XAC DINH Cl/GfNG DO CHjU NEN TINH T O A N CUA BE TONG Cl/GfNG DO CAO
Phan Quang Minh
ABSTRACT The equivalent rectangular stress block for High Strength Concrete is proposed to be used in Vietnain's code TCXDVN 356:2005, which formulae valid only for normal strength concrete. The proposed block has been validated by using
design procedure of RC elements from the code.
PGS.TS Phan Quang Minh Bp mon Cong trinh Be tong cot thep TrUcfng Oai hoc Xay dimg 55 dudng Giai Phong, Hai Ba TrUng, Ha Noi Dien thoai: (04) 38697007 Email: [email protected]
1. Dat van de
Ket cau be tdng edt thep (BTCT) dupe sd dung rdng rai trong cle cdng trinh ed chieu eao vda p h i i VI chi phi thap, dp ben cao v l de b i o dudng. Vdi cle edng trinh cd sd tang Idn, Ung dung vat lieu be tdng cd eudng dp ehju nen truyen thdng cho cle eau kien ehju nen I I khdng kha thi. ThUe te thiet ke v l thi edng tren the gidi eho thay, neu edng trinh nha nhieu t i n g ed he edt BTCT sd dung be tdng eUdng dp cao (gpi t i t II edt be tdng COC) se mang lai hieu qua kinh te tdt hon sd dung he edt holn toan bang thep.
Cudng dp ehju nen eua be tdng COC dViet Nam v l Chiu Au thudng dupe quy djnh trong khoing 50MPa den 90MPa mau tru (cap dp ben t d B60 den B105). Su p h l t trien eua edng nghe bpm be tdng da gdp phan l l m eho edng t i e be tdng, ke e l be tdng COC, dupe de d i n g hPn trong thi edng nha nhieu tang. Cot be tdng COC cd k h i ning ehju lUe t i n g len nhieu so vdi edt BTCT, do do l l m g i l m dupe h i m lUpng edt thep, g i l m kich thude tiet dien elu kien, t i n g dien tich sddung [2,5]. Be tdng COC cd md dun dan hdi Idn hon, do dd dp cdng cua ket eau eung dupe t i n g len. Nhdng yeu td nay cua be tdng COC l l m t i n g dp an toan, k h i ning sd dung v l tinh kinh te cua cdng trinh n h i nhieu t i n g .
Tieu chuan thiet ke TCXDVN 356:2005 [4,6] hien h l n h chi de cap eho cau kien BTCT cd d p dp ben
ehju nen cua be tdng den B60. Oac trdng ed b i n nhl't eua be tdng trong thiet ke cau kien BTCT la cUdng dp ehju nen tinh t o l n eho elc cap dp ben Idn hon B60 chda dupe xlc dinh, do dd ehUa the I p dung be tdng COC trong thiet ke ket eau nha nhieu tang.
Bai bao trinh bay eleh xlc djnh eudng dp chiu nen tinh t o l n cua be tdng COC trong tinh t o l n eau kien theo TCXDVN 356:2005.
2. Quan diem t i n h toan cau kien be t o n g cUorng do cao
Khi thiet ke eau kien be tdng COC, van de dat ra la k h i ning I p dung eua tieu chuan thiet ke hien hlnh. Quan he dng suit - bien dang eua be tdng vdi elc eUdng dp khIe nhau (hinh 1 - [2]) eho thl'y sU l l m viec ciia be tdng COC I I khIc biet so vdi be tdng thudng. Odi vdi be tdng COC, dUdng eong dng suit - bien dang gan nhu I I dUdng thing keo d i i den gia trj eao nhat cua dng suat. Vet ndt ben trong xuat hien khi t i i trpng dat den t i i trpng gidi han, vi the gidi han d i n hdi trong mien nen dupe md rdng ra. Bien dang nen eUc han g i l m khi eUdng dp be tdng t i n g . Be tdng COC ed tinh ddn hon so vdi be tdng thudng.
Ket q u i nghien edu thuc nghiem v l ly thuyet [2] ehi ra ring ed the sir dung bieu dd Ung suit vung nen cua be tdng COC tuang tU nhu eiia be tdng thudng, tuy nhien can cd sU dieu chinh ve eudng dp tinh t o l n 7 4 e « » n i l [ f B l . 7.2009
v l chieu cao v u n g nen quy ddi. Vdi quan diem tren, quy trinh thie't ke cau kien be t d n g COC I I khdng thay ddi so vdi BTCT t h u d n g . Quan diem n l y dugc sd d u n g t r o n g tieu chuan thiet ke ket e l u BTCT cua eac nude tien tien n h u A C I -318-02 (My), CSA A23.3-94 (Canada), Euroeode EC-2 (ehlu Au)... d d l y se de cap cu the hon tieu chuan thiet ke ciia e h l u Au EC-2[1].
Theo EC-2, cudng dp chiu nen tinh toan eiia be t d n g fed (hlnh 2) duoc x l c dinh theo:
140
L
'- nctcc (1)0^^ - he sd xet den su k h I c nhau eiia eUdng dp tren ket c l u va cudng dp eua mau tru.
Y^= 1,5 he s o a n t o l n
f_.i^ - Cudng d p d i e t r u n g ciia be tdng.
Vdi be tdng t h u d n g f^^ < 50MPa:
A = 0,8 (2) n = 1 (3) Vdi be t d n g cudng d p eao 50 <
fek < 90MPa:
A = 0,8 - (f^^ - 50)/400 (4) n = 1 - (f^,'- 50)/200 (5) Vdi viee dieu chinh bieu d d Ung
suat nen n h u tren, quy trinh tinh t o l n dupe I p d u n g t h d n g nhl't eho eau kien BTCT va c l u kien be t d n g COC. Vdi ly do tren, eleh tiep can hop ly ciia TCXDVN 356:2005 I I dieu chinh cUdng dp ehju nen t i n h toan ciia be tdng khi tinh t o l n cau kien be tdng COC. Trong dieu kien ciia Viet Nam hien nay, viec nghien cdu thue nghiem n h i m d i n h g i i su l l m viec ciia e l u kien be t d n g COC I I chUa dap Ung dugc , vi vay sd d u n g ket qua nghien cdu eiia eac nude tien tien I I can thiet, d i e biet I I tieu chuan e h l u Au EC-2.
3. C u d n g d o ehju n e n t i n h t o a n cua be t o n g cUdng d o cao
Tien h l n h so s i n h cudng dp ehju nen t i n h t o l n eua be t d n g cho be t d n g t h u d n g theo TCXDVN 356:2005 v l EC-2.
Cudng d d chiu nen tinh t o l n eua be tdng R theoTCXDVN 356:2005
Cudng dp rat cao
Cudng dp cao Cuong dp trung binh Cuong dp thap
0 0.002 0.004 0.006 0.008 0.01 0.012 Bien dang
Hinh 1 - Bi/ong cong ufng suat-bien dang cua be tong
r//^7V
W
/ ^ / - - J
I
.
, /
\ t
1^
I
' /
n
a
r m
rjfta
F,
Hinh 2 - Bieu do ijfng suat viing nen theo EC-2
B i n g 1 - Cudng dd chiu nen tinh toan ciia be tdng theo TCXDVN356:2005 va EC-2
B
Rb
f..
Rt>/tcd
B20 11.5 10.67 1.082
B25 14.3 13.33 1.075
B30 17.1 16 1.067
B35 19.8 18.67 1.060
B40 22.5 22.22 1.011
B45 25.1 25 1.004
B50 27.7 27.78 0.997
B55 30.5 30.56 0.997
B60 33.2 33.33 0.997
Bang 2 - Cudng dp t i n h toan ciia be tdng cUdng dp cao B
n
Rb
65 0.979
35 70 0.958
37 75 0.938
39.5 80 0.917
40.5 85 0.896
42 90 0.875
43.5 95 0.854
45 100 0.833
46 105 0.813
47
i ^ l l k l f . l S I T . 7.2009 75
THONG TIN
Bt T 6 N G iCr TRAU CO THE GIAM PHAT THAI KHI NHA KINH
Trau ed chda nhieu silic didxft, mot t h l n h phan quan trpng eiia be tdng. Viee sd dung be tdng la mot van de Idn eua bien doi khi hau.
Sin xuat mdi tan xi thai ra mdt tan C02 v I o khi quyen. Phlt thai C02 t d viee san xuat xi m I n g tren the gidi chiem khoang 5% tong lupng CO^ do con ngudi thai ra.
Hien nay, Rajan Vempati thupe Cdng ty ChK, d Piano, Texas va nhdm cle nha khoa hpc d l tim ra bien p h l p dung tro cua trau gan nhu khdng p h l t t h i i caebon. Odt trau d nhiet dp 8000C trong Id khdng ed dxy, t i c h khdi caebon, sau dd tao ra eac hat silic didxft gan nhu nguyen ehl't. Qua trinh nay t h i i ra mdt Idpng nhd CO^, tuy nhien d dang clebon trung tinh.
Tro l l m cho be tdng ehle hon, cd k h i nang chdng an mdn tdt hon.
Theo nhdm nghien edu, tro trau ed the lam t i n g hieu suit be tdng bang viee thay the 20% xi m I n g duoc trpn vao be tdng trong qua trinh x l y dung nha cao tang, c l u v l bat ky cdng trinh kien true n i o tren mat nude hole gan nude.
Hien nay, elc n h i nghien cdu dang t h d nghiem v l hoan thien phuong phap mdi. Neu phuong p h l p nay thanh cdng. Id dot se dupe x l y dung gid nguyen kich thude, cd the s i n xua't 15.000 tan tro trau moi nam.
Tan dung t i t ca trau dupc san xuat d My, mdi n l m cd the tao ra 2,1 trieu tan tro. Tham tri, d nhdng nude dang p h l t trien nhu Trung Qude va An Op, npi cd mdc tieu thu gao v l be tdng Idn se cd nhieu tiem n i n g hpn.
[4,6] II:
Yc
(6) V^^- hesd ket cau:
y^^ = 0.77-0.001 B> 0.27 (7) Y^- he soan t o l n .
Tren b i n g 1 trinh bIy eUdng dp ehju nen tinh toan cua be tdng va Rb v l fed theo elc cap dp ben ehju nentUB20den B60.
Cd the thl'y, tuy 2 tieu chuan dUa ra cac he sd bien ddng va he sd an t o l n khIe nhau, nhung cUdng dp ehju nen tinh toan eiia be tdng I I x i p xi nhau vdi sU sai lech khdng d i n g ke. NhU vay cd the lap lu|n rang vdi be tdng cd d p dp ben Idn hpn B60 cd the dieu chinh eUdng dp chiu nen tinh t o l n eua be tdng b i n g he sd thUc nghiem q (cdng thde 5) theo EC-2.
He sd n dugc viet theo cap dp ben B la:
, B - 6 0 240
Cudng dp ehju nen tinh t o l n eiia be tdng ed cap dp ben chiu nen B Idn han B60 theoTCXDVN 356-2005 duac viet thanh:
7.cB
n-
Yc (9)Gil trj ciia R^ eho cle cap dp ben tuB65den BIOS dupe eho tren b i n g 2.
Odng thdi vdi su dieu chinh cUdng dp ehju nen tinh t o l n ciia be tdng, eIn xac djnh g i i trj chieu cao tuong ddi tdi han cua viing ehju nen ciia be tdng i,^. Gil trj n l y dupe xlc djnh theo tieu chuan [6]:
•^R
OD
U - ^ ( 1 - ^ ) a..„ 1.1
(10)
00 - die trUng tinh ehl't bien dang ciia vung be tdng ehju nen
Lu = a-0.008R^ (11) Cdng thde thUe nghiem (11) la
ciia TCXDVN 356-2005 va I p dung cho be tdng COC vdi g i i trj ciia R^
lly theo bing 2.
0 = 0.85 vdi be tdng nang.
a^^^ - dng suat gidi han cua cot thep trong vung be tdng ehju nen:
o^^^ = SOOMPa ddi vdi t i i trpng
thudng xuyen, t i i trpng tam thdi d i i han v l ngIn han.
a^ ^ = 400MPa ddi vdi tai trpng t i e dung ngIn han v l t i i trpng die biet.
4. Ket luan
Bai bao da de xuat eleh xlc djnh eUdng dp ehju nen tinh t o l n eiia be tdng eudng dp eao theo TCXDVN 356:2005. Vdi sd dieu chinh cUdng dp ehju nen tinh t o l n v l chieu cao vung nen tdi han cua be tdng, quy trinh thiet ke e l u kien BTCT theo tieu chuan dugc md rdng I p dung eho e l eau kien sd dung be tdng
eudng dp cao. •
TAILIEUTHAMKHAO
1. Euroeode 2: Design of concrete strudures, 2003.
2. Priyan Mendis, "Design of High-strength Con- crete Members". The University of Melbourne, 2007.
3. Shah, S. and Ahmad, S., "High performance concrete applications", Edward Arnold, London, 1994.
4. Phan Quang Minh, Ngo The Phong, Nguyen Dinh Cong "Ket cau be tong cot thep - Phan cau kien CO ban"- NXB Khoa hoc va Ky thuat, 2006.
5. Phan Quang Minh, "Hieu gua sir dung be tong cuong do cao trong thiet ke ket cau nha nhieu tang" Tap chi Khoa hoc Cong nghe - Truong Oai hoc Xay dt/ng, so 5/2009.
6. TCXDVN 356:2005 "Ket ca'u be tong va be tong cot thep.Tieu chuan thiet ke; Ha Npi, 2005.
76i»Unfiiail . 7.2009
