M³ng b¨ c»c

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Mng b cc

Sa i o Lch s

Bi vit ny c mt phin bn hon thin hn.

Mng b cc Trung tm H khai thc v PT cc Dch v TH-Vin thng VNPT, 57 HTK H Ni Added by Ketcau

Chung c cao tng s 5 - Nguyn Ch Thanh[1] Added by Ketcau

Mng b cc hay cn c gi l mng b trn nn cc. Mng b cc c rt nhiu u im so vi cc loi mng khc, nh khi chu lc ti trng ln, cng ln, khng gian t do thng thong thun li cho vic b tr tng hm, lin kt gia b v kt cu chu lc bn trn nh vch, ct c cng ln ph hp s lm vic ca cng trnh.

Mc lc[hin]

S dng mng b cc EditCc cng trnh cao tng ca nc ngoi thng s dng gii php mng b cc. Ch yu l mng b trn cc nhi v cc barrette. Chiu cao b mng thng > 2m. Ngy nay mng b cc tr nn thng dng v ph bin v c ng dng nhiu trong cc cng trnh dn dng v cng nghip Vit Nam. Cch b tr cc trong i thng theo nguyn tc trng tm nhm cc trng hoc gn vi trng tm ti trng cng trnh. Gii php ny c u im l ti trng xung cc c phn b hp l hn; tnh lm vic tng th ca nhm cc tt hn. p dng vo tnh hnh Vit nam, liu c th khng; khi m phng n i mng n kt hp vi ging mng vn c hu ht cc n v thit k a dng cho cc cng trnh qui m n 30 tng.[2] Khi i cng th cc cc bin s chu ti ln hn hoc nh hn cc gia ty theo trng hp ti trng. Khong cch cc nn gia tng > 3d, gim bt hiu ng nhm cc. Nu t 6d th coi nh khng c hiu ng nhm. Kh sc chu ti 1 cc n l coi l ng nht vi sc chu ti ca tng cc trong nhm cc. S d phi lm mng b cc v trng hp t yu rt dy, b tr cc theo i n hay bng cc khng . Cn phi b tr cc trn ton b din tch xy dng mi mang ti trng ca cng trnh. Hn na b cc s lm tng tnh cng tng th ca nn mng b p li s yu km ca nn t. Cn v vic chiu dy ca b mng, c bn vn theo nguyn tc chng ct v chc thng ca b b tng ct thp. Mt v d v khch sn Duyt Thin Tn (Trung Quc) mt bng 76x26m; cao 71.8m (20 tng) ; kt cu khung-vch. Cc b tng ct thp c sn 35x35x21m h ti su 26m tng st mn. sc chu ti cc n l 88 Tn. Cc c b tr khp nh. B mng dy 2.0m. ln tnh ton l 8.8cm, chnh ln ln nht 3cm[3]

Cu to mng b-cc EditMng b-cc cu to gm hai phn: b v cc cc. B hay i cc c nhim v lin kt v phn phi ti trng t chn kt cu cho cc cc, ng thi truyn mt phn ti trng xung t nn ti v tr tip xc gia y b v t nn. B c th lm dng bn phng hoc bn dm nhm tng cng chng un. Chiu dy ti thiu ca b c

xc nh theo iu kin b b chc thng do lc tp trung ti chn ct, chn vch, hay do phn lc cc. iu chnh ln khng u c th lm b vi chiu dy thay i. Cc cc lm nhim v truyn ti trng xung nn t di chn cc thng qua sc khng mi v vo nn t xung quanh cc thng qua sc khng bn. C th b tr cc trong i thnh nhm hay ring r, b tr theo ng li hay b tr bt k tu thuc vo mc ch ca ngi thit k, nhm iu chnh ln khng u, gim p lc ln nn y b hay gim ni lc trong b...

Cc quan im thit k EditTi nc ta, trong thc hnh thit k mng b cc, thng quan nim ton b ti trng cng trnh do cc cc tip nhn. ng gp ca i cc thng b b qua, k c khi y i tip xc vi t nn. y l quan im thit k rt thin v an ton, v thc t i c truyn mt phn ti trng xung t nn. Quan im trn c th p dng khi thit k nhng nhm cc nh, c kch thc y i khng ng k so vi chiu di cc. V khi y vng ng sut tng thm trong nn do p lc y i gy ra nh, t nh hng n s lm vic ca cc cc. Tuy nhin, nu b qua s lm vic ca b khi thit k mng b-cc (c kch thc y i ng k so vi chiu di cc) s dn n s m t khng ng s phn phi ti trng ln cc cc v ln ca mng. Thit k mng b-cc c k n s lm vic ca b, theo Poulos (2001) c ba quan im khc nhau:

Quan im thit k th nht: ti trng lm vic cc ch chu ti trng t 35 n 50% sc chu ti cc hn (h s an ton sc chu ti bng 2 n 3), quan h ti trng- ln ca cc vn l tuyn tnh. Gn nh ton b ti trng tc dng ln mng do cc tip nhn. B ch tip nhn phn ti trng rt nh, phn phi ln nn t y b. Quan im thit k th hai: Trong quan im thit k ny, b c thit k tip nhn mt phn ng k ti trng ln mng, phn cn li do cc cc chu. ti trng lm vic sc chu ti ca cc c huy ng t 70 n 100% (h s an ton sc chu ti bng 1 n 1,5), quan h ti trng- ln ca ca cc l quan h phi tuyn do cc c chuyn dch tng i so vi t nn. S lng cc c b tr nhm gim p lc tip xc thc gia b v t nn xung nh hn p lc tin c kt ca t. Cc c s dng vi mc ch lm gim ln trung bnh ca b. Quan im thit k th ba: B c thit k chu phn ln ti trng ln mng. Cc cc ch tip nhn mt phn nh ca tng ti trng, c b tr hp l vi mc ch chnh l gim ln lch (ch khng phi ln trung bnh nh quan im thit k th hai).

Mng b

Sa i o Lch s

Chi tit cu to mng b[1] Added by Ketcau Mng b (cn gi l mng ton din). Mng b l mt loi mng nng, c dng ch yu ni c nn t yu, sc khng nn yu d khng hay c nc hoc do yu cu cu to ca cng trnh nh: di ton b nh c tng hm, kho, b v sinh, bn cha nc, h bi, nh cao tng c kt cu chu lc nhy ln lch ln khng u. Mng b c cu to bng cc vt liu ch yu l b tng v b tng ct thp theo cc hnh thc sau: 1. Bn phng (mng trn nn ph) Thng thng chiu dy ca bn c chn e = (1/6)l vi khong cch gia cc ct l < 9m v ti trng khong 1.000 tn/ct. 2. Bn vm ngc

S dng khi c yu cu v chu un ln. i vi cc cng trnh khng ln, bn vm c th cu to bng gch xy, b tng vi e = (0,03 l + 0,30)m v vng ca vm f = 1/7 l ~ 1/10 l. 3. Kiu c sn Chiu dy ca bn c chn e = (1/8)l ~ (1/10)l vi khong cch gia ct l > 9m. Hnh thc cu to theo 2 cch: a) Sn nm di c tit din hnh thang (kh nng chng trt gia tng). b) Sn nm trn bn. 4. Kiu hp Loi mng b c kh nng phn b u ln nn t nhng lc tp trung tc ng ln n, c cng ln nht v trng lng nh. Tuy nhin, c nhc im l phi dng nhiu thp v thi cng phc tp. Gii php mng p dng cho nh nhiu tng, nh cao tng c kt cu khung chu lc nhy ln khng u (ln lch).Mng b cc

Mng b cc hay cn c gi l mng b trn nn cc. Mng b cc c rt nhiu u im so vi cc loi mng khc, nh khi chu lc ti trng ln, cng ln, khng gian t do thng thong thun li cho vic b tr tng hm, lin kt gia b v kt cu chu lc bn trn nh vch, ct c cng ln ph hp s lm vic ca cng trnh.

S dng mng b c Cc cng trnh cao tng ca nc ngoi thng s dng gii php mng b cc. Ch yu l mng b trn cc nhi v cc barrett e. Chiu cao b mng thng > 2m. Ngy nay mn b - cc tr nn thng dng v ph bin v c ng dng nhiu trong cc cng trn dn dng v cng nghip Vit Nam Cch b tr cc trong i thng the o nguyn tc trng tm nhm cc trng hoc gn v trng tm ti trng cng trnh. Gii php ny c u im l ti trng xung cc c ph b hp l hn; tnh lm vic tng th ca nhm cc tt hn. p dng vo tnh hnh Vit nam liu c th khng; khi m phng n i mng n kt hp vi ging mng vn c h ht cc n v thit k a dng cho cc cng trnh qui m n 30 tng Khi i cng th cc cc bin s chu ti ln hn hoc nh hn cc gia ty theo trn hp ti trng. Khong cch cc nn gia tng > 3d, gim bt hiu ng nhm cc. Nu t 6 th coi nh khng c hiu ng nhm. Kh sc chu ti 1 cc n l coi l ng nht v sc chu ti ca tng cc trong nhm cc S d phi lm mng b cc v trng hp t yu rt dy, b tr cc theo i n hay bn cc khng . Cn phi b tr cc trn ton b din tch xy dng mi mang ti trn ca cng trnh. Hn na b cc s lm tng tnh cng tng th ca nn mng b p li s yu km ca nn t Cn v vic chiu dy ca b mng, c bn vn theo nguyn tc chng ct v chc thn ca b b tng ct thp. Mt v d v khch sn Duyt Thin Tn (Trung Quc) mt bn 76x26m; cao 71.8m (20 tng) ; kt cu khung-vch. Cc b tng ct thp c s 35x35x21m h ti su 26m tng st mn. sc chu ti cc n l 88 Tn. Cc c b tr khp nh. B mng dy 2.0m. ln tnh ton l 8.8cm, chnh ln ln nht 3cm Cu to mng b-c Mng b-cc cu to gm hai phn: b v cc cc B hay i cc c nhim v lin kt v phn phi ti trng t chn kt cu cho cc cc ng thi truyn mt phn ti trng xung t nn ti v tr tip xc gia y b v t nn B c th lm dng bn phng hoc bn dm nhm tng cng chng un. Chiu dy t thiu ca b c xc nh theo iu kin b b chc thng do lc tp trung ti chn ct chn vch, hay do phn lc cc. iu chnh ln khng u c th lm b vi chiu d thay i Cc cc lm nhim v truyn ti trng xung nn t di chn cc thng qua sc khn mi v vo nn t xung quanh cc thng qua sc khng bn. C th b tr cc trong thnh nhm hay ring r, b tr theo ng li hay b tr bt k tu thuc vo mc ch c ngi thit k, nhm iu chnh ln khng u, gim p lc ln nn y b hay gim n lc trong b.. Cc qua n im thit k Ti nc ta, trong thc hnh thit k mng b cc, thng quan nim ton b ti trn cng trnh do cc cc tip nhn. ng gp ca i cc thng b b qua, k c khi y tip xc vi t nn. y l quan im thit k rt thin v an ton, v thc t i c truy mt phn ti trng xung t nn.

Quan im trn c th p dng khi thit k nhng nhm cc nh, c kch thc y i khng ng k so vi chiu di cc. V khi y vng ng sut tng thm trong nn do p lc y i gy ra nh, t nh hng n s lm vic ca cc cc. Tuy nhin, nu b qua s lm vic ca b khi thit k mng b-cc (c kch thc y i ng k so vi chiu di cc) s dn n s m t khng ng s phn phi ti trng ln cc cc v ln ca mng. Thit k mng b-cc c k n s lm vic ca b, theo Poulos (2001) c ba quan im khc nhau: * Quan im thit k th nht: ti trng lm vic cc ch chu ti trng t 35 n 50% sc chu ti cc hn (h s an ton sc chu ti bng 2 n 3), quan h ti trng- ln ca cc vn l tuyn tnh. Gn nh ton b ti trng tc dng ln mng do cc tip nhn. B ch tip nhn phn ti trng rt nh, phn phi ln nn t y b. * Quan im thit k th hai: Trong quan im thit k ny, b c thit k tip nhn mt phn ng k ti trng ln mng, phn cn li do cc cc chu. ti trng lm vic sc chu ti ca cc c huy ng t 70 n 100% (h s an ton sc chu ti bng 1 n 1,5), quan h ti trng- ln ca ca cc l quan h phi tuyn do cc c chuyn dch tng i so vi t nn. S lng cc c b tr nhm gim p lc tip xc thc gia b v t nn xung nh hn p lc tin c kt ca t. Cc c s dng vi mc ch lm gim ln trung bnh ca b. * Quan im thit k th ba: B c thit k chu phn ln ti trng ln mng. Cc cc ch tip nhn mt phn nh ca tng ti trng, c b tr hp l vi mc ch chnh l gim ln lch (ch khng phi ln trung bnh nh quan im thit k th hai).