2. Self-tapping screws as tensile reinforcements perpendicular to the grain

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1 Reinforcements perpeniculr to te grin using self-tpping screws Univ.-Prof. Dr.-Ing. Hns Jocim Blss Dipl.-Ing. Ireneusz Bejtk Lerstul für Ingenieurolzbu un Bukonstruktionen University of Krlsrue 7618 Krlsrue, Germny Summry Tis pper sows te efficiency of screws use s tensile n compressive reinforcements perpeniculr to te grin. Fully tree screws use s reinforcements in notce bem supports, connections wit lo components perpeniculr to te member xis n bems wit oles re esigne using te sme principles s for conventionl reinforcements s e.g. glue-in ros. For reinforce bems wit oles, n extene esign moel to tke into ccount te ser stress peks close to te ole is propose. Furter on, for reinforce bem supports clcultion moel is propose to esign te self-tpping screws s compressive reinforcement. Keywors: Self-tpping screw, reinforcement, bem wit ole, notce bem, connection wit lo component perpeniculr to te member xis, bem support, connection wit multiple fsteners, tensile strengt, compressive strengt 1. Introuction Te tensile n te compressive strengts of timber perpeniculr to te grin re muc lower tn te respective strengt vlues prllel to te grin. Te crcteristic tensile strengt perpeniculr to te grin for soli timber is bout 1/5 to 1/60 of te tensile strengt prllel to te grin. Te rtio of te crcteristic compressive strengt perpeniculr to te grin to te compressive strengt prllel to te grin for soli timber is bout 1/8. Timber structures ence soul be etile in orer to minimise tensile n compressive stresses perpeniculr to te grin. Exmples for structurl etils were tensile stresses perpeniculr to te grin occur re notce bem supports, connections wit lo components perpeniculr to te member xis, bems wit oles s well s connections wit multiple fsteners in row. An exmple for structurl etil wit compressive stresses perpeniculr to te grin is bem support. Glue-in tree ros n glue-on plywoo pltes re tritionlly use s reinforcements. Screws wit continuous tres present n lterntive to te tritionl reinforcement metos n sow new n economic possibilities s reinforcements in connections n bem supports. Wit imeters up to 1 mm n lengts up to 600 mm, fully tree screws my be use in mny structurl members s tensile n compressive reinforcements perpeniculr to te grin.. Self-tpping screws s tensile reinforcements perpeniculr to te grin.1 Stte of te rt Te lo-crrying cpcities for notce bem supports, for connections wit lo components perpeniculr to te member xis s well s for bems wit oles reinforce wit self-tpping screws my be clculte using te sme moels use for conventionl reinforcements. Te clcultion moel generlly ssumes tt te lo component perpeniculr to te grin is entirely trnsmitte by tensile forces in te reinforcing screws. Te tensile strengt perpeniculr to te grin of te timber is not tken into ccount. Consequently, te witrwl cpcity R x, n te tensile strengt of te screw re te only prmeters etermining te lo-crrying cpcity for reinforce structurl etils were tensile stresses perpeniculr to te grin occur. Bse on te esign proceure in [], te lo-crrying cpcity for reinforce notce bem

2 supports (Fig. 1) is clculte ccoring to eqution (1): 1,c α 90 l, l,1. V n Rx, 3 13, 3 ( 1 α) ( 1 α) (1) V Fig. 1: Reinforce notce bem support Similrly, te lo-crrying cpcity for reinforce connections wit lo components perpeniculr to te member xis (Fig. ) my be clculte ccoring to eqution (): α l l,,1 l S,c,c 1 F 90, n Rx, 1 3 α + α 3 [ ] () F 90 Fig. : Reinforce connection Finlly, te lo-crrying cpcity for reinforce bems wit roun or rectngulr oles (Fig. 3) cn be clculte ccoring to eqution (3): r l,1 M V r l,,c,c V M r V M +, n R Fig. 3: Reinforce bem wit rectngulr ole Te esign vlue of te xil lo-crrying cpcity R x, is clculte ccoring to eqution (4): l f1, R (4) x, = min wit l = min{ l, 1 ; l, } Rt,u, wit Nominl or outer imeter of te self-tpping screw f 1, Design vlue of te witrwl cpcity prmeter ([]) n Number of screws in row perpeniculr to te grin irection R t,u, Design vlue of te tensile cpcity of te screw For self-tpping screws loe in xil irection te minimum spcing, en n ege istnces in Fig. 1 to Fig. 3 re efine s: 5, ,c,c x, (3)

3 In [1] bout 140 test results for reinforce n non-reinforce structurl etils, were tensile stresses perpeniculr to te grin occur, re presente. For reinforce notce bems n for reinforce connections wit lo components perpeniculr to te member xis te clculte locrrying cpcities were lwys lower tn te test results n ence conservtive. Only for some reinforce bems wit oles te clculte lo-crrying cpcities significntly extene te test results. Te filure of tese bems wit oles ws controlle by lrge ser stresses close to te ole.. Extene eqution for reinforce bems wit oles Due to te existence of te ole, te istribution of te ser stresses in te vicinity of te ole consierbly evites from te istribution ccoring to bem teory. Fig. 4 sows scemtic representtion of te ser stresses were τ 1 n τ 4 re ser istributions ccoring to bem teory n τ n τ 3 result from te influence of te ole. In generl τ > τ 3 > τ 1 > τ 4. Te mximum vlue of τ is close to te corner of te ole n of τ 3 in te bem xis t istnce of bout from te ole. τ τ τ τ Fig. 4: Distribution of te ser stresses in bem wit ole In orer to estimte te mximum ser stresses roun ole bout 000 FE clcultions wit ifferent geometries were performe n n empiricl eqution ws erive [1]. For constnt ser force n for ole geometry wit 0,1 / 1,0 n 0,1 / 0,4 te mximum ser stress τ mx in te vicinity of ole my be estimte using te following eqution: V 1 5 wit 1 84 = = 1+ τmx κmx, κmx, b ( ) 0, (5) Prticulrly for lrger rectngulr oles te ser esign ccoring to eqution (5) governs te locrrying cpcity n reinforcement using glue-on plywoo les to iger cpcities. Clculte vlues V,F 90 [kn] 60 Connections wit lo perpeniculr to te xis - non-reinforce Connections wit lo perpeniculr to te xis - reinforce Bems wit ole - non-reinforce 50 Bems wit ole - reinforce Notce bem supports - non-reinforce Notce bem supports - reinforce Test results [kn] Fig. 5 sows te comprison between te test results n te clculte lo-crrying cpcities V n F 90 for reinforce n nonreinforce structurl etils were tensile stresses perpeniculr to te grin occur. Te vlues for nonreinforce etils were clculte ccoring to []. Te vlues for reinforce etils were clculte tking into ccount eqution (1) to (5). Fig. 5: Comprison between tests results n clculte vlues 1003

4 .3 Reinforce connection res Tensile stresses perpeniculr to te grin lso occur in multiple fsteners connections wit oweltype fsteners were te timber is prone to splitting long te fstener row. Te splitting tenency increses wit ecresing fstener spcing prllel to te grin n ecreses te effective number of fsteners. Splitting my be prevente by reinforcing te connection n consequently, te effective number of fsteners increses. Self-tpping screws wit continuous tres represent simple n economic reinforcement meto. Te screws re plce ner te owel-type fsteners, perpeniculr to te owel xis n to te grin irection (Fig. 6). Tey trnsfer te tensile forces perpeniculr to te grin irection. At te University of Krlsrue reserc project wit reinforce multiple fstener connections is presently crrie out. First test results sow tt in connections wit sufficient reinforcement te effective number equls te ctul number of owels. 1 R R R R R Fig. 6: Reinforce connection wit owel-type fsteners 3. Self-tpping screws s compressive reinforcements perpeniculr to te grin 3.1 Tests wit reinforce bem supports Tests were me to compre te lo-crrying-cpcity of reinforce n non-reinforce bem supports. As reinforcement, self-tpping screws wit continuous tres were plce t te bem supports (Fig. 7). Te ngle between te screw xis n te grin irection ws 90. To evenly pply te support lo on te screws n on te timber, steel plte ws plce between te lower bem surfce n te support. Type A Type B lo istribution F90 lo istribution F 90 lef, lef, 45 l S 45 l S steel plte F90 F90 l A l A lef,1 lef,1 b 1 1,c,c,c b 1 1,c,c,c Fig. 7: Teste bem supports In tese tests, te type of screw (screw imeter n lengt l S ), te number n of screws s well s te imensions b x l ef,1 of te support re were vrie. A prt of te test results for ifferent influencing prmeters is sown in Tb. 1. Compring te test results, te lo-crrying cpcity of te reinforce bem supports ws between 31 % n 49 % of te lo-crrying cpcity of te non-reinforce bem supports. Depening on te type of screw, tree ifferent filure moes (FM) were observe:

5 FM 1: Te lo-crrying cpcity of te reinforce bem support ws crcterise by pusing te screws into te timber. Simultneously, te compressive strengt perpeniculr to te grin t te lower bem surfce ws rece. For screws te pusing-in cpcity is consiere equl to te witrwl cpcity. Tis filure moe ws observe t bem supports reinforce wit sort screws. FM : Te lo-crrying cpcity of te reinforce bem support ws crcterise by buckling of te screws. Simultneously, te compressive strengt perpeniculr to te grin t te lower bem surfce ws rece. Tis filure moe ws observe t bem supports reinforce wit slener screws. FM 3: Te lo-crrying cpcity of te reinforce bem support ws crcterise by recing te compressive strengt perpeniculr to te grin in plne forme by te screw points. Tis filure moe ws observe t bem supports reinforce wit sort screws. Tb. 1: Test results Specimen Screws Bem support Type Number Dimeter Lengt Number Effective lengt Wit Lengt Lo Lo Filure A/B n l S n l ef b l F c,90 increse Moe [mm] [mm] [-] [mm] [mm] [mm] [kn] [%] A ,1 - - A 10 6, % 3 B ,4 - - B 3 7, % 1/3 B % B % 3. Design equtions for reinforce bem supports Tking into ccount te ifferent filure moes, te lo-crrying cpcity R 90, of reinforce bem support my be clculte s follows: n R + kc, 90 lef, 1 b fc, 90, R90, = min bl ef, fc, 90, were { ; } R = min R R R = l f x, c, x, S 1, N pl,k R c, = κc wit γ M = 1, 1 γ M κc = 1 for λk 0, 1 κc = k+ k λ k for λk > 0, wit k = 05, 1+ 0, 49 ( λ 0) + k k, λ λ k = N p ( ) l,k 07, wit N = π f Nki,k 4 n pl,k y,k (6) (7) 1005

6 Screw imeter n Number of screws l S ; b; l ef,1 ; l ef, see in Fig. 7 f 1, f c,90, Design vlue of te witrwl cpcity prmeter Design vlue of te compressive strengt perpeniculr to te grin f y,k Crcteristic yiel strengt (for conventionl screws: f y,k = 400 N/mm ) k c,90 Coefficient k c,90 [1 ; 1,75] for te lo istribution, see []. N ki,k Crcteristic buckling lo for screw tking into ccount te elstic fountion perpeniculr to te screw xis, tringulr norml lo istribution long te screw xis s well s te support conition of te screw e. For inge e supports te crcteristic buckling los re summrise in Tb.. For clmpe e supports see [3]. For xilly loe self-tpping screws te minimum spcing, en n ege istnces in Fig. 7 re efine s: 5, ,c,c Tb. : Crcteristic buckling los Screw lengt l S in [mm] N ki,k in [kn] Screw imeter in [mm] Screw imeter in [mm] Screw imeter in [mm] Screw imeter in [mm] ,34 5,55 5,58 5,59 5,59 6,47 6,78 6,83 6,84 6,85 6,94 7,31 7,37 7,38 7,39 7,57 8,0 8,09 8,10 8, ,85 15,0 16,3 16,6 16,7 9,84 17,8 19,8 0,3 0,4 10, 19,0 1,3 1,8,0 10,8 0,4 3,3 3,9 4,1 60 8,77 18,4 6,8 9,4 30,1 9,76 0,6 31,5 35,6 36,8 10, 1,5 33,4 38, 39,6 10,7,6 35,8 41,6 43,3 80 8,71 18,5 30,6 40,3 43,9 9,69 0,7 34,4 47,3 53,0 10,1 1,6 36,0 50,1 56,7 10,6,7 37,9 53,6 61, ,6 31,0 44,9 55, 0,8 34,8 50,9 64,9 1,6 36,3 53, 68,7,7 38,3 56, 73, ,4 45,8 60,8 35, 51,7 69,4 36,8 54,0 7,7 38,7 57,0 77, ,7 46,6 6,6 35,4 5,5 70,8 36,9 54,8 74,1 38,8 57,9 78, ,3 63,7 53,1 7,1 55,4 75,4 58,4 79, ,67 18,6 47,6 64,8 9,64 0,8 53,4 73, 10,0 1,6 55,7 76,5 10,5,7 58,7 80, ,8 65,7 35,5 73,9 37,0 77, 38,9 81,5 0 47,9 66, 53,6 74,5 55,9 77,7 58,9 81,9 >40 66,7 74,9 78,1 8,3 Note: Interim vlues by liner interpoltion 4. Conclusions Te efficiency of self-tpping screws use s tensile n compressive reinforcements perpeniculr to te grin is sown. Self-tpping screws re crcterise by simple ssembling witout prerilling te timber. Compre wit conventionl fsteners, self-tpping screws wit imeters up to 1 mm n lengts up to 600 mm sow ig tensile strengt n ig witrwl cpcity. Generlly, self-tpping screws cn be use in timber-to-timber connections to trnsfer tensile n compressive forces or to compenste te low tensile n te low compressive strengt vlues of timber perpeniculr to te grin. Self-tpping screws bot s fsteners n s reinforcements open new opportunities for numerous pplictions in timber structures. 5. References ρ k = 310 kg/m 3 ρ k = 380 kg/m 3 ρ k = 410 kg/m 3 ρ k = 450 kg/m 3 [1] Blss H.J., Bejtk I., Querzugverstärkungen in gefäreten Bereicen mit selbstborenen Holzscruben, Forscungsberict 003. Versucsnstlt für Stl, Holz un Steine, Universität Krlsrue. [] Blss H.J., Elbeck J., Kreuzinger H., Steck G., Entwurf, Berecnung un Bemessung von Holzbuwerken, Sclussentwurf Bemessungsnorm Holzbu BEKS - 00 in Tgungsbn 00 - Ingenieurolzbu - Krlsruer Tge. [3] Bejtk I., Querzug- un Querruckverstärkungen - Aktuelle Forscungsergebnisse, in Tgungsbn Ingenieurolzbu - Krlsruer Tge.