T H I E T K E C O N G T R I N H S U f D U N G K H U N G B E T O N G C O T T H E P C H E N G A C H
C H j U T A I T R O N G O O N G D A T
T H E O T C X D V N 3 7 5 - 2 0 0 6
Vuong Ngoc Luu
ABSTRACT Frame systems of multistory build-
ings are popular in Viet Nam currently.
However the masonry infills are not usually included in designing because they are considered as no-structural elements of the building. Consequently, this leads to some inaccuracies consider- ably in calculation, especially in design
for earthquake resistant building. This article recommends a new method that considers ability in supporting of masonry infills and the method is based
on TCXDVN 375-2006 code.
TS VirorigNgoc Lull
Pho hieu trudng, TrUdng Dai hoc kien true Ha Noi Km 10 dudng Nguyen Trai, quan
Thanh Xuan, Ha Noi Di dong: 0913579502
T
CXDVN 375-2006- Thiet ke cong trinh chiu ddng dat duoc ban hanh thang 9 nam 2006.Day la mpt tai lieu rat quan trpng doi vdi cdng tac thiet ke cdng trinh d nhu'ng vung cd ddng dat.
Viec van dung tieu chuan nay trong cdng tac thiet ke cdng trinh da va dang ddpc cac nha khoa hpc, cac ky sd nghien cdu va trien khai mdt each tich cdc.
dViet Nam da so cac cdng trinh xay ddng deu sd dung ket cau be tdng cot thep (BTCT) la ket cau ch|u Idc chinh, trong dd cd ket cau khung BTCT chen gach.
Trddc day da cd mdt so cdng trinh nghien cdu xet den sd lam viec cua khung BTCT cd tddng chen ch|u cac tai trpng tinh (ddng va ngang).
Tuy nhien, ddi vdi tai trpng ddng dat thi chda cd cac chi dan cu the ve mat tinh toan va cau tao.
Vi vay, viec ap dung tieu chuan nay vao thiet ke cdng trinh cd khung BTCT chen gach ch|u ddng dat can ddpc nghien cdu mdt each cu the ca ve ly thuyet va tinh toan thdc te.
1. Anh hddng cua khdi xay chen trong khung tdi kha nang khang chan cua cdng trinh.
Khdi xay chen trong khung thddng cd cac loai sau: khdi xay gach dat set nung, rdng hoac dac chieu day 220; 200; 110mm, khdi xay bang gach be tdng xi mang cd nhieu Id
rdng hoac cd trpng Idpng nhe (BT nhe) chieu day 200 - 300mm.
Khdi xay chen khdng chju lUc, chi cd tac dung bao che, ngan each, each am, each nhiet. Nd la cac bp phan phi ket cau cua cdng trinh
Khdi xay chen trong khung cd tac dung:
- Lam tang dp cdng cua cdng trinh nen chu ky dao ddng cua nha giam, he qua lam tang trj so lpc cat day.
- Lam thay ddi sp phan bd dp cdng ngang cua he ket cau trong mat bang cung nhutheo chieu cao.
- Tham gia chju tac ddng cua ddng dat cung vdi khung.
- Lam tang kha nang phan tan nang Idpng cua cdng trinh.
Sd tham gia chju tac ddng ddng dat cua khdi xay chen phu thupc vao cac yeu td sau:
- Cddng dp va dp cdng cua khoi xay: Day la cac chi tieu cd dp tin cay thap do tinh chat khdng ddng nha't va sd bien thien cac tinh chat co ly cua chung.
- Mdc dp lien ket cua khdi xay chen vdi khung.
- Mdc dp pha hoai cua khdi xay chen trong thdi gian ddng dat.
- Kha nang thay ddi tinh toan ven cua khdi xay chen trong qua trinh sd dung.
Theo tieu chuan EN 1998-1: 2004 va TCXDVN 375 - 2006 khdi xay chen ddpc xet den khi thoa man cac dieu 90 (SUfil!** . 3.2009
kien sau:
- Duac thi cong sau khi khung BTCT da kho cung hoac khung thep da lap dpng xong.
- Chung tiep xuc vai khung khong co ke hd va khdng cd lien ket dac biet nao.
- Ve nguyen tac chung dupe xem la cac ca'u kien phi ket ca'u.
Cac quy dinh ve xac dinh chieu dai tdi han cua cdt khung khi cd tddng chen.
Ddi vdi cdt tang tret: Do tudng chen cua tang tret de bi hu hdng nen toan bp chieu dai cac cdt tang tret can ddpc xem nhu chieu dai tdi han va phai cd cot dai han che bien dang mdt each hop ly.
Ddi vdi cac cdt tang tren:
- Neu chieu cao tudng chen nhd han chieu dai thdng thuy cua cac cdt lien ke thi can ap dung cac bien phap sau:
+ Chieu dai toan bp cdt dupe xem la vung tdi han va can dupe dat cdt thep vdi so lupng va hinh dang cdt thep dai nhd ddi vdi yeu cau cho vung tdi han.
+ Thiet ke cdt chju ddpc cac Idc cat do md men d cac dau cdt lay bang chieu dai cua phan cdt khdng tiep xuc vdi tddng chen.
- d nhdng vi tri cd tddng chen kin toan bp chieu dai thdng thuy cua cac cdt lien ke va tddng chen nay chi nam ve mdt phia cua cdt (cdt d gdc ) thi chieu dai toan bp cdt dd can ddpc xem la vung tdi han va
phai dupe dat cdt thep vdi so lupng va hinh dang cot thep dai nhd yeu cau ddi vdi vung tdi han.
Khi thiet ke cdng trinh cd ket cau khung chen tudng can phai:
- Xet tdi hau qua cua tinh khdng deu dan cua khdi xay chen gay ra trong mat bang nha. Khi dd can tranh bd tri cac khdi xay khdng deu dan, khdng ddi xdng hoac khdng ddng deu trong mat bang. Trong trddng hpp cac khdi xay chen bd tri khdng ddng deu gay ra tinh mat ddi xdng Idn thi can phai phan tich ket ca'u theo md hinh khdng gian trong dd cd ke den sd lam viec cua tddng chen nhd nhdng thanh xien hoac nhd nhdng phan td tam cd md dun dan hdi tdong dng
- Xet den tinh khdng deu dan theo chieu cao.
Trong trudng hop khdi xay chen gay ra sp khdng ddng deu theo chieu cao mdt each dang ke, phai tang he qua tac ddng cua ddng da't trong cac ca'u kien thang ddng chiu tai tai cac tang dang xet bang he so khuyech dai n, xac dinh nhd sau:
AV - Tdng chenh lech sdc chiu tai cua khdi xay chen trong tang dang xet so vdi tang cd nhieu khdi xay chen han d phia tren
IVs d - Tdng cac ldc cat tac ddng len tat ca cac cau kien khang chan chinh theo phdong thang ddng cua
tang dang xet.
q - He so Ung xd:
Neu n, < 1,1 lay 11 = 1 cd nghia la khdng can xet den tinh khdng deu dan theo chieu cao.
Can cd cac bien phap cau tao de cac khdi xay chen khdng bi nut sdm va bi pha hoai gidn, nhat la ddi vdi cac khdi xay cd Id cda, cac khdi xay cd dp manh >15 nhu lam cac giang ngang, giang dung bang BTCT, dat sSn thep lien ket cdt vdi tudng xay, gia cd Id cda bang cac giang ddng va giang ngang.
2. Tinh toan anh hddng cua khdi xay chen den sU lam viec cua khung
2.1. Md h'inh ket cdu
Md hinhl: Khdi xay chen ddpc thay bang thanh cheo tddng duang chiu nen cd dp cdng ddpc tinh theo Poliakov
Dai khdi xay tuang duang dd dupe xac dinh theo cac yeu to va nhan xet sau:
- Op cdng cua dai chiu khdi xay tuong dddng thay ddi phu thupc vao tai trpng ngang tac ddng len he khung chen va dp cdng cua khung.
- Dac trdng co hpc cua vat lieu khung va khdi xay chen thdng qua md dun dan hdi cua chung.
- Phai dam bao sd tiep xuc tdt gida khung va khdi xay chen.
Viec xac djnh dp cdng cua giai tddng dddng dda tren co sd va gia thiet sau:
iWilTlJi? . 3.2009 91
- Xac dinh ddac vung tiep xuc cua dam va cot vdi khdi xay chen la I va h thdng qua bai toan tren dam tren nen dan hdi tuan theo md hinh cua Winkler.
- Khung chen gach nhieu tang khi chiu tai trpng ngang lam viec nhd mdt cdng xon ngam vao mdng.
- Ket cau roi vao trang thai gidi han khi xuat hien vet nut xien dau tien trong khdi xay chen.
- Cac nut khung ddpc cau tao la nut cdng va cac bien dang dupe coi la be.
Xac dinh doan tiep xuc gida cdt vdi khdi xay chen.
\ =—arctg
rrv 2m, xh + 1 Trong 6d:
mc dac trdng dp cdng cua cdt tren nen dan hdi.
Xac dinh chieu dai tiep xuc gida dam khung vdi khdi xay chen.
md
dieu kien: l.md >3 h.mc >3 I - chieu dai dam khung h - chieu cao cdt
m„ = 4 4(EJ)d
Kpxbc
4(EJ)C
bd, bc be rdng dam khung va cdt khung.
K he so nen Winkler cho khdi
xay chen.
(EJ)d, (EJ)c dp cdng cua dam va cdt khung.
Be rdng dai khdi xay tuong duong cd the xac dinh gan dung nhu sau:
W = Id.since + hc.cosa
0 0 0 ao gdc nghieng cua dddng cheo chiu nen trong khdi xay chen vdi phuong nam ngang.
L la chieu dai khdi xay chen.
H la chieu cao khdi xay chen.
Od cdng cua dai khdi xay tddng dddng ddpc xac dinh bang cdng thdc sau:
C = W .t.EKX o
C - dp cdng cua dai khdi xay tdong dddng
t - chieu day khdi xay chen EKX - mddun bien dang cua khdi xay chen.
Mddun bien dang cua khdi xay dpc theo dddng cheo cua khdi xay chen cd khung bao quanh tddng dddng vdi mddun bien dang cua Idc khdi xay khi Ipc nen tac ddng vudng gdc vdi mach vda va mdi quan he the hien nhd sau:
EKX=EO 1
1,1.R
a dng suat trong khdi xay.
R eddng dp chiu nen gidi han trung binh cua khoi xay ddpc xac dinh nhd sau:
R=k.R
k la he sd an toan cua khdi xay ddpc lay nhd sau:
= 2,5 - cho khdi xay chiu nen
bang gach nung.
= 2,0 - cho tat ca cac loai khdi xay chiu nen.
= 2,25 - cho tat ca cac loai khdi xay chiu keo.
R eddng dp chiu nen tinh toan cua khoi xay.
Op cdng C luc nay cd dang:
C=W0.t.Eo{l r 1,1.k.RJ
Ky hieu r\ la he so thay ddi dp cdng cua dai khdi xay chen tddng ddong.
Khi cr = 0 thi n = 1 la Idn nhat.
Khi cr = R thi i"i la nhd nhat.
Op cdng cua dai khdi xay tdong duong ddpc dieu chinh bang he so n thdng qua sd thay ddi ti so dng suat o7R.
Md hinh 2: Khi phan tich ket cau bang cac phan mem chuyen dung SAP, ETAB, STAP... cac tam tddng chen ddpc md ta bang cac phan td tam (sell) cd md dun bien dang tddng Ung
2.2. Anh hudng 6en 6dc trUng 66ng hoc va tdc ddng ddng 6dt thiet ke.
Khdi xay chen lam giam chu ky dao dpng td nhien cua ket cau, do vi tri sd tai trpng ddng dat tac ddng len cdng trinh se bi thay ddi. Sp thay ddi nay ddpc xet tdi khi xac dinh tri so tac ddng ddng dat thiet ke (Phd phan dng ddng da't thiet ke hoac he so trong cac tieu chuan thiet ke khang chan). NhU da biet tri so tac ddng ddng dat thiet ke phu thudc vao chu ky dao ddng co ban cua ket cau, trong trddng hpp nay chu ky cua dao dpng dung de xac dinh tac ddng ddng da't thiet ke ddpc xac
i t
7 2m
:4m 3.9m 3 9m 3.9m 3.9m 4.2m 4.2m 4.2m 39m 39m 3 9m 39m Mat bang cong trinh
92 iSMnTlffif . 3.2009
dinh nhu sau:
Trong do T|b la chu ky dao dpng cua khung khong co khoi xay chen va Th la chu ky dao dpng cua ket ca'u co khoi xay chen cung tham gia chiu tac dpng dpng da't.
Chu ky T„ co the ddpc xac dinh theo mpt trong cac bieu thdc sau:
I (l+T^C^g/(16HW)
Hoac
Ty = min
0,065n
0 , 0 8 0 ( H / V B ) [ H / ( H + B ) ]
0.075H3'4 Trong do:
Aki< - Dien tich tiet dien ngang trung binh cua khoi xay chen d moi tang;
Gkx - Mddun cat cua khoi xay chen;
g - Gia toe trpng trddng;
H - Chieu cao nha (tinh theo met)
B - Be rdng cua nha trong hddng dang xet (tinh theo met);
W - Trpng Idpng nha ddpc xet tdi trong tinh toan khang chan;
n - Sd tang nha
Theo tieu chuan cua mdt so nddc, khi cd khdi Idpng xay chen tat ca cac he qua tac ddng dpng da't xac d|nh d md hinh khdng cd khdi Idpng xay chen ddpc nhan vdi he so sau:
S*j(T|)
Trong dd Sd la phd ddng da't thiet ke. Thdng thddng ddi vdi nha tha'p va cdng X cd the lay bang 1. Ddi vdi cac nha cao va ket cau mem he so A.,
>1, dieu nay cd nghTa la khi cd khdi xay chen he qua tac ddng ddng dat bi tang len.
3. Vi du tinh toan tan so dao dpng rieng nha khung co va khong co tddng chen theo hai mo hinh
Nha khung 2 nhip (7,2m va
2,4m), cd 12 khung ngang, 10 tang cao tdng cdng 39m.
Trddng hop 1 : Khdng ke den sd lam viec cua tddng chen
Trddng hpp 2: Chi cd khung dau hdi cd tddng chen
Trddng hpp 3: Them 6 khung trung gian cd tddng chen.
Vat lieu xay tddng chen: Gach dac dat set nung ep deo mac 75, vda XMmac50.
Tddng chen ddpc md ta theo 2 md hinh: cac dai khdi xay chiu nen va cac phan tdtam.
Ket qua :
Trddng hpp 1: Tan so dao ddng rieng co ban f = 0,576 Hz, chu ky dao ddng T.= 1,7361 s
Trddng hop 2: Chi cd khung dau hoi cd tddng chen
Theo md hinh 1 :Tan so dao ddng rieng co ban f = 0,694 Hz, chu ky dao dpngT,= 1,44s
Theo md hinh 2 :Tan sd dao ddng rieng co ban f, = 0,759 Hz, chu ky dao ddng T = 1,317 s
Trddng hpp 3: Hai khung dau hdi va them 6 khung trung gian cd tddng chen
Theo md hinh 1 :Tan so dao ddng rieng co ban f.= 0,944 Hz, chu ky dao ddngT,= 1,059s
Theo md hinh 2:Tan so dao ddng rieng co ban f.= 0,980 Hz, chu ky dao dpngT,= 1,02s
4. Ket luan
Cac ket qua tinh toan cua vi du tren cho thay:
- Khi ke den sp lam viec cua tddng chen, dp cdng cua cdng trinh tang len rat dang ke. Tan so dao ddng rieng co ban d trddng hop 3 tang 26°o so vdi trddng hpp 2 , tang 70% so vdi trUdng hop 1.
- Viec md hinh hoa tudng chen thanh cac dai khdi xay chiu nen (md hinh 1) hay md ta la cac phan td tam (md hinh 2) khi phan tich ket cau cho ket qua chenh lech khdng qua Idn, chi khoang 10%.
Nhu vay cac ket qua ly thuyet cung vi du tinh toan cho thay anh hudng rd ret cua tddng chen den sd
lam viec cua khung khi chiu tai trpng ddng dat. Dieu nay khi thiet ke cdng trinh khung BTCT cd tddng chen can luu y den. Tuy nhien viec md hinh hoa ket cau tddng chen thanh cac dai khdi xay tddng ddong chiu nen, viec tinh toan dp cdng cua dai khoi xay kha phdc tap de lam ngddi thiet ke bd qua anh hddng tddng chen.
De thuan tien khi thiet ke cd the md hinh hoa tudng chen la cac phan td tam cd md dun dan hdi cua khdi xay ma khdng gap phai sai so qua Idn. •
TAI LIEU THAM KHAO
1. Eurocode 8. Design of Structures for Earth- quake Resistance- Part 1 (BS EN 1998 -1: 2004) 2. Ly Tran Cudng, Su lam viec dong thdi cua khung BTCT vdi khdi xay chen dudi tac dung cua tai trgng ngang, Luan an tien sy khoa hoc ky thuat, 1991.
3. TCXDVN375:2006 - Thie't ke cdng trinh chiu ddng dat. NXB Xay dung. Ha Ndi 2006.
4. Nguyen Le Ninh, fldng dat va thiet ke cdng trinh chiu ddng dat, NXB Xay dung, Ha Ndi 2007.
CWrt¥*3» . 3.2009 93
