trng i hc xy dng b mn c hc t - nn mng.

*********

tm tt nn mng

h ni 2005

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chng i

mt s vn c bn $.I .Khi nim chung: 1.1. nh ngha:

+ Mng: B phn ko di xung ca cng trnh (CT) v nm ngm di t. Nhim v nhn ti t CT v truyn xung t nn( N) Kh nng ca vt liu CT >> kh nng ca N: (thng l 100-150 ln) nn mng c kch thc m rng thm. Cu to: G mng tc dng:

phng sai lch khi thi cng thi cng phn trn tit kim vt liu cu to bc mng. + Nn: Vng t di y mng chu tc dng trc tip ca ti trng CT. + Nu CT t trn cc lp t t nhin gi l nn t nhin. + Nu khi xy dng ngi ta dng bin php no nhm lm tt hn tnh nng xy dng ca nn nh: thay th t (lm m), tr vt liu ri, ging ct+nn trc, in thm, h mc nc ngm gi l nn nhn to.

1.2. ngha ca cng tc nn mng:

Thit k Nn mng kh phc tp: nhiu yu t: c im cng trnh. Nn mng cng trnh bn cnh. CCT. CTV.

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+ Nn t rt a dng nn c nhiu phng n v mng. + Ngi thit k phi la chn c phng n m bo:

/k kinh t, k thut. + Theo tng kt: gi thnh v nn mng chim khong 20-30% trng hp c bit n 50-60%. + Kinh nghim thc t cho thy phn nhiu cng trnh h hng l do vic gii quyt cha tt vn nn mng. T hin tng nt tng gim tui th CT. Ln nhiu, nghing cn tr s dng. Cng trnh sp . + Sa cha kh hn nhiu so vi xy mi.

CT phi ngng lm vic, thi gian thi cng ko di.

Tm li ni dung cn: + Nghin cu cc nguyn l v cc phng php tnh ton vi tng loi mng. + Nghin cu cch bo v mng trong mi trng phc tp. + Nghin cu cc bin php x l nn t yu, phc tp. : + m bo s lm vic bnh thng ca CT trong s dng + Bo m cng tng b phn v ton b CT. + Bo m thi gian xy dng CT ngn nht, gi thnh r nht.

tho mn c cc yu cu trn cn phi lm cht ch t cc khu: Kh/st; Thit k; Thi cng v s dng CT.

Kh/s: La chn phng php kh/st ph hp vi tng loi t, tng cng cc th nghim hin trng. Th/k: Trn c s tm hiu v CCT, CTV, c im CT xut phng n nn mng hp l v tnh ton chnh xc. Th/cng: Bin php tt nht hn ch cao nht s ph v kt cu ca t nn v nh hng n cc CT ln cn.

$.2. Phn loi mng v phm vi p dng: Phn loi da theo:

Phng php thi cng: + Mng nng: o ton b h mng trc khi xy mng.

C th xem: H/D < 6; H < 3m Mng nng + Mng su: khng cn o hoc ch o mt phn ri dng cch

no a mng xung su th/k. Vt liu: Gch, , b tng, b tng ct thp, thp... Cch ch to: Ton khi , lp ghp. c tnh chu ti: Tnh, ng. Hnh dng: n, bng, b , hp, v. Hnh thc kt cu: Dm, gii, bn, khi. cng kt cu: Cng, Mm.

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1.Mng Nng: ( vi ti nh) a, Mng n: di m tr cu nh, chn ct nh, chn thp nc... b, Mng bng, bng giao nhau: di tng nh, di hng ct nh, di tng chn... c, Mng b: dng ni t yu, hoc do cu to cng trnh nh: B, kho cha... 2.Mng su: ( ti ln - t tt su) a, Mng ging chm: Hnh dng v cch h tng t nh h ging nc. Dng cho mng m tr cu, cc phng ngm k thut... Hin nay, ch yu do vic thi cng ging chm kh khn m nhng dng cc khc hon ton c th thay th c cho ti rt ln nn mng ging chm t c s dng. ( Trong xy dng ng vng su v xa do thit b thi cng cc a n kh; hoc do yu cu v mi trng mng Ging chm vn c dng ) b, Mng cc: Gm cc cc ring r a xung t ni li bng bn i cc S dng rng ri.

$.3. Khi nim v tnh ton nn mng theo Trng thi gii hn 3.1 Khi nim v Trng thi gii hn: Trng thi gii hn: L ngng cui cng cng trnh hoc ngi s dng cng trnh c th chp nhn m khng c bt k s c no cho ton b CT hay mt vi b phn CT c v s ton vn ln vic s dng, khai thc bnh thng. Chia ra: 2 nhm: Nhm trng thi gii hn v cng v n nh ( TTGH I) Nhm trng thi gii hn v khai thc v s dng CT mt cch bnh thng (TTGH II). 3.2 Tnh ton nn mng theo TTGH: Trng thi /s v B/d trong t nn v trong bn thn mng lin quan cht ch n CT bn trn nn vic tnh ton hin nay thng hng ti p dng trong quan h tng th N-M-CT. Tuy vy, tin cho vic trnh by ni dung c bn ca vn , ta chp nhn xem xt ring cc trng hp tnh ton ri rc ho. Ni dung: 1. T/ton nn theo TTGH th nht (Theo cng v n nh)

N SF

F

Trong : N: ti trng hoc tc ng t CT ln t. F: kh nng chu ti ca dt nn theo phng tc dng ca N. Fs: h s an ton tng ng.

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Thng thng: * Ti ln nn gm:

Ti trng theo phng ng. Ti theo phng ngang gy trt phng theo y mng. M men gy trt su hoc m men gy lt. * Kh nng chu ti tng ng:

L sc chu ti gii hn ca t nn: Pgh. Kh nng chng trt phng: Tgh. M men chng trt, M men chng lt: Mgh. Cc gi tr c th ca N v F tng ng s c xc nh cho tng trng hp tnh ton thit k ring cng vi h s an ton thch hp. C th: V cng : Rtb ; R2.1max ; ngng R

Trong : tb ; max ng sut trung bnh v ng sut ln nht ti y mng.

R sc chu ti theo phng ng ca t nn. ng ng sut ln nht theo phng ngang ti mt bn mng.

Rng sc chu ti theo phng ngang ca nn.

V n nh trt:

[ ]tt

gitr KT

TK =

Trong : Kt - h s n nh trt. Tt - tng lc trt. Tgi - tng lc gi. [ Kt] - h s n nh trt cho php nh nc quy nh. V n nh lt:

[ ]gil ll

MK K

M=

Trong : Kl - h s n nh lt Mgi - m men gi. Ml - m men lt [Kl] - h s n nh lt cho php nh nc quy nh. 2. T/ton nn theo TTGH th hai: ( Theo Bin dng) Nguyn nhn ch yu dn n trng thi gii hn l bin dng v chuyn v vt ra ngoi phm vi chp nhn c. Vy vic tnh ton nn theo TTGH II l hn ch ln, ln lch v nghing (hay chuyn v ngang) ca mng m bo cho cng trnh khng b ph hng hoc m bo s lm vic bnh thng ca n.

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S Sgh S Sgh .... Trong : (S; S ln v ln lch ca mng) (Sgh ; Sgh ln gii hn; ln lch gii hn l gi tr ch u t nu ra hoc vi cc tiu chun xy dng tng ng). 3.3 Cc loi ti trng v t hp ti trng: Gm 2 loi: Ti trng thng xuyn. Ti trng tm thi. Ti trng thng xuyn: (Tnh ti) l ti trng khng bin i (v tr t ti, ln v phng chiu) trng lng bn thn, p lc t, t p... thuc ti tc dng di hn. H s vt ti ca trng lng bn thn bin i t 1,05 n 1,3 theo TCVN 2737 - 95. Ti trng tm thi: ( Hot ti) ti c th thay i v tr, ln v chiu Ti trng s dng: ngi , thit b, dng c, cu trc , gi... chia ra: Tc dng di hn: trng lng Tc dng ngn hn: gi. H s vt ti ca cc loi ti trng ny c quy nh trong TCVN 2737-95 " Ti trng v tc ng" Ni lc dng tnh ton S l tng i s ca cc ni lc ln nht do hot ti v tnh ti. Vic xp xp v tr ca hot ti tm gi tr ln nht gi l t hp ti trng. Theo TCVN 2737 -95 phi phn bit: + T hp ti trng c bn: gm thng xuyn + tm thi ngn hn v di hn. + T hp ti trng c bit: thng xuyn, di hn, ngn hn v mt trong cc ti trng c bit ( ng t hoc ti trng dng tnh kh nng chng chy ca kt cu....) Khi tnh ton ta dng h s t hp xt n kh nng tc dng khng ng thi ca cc loi ti trng ngn hn. Ti trng tiu chun c xc nh theo cc thng s trn bn v thit k

hoc theo l lch thit b. Ti trng tnh ton l ti trng xt n nhng sai khc do qu trnh thi

cng, ch to hoc khai thc khng ng nh thit k m nhng sai khc gy nguy him cho cng trnh.

Khi tnh nn theo TTGH 2 - Bin dng - do qu trnh bin dng ca t din ra rt lu nn nhng ti trng tc dng lu di mi c ngha trong tnh ton Khi tnh theo TTGH 1- Xc nh cng v n nh ca nn bi vy iu quan trng khng phi l thi gian tc dng ca ti trng m chnh l tr s ln c th lm cho nn mt n nh.

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$4. Cc phng php tnh ton a k thut vi h s an ton tng th v an ton ring phn: 4.1 Phng php dng h s an ton tng th: 4.1.1. Trng thi gii hn v cng v n nh: ULS (ultimate limit state) * Kim tra n nh ca nn: Trng thi gii hn v cng v n nh ca nn c kim tra theo iu kin:

s

u

FP

RP = (1)

trong : p - cng ti trng tiu chun tc dng di y mng R - cng ti trng cho php ca nn di y mng.

uP - sc chu ti cc hn ca nn di y mng.

sF - h s an ton tng th.

ccqqu cNshNsbNsP ....5,0 ++= Trong : cq NNN ;; cc h s sc chu ti ca nn ph thuc vo: tra bng.

y khng c h s no tc ng vo ti trng, cng nh khng c h s no tc ng vo gi tr ca c v . Trong iu kin (1) ch c h s duy nht Fs, gi l h s an ton tng th.

thng thng: Fs = 23, ty thuc vo thnh phn ti trng t hp: nu trong t hp ti trng tnh ton khng c gi th Fs =3; nu c gi hoc ng t th Fs =2. i vi cng trnh khng quan trng lm hoc cng trnh tm thi , c th cho Fs =1,5 1,8

h s an ton trong trng hp ny bao hm c cc yu t khng xc nh nh hng ti ti trng, tc ng v cc ch tiu c l ca t... phng php ny c dng ph bin trong cc tiu chun ca Ty u, Bc M. * Xc nh sc chu ti cho php ca cc:

[ ]s

u

FP

P = hoc [ ]s

xumu

FPP

P ,,+

=

Trong : [ ]P sc chu ti cho php ca cc. uP sc chu ti cc hn ca cc theo t nn

muP , - sc khng cc hn ca t di mi cc

xuP , - sc khng cc hn (ma st) ca t xung quanh thn cc

sF h s an ton tng th sc chu ti ca cc. Trong thc hnh tnh ton phi bo m sao cho:

[ ]PP trong P tr tiu chun ca ti trng tc dng ln cc Thng thng 32 =sF hoc ln hn, ty thuc vo thnh phn ti trng trong t hp, phng php xc nh sc chu ti cc hn ca cc.

uP sc chu ti cc hn ca cc theo t nn: c xc nh bi cc phng php khc nhau nh phng php th nghimgia ti tnh, th nghim va chm ng, cc phng php da vo kt qu th nghim t hin trng v trong phng. 4.1.2. Trng thi gii hn v tnh s dng: SLS (serviceability limit state) (Cng c xem xt tng t)

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4.2. Phng php dng h s an ton ring: 4.2.1. Trng thi gii hn v cng v n nh: ULS Phng php h s an ton tng th trn c u im l gn. Tuy nhin ngi ta khng th xc nh c mc an ton ca tng thnh phn ring nh ti trng v cc thnh phn sc chu ti ca nn. Vic tnh ton theo phng php ny c cc nh chuyn mn Nga(Lin x c) dng t rt lu do n khng xa l vi cc k s Vit nam, vn mt thi gian di dng cc Tiu chun Nga. Cc nc Ty u cng ang chun b dng tiu chun thng nht EUROCODE 7, trong khng dng h s an ton tng th m dng cc h s ring. * Kim tra n nh ca nn: Vic kim tra n nh theo phng php ny l a vo tnh ton nn mng cc h s ring. Theo bin thin ca ti trng v cc c trng ca t c k bi cc h s. Hay ni cch khc, trong tnh ton thit k ngi ta khng dng cc tr tiu chun m dng cc tr tnh ton ca tc ng, cc tr tnh ton ca cc ch tiu c l t nn. 4.1.2. Trng thi gii hn v tnh s dng: SLS (Cng c xem xt tng t)

Tm li: Theo Snhip II-15-74 ( Lin x): Cc ch tiu c l ca t nn c chia ra: Khi tnh nn theo trng thi gii hn th nht ta dng: ttI

ttIc ; .....

Khi tnh nn theo trng thi gii hn th hai ta dng: ttIIttIIc ; .....

l cc ch tiu tnh ton c s l da trn nhiu kt qu th nghim mu. Theo EUROCODE 7: Cng nu rt nhiu cc tip cn tnh ton tm ra cc tr tnh ton ca tc ng, cc tr s tnh ton ca cc ch tiu c l t nn. Tuy nhin: Do cch s dng phng php h s an ton ring phn i hi s lng mu phi nhiu, vic x l cng rt phc tp, i khi kt qu rt phn tn ( tham kho Tnh ton Mng nng V Cng Ng) do trong thit kt thc t ta hin nay vn a s dng phng php h s an ton tng th. Vic dng cc h s ring trong tnh ton thit k Nn mng l cp thit i vi ngnh a k thut nc ta hi nhp vi cc nc tin tin trn th gii. Nhng cn c xem xt y trc khi a vo thit k thc t trnh trong mt tiu chun li dng c hai loi h s ( h s an ton tng th v h s an ton ring) nh Tiu chun mng cc hin hnh.

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$5. Cc ti liu cn thit cho thit k nn mng:

1. Ti liu v a cht cng trnh v a cht thu vn:

*Cc bn v mt bng: + Bn a hnh, bn v san nn. + Mt bng v tr: th hin c cc cng trnh ln cn. + Cng trnh ngm: cp in, ng cp thot nc, cp thng tin...

*Cc ti liu v CCT; CTV:. Tng hp cc s liu CCT; CTV phn tch s l s liu nh gi c tnh cht ca t nn a vo trong tnh ton y l ti liu quan trng nht:

Theo TCVN 4419: 1987 kh/st cho xy dng. TCXD 160: 1987 kh/st a k thut phc v cho thit k

v thi cng mng cc. + Khong cch cc h thm d 10m 20m 100m tu thuc mc

phc tp ca a cht. + Chiu su khoan thm d cn thit:

5m 30m 60m hoc hn. : ti cng trnh, loi cng trnh, mc phc tp ca iu kin a cht.

Thng thng: Mng bng hks > 3 b b: b rng mng bng. Mng b: hks > 3 B B: b rng nh Mng cc: hks > 5m di mi cc. + Nghin cu t/cht t trong phng: Ni dung:

+ Xc nh cc ch tiu c bn nh sau: Thnh phn ht, dung trng, m, gii hn nho, gii hn do, h s rng, E,k,c, ... Hm lng hu c trong t, tan r, trng n, gc mi thin nhin ca t.

+ Th nghim hin trng bao gm: Th nghim bn nn, cc phng php xuyn: Xuyn tnh, Xuyn ng, Xuyn tiu chun, Ct cnh...

+ Quan st mc nc ngm ti hin trng: nh gi kh nng thm, mc xm thc ca nc n cng tc thi cng mng. y cn phi ch n cch chn cc ch tiu ca t: ch tiu cc b (ch tiu ring)

3loi: ch tiu chung (ch tiu tiu chun) ch tiu tnh ton.

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Ch cc b: l tr s ca mt c trng c hc hoc vt l no y ca t xc nh theo ring 1 mu th nghim (tc l cho 1 im no )

Ch tiu chung: l tr s ca mt c trng c hc, vt l no ca 1 lp t thng ly gi tr trung bnh ca mt s gi tr cc b.

Ch tiu tnh ton: l tr s 1 c trng c hc, vt l no ca lp t dng trong cc thit nn mng n c xt n s thay i v tnh cht ca t trong qu trnh thi cng v s dng CT

Theo Snhip II-15-74 C hai phng php hay dng xc nh ch tiu tnh ton. Phng php sai s bnh phng trung bnh Phng php tr s trung bnh cc tiu.

(Tuy nhin s mu phi nhiu, ti thiu phi c 6 mu cho 1 n nguyn a cht th mi dng c phng php thng k ton hc x l cc ch tiu Tham kho TK v TTMN)

Mt Bng nh V H KHOAN

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Mt ct a cht cng trnh:

Cc k hiu

Bn Cui , siCtHu cCt phaSt phaSt

2 Ti liu v CT: c im cng trnh: mt bng, dy chuyn cng ngh. Bn v kt cu + ti trng. Mng cc cng trnh ln cn. 3. Vt liu xy dng, my mc thit b thi cng:

Tn dng v/ liu v th/b thi cng ti ni xy dng cng trnh gim gi thnh.

$6. xut, so snh v chn phng n nn mng: Khi thit k nn mng cn phi ra mt vi phng n chn la, mt phng n ti u da vo: Ch tiu kinh t. Ch tiu k thut. Ch tiu thi cng. Thng c mu thun trong ni b cc yu cu trn, nhim v thit k l phi thng nht c cc mu thun la chn phng n tt nht.

Phng n thit k nn mng l: xut hm . Loi mng. Hnh dng. Kch thc. Vt liu.

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1. Chn chiu su chn mng: L khu quan trng, n ph thuc cc yu t: + iu kin CCT v CTV. + Tr s v c tnh ca ti trng, ti ln v chu lc phc tp chiu su chn mng cng phi ln, ti trng ng cng thng dng mng su. + Cc c im cu to ca CT: nh c tng hm nn y mng phi t kh su, trong khi thc t tnh ton th khng cn. + iu kin v kh nng thi cng. Sau khi phn tch c im ca CT c thit k, xc nh ti trng tc dng xung mng, nh gi iu kin CCT; CTV ta tin hnh chn loi nn v mng. + S 1: Trng hp nn gm ton t tt: Ta c th dng mng n, bng, nu ti ln c th dng bng giao nhau, b di nh khung hoc tng chu lc. Trong trng hp ny chiu su chn mng ch yu do tnh ton quyt nh, hm khng < 0.5m m bo lp t t lc c kt cu nguyn dng khng b ph hoi do nguyn nhn bn ngoi v mt mng phi cch mt t t nhin 20-25cm trnh cho mng khi tc dng va chm bt thng. + S 2: Trng hp nn gm ton t yu: Sc chu ti nh, kh nng bin dng ln, khng tip thu ti CT. Trong trng hp ny chiu su chn mng ph thuc vo bin php x l nn nh: m ct, cc tre, cc ct, cc BTCT hoc gia c bng phng php ho l. + S 3: Trn l lp t yu c chiu dy h1 v bn di l lp t tt. Xy ra 3 trng hp: Trng hp 1: h1 nh: o b t mng ln lp tt, t mng vo lp tt 10-50cm. quay v s a.

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Trng hp 2: h2 kh dy nu t trc tip ln lp t tt di khng kinh t. c th dng mng bng, mng bng c ging, mng b cc su khc nhau th cng gim c ln v chnh ln. quay v s b,c Trng hp 3: Nu lp t yu bn trn c chiu dy rt ln, khi khng t c mng ln lp t yu Dng mng cc, m ct hoc cc phng php nhn to khc. quay v d,e,g (hnh di) + S 4: Lp t yu c chiu dy h2 trn v di l lp t tt. Vic x l mng ph thuc chiu dy h1,h2: Nu h1 kh ln th nn tn dng lp nhng phi m bo ln cng nh sc chu ti trn mt lp t yu. a h1 qu nh khng t c ln lp 1 dng bin php x l nn b,c,d,e CTV: Nu mng t trong vng mc nc ngm ln xung theo ma th mng c th b bin dng ln v m thay i, nu di mng l ct nh th s ln xung mc nc c th lm tri ct i do c gng trong mi trng hp nn tn dng t mng trn mc nc ngm. Xt n CT bn cnh: trnh nh hng tc dng ca hai mng cnh nhau c chiu su chn mng khc nhau, khi thit k yu cu phi bo m /kin sau:

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2. So snh v chn phng n mng:

Cn phi chn c ph/n tt nht c v kinh t ln k thut. + Vi nhim v thit k cho ta c th ra nhiu phng n, cc

phng n c th khc nhau v c bn nh: Mng nng trn nn thin nhin.

Mng nng trn nn gia c. Mng cc.

+ Trong mi ph/n ln li c nhiu ph/n nh: V d: Mng nng c th: mng n; mng bng; mng b. Mng cc c th: cc di, cc ngn, cc ng, cc p hay cc nhi... * Tuy nhin do kinh nghim thit k ta c th gt b ngay nhng ph/n bt hp l ch li vi ph/n tnh ton so snh. * Khi tnh ton s b ngi ta thng da vo ch tiu kinh t quyt nh. * Khi quyt nh chnh thc ngoi ch tiu kinh t cn da vo iu kin k thut, iu kin thi cng v yu cu v thi gian thi cng. S : Trnh t thit k nn mng ni chung.

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