shll agas bucklg afr a -srvc prod of m was also valuad cosdrao of h p, ad a lar rlaoshp was also foud bw h bucklg load ad h logarhmc srvc m.. Iroduco Lamad vr lumbr (LVL) s a grd wood produc wdly usd cvl grg. I s suabl for cosrucg larg spa srucurs lk rculad shlls. Y bucklg of such srucurs s a problm ha ds o b addrssd wh car. Crp s o of h characrscs ha dsgush mbr ad wood-basd composs from ohr srucural marals. Effc of p of LVL o h srucurs s worh vsgag o prdc h log-rm prformac, cludg bucklg. Ths sudy ams o vsga xprmally h p of LVL o facla sablshg of h cosuv modl of LVL, ad o vsga umrcally h bucklg bhavour of rculad LVL shlls akg p o cosdrao. Suds of p of wood da back o as arly as 740 wh Frch aval archc Buffo [] vsgad h srgh loss of Oak bams udr susad load. Evr sc, p of wood ad

2 wood srucurs has log b a focus of sudy, whch prom work cluds h rsarch by Wood [2, 3], Lska [4] ad rsarch by Mads ad Barr [5]. Mor rcly, rsarchrs hav coducd sg of p bhavour of bdg mmbrs maly, cludg bams or bam sysms mad of grd wood producs [6-0]. Ths suds usually provdd a facor rflcg h rao of dflco du o p o dflco udr sa loadg. Th log-rm srucural bhavour ca hus b valuad by applyg such a facor. Bucklg of srucurs s also qu old a opc, wh Eulr s porg aalyss of h bucklg of a axally comprssd lasc rod publshd 757 []. Takg p of maral o cosdrao, h da of p bucklg was frs roducd o aalyss of srucurs by Hlo ad Hoff h 950s [2], rspcvly, ad currly aalyss of sl srucurs udr fr [3]. Alhough hr ar som lmd publshd suds of bucklg of rculad wood shlls [4], so far hr ar hardly ay publshd suds o h p bucklg of LVL shlls, dsp h fac ha p of LVL dd poss sgfca ffc o h log-rm srucural prformac. I hs sudy, so ad comprsso ss o h p of LVL wr coducd udr a door clma for a prod of o yar. Th cosuv modl dalg wh p ad suabl for umrcal modlg of srucurs was sablshd, ad FE modls corporag ABAQUS o prdc h log-rm srucural prformac of LVL srucurs wr dvlopd. Aalyss of bucklg ad p bucklg of a rculad Kw-6 LVL shll was coducd ad h rmag load poal agas bucklg afr a -srvc prod of m of h shll valuad. 2. Tso ad comprsso s of LVL 2. Ts of LVL Th Kro LVL, a prom Fsh wood produc, was usd h sg. Shor-rm s of LVL was frs coducd o oba h basc mchacal proprs lk modul of lascy (MOE) ad h ulma srghs, hus o provd rfrc for log-rm sg. 5 so ad 5 comprsso spcms wr prpard. I accordac wh ASTM D98-05 [], h spcms for comprsso wr a sz of mm; h ovrall lgh of so spcms was 500 mm, wh a oss-sco h wasd sco of 5 5 mm; h gaug lgh for boh so ad comprsso was all 50mm. All spcms wr codod o cosa mass a a rlav humdy of (65±5)% ad a mpraur of (20±2) C. Boh h so ad comprsso ss wr coducd o a vrsal sg mach. Accordg o h sg rsuls, h avrag comprsso srgh was 43.6 MPa, ad h MOE was MPa; h avrag so srgh was 49.8 MPa, ad h MOE was MPa. Th avragd valus of so ad comprsso proprs wr usd as h rfrc for log-rm sg ad for sablshg h cosuv rlaoshps. Log-rm ss wr coducd accordac wh ASTM D685-02a [5]. Boh so ad comprsso spcms wr subjcd o hr srss lvls,.. 0.2, 0.4 ad 0.6 m of h avragd srgh, ach srss lvl coag 3 spcms. Toally 9 so ad 9 comprsso spcms wr usd. Th ss, udr a ormal door clma Harb, Norhas Cha, lasd for o yar, from h d of Ju 2006 o arly July 2007, durg whch h room was had from md-ocobr o md-aprl.

3 (a) Tso s. (b) Comprsso s. Fg. Log-rm s s-up. To facla h sg, 6 ss of s apparauss wr dsgd ad maufacurd, of whch 3 ss wr for so ad 3 ss wr for comprsso. As show Fg., ach apparaus accommodad 3 spcms udr a cra srss lvl. For so s Fg. a, h hr spcms wr arragd a paralll way. A bol hol was drlld a boh ds of spcm o coc o h apparaus; for comprsso s Fg. b, h hr spcms wr arragd srs. A udrcoal l hg was provdd o ach d of spcm ad h wo hgs wr prpdcular o ach ohr, hus assurg wo-way roaos of spcm. Th daphragms bw spcms wr dsgd o provd laral rsras. A dal gaug covrg a lgh of 50 mm was appld o ach spcm o dca dformao. Load was appld va a jack ad maad by h sprg sysm. Th load was moord hroughou h s by a srss rg moud o ach apparaus. If ay loss of load was dcd, compsao of load was mad o maa h cosa srss lvl. Tmpraur ad rlav humdy Tmpraur ( o C) Rlav humdy (%) 0 Jul. Sp. Nov. Ja. Mar. May Tm (day) Sra Ts. SL=0.2 Comp. SL=0.2 Ts. SL=0.4 Comp. SL= Ts. SL=0.6 Comp. SL= σ f σ f σ f Jul. Sp. Nov. Ja. Mar. May Tm (day) Fg. 2 Aual varaos of mpraur ad h rlav humdy. Fg. 3 Crp of LVL vrsus m. Th mpraur ad h rlav humdy wr rcordd by a hygrohrmograph. Fg. 2 shows h rcords dcag h door mpraur ad humdy varaos/flucuaos all yar roud. Th

4 rcords dmosra ha mpraur varaos wr grar summr ad lssr wr du o hag of h buldg, whl h rlav humdy was low wr. Fg. 3 shows p sra of LVL udr h hr dffr srss lvls, whr a sgl curv s h avragd valu from 3 spcms udr h sam srss lvl. Grally, h p sra ass wh m, alhough hr ar flucuaos du o varaos mpraur ad rlav humdy; h magud of p sra ass wh srss lvl; a h al sag of abou wo mohs, p sra clmbs up rapdly, ad h rs h scod sag of sady as. I s worh og ha hr s vrually o dffrc of p bw so ad comprsso. 2.2 Emprcal modl of p of LVL Rlav p Ts. SL=0.2 Ts. SL=0.4 Ts. SL=0.6 Rgrsso Comp. SL=0.2 Comp. SL=0.4 Comp. SL= Tm (days) I ordr o prdc h log-rm prformac of srucurs aalycally, umrcal modl of p of LVL ds o b sablshd. Basd o h characrscs of p from sg, p ca b xprssd as a fuco of srss ad m, ad ca b rad as h sam rgardlss of so or comprsso. Furhrmor, f h rlav p sra s dfd as ε r ε ε =, whr ε s h p sra ha s h oal Fg. 4 Rlav p sra vrsus m. sra mus h sa lasc sra; adε s h sa lasc sra rspos o h srss, ad f h rlav p sra s roducd o Fg. 3, h p bhavour ca h b llusrad Fg. 4. Rlav p sra of dffr srss lvls s show Fg. 4. Th Klv cha modl was usd o f h rlav p curvs. By rgrsso h rlav sra of LVL s xprssd as ε r ε = = c ( τ ) () ε = whr c s h coffc of h srs; ad τ s h m coffc h Klv cha modl, τ was s as 0.,, 0, 00, 000, 0000 days. Th p complac ca hus b obad as J () = c( τ ) (2) E = Th muldmsoal p sra ca b xprssd as d σ( ) ε = S ( ) d (3) d 0

5 whr S ( ) s h p complac marx [6] u T () = φ φ φ ( τ ) = S S (4) whr u T φ s h mosur facor; φ s h mpraur facor; φ s h facor of h srs ms. Subsug Eq. (4) o Eq. (3) ad usg covoluoal grao, h p sra ca b rwr as u T d σ( ) τ ε = φ φ φ S ( ) (5) = d whr s h opraor for covoluoal grao. Th p sra ra s gv as ε d σ( ) τ u T = φ φ φ S = d τ τ = r () (6) τ = u T d ( ) σ () = τ φ φ φ d d 0 r S (7) r () ca b gv by h rcurso formula as u T, r( ) = r ( ) + τ φ φ φ Δε (8) Th mal p sra s hus gv as N τ τ u T, (( r ),) = τ Δ ε = + φ φ φ Δε Δ (9) 3. Soluo procdurs 3. Iraos of srss ad sra Th mal oal sra Δε ca b dvdd o h lasc compo Δε, ad h p compo Δε, hus Δ ε = Δ ε +Δε (0) Th m of lasc sra du o srss m s xprssd as Δ ε = S Δσ () whr S s h maral complac marx. Subsug Eq. (9) o Eq. (0), h par dpd o srss m ca b gv as N τ oal r = τ N u T Δ ε ( )Δ Δε = (2) + φ φ φ Δ = τ Th srss m ca hus b gv as

6 Δ σ = S Δε (3) Th curr srss sa s updad as σ = σ +Δσ (4) Soluo of h qulbrum quaos A a arbrary m, h olar f lm qulbrum quao of a srucur s xprssd as K d δ d R = ( K0 + Kσ + KL) d δ d R 0 (5) T = whr d δ s h m of dsplacm of h srucur; d R s h rsdual forc; KT = K0+ Kσ + K L, h ag sffss marx of h srucur, of whch K 0 s h lar lasc sffss marx, K σ s h srss sffss marx, ad K L s h lar dsplacm sffss marx. Th ag sffss marx s updad afr ach m of load or ach m of dsplacm du o p, whr load ca b cosa ad m s a ral varabl. Ths sudy covrs wo yps of olar aalyss of srucurs ha d dffr rav srags: lasc olar bucklg aalyss ad p bucklg aalyss. I ordary lasc olar bucklg aalyss, bucklg load ad qulbrum pahs ar sough by solvg h qulbrum quao a ach load m. To ovrcom h dffculs causd by h sgulary of h ag sffss marx a h cal po, h arc-lgh mhod or h Rks [7] mhod s roducd. Usually lar gvalu bucklg aalyss s coducd frs, o oba h bucklg mods. Th lows bucklg mod s h roducd as a kd of gomrcal mprfco o h srucur o mplm h olar aalyss. Wh a srucur s subjcd o a cra load, h dformao cous o as du o p of h maral. Wh h dformao s larg ough, h srucur loss h sably - p bucklg happs whl h load rmas uchagd. Numrcally spakg, h procss of pg h srss sffss marx ad lar dsplacm sffss marx vary wh m. Wh h ag sffss marx approachs sgulary, h srucur approachs a sa of p bucklg, rsulg hr abrup as of dformao or dvrgc of aalyss. I h aalyss, h m s chos as h varabl, whl load s kp cosa. Dformao s sough accordg o updad qulbrum wh as h p sra, whch s a fuco of m as xprssd by Eq. (5). Basd o h abov mod hory ad mhods, FE modls wr dvlopd corporag ABAQUS [8]. Th Rks mhod was roducd o coduc lasc olar bucklg aalyss. A usr dfd subrou, Uma, was codd o dal wh h p bhavour of LVL, hus o coduc p bucklg aalyss. 4. Bucklg of a rculad Kw-6 LVL shll Fg. 5 shows a rculad Kw-6 LVL shll subjcd o uformly dsrbud vrcal load, wh a spa of 30 m ad a hgh of 6 m, All LVL bars, wh a oss-sco of mm, ar rgdly jod a ods. Th fxd boudary codos ar assumd,.. all dsplacms ar cosrad.

7 (a) Gomry of shll. (b) Lows bucklg mod. Fg. 5 A rculad Kw-6 LVL shll: E=3750 MPa. 4. Elasc olar bucklg Egvalu bucklg aalyss was coducd frs. I h FE modl, ach LVL bar was modlld wh 0 bam lms, B3 [8]. Th lows bucklg load s 0.85 kn/m 2, ad h bucklg mod s show Fg. 5b, dcag ha dformao maly happs o h ma rbs. I s wll kow ha h bucklg load s usually ovrsmad by h gvalu bucklg aalyss. Thrfor, h lows bucklg mod, magud of /000, 3/000 ad 5/000 of h spa, rspcvly, was roducd as gomrcal mprfcos o h shll o coduc h olar bucklg aalyss. Fg. 6 shows h load-dflco curvs of h shll. Load (kn/m 2 ) Dflco (m) f=/000 f=3/000 f=5/000 Fg. 6 Load-dflco curvs of h shll. Th shll s mprfco ssv, wh h bucklg load sgfcaly lowr ha h gvalu bucklg aalyss. Th bucklg load from Fg. 6 s 7.63, 4.84 ad 3.35 kn/m 2 wh rspc o dffr magud of h mprfcos. Sc 3/000 of h spa s h grally rcogzd magud of gomrcal mprfco olar aalyss, h corrspodg bucklg load of 4.84 kn/m 2 s ak as h rfrc load-carryg capacy of hs shll. 4.2 Crp bucklg If h p bhavour of LVL s ak o cosdrao, h bucklg load wll b m dpd ad wll b much lowr ha prdcd h las sco. To prdc h log-rm bucklg bhavour, h lows bucklg mod was aga roducd o h shll as h gomrcal mprfco wh magud of 3/000 of h spa; a uform dad load a cra prcags of h sa olar bucklg load was appld o h shll. Th aalyss was coducd followg wo sps: (a) Gral sac aalyss. Th load was appld o h srucur ad sa rspos lk dformao ad ral forcs of h shll wr obad. (b) Gral ras sac aalyss. Rag h appld load, p of LVL was cosdrd by mas of Uma. Th log-rm bucklg bhavour of h shll was hus prdcd.

8 Dflco (m) P /P E =0.80 P /P E = P /P E =0.50 P /P E =0.45 P /P E = Tm (day) Load rao Safy zo Crcal m (day) Smulao Rgrsso Fg. 7 Log-rm bucklg bhavour of h shll. Fg. 8 Bucklg load vrsus bucklg m. Fg. 7 shows h p bucklg bhavour of h LVL shll wh rspc o dffr load lvls, whr P s h p bucklg load; P E s h sa bucklg load; P / P E s dfd as h load rao. If h load rao s blow a cra lvl, 0.40 hs xampl, h dflco grows sadly wh m passg, bu whou prcpag bucklg wh h dsgd basc srvc lf, 50 yars Cha. A a rao of 0.45, or abov, h dflco grows wh m. Gradually, h dflco clmbs up sharply ad a a cra m, abou 000 days for xampl, urs o b dvrg. Hc h rdpd bucklg load ad h bucklg m. Th largr h load rao, h shorr h bucklg m. If h load rao s ovr 0.8, bucklg of h shll ca b a mar of oly a fw days. Fg. 8 shows a arly lar rlaoshp bw h bucklg load ad h logarhmc bucklg m of h shll. Th load blow h rsco of h bucklg load-m l ad h l draw vrcal a h 50-yar srvc m, whch wll o prcpa p bucklg wh h srvc lf, s dfd as h safy load. Th shadowd ara h fgur s h safy zo, h shll udr h load fallg hs zo wll b saf agas bucklg. Th ffc of p s so sgfca ha h log-rm load carryg capacy (h safy load) s abou 8% of h olar lasc bucklg load. Th bucklg load-m rlaoshp of hs shll ca b mahmacally xprssd as P / P = lg( ) (6) E whr h u of s days. 4.3 Rmag load poal of h shll srvc agas bucklg Th abov aalyss dcas ha p of LVL poss sgfca ffc o h bucklg bhavour of h shll. Havg b srvc for som m, h bucklg rssac of h shll shall b lowrd. Evaluao of h rmag rssac wll b of a cocr. To prdc hs, h shll s supposd o b udr h aco of load,.0 kn/m 2, ormal srvc codos. Th srucural rspos o hs load afr a prod of m s obad va aalyss followg h wo sps dsbd Sco 4.2. Th rmag load poal agas bucklg s h valuad by coducg h ordary olar bucklg aalyss, bu akg h load ad dformao, cludg dformao du o p as h sarg po.

9 5 4.0 Load (kn/m 2 ) yars 5 yars 0 yars 30 yars 50 yars Bucklg load (kn/m 2 ) Dflco (m) Tm (yar) Fg. 9 Rmag load poal of h shll. Fg. 0 Load poal vrsus logarhmc m. Fg. 9 shows h rmag bucklg load rgard o a srvc m of 0, 5, 0, 30 ad 50 yars, rspcvly. Th bucklg load dass wh srvc m. Th mos svr rduco, as much as 5% happs fv yars hs aalyss du o h fac ha much of p happs h al sag of loadg. Irsgly, a lar rlaoshp was also foud bw h bucklg load ad h logarhmc srvc m, as show Fg Coclusos Comprsso ad so ss of LVL wr coducd o valua h p bhavour. Basd o h s rsuls, h p cosuv rlaoshp of LVL rms of rlav p, was sablshd usg h Klv cha modl. Icorporag ABAQUS, FE modls o prdc h log-rm prformac of LVL srucurs, bucklg parcular, wr dvlopd. Bucklg aalyss of a rculad Kw-6 LVL shll was coducd. Du o h ffc of p, h p bucklg load ad bucklg m ar rdpd. Th largr h load rao, h shorr h bucklg m. A arly lar rlaoshp bw h bucklg load ad h logarhmc bucklg m of h shll was rvald hrough h aalyss. Usg h rlaoshp, h safy load was dfd, blow whch h shll shall o buckl wh h srvc lf. I rms of bucklg, h rmag load poal of h LVL shll afr a -srvc prod of m was also valuad cosdrao of h p. Th bucklg load dass wh srvc m ad a lar rlaoshp was also foud bw h bucklg load ad h logarhmc srvc m. Ackowldgms Th wrrs ackowldg, wh graud, h facal suppor of hs sudy from Harb Isu of Tchology, Cha. Thaks ar also du o Profssor Alpo Raa-Mauus from VTT, Flad, who hlpd oba h sg maral, ad o Mr Vcor Ta from Ffors who coordad rasporao of h maral. Rfrcs [] Hoffmyr Prb, Srgh udr log-rm loadg, Corbuo o Wly xbook o Tmbr Egrg, Novmbr 200.

10 [2] Wood L. W., Bhavour of wood udr coud loadg, Eg. Nws-Rcord, 39(24), 947, pp [3] Wood L. W., Rlao of srgh of wood o durao of srss, USDA Fors Producs Laboraory, Madso, WI, USA. Rpor No. 96, 95. [4] Lska J. A., Effc of rapd loadg o h comprssv ad flxural srgh of wood, USDA Fors Producs Laboraory rpor No. 767, 950. [5] Mads B., Barr J.D., Tm srgh rlaoshp for lumbr. Srucural Rsarch Srs Rpor No. 3. Uvrsy of Brsh Columba, Vacouvr, Caada, 976. [6] Nur Yazda P.E., Johso Erc, Duwad Shla, Crp ffc srucural compos lumbr for brdg applcaos, Joural of Brdg Egrg, Jauary/Fbrury 2004, pp [7] Raa-Mauus Alpo, Korsmaa Markku, Crp of mbr durg gh yars aural vroms, WCTE [8] Frdly Kh J., Hog Pyoyoo, Rosowsky Davd V., Tm-dpd srvc-load bhavor of wood floors: xprmal rsuls, Joural of Srucural Egrg, Ju 997, pp [9] Frdly K. J., Rosowsky D. V., Hog P., Tm-dpd srvc-load bhavor of wood floors: Aalycal Modl, Compurs & Srucurs, Vol.66, No. 6,997, pp [0] Wswsk Bjam, Mabck Harvy B., Rsdal Floor Sysms: Wood I-Jos Crp Bhavor, Joural of Archcural Egrg, Vol. 9, No., March 2003, pp [] Callad C. R., Udrsadg mprfco-ssvy h bucklg of h-walld shlls, Th-Walld Sruc, Vol. 23, 995, pp [2] Mulaa Aasasa H., Haj-Al Ram M., Aalyss for p bhavor ad collaps of hck-sco compos srucurs, Compos srucurs, 73(2006), pp [3] Zg J. L., Ta K. H., Huag Z. F., Prmary p bucklg of sl colums fr, Joural of Cosrucoal Sl Rsarch, 59 (2003), pp [4] Fujmoo M., Ima K., Furukawa T., ad Ikmoo M., Exprmal sudy of sgl layr hr-way grd dom composd of K-wood spac russ sysm wh roud mbr, IASS Symposum 2004, Mopllr. [5] Aual Book of ASTM Sadards 2005, Sco Four, Cosruco, Volum 04.0, Wood. Prd Balmor, MD, U.S.A. p [6] Ormarsso S., Numrcal Aalyss of mosur-rlad Dsorsos Saw Tmbr, PhD Thss, Chalmrs Uvrsy of Tchology (Swd), 999. [7] ABAQUS hory maual Vrso 6.. Hbb, Karlsso & Sors, Ic., Pawuck, R.I. 2000a. [8] ABAQUS usr s maual Vrso 6.. Hbb, Karlsso & Sors, Ic., Pawuck, R.I. 2000b.