TiNH TOAN ICHA NANG CHjU CAT CUA B l TONG TRONG DAM BE TONG UfNG LlTC TRU6C THEO MOT SO
TIEU CHUAN THIET KE
TS. Nguyen Nggc Phddng*
K it clu bg tdng dng lUc truorc (l/LT) vdri nhi^u uu di^m dang dddc dlig dung kha rong rai d Viet Nam trong ITnh viicxaydiTng.Ly thuyet tinh toin ket cau l/LT da tUOng ddi hoin chinh v i duoc uTng dung nhieu trong thUc ti.
Rieng vdri khi nang chiu cit cua dam be tong l/LT md^ dang dr muTc do nghien cdu ly thuyet. Tidu chuin thiet ke ket clu bi tong cot th^p (BTCT) cda Viet Nam TCVN 5574:2012 ve khi nang chju cit cda dam be tong l/LT tuy dip i]rng diTorc cic yeu clu ve thiet ke nhUng c6n cd nhihig van de khic biet so vdri mot so tieu chuan nUdrc ngoii va c6n chUa phin inh mot cich ro ring, dly dd cac yiu to inh hiTdmg de co the danh gii dung muTc va hieu qui. Vifc dng dung trong thiet ke cdn co nhiirng han che nhat dinh. Bii bao nhim lim ro hOn vln d^ niy.
1. Ket clu bi tong iimg life trUdc
BTCT Id su k i t hop don thuin glQa be tdng va cdt thep 3e chung cCing Idm viec mdt d e h bj ddng, edn be tdng ULT Id su ket hpp mdt cdch tich cue, cd chu y giQa be tdng va cdt t h i p . liLT chfnh Id vile tao ra cho kit clu mpt d e h ed chil y cdc ULT nhim t i n g cudng sQ lam viec cua vat lieu trong cdc dieu kien sQ dgng khdc nhau. Ndi d c h khdc, trUde khi cau kiln chju tdi trong sQ dgng, cdt thep dd bj cdng trQdc, cdn be tdng bj nen trQdc. l?ng suit nen trQde se t r i i t tieu hay lam gidm thig suit keo do t l i trpng sCt dung gay ra, do ; v l y idm t i n g khd ndng chju lUc cua cdu kiln khi bi nQt vd lam han c h l su phdt trien cOa khe nOt (hlnh 1).
2. Anh hirdmg cda l/LT v i cic yeu td khic den kha nang chiu cit cda dam BTCT
Khdo sat mpt dam BTCT chju udn (hlnh 1). Xlt 1 phdn td vat lieu trong giai doan ddn hoi d vung g i n goi tUa. O l don gidn, xet bdi todn Qng suit phdng, bd qua thdnh phan Qng suit theo phuong vudng gde vdi true dam.
Qng suat keo chinh xae djnh theo cdng thQc:
fr- 2
\' \l/ \
- 4 0 ^ -
f \'
!^\ 1^
Hlnh I Trang thdi dng sudt trong ddm cdlUc doc tdc dung doc true
vdi V - Qng suit t i l p trong be t d n g d o l U c d t V g l y r a ; fh -dng suit phdp theo phuong dpc d i m (quy Udc lay dau (+) khi nen).
Khi Qng suit keo chinh vUpt qua cudng dp ehju keo cua be tdng fer thi tren d i m se xult h i l n vet nut va b i t dau bi p h i hoai theo t i l t dien nghieng.
Cfng suit t i l p trong b l tdng tai thdi d i l m nay dat d i n gid tri gidi hanv^ ta cd:
A /,= W] 2
se35 2014 x A v o y N G & €X5 T H 1 5 7
VANDE
Khi khdng co Crng suat phap doc truc(./;, = 0 ) : / , = i; I
Khi ed Ung sudt phap dpc true:
Neu Qng suat f^ la nen
Ro rang, Lfng suat nen doc true da lam tang Qng suat eat gidi han trong eau kien, kha nang ehiu cat cua dam dupe cdl thien, ngUoc lai, se bi giam yeu khi co su tac dung cCia Cfng suat keo doe true. Co the thay: Luc nen doe true CO khuynh hudng lam tang tai trong gay nut xien, lam cho eae vet nut do udn khong phat trien sau vao trong dam; do do, dam se chiu duoe lUc c3t tdn hon tai thai diem iJog suat keo chinh bdng cudng do ehiu keo cua be tdng. Tuy nhien, vdi dam be tong L/LT, khi da hmh thanh vet nOt do uon thi eac vet nOt phat trien tuang tu nhu dam BTCT thuang Si/ tang tai trong gdy nQt xlln ddi vdi dim b l tdng ULT se ldn hOn do sQcham hlnh thdnh v l t nQt do udn.
Khd ndng chju d t cQa dam BTCT phu thudc nhieu yeu td nhu cQdng dd eCia b l tdng, kfch thudc t i l t diln, iQc dpe, hdm luong cdtthep doc, cot t h i p ngang, tdi trpng tdc dung... Trong ndi dung cua bdi bdo nay, d l cap den dnh hudng eua ti sd a/h^ vdi a la khodng edeh tQ dilm ddt iQc tdp trung d i n gdi tua cCia ddm (nhjp chju cdt) vd h^
(d) Id chilu eao Idm viec hQu fch cua t i l t diln. Kani (1967) da nghien cQu dnh hudng ndy va thay ring cd dilm chuyen tilpdejAg = 2,5; tQdd xdc dinh dupc luc d t tdi han.
3. Khi ning chiu cit cda be tdng vung nen theo mdt so tieu chuin thietke
Cac t i l u chuan t h i l t ke d l u xdc djnh: Khd ndng chju d t eua d i m be tdng ULT Id tdng khd nang ehiu d t tfnh todn eCia be tdng vd eCia cdt t h i p ngang.
3.1. T i l u chuan cua Viet Nam TCVN 5574:2012
Co sd d l xdc djnh khd ndng chju d t cCia be tdng trong t i l u chuan sir
dung cong thUe thQc nghiem:
O, = k,Ri,bho/c vdl A^ = 0,15 xae dinh theo thUc nghiem. Cdng thQc nay duoc dieu chinh bdng each thay fl^ bang Rj,, va /;_ bang 'Ph2 va kha nang eh|u cat eua be tdng vung nen duoe xac dinh:
Qu = iph/i + (pt + (piJRbihhJ/c vdi ^62-he soke den dnh hudng cua loai be tdng (vdi be tdng thudng
<Pb2 -2); e - hinh ehilu cua t i l t dien nghieng len true dam; <Pt =0);
<p„ - he so ke den anh hudng eua lUc doc, trudng hop lUc doc la li/c nen
Khi khong cd cdt dai chju lUc cat, kha nang chiu cat eiia be tong dLfpe xae dinh'
Qi, = (PhJi -^ <p,jRiM:/c
Qh lay khong lan hon va khong nho han Qb,ntm ='Phi(^ + <Pn}l^h!^fh) vdi (pi,T. - he so phu thuoe loai be tdng [vdi be tong thudng 'PHT^ = 0,6);Vb4 - he sd phu thudc loai b l tdng (vdl b l t d n g t h u d n g ^ 6 4 = 1,5).
3.2. Tieu chuan cua C h l u Au EC212004
Oilu kiln khi b l tdng du khd nang chju d t vd chi can ddt cdt dai, cdt xien theo cdu tao: Vsd S VRJC vdi Vsd - luc d t tinh todn;
Vkd.c' kbd ndng chju d t eua be tdng.
Gid trj F/ucdUdc xdc djnh theo cdng thQc thuc nghiem:
*'lU.t = 0,184(100/^/^)3 + *|cr^ L ^ 2 yxd.cmm
J 1 vdi ' ' W , . ™ B = ( 0 , O 3 5 A V J + V < P M Trong cdc cdrig thQc t r l n : k -he sd: *=i+J^I^vdi d Id chilu cao lam viee hQu h i l u cQa d i m (mm);
- A hdm iupng cdt t h i p chju klo:
P}=A,i/b^< 0,02 ,vdi^j,-dien tich cdt thep chju klo; 6^ - b l rpng nhd nhat cua t i l t d i l n trong vQng keo (mm); kj =0,15 - h i sd xet d i n I n h hudng cua lUe dpc trgc; ^cp - Qng suit nen do lue cdng trUdc:
^cp=ypkPo/A<0,2f^, vdi/p-he sd an toan lay eho luc cdng trudc (=
0,9).
3.3. Tieu chuan cua My ACl 318:2002
Kha nang ehiu cat cua be tdng K khi cd luc nen dpc trgc tde
^ ^ " 9 : , ; = O . 1 7 { 1 . 3 - ) ^ ( Z M i4Ag
Khi ed Ipc k l o dpc trgc tdc dung:
^;-o.l7(l+^:^)7}^M
Trong dd: N^/A^ tinh bang MPa, vdi - V iQc dpe tac dung len cdu kiln;'^z - dien tieh mat eat ngang cua t i l t dien; -^^ cd gia tn (-) trong trUdng hop luc dpe true la lUC keo; fc- cudng dp ehiu nen eua be tong.
3.4. Ly thuyet mien nen d i tien MCFT
MCFT duac dua ra bdi Vecchio va Coilins (1985), dau tien duae nghien cQu eho nhQng tam BTCT duoc xem la nhUng cau kien hai chieu, sau do duoe ap dung eho cot va dam duoe xem nhu bao gom nhieu Idp tam be tong. M6 hinh be tong truyen thdng eho ring be tdng bj nQt khdng dQpc chju k^o, MCFT cd k l tdi dnh hQdng cua Qng suit k l o trong vung b l tdng bj nut. Khi nOt, (JTng suat cat truyin qua vet nOt thdng qua cdt thep liln k i t vet nOt, mien Id cdt thep khdng bi chdy. Be tdng giQa cdc v l t nOt coi Id cd h i l u q u i sau dd. TQ ly thuyit ndy, Bentz (2001) dd xdy dUng phan mIm Response 2000 de p h l n tfeh d i m vd cdt. Phan m I m ndy dUa ra dUde mdi quan he giQa chuyln vi todn phan vd lUc cat, b i l n dang d t t r l n toan bp chieu dai d i m , sU Idm vlec cua dim d cdc giai doan tdi trpng. Cac nghiin cQu eho t h i y MCFT cd tinh dU bdo khd ehinh xde khi so sdnh vdi cdc kit ' qua thi nghidm.
4. Khio sit tinh toan
Khdo sit d i m b l tdng ULT nhU hinh 2 vdi d e s d lieu:Be tdng cd e l p b i n B40:
R^ = 22MPa-, /ift,-1,4 MPa;
/ , i = 32 M P a ; / ; = 3 2 MPa.
Kich thude tilt dl#n
bxh = 25x50 (cm); Ao = d = 46 cm;
a= 150 cm; eo = 21 cm.
5 8 HQCVieNCANBOQUANLyXAYDl^NGVADOTI
Hinh 2 Thi du khdo sdt
i Gia thiet A ^0,0026.
j Cho lue cang trudc P thay ddi, irng voi Qng suat nen trudc <^p bien thien
^ tQOden 1,5 MPa.
Khao sat tinh toan kha ndng chiu eat eua be tdng 2/jtheo TCVN 5574:2012, EC2:2004, ACl 318:2002 va MCFT theo cac trudng hop
a/}to> 2,5 \'a a/ho < 2.5.
Cac k i t qua tinh todn the hien d bang 1,2 va hinh 3,4.
Bang 1. Tnrtmg hgp a/ho > 2,5
CTp
(KG/cui') 0 0.1 02 03 0.4 5 6 7 8 9 10 11 12 13 14 15
a (KG) T C V N
14812 14927 15042 15157 15272 15387 15502 15617 15723 15847 15962 16077 16192 16307 16422 16537
EC2 6963 7273 7584 7894 8205 8515 8826 9136 9447 9757 10068 10378 10669 10999 11310 11620
ACl 11059 11138 11217 11296 11375 11454 11533 II612 1169!
11770 11849 11928 12007 12086 12165
!2244 M C F T
16600 16900 16900 16900 16900 16900 17100 17400 17500 17500 17900 18000 18100 18300 18400 18700 Bangl. Trucmg hop a//io< 2,5
(KG/coi-) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 IS
a (KG) TCVN
24687 24878 25070 25262 25453 25645 258^3 26028 26220 26412 26603 26795 26987 27178 27370 27562
EC2 6963 7273
^584 7894 8205 8515 8826
ACl 11059 11138 11217 11296 11375 11454 11533
M C F T 23400 23500 24000 24200 24400 24900 24S00 944" 11691 Z--:-d 975"
10068 10378 10669 10999 11310 11620
117'0 j Z-.'-JO 11849 11928 12007 12086 12165 12244
25400 25400 25500 25500 26400 26400
TQ ket qud tren lap bieu do quan he giUa Up va Of, bl\ cua 2 trudng
;p nhu hinh 3 vd4.
Nhdn xet: Kha nang chiu cat eua be tong viing nen theo eae tieu chuan dUa tren cac md hlnh tinh toan khac nhau, tuy nhien theo EC2, gia tn nay chi xet den khi be tdng vung nen dii khd nang ehiu eat ma khong phai bo tri edt dai.
Khd ndng chiu cat ciia be tong vung nen xae dinh theo TCVN eo gia tri gan v6i MCFT, theo EC2 la nho nhat vl bdn ehat gia tri nay tuang tU nhU theoTCVN TCVN ve kha nang chiu cat eiia dam BTCT edn ehua de cap den mot sd yeu to anh hudng nhu ham luong edt doe, be tong viing keo...
5. Ket luan
Ty so ci/h^, eo anh hudng dang ke den kha nang chiu eat ciia dam be tdng ULT. Khi ty sd nay tang, nhat la khi gia tri nay > 2,5 thi khd nang chiu cat cua be tong viing nen giam,
Luc nen trude lam tang kha nang chiu cat eiia be tong, tiic la tang kha nang ehiu catciiadam, tuy nhien mQc do dnh hudng tren dien tieh dan vj ciia tiet dien la khong qua Idn khi Qng suat nen trudc trung binh ciia be tong trong khoang 0,75 den 1,5 MPa - la gia tn thudng duac sQ dung trong thiet kedam betdng LfLT.
*BH Kien true Ha NOI
I XAV DLr^C A CJO T H ' 5 9
