Bài Giảng Kết Cấu Gỗ - Huỳnh Minh Sơn

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Transcript of Bài Giảng Kết Cấu Gỗ - Huỳnh Minh Sơn

1. Bai giang Kt cu g Huynh Minh Sn -------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------------------------- Trang 1 CHNG M U: AI CNG V KT CU G 1. C IM VA PHAM VI S DUNG Cac loai cng trnh xy dng hay b phn cua cng trnh chu c tai trong lam bng vt liu g hay chu yu bng vt liu g goi la kt cu g. 1.1 u, nhc im cua kt cu g 1. u im: - Nhe, khoe. Tnh cht c hoc tng i cao so vi khi lng ring. C= R ; CCT3=3,7. 10-4 (m-1 ) < C G nhom VI =4,3.10-4 (m-1 ) < CBT200= 2,4.10-3 (m-1 ) - Chu nen va un tt. - Vt liu ph bin, a phng ha gia thanh vn chuyn. - D ch tao: Ca, xe, khoan, bao, ong inh... - Chng xm thc cua mi trng hoa hoc tt hn so vi thep va b tng. a s axit nng thp nhit thng khng lam g hong Nha may ep hoa qua, nha may ng, trai chn nui... 2. Nhc im: - Vt liu khng bn, d muc, mi, mot, chay khng s dung c trong cac kt cu vnh cu. - Vt liu g khng ng nht, khng ng hng. Cung mt loai g nhng cng R co th khac nhau tuy theo ni moc, tuy v tr trn thn cy (gc, ngon), tuy theo phng tai trong (doc thn, tip tuyn, xuyn tm) khi tnh toan ly h s an toan cao. - Co nhiu khuyt tt (mt, khe nt, th veo) giam kha nng chu lc. - Kch thc g t nhin han ch (G xe: 30 ld: sm hay mun g cung b pha hoai. Do o xac nh Rtc ta da vao ld: ld = ( 0,5 0,6) b = K0 .b (1.1) K0 la h s lu dai Rtc = ld = K0. b (1.2) 3.2. S lam vic cua g khi chu keo - Keo doc th mu th tiu chun, ve biu quan h ng sut va bin dang, nhn thy quan h gia - gn nh thng. Pha hoai xay ra t ngt khi bin dang tng i kha nho 0,8% , qui c ly tl = 0,5 b Khi chu keo g lam vic nh vt liu don khng th phn b lai ng sut nn chu anh hng nhiu bi cac yu t khuyt tt va khng ng nht. - Cng chu keo khi th nghim cua g kha cao, thng b k = 800 1000 kG/cm2 (g si gai b k =1700kG/cm2 ). Nhng thc t cng g chu keo giam i nhiu do chu anh hng cac yu t khuyt tt cua g (mt g, th cheo, k n h s K2< 1) hay do kch thc tuyt i cua thanh g ln a n mc khng ng nht cang cao ( k n h s K1 < 1) H s ng nht: K = K1 .K2 Rk = K .R tc (1.3) - Cng chu keo ngang th cua g rt nho: ( ) KK RR = 15 1 20 10 90 (1.4) RK: Cng keo doc th http://tailieuxd.com 8. Bai giang Kt cu g Huynh Minh Sn -------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------------------------- Trang 8 G khng phai la vt liu chu keo tt chu keo phai chon thanh g tt, t khuyt tt. 3.3 S lam vic cua g khi chu nen: - Nen mu th tiu chun, ve biu nen - , nhn thy: Biu la mt ng cong thoai. Phn u thng: tl = 0,5 b. Phn sau cong ro rt, bin dang tng nhanh so vi ng sut. G pha hoai bin dang, tng i khoang 0,6 0,7 % khi chu nen g lam vic nh vt liu deo, ng sut c phn b u trc khi pha hoai nn t chu anh hng bi cac yu t khuyt tt khi chu nen g am bao c cng . Mc du k b = 0,4 b k ( K b = 300 450 kG/cm2 ) nhng Rn > RK Nen la hnh thc chu lc phu hp nht vi g. 3.4 S lam vic cua g khi chu un: - Tit din g gm 2 vung keo va nen. Cng chu un cua g vao khoang trung gian gia keo va nen t 700 900 kG/cm2 . Anh hng cua khuyt tt cung trung bnh. - Khi chu un, g lam vic qua cac giai oan: http://tailieuxd.com 9. Bai giang Kt cu g Huynh Minh Sn -------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------------------------- Trang 9 Khi M con nho: ng sut phap phn b doc chiu cao tit din theo qui lut ng thng, biu co dang tam giac: W M u = (1.5) - Khi M tng ln: ng sut nen tng chm theo ng cong, ng sut keo tng nhanh theo ng thng, truc trung hoa lui xung pha di. Khi vung nen xut hin bin dang deo Mu th bt u b pha hoai khi n = n b ( cac th nen b gay). Mu th b pha hoai hoan toan khi k = k b Trong giai oan sau khng th xac nh u theo cng thc bn ma y ch dung cng thc nay theo tnh cht qui c co k anh hng cua hnh dang tit din: Wm M u u = (1.6) mu: H s phu thuc hnh dang tit din (theo thc nghim) - Mduyn an hi E: c xac nh bi h s goc trong phn ng thng ban u cua biu ng sut - bin dang tc la trong pham vi gii han t l. V phn ng thng ban u cua nen, keo, ung cung xp x nh nhau: G thng Lin X va g nhom V, VI, VII cua ta ly E = 105 kG/cm2 3.5 S lam vic cua g khi chu ep mt: 1.Khai nim: Ep mt la s truyn lc t cu kin nay n cu kin khac qua mt tip xuc. ng sut ep mt tai mt tip xuc: em em F N = Rem (1.7) N: Lc ep mt Fem: Din tch chu ep mt ( din tch tip xuc) - Tuy theo phng tac dung cua lc i vi th g ta co: + Ep mt doc th. + Ep mt ngang th. + Ep mt xin th. http://tailieuxd.com 10. Bai giang Kt cu g Huynh Minh Sn -------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------------------------- Trang 10 2. Cng ep mt cua g: a. Ep mt doc th: Cng tng ng nh nen doc th Rem (00 ) = Rem = Rn (1.8) b. Ep mt ngang th: - Ban u cac th ep vao nhau trn biu la ng OA. Sau o cac thanh t bao cua g sm b pha hoai, bin dang tng nhanh (AB). Cui cung cac thanh t bao b pha hoai lai ep vao nhau va g lai co th chu c lc (s cng lai). G pha hoai la do bin dang ln qua gii han cho phep. Cng gii han chu ep mt ngang th c cn c vao ng sut ty l tl (im A khi g bin dang nhiu) 0)90( .0 KR tlem = (1.9) K0 : H s lu dai - Ep mt ngang th lai c phn thanh: + Ep mt toan b (H.a): Cng nho nht, thc cht la nen ngang th + Ep mt 1 phn chiu dai (H.b): Cng tng theo ty l l/lem khi l/lem 3 + Ep mt 1 phn din tch (H.c): Cng ln nht do co s tham gia cac phn g xung quanh. Din tch tip xuc cang nho, cng ep mt cang cao. http://tailieuxd.com 11. Bai giang Kt cu g Huynh Minh Sn -------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------------------------- Trang 11 c. Ep mt xin th: - Cng phu thuc goc gia phng cua lc va th g: ( ) ( ) 3 90 sin11 0 + = em em em em R R R R (1.10) Rem: Cng ep mt doc th Rem(900 ): Cng ep mt ngang th 3.6 S lam vic cua g chu trt: 1. Khai nim: Khi lc trt nm trong mt phng tip xuc 2 cu kin se gy trt. ng sut trt tai mt trt: tr tb F T = Rtb tr (1.11) T: Lc trt Ftr: Din tch mt trt - Tuy theo v tr lc ct i vi th g, co 04 trng hp chu trt cua g nh sau: + Ct t th. + Trt doc th + Trt ngang th + Trt xin th. 12. Bai giang Kt cu g Huynh Minh Sn -------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------------------------- Trang 12 2. Cng chu trt: Rtb tr = e l R tr tr +1 (1.12) ltr : Chiu dai mt trt e: Canh tay on cua cp lc trt - Rtb tr tuy thuc vao s phn b ng sut nn chu anh hng bi cac yu t: , + Loai trt: 1 pha hay 2 pha. Trt 2 pha th cng trung bnh cao hn v ng sut u hn. = = phia phia 2:125,0 1:25,0 + Ty s ltr/e, cao nht khi ltr/e = 3 4 (nu qua ln th ng sut phn b khng u. Nu qua nho anh hng ni lc boc ngang th g). + Co lc ep vung goc hay khng: Lc ep lam tng kha nng chu trt (v giam ni lc boc). a. Ct t th: Kha nng chng ct t th cua g rt ln nn hu nh khng th xay ra v nu co g se b pha hoai bi ep mt hay un trc. b. Trt doc th: Vi mu th tiu chun, ng sut trt doc th vao khoang 70 100 kG/cm2 , ln nht so vi cac trng hp khac. c. Trt ngang th: Vi mu th tiu chun, ng sut trt ngang th vao khoang 3550 kG/cm2 , 13. Bai giang Kt cu g Huynh Minh Sn -------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------------------------- Trang 13 d. Trt xin th: (t xay ra) ( ) 3 90 sin11 + = tr tr tr tr R R R R (1.13) V Rtr90 = 0,5 Rtr nn: R tr = 3 sin1+ trR (1.14) 3.7. Cac nhn t anh hng n tnh cht c hoc cua g: 1. m: %100 2 21 G GG W = (1.15) G1: Trong lng g m G2: Trong lng g sau khi sy kh - m cua g thay i theo mi trng xung quanh va anh hng ro rt n cng cua g. Khi m tng t W = 0 n Wmax = 30% ( m bao hoa th ) th tnh cht c ly cua g giam. m thay i 1% th ch tiu c ly thay i 3 5%. Khi W > Wmax th khng con anh hng n cng cua g. - nc ta m thng bng cua g la 17 20% nn m tiu chun c qui nh la 18% va cng cua g c quy i v cng tng ng: ( )[ ]18118 += Ww (1.16) 18: Cng m tiu chun 18% : H s iu chnh m tuy thuc loai g = 0,05: nen doc th; = 0,04: un; = 0,035: nen ngang th; = 0,03: trt; = 0,015: keo doc th - Khi tnh toan cac kt cu lam vic trong iu kin chu nhiu ma gio th ta ly W% ln hn theo qui pham ng thi E phai nhn vi h s iu kin lam vic: + Vi kt cu c che ma nhng v tr khng thoang th ly W = 20% + Vi kt cu khng c che ma nng th ly W = 25% 2. Nhit : - Khi nhit tng du W nao th cng cua g giam, mun an hi giam nhiu va vong tng. Mt khac, nhit tng, g gian n gy ni ng sut cuc b ln, nu co mt co th ct t th. Kt cu chu nhit > 500 C khng dung vt liu g. - Khi tnh toan quy i v cng nhit tiu chun c chon la 200 C. ( )2020 += tT [MN/m2 ] (1.17) : h s iu chnh nhit tuy thuc vao trang thai ng sut = 3,5: nen doc; = 4,5: un; = 4,0: keo doc; = 0,4: trt doc 14. Bai giang Kt cu g Huynh Minh Sn -------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------------------------- Trang 14 3. Khi lng ring: - Noi chung khi cang ln th cng cang cao va cang kho mi, mot, muc, chay. 4. Cac bnh tt: - Mt cy: Ch gc cua canh m t thn ra, cac th b ln veo, mt tnh cht lin tuc (cac th khng trung phng cua lc tac dung) gy nn hin tng tp trung ng sut lam giam kha nng chu lc, gy kho khn trong ch tao. c bit anh hng n cu kin chu keo hay cu kin chu un co mt nm trong vung keo. G loai A: Khng cho phep co mt G loai B: Khng cho phep qua 1 mt trn 1m dai - Th nghing: Khng nm trung phng truc doc th g, lam giam cng cua g rt nhiu, nht la i vi cac ng sut tac dung doc thanh. - Khe nt (thng d