Anh hu'O'ng cua cot dai den kha nang chju
ILKC cua cot be tong cot thep
Ngay nhan bai: 9/7/2014 Ngay su^ bai: 23/8/2014 Ngay chap nhan dang: 27/8/2014
Nguyen Ngoc Phu'dng
TOM T A T :
Bai bao trinh biy vi?c khao sat bi^n dang va idia nang chiu lUc cua cgt be tdng cot thep xet anh hUdng cua cot dai theo tieu chuan thi^t EC2, ACt318 va mdt so mo hinh, qua do lam rd hon sU lam vigc ciia cot BTCT va ki^n nghi v6i tieu chuan Viet Nam.
ABSTRACT
The article analyzes the deflection and load capacity survay of reinforced concrete columns with the effect of stirrup according to the designed codes such as EC2, ACI 318 and some models.
Consequently responses of reinforced concrete columns can be seen much more clearly and suggested to the Viet Nam codes.
TS. Nguyen Ngpc Phiitfng TrUcfng DH Kien triic Ha Ndi Di dong: 0903405450
Email: Phuong.nguyenngocphuong@
gmail.com
Dat van de
Cdt BTCT la cau kien chju lUC chinh trong ket cau cong trinh, la cau ki6n chju nen, nen - udn. Kha nang chiu k^o eiia be tdng kem n l n toan b d lUc keo se do cot thep chiu, trong vUng nen cd ca be tong va edt thep ciing cdng t i e chiu tuc Do vay, neu nang cao dUOc eUdng d d cua thep sir dung trong cdt se dan den hieu q u i dang ke khi cdt lam viec. Theo tieu chuan t h i l t ke ket eau be tong cot thep (BTCT] ciia Viet Nam TCVN 5574 2012, vdi be tdng thUdng, bien dang eye han s* = 2.10 ^ mddun dan hoi e6a edt t h e p E = 2.10^ nen de dam b i o sU lam vi^c chung giUa be tdng va cdt thep va han che lang phi, Ung suat trong edt t h i p khdng qua 400 MPa (cudng do tinh loan ve nen ciia cdt t h i p khdng Idn hon 365 MPa) [1,2]. Vi vay, k h i nang chiu lUc cda cdt BTCT khdng eao trong khi y l u cau ve k h i nang chiu nen eiia edt, d i e biet vdi c6t nha cao tang la lat Idn, dan den kich thUdc cdt Idn, anh hudng den edng nang va thi'm my. Do dd, k h i o sat i n h hudng ciia cot dai den sU thay doi bien dang ngang ciia be t d n g , qua d d lam rd hon sy lam viec cua edt BTCT ed x^t den bi^n dang kiem che n d ngang va d i n h g i i k h i nang c h i u l U c c u a e d t t h e o m o t so tieu chuan t h i l t k l va m d hinh la can t h i l t , de t i f dd c6 nhCing luu y vdi cac ky sU ket eau va k i l n nghi vdi TCVN.
1 . Tieu chuSn M ^ ACI 318-2005 va chau Au EC 2-2004
1.1. Tieu chuan ACI 318-2005(3,6}:
- Ong suat eua be tdng khi cd kiem che n d ngang- t\^ = V^+4,]o^ vdi: f'^ - cUdng d d chiu nen ciia b l t d n g ; o, - iing suat ciia cot dai:
sb. i: A - dien tich t i l t dien cot dai, f - cudng d o cot dai, a - gdc nghieng cua cot dai, s - k h o i n g each giCia cac cdt dai, b - kich thudc canh eua Idi. sb^ - dien tich be mat l d i .
- Bien dang eua be tdng khi c6 Ung suat kiem che n d ngang. e „ = - ^ vdi E - md d u n d i n hdi ciia be tdng ^
- Kha nang chiu lUc cda cdt:
[N]=0,8(0,85f'^i^^A^ -(- A^f^) - vdi \ - dienlkk t i l t dien cdt.
1.2. Tieu chuan EC2-2004 [5,6]:
- Qng suat ciia be t d n g khi c6 kife cM n d ngang: f.k.c ^ ^ . k ^ + S ^ ) khi o^ s 0,OS|vi f.k,. - f ^ ( 1 , 1 2 5 + 2 , 5 y i ) khi O; > 0,05f^^ \ffflfj\
cUdng d o chiu n l n ciia b l t d n g ; o^ - tiKingU n h u Oj cda ACI.
- Bien dang ciia be t d n g khi c6 iing sui kiem che n d ngang: ^ciZf-^-
= 0.002
- K h i nang chiu lUc cua c6t:
[Nl=0,8(0,85f ; ^ / ^ + fiif) 2 . M o t so m o h i n h
2.1. Mo hinh cua Legeron vd Paultre (199SI cua cdt: 7 ^
Hinh 1 Quan li& fing suat-bien dang cia be tong thfi liinh cua Legeron va Paultre
Md hinh i p dung cho cdt BT duoc kiSmc n d ngang bdi c i c cot dai hinh chU nhStlK hinh vudng. Vdi cot BTCT, Cfng suat k i l m die ngang khong phai la m o t h i n g so ma phu thi vao Cfng suat trong cdt t h e p kiem che nfings cd lien quan t d i sU n d n g a n g ciia b l tong chiu nen - hieu Ung Poisson (he sd Poisson 0,5 tai giai doan khi cdt dai c h i y d I o ) . KhicA dai se e i n t r d n d ngang eiia be t d n g , larnt dang cUc han cua be t o n g khi chju n l n tang dan d i n b i l n dang cUc han t r o n g cdt th^pl len, ket q u i cd t h e nang cao dUOC Cfng suSt trong cdt t h e p chju n l n . Bieu d o Ung sucit-' dang ciia be t d n g theo m o hinh n i y dUOi
58|BHnK[IBl 09.2014
1 0 B i n g 1. B i l n dang eiia b l t o n g khi cd kiem che n d nqanq
1 2 3 4
Tieu chuan va m d hinh
ACI 318-2005 EC2-2004 Legeron va Paultre Salim Razvi v i Murat Saatcioglu
" Sai so idn nhat glQa c i c t i l u chuan v a m d hinh
B i l n dang eye han cda b l tdng £ „ M l
0,002 0,002 0,002 0,002
M2
0,0022 0,0024 0,0023 0,0030
36,4%
Sai sd so vdi M l 1 0 % 2 0 % 1 5 % 5 0 %
M3
0,0023 0,0027 0,0028 0,0037
50,9%
Sai sd so vdi M l 1 5 % 3 5 % 4 0 % 8 5 %
M4
0,0025 0,0030 0,0033 0,0045
8 0 %
Sai sd so vdi M l 2 5 % 5 0 % 6 5 % 1 2 5 %
B i n g 2. K h i nang chiu lye cOa cdt
VH*,''"''
- W K ( 1 lon9l}i[2
^fm;3
' '4 khi(i,)M
T i l u chuan va m d hinh
ACI 318-2005 EC2-2004 Legeron va Paultre Salim Razvi va Murat Saatcioglu betOKiiSai sd Idn n h i t giCfa c i c t i l u
' c h u a n v i m d h i n h
K h i nang chiu lye cua cdt (KN) M l
2665 2438 2565 2665
M2
2846 2589 2884 2871
11.4%
Sai sd so vdi M l 6 , 8 % 6 , 2 % 8,2%
7 , 7 %
M3
3027 2740 3054 3032
11,5%
Sai so so vdi M l 13,6%
12,4%
14,6%
13,8%
M4
3231 2910 3167 3196
1 1 %
Sai so so vdi M l 2 1 , 2 % 19,4%
18.8%
19,9%
=( . ( i l n 6 h i n h 1 . [ 3 , 4 ]
- iJng suc!t cda be t d n g khi cd kiem che
^ ^ b ngang: r^ = [ 1 + 2 , 4 ( r / ' ] . f ; vdi i ; - chl so
leraiiriW -^'~^^*^ l3 3p iye han c h l n d ngang hieu ung, trong d d : A^^ - t o n g dien tich mat cat gang ciia c i c tam v d boc theo phUdng tryc y ong k h o i n g c i c h giUa cac cot dai s, c - dUdng
^, nh Idi b l t o n g han che n d ngang, f ^ - irng s u i t
\ Sng t r o n g cdt t h e p dai, K^ - he sd kiem c h l n d
- ' \ ^ ^ (1-sy4c)' .. ^ _t^
»^iB\gang hieu q u i : K=—~ voi M « ^ - -: ty' sd d i l n tfch cot t h i p doc tren dien tich b l
j n g han che n d n g a n g .
BitifS' . Bi^n dang cda be t d n g khi cd <ing s u i t i l m che n d ngang: i\^ = [1 +3,5(\\y-^].E\
icotSTW 2.2. Mo hinh cua Salim Razvi vd Murat iih'^^aatcioglu(1999):
Signs' / • . ! hinga"* ,
;p|[ie(iic''~v
dij?**'
jjiJ^* tiinli 2. Quan hS ling suSt-bien dang cua be tong theo mo
0^'
hinh cua Salim va Murat Saatcioglu
Md hinh nay xet tdi t i t ca c i c yeu t d i n h hudng n h u hinh dang cdt dai, ty- sd t h e tich cot dai tren the tich Idi b l tdng, k h o i n g each va gidi han ehay cua cot dai cung n h u cudng do b l t d n g va kieh thUde hinh hoc cua cdt. Bieu dd Ung s u i t - b i l n dang ciia be tdng theo m d hinh nay duoc the hien d h i n h 2. [3,4]
- Cfng s u i t cua be t d n g khi cd k i l m che n d ngang: f'^ - f'^^ + k^f,^ vdi f ^^ = f'^
k,=5,7(fie)"''"; Ung s u i t kiem c h l nd ngang tUdng dUOng f,^ = k^f, vdi f, - iing suat kiem che n d ngang bi d d n g , \hc djnh nhU o^ trong ACI;
- 0 , 1 5 , <1,0
- Bien dang ciia be t o n g khi eo Ung s u i t kiem , 40 , „ e h e n d n g a n g : £ = E (l-i-5k K) v d i k j = - : - < 1 . 0 v a K = -3—
3. K h i o sat tinh t o a n :
Khao sat b i l n dang eiia be t d n g cot va kha nang chiu lUc cua cot cd hinh dang, kich thudc, c i u tao va bo tri cot thep doc. dai n h u hinh 3.
Kich t h u d c tiet dien edt 400x400 m m . Tdng so 4 mau cot:
-I- Mau 1 ( M l ) chi cd edt doc, khong e d e d t dai;
•1- M a u 2 ( M 2 ) : b 6 t r i c d t d a i , 4 ) 8 a l 0 0 + Mau 3 (M3): b o tri cdt d a i ; 4.8a50
•I- M a u 4 ( M 4 ) : b 6 t r i c d t d a i ( t > 1 0 a 5 0
-Thep doe chiu lUe ciia cdt sU d y n g 8iji20 va edt dai sU dung t h i p cd cUdng d p c h i y deo f
= 295MPa (theo TCVN t u o n g d u o n g nhdm t h e p CII).
- B l tdng:-)-Theo TCVN 5574-2012, si( d u n g 625: R^, = 25 MPa; R,, = 18,5 MPa
•H Theo ACI 318-2005: ^c ^ T ^ = T : ; ^ = ^0 MPa ^'^^ ' ' ^ ^
•1- Theo EC2-2004: C20/25 cd f^^ = 20 MPa, ''icb* = 25 MPa, E,^ = 30x10^ MPa
Tinh toan theo ACI 318, EC2va eie m o hinh neu tren. Cic k i t q u i tinh t o i n t h e hien trong c i c bang 1 va bang 2.
Cd t h e thay rang khi CO su kiem ehe n d ngang, bten dang cUc han eua be t d n g tang len, tang it nhat la theo ACI va tang len Idn nhat la 125%. GiCta c i c tieu chuan va m d hinh cd sy chenh lech nhau kha Idn ve gia tri bien dang nay, Idn nhat la vdi m i u 4 (80%).
Cd the thay rang k h i nang chiu lUc ciia cdt theo EC2 la n h d nhat. Khi cd sy kiem c h l n d ngang. k h i nang ehju luc ciia cot tang len, Idn nhat la 21,2%. GiUa c i c t i l u chuan v i m d hinh cd sy chenh lech nhau ve k h i nang chju lUc Idn n h a t l a 1 l , 5 % .
Ket luan;
Khi cd sU kiem che nd ngang. bien dang va cudng d o ciia be tdng duoc tang len. k h i nang chiu lye cua cdt BTCT cung tang len, ed t h e tang len k h o i n g 18 den 21 (%) va cd t h e sit d u n g t h e p cd cudng d o cao han.
Gidi han c h i y ciia edt t h i p dai cd i n h h u d n g
09.2014B«EIE[nail59
B«20
BfflgQ . ? ^ \ *
f V |
Beoioo
SfflgQ
D
f...="+
aesa
fl8Q50 8ggo
020 aim J 400 1
eiOo50
8050
i
Hinh 3. Cac man cot khao sat
dang ke den su k i l m che nd ngang ciia be tdng.
Viec cau tao cdt dai n h u dUdng kinh, khoang each cdt dai theo phuang dUng va phuong ngang, t J so giCfa ham lUOng edt dai va di&n tieh Idi be tdng cd i n h hudng den viec kiem che nd ngang cUa b§ tdng.
Cic k i t qua tren gdp phan cho cac ky sU t h i l t k l ket cau trong nghien eUu ve sU lam viee va tfnh toan, d i n h gia kha nang chiu lUc eiia cot BTCT, ddng thdi kien nghi trong t i l u chuan thiet ke ket cau BTCT eua Viet Nam can cd nhUng nghien cUu ly thuyet, thuc nghiem ve van de nay de bd sung va lam rd han nham hoan thien TCVN.
T A I L I E U THAM KHAO
LPhan Ouang Minh, Ngo The Phong, Nguyen Dinh Cong (2011), Ket ca'u be long cot thep, M. Khoa hot va ky thuat. Ha NOI
2. Tieu chuan Viel Nam (2012), TCVN 5574 2012, Ke't cau be tong va be tong t o i thep -Tieu chuan thiet ke, Nxb. Xay dung, Ha Noi.
3. ACI committee (2005), Building code reguirements for structural concrete (ACI 318-05), Structrural Engineering Handbook
4 Ed Chen Wai-Fah (1999), Structural cono¥te Des[gn, Structrural Engtneering Handbook.
5 Eumtode 2. Design of contrete structures 6. MC Graw-Hill (1999), Ciifil Engineering Formulas, Structrural Engineering Handbook.
