Danh gia kha nang chong cat cua dam bet be tong cot thep
Nguyen Ba Thach Phan Quang Minh
ABSTRACT
The paper presents estimate shear strength of a RC beams, analyzed by MCFT, which takes into account the simultaneous etTects of amount of longitudinal steel reinforcement,, width to depth ratios b/h.. Some remarks are suggested on designing of wide beams to TCXDVN 356-2005.
ThS. KS Nguygn Ba Thach Ban Quan ly Xay dimg cac cong trinh trpng diem Gia Lai Email: [email protected] GS.TS Phan Quang Minh Truong Dai hoc Xay dung
Vdi vi$c han c h ^ chiSu cao t3ng cda cdc edng trinh nhd nhi^u tSng nhdm dat hi^u qud trong ki^n true, surdung.., k i t c i u ddm bet bg t d n g cdt t h i p thudng duoc sUdi^ng khdng chl ddp Ung cdc ydu cdu t r i n md cdn Id phuong dn phu hop vdi d i l u k i l n thi cdng d V i | t Nam,
Bdi bdo nghidn cijru dnh hudng cua be rdng t i e i d i l n , hdm luong cdt t h i p dpe eua dam be t d n g cdt t h i p den khd nang chdng cdt cua cau kien vd dUa ra ede lUu y khi t h i l t k l dam bet theo TCXDVN 356:2005.
1 . D d t v £ n d e
Theo t i l u chu^n t h i l t k l k i t edu b l tdng cdt thep cCia V i l t Nam TCVN 356:2005 [1,3], khd nang chiu cdt cOa bS tdng vCing n i n Qb cCia dam BTCT khi khdng bd trf cdt dai vd khdng ed lUc dgc true Id:
( p , , R ^ b h ; Trong dd:
(1)
: bdng 1.5 vdi b l tdng ndng;
hjj - c h i l u cao lam viec cda d i m ; c - c h i l u ddi hinh chieu tiet d i l n nghieng tren true cdu k i l n tinh tU m i p gdi tMa;
Ry - cudng dd chju k l o tinh todn cOa be tdng,
Q ^ „ , „ s Q , s Q ^ ^ (2) Gid tri Q^^,^ duoc xac dmh phu thude vao
cau k i l n dSm (3a) hay bdn (3b):
tuong iJng vdi c=2,5hg,
Q t ™ = 2,5R„bh^, (4) Cd t h i nhdn thdy rdng neu til sd c/h^ vd
chieu cao dSm Id khdng doi, gid tri Qj, t? I I thuan vdl be rdng dSm. Cdng thu'c (1) dUOc xSy dung tr^n CO sd ly t h u y i t va thye n g h i i m nhung t i l t dien cSc dSm thi n g h i i m d i u cd t^ sd b/h < 1 . Dieu d d d | t ra cin cd sU khdo sdt v l khd ndng chdng c3t cua dam khi cd b l rdng t i l t d i l n thay doi nhdm cd nhOmg luu y c3n t h i l t khi t h i l t k l k i t cau ddm bet.
2. Khdo s i t sd theo Ly thuydt IVlien nen e d i t i o n (MCFT)
Gid t h i l t Id lue edt hay iJng suSt edt ddng vai t r d quan trong trong viec gay ra sU phd hoai nen nhieu ndm qua ngudi ta quan n i l m phd hoai
cdt cOng tuong t y n h u nhu pha hoai do udn.
O i l u dd cd nghTa Id su phd hoai do cdt dat d i n cudng do chju cdt cua b l tdng. Cae nghien eu>j ly thuyet va thUe n g h i i m da neu ra sai lam dd khi chl ra rdng Ung su^t cdt phd hoai khdng phdi Id hdng sd vat lieu ma phu thudc rat n h l l u vdo t j s6 a/hg (hlnh 1), dang tdi trgng, kieh thudc t i l t diln,,, Khi h^ £ a £ 2,5h^ khd nang chiu cdt cda d i m chCi yeu theo c o c h i l cCia tde ddng vdm, do vdy hinh thUc phd hoai id sU nen v d v i j n g nen t r i n v i t nUt nghieng. Khi a > 2,5hg hoac khi dam chju tdi trong phan bd deu, sU pha hoai do tdc dung ddng thdi cua md men udn M va lUC cdt Q [2], Cde thuc nghiem eho thay rang khi ty s6 a/hji tdng, kha ndng chiu eSt eua be tdng vung n i n cung gidm [4,6]. Trong TCXDVN 3562005 quy dinh gid tri e trong cdng thUc (1) lay khdng Idn hon gid t n nhd nhat cua 2,5hu va a:
c £ min(a,2.5h,) (5) Ddi vdi ket eau ddm bet, [thudng chl chju tdi
trgng phdn bo deu) n&n viec khdo sat sd ed the chi x l t den trUdng hop khi a 2 2,5h^.
Vdi sy phat trien manh me cda cde md hlnh ttnh todn khd nang chiu cat cua cau k i l n trong nhUng nam qua vd dac biet la LJ t h u y i t mien n i n edi t i l n (MCFT) [4,5] cho phep cac ky sU tiem edn gan hOn sU Idm viec that cda cau kien, Gid t n 0,^ theo MCFT dUOc xac djnh cd x l t d i n c3c y i u t d dnh hudng n h u md men udn, hdm luong edt thep doe, t j sd a/h^... L;? t h u y i t mien nen cdi t i l n (MCFT) ed d d tin cay cao vd dd duoe kiem chOrig bdng rat n h i l u cae thuc n g h i i m . Quy trinh tinh todn MCFT la quy trinh lap, khd phdc tap nen v i l e sil dung phan m i m Response 2000 [5] xdy dUng tren co sd ly t h u y i t cCia MCFT, cd t h i de ddng khdo sat s6 vdi cdc trudng hop tinh todn khac nhau.
SCr dung Response 2000 d l khdo sdt dam chju tdi trong tap trung n h u hinh 1. CSp dd b i n nen cOa b l tdng Id B25, t^ sd a/h^ = 2.5.
Cdc thdng sd khao sdt:
Chilu cao dam: h = 40 cm; 50 cm va 60 cm (Id cdc c h i l u eao dam bet hay gap trong thye t l ) ;
B l rdng dam- b/h = 0.5; 1; 2; 2.5 va 3;
2.1. Trudng hap hdm fUcfig cdtdgckhong dd;
U y hdm luong cdt doc \i = 0,5% = const.
05.20i2«nni![|oi|65
Hinh 1, Kh^o sat khi nSng chju c3t cua dSm
ft
1100 10.00
2 00 000
" ^1
' » • h = 4 0 c m a!*
- • - h = 5 0 c n l ^
•=*=•=•=*=• ^
3S6:200S h=60cmHlnh 2, iJng su3t c3t giiJi h?n vb (kg/cm') Bdng 1 Gid trj Q^ (KN) trUdng hgp a/\
b/h OJ 1 1.5
2 2.5 3
h=40cm MCFT
49.4 98.8 148.3 197.8 247.2 296.7
356:2005 69.12 138.24 207.36 276.48 345.6 414.72
= 2,5 h=50cm
MCFT 72,7 146,1 219,2 292.2 365.3 438.4
356'2005 108 216 324 432 540 648
h=60em MCFT 104.7 209.5 317 422.7 528.4 634,1
356:2005 155.52 311.04 466.56 622.08 777.6 933.12
Bdng 2. Gid tri Q^ (KN) theo ham luong edt doc b/h
0.5 1
^3 2 2S 3
As=0.5%
72 7 146.1 219.2 292.2 365.3 438.4
As=1%
97 194.1 291.1 388.1 485.2 582.2
As=1,5%
101.4 200.6 300.9 401.2 498.6 598.3
356:2005 108 216 324 432 540 648
k i t qud tinh todn khd nang chju edt cua dam Q^
(khdng x l t den h i sd an todn cOa v^t li^u) theo MCFT vd TCXDVN 356:2005 khi khdng b d t r i edt dai cho cdc trudng hop khdo sat dugc trinh bdy trong bdng 1, Luu y la hdm luong edt dge cOa dim khdng duoc x l t d i n n i n khdng dnh hudng den gia trj Q^ khi tinh theo TCXDVN 356:2005.
T r i n hinh 2 Id do thj the hl^n gid trj ilTig suat cat tdi han cua b l tdng vb tinh theo TCXDVN 356:2005 vi MCFT t y k i t qud trong bdng 1;
Vt, Q ,
(6) 2.2. Tn/dng hgp ham lugng cot dgc thay doi
Khdo sdt khi hdm luyng c6t dgc thay ddi p=0.5%; 1 % vd 1.5% cho dSm cd c h i l u cao t i l t d i l n h=S0 cm, tj- sd b/h thay ddi tU 0.5 d i n 3.
Cdc gid tri tinh todn cua Q^, tinh todn theo MCFT vd TCXDVN 356:2005 dUOC trinh bdy d bdng 2.
Ty cdc k i t qud khdo sdt t r i n cd t h ^ rut ra cdc nhdn x l t sau:
- Ty hinh 2 cho thay khd ndng chju cdt b l t d n g viing n i n Q,^ cOa dSm Id khdng thay ddi t r i n I m b l rdng t i l t d i l n [cdc tiet di^n cung hdm lucmg cdt t h i p ) . D i l u ndy cung trCing hop vdi ket qud cua t h i nghiem [4].
- Khl hdm luong cdt t h i p dge tang, gid tri Q^_
cung tang, nhUng khi hdm lUOng ldn hon 1 % ,
gid trj ndy tdng l i n khdng ddng k l . - Khi dim cd hdm luong cdt thep doc \i=
1.5%, gid tri Q^ tfnh theo TCXDVN 356:2005 vd MCFT Id xdp xi nhau, nhung khi ham l u ^ p=0.5%, TCXDVN 356:2005 d y bdo khd ndng chiu cdt Qb Idn h o n so vdi MCFT x3p xl 1.513n, -Tai c d c g d i tya, can t h i l t phdi ki6ntrakhd ndng chdng choc t h u n g cua dam bet. "*^
3. K A l u | n
Ap dung d a m bet be t d n g cdt t h i p trong k i t cSu nhd n h i l u tang Id m d t gidi phdp co the dUa d i n cde h i l u qud ve ki^n tnJc sir dung, phu hop vdi d i l u k i i n thi cdng dViet Nam Khi ddnh gid khd ndng chju c i t cda dam bet d n xem xet d i n hdm luang cdt doc, ddc b i l t luu y khi ham l u y n g ndy n h d hon 1 J % . Vile lua chqn b l rdng t i l t d i l n ddm bet phdi thda man cic y i u cdu v l chiu l u c b i l n dang cua ddm cung n h u yeu c i u v l c i u t^o theo t i l u chuan thilt k l khdng c h i n .
TAILI|UTHAMKHAO
1. Phan Quang M i n h , Ngo Th^ Phong, Nguyen Dinh Cdng. M t A H tong 1:61 th^p, Nhd Xua't b i n Khoa hoc Ky t h u i t , 2006, 2. Phan Quang M m h , NguyJn Ngoc Phucmg Mo hinh ti'nli WJn k h i n i n g chiu c i t cua dam bS tong dng lUc Wide Tap dil til dung, s6 2 , 2 0 0 7 .
3. TUDVN 356:2005, Kel cSu W tong va b l tfing cot th*p - Tieu c h u i n thiet ke.
4. Edward G. Sherwood. One-Way Shear Behavioui of U i g t Lighlly-Reinfbrced Concrete Beams and Slabs, Univeisty it Toronto, Canada, 2008.
5 Evan Bentz. Response 2000 - User Manual, Toronto,Canaila, 2001.
6Wassim M Ghannoum, Size effect on Shear Strength 1^
Reinforced Concrete beams. Department of Civil Engineering and Applied Mechanic, McGill University, Montreal, Canada, 199!t.
