QUY CHUAN - TIEU CHUAN
PHAN TICH VA THI^T KE D A M BE T O N G C 6 T THEP CHjU U 6 N TREN TI^T DIEN NGHIENG THEO ACI 318, EUROCODE 2 VA TCVN 5574:2012
TS. PHONG NGOC DUNG Dgi hoc K i l n true Tp. Hb C h i Minh ThS. Le THj T H A N H H A D^i hgc K i l n true Ha Nfii
Tdm tdt: OYig xu- cit cua dam be tdng edt thep (BTCT) vdi vit ndt hinh thinh tren tiit dign nghieng ii hi$n tuang phifc tap. Vigc thiit kS dim chiu Iyc cit trong eic tiiu chuan deu dy-a nhiiu tren kit qua nghiin ciru thye nghiem. Vdi myc dich trinh biy mgt cich ro ring han vi img xie cit dim BTCT eho sinh vien. ky sw, nhi thiit ki, nhiiu nghien cdu ca ban vi ung xO- eOa dim BTCT da duae tong kit. Bii bio niy se gidi thigu mgt trong sd eic tong kit do: so sinh eich thiit ki cot thep dai theo 3 tidu ehuin 5574, ACI vi EC2.
1. Gidi thigu
Trong bdi bdo ndy, dya tren s y t i n g k i t nghien cdu cua nhieu tdc gid, dng x d cua dam BTCT chju uon vd edt se d y g c gidi thigu, sau dd mgt s6 van de thiet ke eg ban cho dam BTCT cd va khdng cd col thep dai thing ddng theo ba tlgu chuan 5574, ACI va EC2 se dugc tong kit. S y giong nhau vd khac nhau v l edch t h i l l ici e l l thep dai cCia ba tigu ehuin ndy cung sd d y g c trinh bdy. Cdc v i dy vd ydu eau ve cau tgo se dygc trinh bdy g bdi bdo tiep theo.
2. SIP lam viec cua dam BTCT chju uon va edt Z1 Sy lim wgc cua dam dan hoi ding chat dang hudng chju uon va cat
Sy Idm vigc cua d i m ddn hoi d i n g chit dang hydng ehju uon vd edt da di^iyc trinh bdy ky trong nhilu tdi ligu [1,5,6,7]. D l , giai thieh s y Idm vigc cua dam lam bang BTCT, vat ligu khong dong chat vd ddng hadng, dng x d eua dam don gidn bang vgt ligu dong c h i t , dang hydng cd kha nang chju kdo, ndn vd c l t deu Idn, chju lai trong phdn b l deu (hinh 1), sd dygc trinh bdy. Neu vat lieu Id ddn hoi, dng suat tiep vd phdp tgi b i t k^ d i l m nao tren m0l tilt di$n ngang (cd Iyc c l t khdc khong) cd t h i d y g c xae djnh t d hai cflng thde cOa sdc ben vgt ligu [1]:
vd
I (2)
trong dd: vvaf\a gid trj dng suit tiep vd dng suit phap tgi diem do; Vvh M la lye c l t vd moment uon tai tiet dign dang xet; Q vd / Id moment tTnh vd momen qudn linh eua tilt dign; y Id khodng edch t d true trung hda ldi diem dang xet dng suat; b„ la be rgng ciJa tiet dign. Phdn bo dng s u i t phdp, f, do md men uon Id tuyen tinh, eon dng suit tiep v cd dgng bgc hai. Tgi vi tri till dign di qua hai phan to 1 vd 2 nha tren hinh 1, gid tri M vd V ddu khd Idn. Phdn to 1 tai vj tri trye tmng hda se chju eat thuan tOy, nha the hign d hinh 1b, vdi gid tn Id Idn nhat so vdi ede vj tri khde trdn eiJng t i l t dien do vd bdng 3/2 Vgv. Vdi Vav Id dng suit t i l p trung binh, cd gia trj bang lye eat chia eho didn tieh till dign ngang Vav = V/bwh, trong dd b„ vd h Id Id chilu rgng va ehilu eao eOa till di#n. Cfng suat ehinh eua phdn to ndy se cd goe nghidng 45" nhu d hinh 1c, gom chi 1 dng suat keo vd 1 dng suat ndn. Phan l l 2 se chju ed hai dng suit phdp vd tilp do M va Q, nha hinh I d . Phaang dng suat ehinh cila phdn to ndy se nghidng mgt gdc a so vdi tr\ie hodnh, nha the hign d hinh I d . Gid tri eiia dng suat ehinh ndy se xdc djnh theo phaang trinh (3) va goe nghieng xdc dinh bdi phaang trinh (4):
-WF
tan(2a) 2v
" f
(3)
(4)
vg
lb. (1)
VI f v d f thay doi dpc theo dam vd theo ehilu eao eua till didn ke t d trye tnjng hda, gdc nghieng cung nha dg Idn eOa dng suat ehinh t eung thay doi. hinh 1f the hign quy dao cua cdc dng suit chinh nay cho dam dan gian chju tai trgng phan bo deu. Cd the thay phygng cac dng suat chinh t i l p tuyen vdi quy dgo cua ehung. Tgi eae thd ngodi cung eua tiet dign giya nhjp, Iyc c l t bdng khdng, dng suit chinh cd phaang tnjng vdi trye hoanh.
T^p chi KHCN Xay dung - so 3/2014
QUY C H U A N - TifeU C H U A N 111111111 n u l l
fli' .>:S'''^ V^^
Hinh 1. Hinh dgng Cmg suit chinh vi hinh chiiu img suit trong dam chO- nhat ding chat chju uon cit [1, 5, 6}
2.2 Dam be tong cot thep khong eo cot ngang chju cat l/ng x d cua dam dan hoi dong chat ed the dp dyng eho dam be long khdng co c l t thdp. Khi tai trgng dage tang Idn, vet ndt keo se hinh thdnh lai vi tri c6 dng s u i t keo Idn nhlt v d Idm d i m syp do nhanh chdng. Neu cot thdp chju kdo dage b l tri trong d i m , dng x d cOa nd se khdc bigt so vdi dam khong cd col thdp chju kdo. Mgc du cdc v l t ndt keo hinh thdnh trong be tdng, cadng dg keo uon yeu c l u du/q/c chiu bdi cot thdp dge, vd dam eo the chiu d\j-ac cdc tai trgng Idn hgn. Lye cat lang ty Ig vdi tdi trgng vd ede dng s u i t keo xien vdi gia tri Idn se xuat hign tgi cde vCing ed lye edt Idn. Cot thep dgc ehju kdo d y g c tinh todn vd bo tri sao eho n6 higu qud trong vige ching lai Iyc kdo gan mgt chiu keo. Nd khong ldm tang kha nang ehju keo cua be tong chong lai dng suat keo xidn x u l l hign d ddu dd, gdy ra bdi ehi lye cat hoac bai s y k i t hgp gida lye edt vd mo men uon. Khi eae dng suit ndy dgt du Idn Idm cde vet ndt keo xien m d rgng theo hudng vuong gdc vdi dng suat keo cyc bg, do la cdc vet ndt xien, khde vdi cdc vet ndt uon thing gdc.
TiSu chi hinh thinh vet nirt xien:
Phaang trinh (3) cho thay dng s u i t kdo xien t the hign dnh hydng k i t hgp gida dng s u i t phdp f vd dng s u i t tilp v. Nhdng dng suat ndy phy lhu0e vdo gid trj cua M vd V. Tgi cdc vj trf tiet dign cd Iyc cat V Idn vd md men u l n M nho, vet ndi do u l n nhd. N l u x u l t hign v l t ndt ndy thi nd se xay ra t n j d c khi hinh thdnh v l t ndt xidn do cat. Cfng suat t i l p trung binh trydc khi hinh thdnh v l t ndi la:
V
~ bd
Vdi d la ehilu cao Idm vige cOa till dign. S y phdn bo ehinh xde cCia dng s u i t t i l p tren todn bg m$t cat t i l p dign la khdng b i l l , khong t h i s d dyng phaang trinh (1) d l xdc djnh vl nd khdng ke den s y c6 mat eua col thdp v d bd long khdng phdi Id vgt ligu ding c h i t ddn hoi. N l u ede dng s u i t do u l n nhd, cdc i>ng s u i t kdo xien, nha trong hinh l b , c, nghieng mgt gdc 45° vd bang dng suat tilp vdi gid tq Idn nhat tgi trye trung hda. Do dd eae vet ndt xien thadng hinh thanh hau het tai hoac gan tn,ic tnjng hoa vd lan tmyen tir vi tri do, nha trdn hinh 2a. Vdt ndt do cat thuan tCiy tai than dam (w^eb-shear crack) cd the xuat hign khi ung suat kdo Xidn t d trye trung hda bang vdi khd nang chju kdo eua bd tdng. Thdng qua thi nghigm ngudi ta xdc djnh dage rang Irong eae viing vdi V Idn vd M nhd, cdc v l t ndt xien do keo hinh thdnh vdi mgt gid trj dng s u i t tiep trung binh hay danh nghTa bang [6]:
^cr = KrKKd) = 0,29^ (6) vdi fc la cadng dg chju ndn dgc trang eOa be
tdng; Ver Id gid tri Iyc cat gay ra v l t ndt xidn. N6i ehung, v l t not ndy thydng it, chu yeu xay ra Igl cdc vj tri gan goi t y a eCia dam eao vdi b l rgng than d i m nho hoac tai vj tri md men doi dau trong d i m lien lye. CTng x d eua dam trd nen khde di khi tiet dien chju md men uon va Iyc eat d i u khd Idn. Tgi cdc vj tri nhy vay, trong m^t dam ma cot thep dge d y g c bo tri hgp \%
cdc vet ndt do lye kdo khi uon hinh thdnh d i u tien.
Chieu rOng vd chieu sdu cua chung d y g c khing chi bdi cot thep dgc. Tuy nhidn, khi dng suat xien do keo tgi phan ben tren cOa cdc vet ndt ndy v y g t qud cudng dp chju kdo eua be long, vet ndt eong theo phuong dadng xidn v d tilp tyc phat t r i l n ve dg Idn eung nhu be rgng (hinh 2b). Cdc vet ndt ndy ggi Id vet ndt do c l t khi u l n (flexure-shear crack) vd thing dyng hon so vdi v l t ndt do cat thuan tuy. Rd rang, tgi thdi dilm ma v l t ndt xidn do kdo hinh thdnh, dng s u i t tiep trung binh se Idn han gid tri eho trong phygng trinh (5) v i v l l ndt do keo san co Idm giam dign tich cua vLing bd long chya bj ndt (vCing ndy dCing d l ching Igi Iyc cat) tdi mgt gia trj nho h a n dign lieh chaa bj nut
"bwd" dung trong phygng trinh (5). Cdc thi nghi§m [6]
cho t h i y vdi s y c6 mgt cua md men uon khd Idn, dng s u i t t i l p danh djnh md tgi dd dng s u i t gay ra v l t ndt xien do kdo hinh thdnh vd phdt trien, trong da so cac tradng hgp, dage xae djnh khd an toan bdng:
(5)
bd
U)
64 T$p chi KHCN Xay dtjrtg - so 3/2014
QUY C H U A N - TIEU C H U A N
^—4"—^-^-
Web-Shear crack Flexure crack (a) Web-shear cracking1;-"^'^+^
Flexure-stiear cr&t* Rexural cracks (6) Rexure-shear craddng Hinh 2. V^( nOt keo xien tmng dim BTCT[6]Hinh 3. Cdn bing Iyc tgi vit niit kio xHn trong dim BTCT khdng ed cit dai ch^ dt [6]
Nhy v$y, dng s u i t t i l p tgo ra cdc v l l ndi xidn phg thu$c vdo t^ so gida Iyc edt vd mo men uon hay cnlnh xdc han Id vao t? so » f tgt vj tr\ gan vdi dinh cua vet ndt do uon. Gid trj dng s u i t tiep tmng binh phy thugc vdo chieu sdu ciJa vet ndt do uon nen nd co I h l xde djnh bdi v = ki(V/bd), trong ki Id hg so phy thuOc vdo ehilu sdu cua vide thdm nhgp cua vet niji uon [5, 6, 7]. Mgt khac, dng s u i t phdp do md men uon co the xde djnh theo f = ka (M/bd), trong dd ka cung phy Ihuflc vdo hinh dgng v l l ndi [5, 6, 7]. Do do, ty" so
— = - ^ — s e anh h y d n g den ldi trpng md dudi tdc / ^ A/
dyng cua nd vet ndi do u l n se phdt trien thdnh vet ndt do uon-cat. dgi l y g n g kilk2 se d y g c xdc djnh bing thye nghigm. Phygng trinh (6) vd (7) dung de xdc djnh dng s u i t gay ra v l t ndt xien trong trudng hgp gid trj r i t Idn vd rat nhd cua \/<HM. Vdi cdc gia trj khdc cua V v d M, dng s u i t gay ra vet ndt xidn se nam trong khoang hai gid trj trdn. Nhieu thi nghigm da dygc thye hi^n d l xdc djnh m i l quan hg gida dng suat gay ra v e j ndt xien vd gia trj M, V. Cfng s u i t tilp danh ^ n h md tgi dd cac v l t ndt xidn do udn-cit phat t r i l n c6 I h l xdc djnh t d :
K, ^.^ IV . . ' 7 p f ^ < o , 2 9 ^ (8) bd ^-^^ M
T^p Chi KHCN Xiy dyng - so 3/2014
trong dd p=As/bd vdi As la dign tich tilt dien c l t thep chju kdo.
lyng xir eua dim BTCT bi ndi xien:
Neu cAi thep dgc d y g c thiet k l hgp I;? thi v l l ndt t h i n g gdc do uon Id vo hai doi vdi sy Idm vige cDa dam. VI d i m BTCT trong Injdng hgp ndy khdng cd cot thdp dai chju c l t ndn ede vdt ndt xidn do lye cat gay ra se quylt djnh den dng x d cua dam hon Id vet ndt thang gdc do u l n . Thdng qua thye nghigm [6], dng x d cua dgng dam ndy cd hai dgng nhu sau:
Dgng 1: Vet ndt xien khi bat ddu hinh thdnh lan t m y l n ngay ldp tdc khi tdi trgng ngodi tdng len mgt lagng nhd. I d vOng e l l thdp chiu keo d i n mgt trdn cua vung bd tdng chju nen vd lam dam phd hogi. Qud trinh nay thadng dot nggt vd khdng cd cdnh bdo ndo trade do va xay ra ehu yeu d eae dam cd ehieu eao till dign khd thap (vi dy nha sdn hay dam ed ly so nhjp/chilu eao till dign > 8). VI khong cd cot thdp dai ehju edt ndn cdc d i m dgng nay r l i d l bj phd hogi gidn neu cd mgt tde dgng dot nggt ndo dd. VI vdy, trong thye l l ngodi ta thydng b6 tri c l t thdp chju cat toi thieu cho dam ke ca khi vi^c tinh todn khdng c i n d i n chdng. Cdc d t thdp dai ndy se hgn c h i vi$c tang v l t ndt xidn va Idng d$ ddo cho dam, t d dd cd t h i x u l t hign cde canh bao t r y d c khi dam bj pha hoai. Trong mgt s6 tradng hgp, nha ban sdn,... khi dng suat tiep thye t l nho hgn rat nhieu so vdi dng suat gdy ndt Vd a phygng trinh (8) thi ngydi ta co t h i bd qua cot thdp chju cdi.
Dgmg 2: V l l ndt xien khi hinh thdnh se lan fruyen ve phfa vung ndn vd den mgt phia dudi eua vung bd long chiu ndn thi ddng lai. Trong tnidng hgp ndy khong co s y phd hogi dpt ng$t ndo xdy ra vd tai trgng phd hogi cd the Idn hgn n h i l u lan so vdi tgi trong gdy ra v l l ndt ddu tidn. l}'ng x d ndy ggp nhilu d cdc dam cd chieu cao tiet dign Idn, vdi ty s6 chieu dai nhjp/chieu cao liet di$n nhd. Day Id loai dam thadng g i p trong thye t l . hinh 3 I h l hidn mgt doan d i m chju tai trgng bat ky, ti-ong do mOt vet ndt xidn da hinh thdnh. Phan bdn trdi vet ndt cua dogn d i m ndy, tai trpng tdc dpng Idn nd co chieu hodng Idn vdi gid lii Vext = Ri - P i . Khi v l l ndt dage hinh thdnh, khdng cd Iyc kdo nao vuSng gdc vdi v l t ndt c6 the d y g c tmyen qua nd, luy nhidn, n l u v l t ndi ndy hgp nd c6 the tmyen Iyc dgc theo b l mat cOa no ttiGng qua moi lidn k i t gida cac vdt ligu trdn b l m§t g6 ghe. Lye lien k i t
65
QUY CHUAN - TI^U C H U A N
V„ cd the duae chia thanh hai thdnh phan V,x va V,y nhu hinh 3a, thadng dage xae dinh thdng qua thi nghigm, c6 gid tri bang khoang 1/3 gid trj t i n g lye edt [5, 6]. Cdc ngi lye ddng khae tgi vet ndt la lye tgi vung be tong chju ndn ehua b] ndt Vcz va lye ngang qua cot thep chju keo Vd, cdn gpi Id Iyc chit. Theo dieu kign edn bang, tong ngi Iyc, Vm = Vcz + Vo + V,y, se bang vdi ngoai Iyc tdc dyng, do vgy phan be tong ehju nen se chiu mpt lugng lye edt Id:
P'^ =I^„,-F^-J^j,(9).Trong cdc dam md ehi ed cot thep dge ehju kdo, kha nang ehju eat cua cot thep dpc, Vd, thadng nho vi Iyc V^ ndy phy thugc vdo chilu ddy cua Idp be tong bao vg cot thdp dpc, dung de ngan can chuyen vj ddng cua nd. Ldp bd tdng bdo vg ndy thadng nho, do do, Iyc Va trong tradng hgp nay eung nho. Thye ra lye V^ nay tao ra trong bd tdng bao ve va bd tSng d phia tren cot thep dpc mgt dp lye kdo ddng, nha d hinh 3b. Do nhdng dng suit nay, cdc vet ndt xien thudng ed xu hadng lan truyen dge theo cot thdp dgc ehju kdo. D i l u ndy lam gidm V^vd lam cho v l t ndt xien m d rgng, do do, Iyc lien k i t Viy cung bj giam di vd thudng dan den phd hogi dgt ngpt cho dam. Xdt mo men xung quanh diem a nhy d hinh 3a, id dilm eat giya C vd Vcz; mo men ngogi lye Mext,a tdc dyng tgi a vd co gid tri bang Me^^a = RiXa - Pi(x3 - Xi) eho trydng hgp tdi trpng tren hinh 3a; m l men npi Iyc bang M|„,a = TbZ + V^p - V,m. d day, p Id hinh chieu ngang cCia vet ndt xidn vd m Id cdnh tay ddn mo men cOa Iyc Vj doi vdi diem a. Vigc sd dung ky hieu Iyc keotrong c l t thep Id Tb thay cho T d l nhan manh rang Iyc trong cot thep nay tdc dyng tai d i l m b, khdng phai tai a. Cdn bang mdment ngi vd ngoai Iyc Mir,i,a = Mejs,a ta cd Iyc keo dpc trye trong cot thdp tgi diem b Id:
Neu v l l ndt xidn phdt triln thi cac lye Vd vd V, nho dl nhieu, ndn iyc keo trdng c l t thdp lai d i l m b co the xdc djnh gan dung theo:
T,=- (11)
Sy phan bo dng suat vd npi Iyc sau khi xuat hign v l t ndt xidn c6 the torn tat nhy sau:
- Tgi tiet dign thang ddng qua diem a, dng suit tiep tmng binh trydc khi xuat hi^n v l t ndt Id Ve^/bv^d.
Sau khi hinh thdnh vet ndt, Iyc cat d y g c chong lai bdi
so k i t hgp cCia lye chot V^, Iyc lidn ket V, vd khd nang ehju cat tren vung tilt dien nhd han ma be tdng chaa bj ndt. VI dng suit keo dge theo cot thdp (hinh 3b) lam v l t ndt m d rgng ndn gia trj Vd va V, gidm dl, va dieu ndy lam lang lye eat vd dng suat tiep tren phan bd tdng cdn Igi chaa bj ndt.
- v l t ndt xien md ta d tren thodng xuat hign phia tren trye trung hda vd cat ngang qua mgt vdi phan cua vung nen trade khi nd bj ddng lai bdi dng suit nen, Nha vay, lye nen C eung tae dyng Idn mgt dign tich nhd han phan dign tieh trade khi xuat hign v l t not, do do, vide hinh thanh vet ndt xien eung lam tang i>ng sudt ndn trong phdn be tdng chya bi ndt.
- Trydc khi hinh thdnh v l t ndt xidn, lye keo trong cot thep tai diem b d y g c gay ra va ty Ig thugn vol gia trj mo men uon chinh tgi till dign di qua b. Vi cd sy hinh thanh vet ndt *xien, t d phaang trinh (11) ta thay lye keo tai diem b tdng thdm vd l^ Ig thugn vdi mS men tai tiet didn di qua i3ilm a. Vi m6 men tai tiet dign qua d i l m a thodng Idn han tai tilt dign qua diem h nen ro rang rdng vigc hinh thanh vet ndt xidn Idm tang dgt ngpt dng suit kdo trong cot thdp tgi b.
- N l u ed hai vgt l i g u ' d i u co kha ndng t i l p nhdn cae dng suat tang lgn nay, can bdng se t y thiet igp sau khi co sy phan phoi Igi npi Iyc va tai trpng 16n hgn ed the lac dyng len dam trade khi no bj phd hoai.
Sy phd hogi eo the xdy ra theo mgt so tinh hulng khdc nhau. N l u cot thdp tai t i l t didn b duac thiet ke chi du tai tilt dign dd thi vigc tang Iyc kdo Igl b do hinh'thanh v l t ndt xien se gdy ra s o ehay ddo tgi cot thdp d b do md men Idn hon d a tde dyng, nhy m l ta d tren, vi vgy gdy ra s y phd hogi cCia dam n l u cot thep bi kdo ddt. N i o dam doge thilt ke hgp IJ fie tranh xdy ra van d l nay thi thong thodng bd.tong tai dinh vet not se pha hoai do bj dp v d . Be tdng d vijng ndy ehju d i n g thdi dng s u i t do ndn vd do eat vd i>ng suit k i t hgp ndy thodng gay ra sy phd hoai sdm hon la mgt trong hai dng suat nay tae dyng ridng re. Cuoi eung, neu co s y ldch dge cot thep ehju kdo, nd se lam gidm Iyc dinh k i t giya cot thep vd be long ldi mgt mde md col thdp se bj kdo lugt khdi be t6ng. Dieu ndy cung cd the gdy ra s y phd hogi eua dam hogc n6 d i n g thdi vdi vide be tong bj dp v d .
2.3 Sy iam viec cua dam be tong cot thep cd cot tfiep dai thang ddng chju cat
Sy ed m0t cua c l t thdp dai khflng c6 dnh hodng dang ke nao taFdc khi hinh thdnh cdc vet ndt xidn.
Sau khi v l l ndi xien hinh thdnh, c l t thep dai Idm tang
Tap chi KHCN Xay dyng - s6 3/2014
QUY C H U A N - T i e u C H U A N
khd nang chju c l t eCia dam BTCT theo bon edeh khde nhau [5,6,7]:
- Mpt p h i n cua Iyc edt d u g c chong Igi thdng qua cdc thanh thep di ngang qua mpt v l t not ey the. C o ehe cua khd ndng chju cat lang them ndy dage t h i hign ky han nho dodi ddy;
- Sy cd mgt cOa cdc thanh thdp ndy han ehe sy phdt trien cua cdc v l t ndt xien vd Idm gidm sy tham nhgp cOa nd vdo vCing ndn. Dieu nay tao ra nhieu bd long chya bj ndi han tgi dau vet ndt eho vige ching Igi dng suit ket hgp ci}a dng suat kdo va nen;
- c l t thdp dai cung Idm eho cdc vet not khd m d rgng hgn vd nhy vgy hai m§l cua mgi vet ndt v i n tiep xuc vdi nhau. Dieu ndy ldm tang len ddng ke gid trj Iyc lidn k i t Vc
- Cot thdp dpc thydng d y g c bo tri d goe eua c l t thdp dai, do v^y c l t thdp dai se kdo cot thdp dgc lidn k i t chgl vdi viing bd tdng hgn Che phia trong. D i l u ndy cung hgn c h i p h i n ndo vigc ldch cOa be long dpc theo c l t thdp dpc ehju kdo va do do lam tdng khd ndng chju cat cCia Iyc chot col thdp dge.
Nhy vgy, sy phd hogi do lye edt se xay ra khi c l t thep dai b i t d i u chdy ddo. Dieu ndy khong ch! Idm g i ^ kha nang ehju Iyc eua chfnh c6l thdp dai md no cung Idm cho cdc v l l ndt m d rgng hgn va Idm gidm kha nang chju Iyc cdi.
Vi cot thdp dai khdng hidu qua trong cdc d i m khdng ndt ndn dO Idn cCia Iyc cat hogc dng suat tilp md gay ra ndt Id g i l n g nhy dam khong cd cot thdp dai vd CO the xdc ^ n h g i n dijng theo cong thde (8). Cae Iyc tdc dyng Idn m$l p h i n dam co cot thdp dai ddng gida v l l ndt vd goi tya doge the hign d hinh 5.
Chung g i n giong nho d hinh 3, eh! khde d chS mSi cot thdp dai edt qua vet ndt tgo ra mgt Iyc Avfv trong phan d i m dang xdt. 0 diy Ay Id dign tieh t i l l dign ngang cda todn b$ cdc nhdnh dai eua mgi col thdp dai vd fy Id dng s u i t kdo trong col thdp dai. Cdn b i n g theo phygng thang ddng: Vaxi = Vcz + V j + V]y + V5(12); trong dd Vg = n/Kjv Id Iyc thang ddng Irong cdc col thdp dai, n Id s6 lygng c l t thep dai cat qua v l l ndt. Neu s id khodng cdeh gida cdc cot thdp dai vd p Id hinh c h i l u cua v l t ndt nghieng thi n = p/s.
Sy phdn b l gan dung eua bon thdnh phan Iyc cdi nOi iyc (ve phdi cOa phygng trinh (12) vdi sy lang dan cQa Iyc c l t ben ngodi Vext doge t h i hign theo sg
d l nho hinh 4. Cd the thay, sau khi hinh thanh vet ndi xign, phan Iyc edt V^ = nAyfv chju bdi cac cot thep dai tang tuyen tfnh vdi viec tai trgng tang, cdc thdnh phdn V M + V,y + Vd gdn nho Id khdng doi. Khi cdc eot thep dai chdy deo, s y ddng gdp eua ehOng Id khong doi lai gid tri chay deo V5 = nAi,fy,, trong do fyi la cudng dp chay ddo cua cot thep dai. Tuy nhidn do eo sy m d rgng eua v l t ndt xien vd vigc tach eot thep dpc khdi be tdng nen V,y vd Vd giam dl rat nhanh. D i l u lam eho vCing be tdng chju ndn se chju dng s u i t Idn do Iyc cat vd uon vd dam se d l ddng phd hogi. Trong khi khd nang ehju Iyc cat long cgng eua cot thep dai da b i l l , dp Idn rigng Id eua ba thdnh phan cdn Igi trong eong thde (12) la chaa bilt. Rat it ede thi nghigm vd nghien cdu ly thuyet cho den ngdy hom nay eo the xdc ^ n h gid trj cCia ba thdnh phan nay [5,6,7]. Ngodi ta thadng d y g c gia sy mgt each an todn rdng Irudc khi dam eo cot thdp dai phd hogi long cua ba thdnh phan ndy bdng Iyc eat gay ndt bd tdng Vcr, xdc djnh theo phygng trinh (8). Tong ndy thudng dygc coi mpt cdeh g i n dung Id ddng gdp cua bd t6ng tdi khd nang chju eat long cgng vd dygc d i l n ta b i n g Vc. Do do V^
= V „ vd Vc = V<a + Viy + Vd (12).
Hinh 4. Sy-phin tio cua cic luc cit ndi lye trong dam voi cit dai dung [6]
1 1 1 1 1 1 1 1 H ' l - I i J - l l l l l - f V, 1 1 1 L < "1
i bi::lS
1 c
l l
z
Hinh 5. Cin bing lye tgi vit niH kio xiin tmng dim BTCT CO cot dai ddng chiu cit [6]
So lygng col thdp dai n vdi khodng edch s phy thuije vdo chieu ddi hinh c h i l u p cua vet ndt xidn.
C h i l u dai ndy phu thupc vdo gdc nghidng eua vet ndt xidn. N6 co the d y g c gia s d bdng ehieu cao lam vide
T^p chi KHCN Xay dyng - so 3/2014
67
QUY C H U A N - TIEU C H U A N
cua dam, d [6]: vi vay n=d/s, co nghia la goe nghieng con gpi Id goe nghieng cua phan be B n g chju nen ciia v6t nilt nho hon 45°. Nhu' vay, tai trang thai gloi ,|.(,„g ^g| ,|gj, ^|^j,jj, ,i.gj^ ligp ^ugp m$t t h i i t toi vifc han ve cuimg do, kh\ V „ = V„, phu-ong trinh (12) va ^^^ g.^^^ ^^. . ^ ^^^ , j , j „ ^ p ^ j n cua dam khi
it H ' l n h rlinh r-i la H a m ' . " ' ' . i (12) cho ta cadng dg chju eat danh djnh cua dam
Kfy,^
(13); trong do V^ doge lay bang lye gdy ndt do eat cua dam chju uon va cat, theo cdng thde:. . ^ - ^ = 0 , 1 6 ^ ^ - 1 7 ^ ^
= 0 , 1 6 J / + 1 7 . ^ ^ < 0 , 2 9 7 Z (14) bd M 3. Cdc each tiep can d l t h i l t ke cot thep dai chju eat cho dam chju uon theo ACI 318, EC2 va TCVNVigc thiet ke d i m chju cat cua EC2, ACI 318-08 vd TCVN : 2012 d i u dya trdn nhieu ket qua phan tich va thi nghigm. Chung dya tren tieu ehi ve sy hinh thdnh vet ndt xidn do lye edt va md men gay ra. Gid tri gdc nghidng cua vet ndt ndy doge quy djnh khdc nhau trong ede tieu chuan. Vdi ACI, goe nghieng 9 cOa v l l ndt xien dage gid thiet thien v l an todn vdi gid trj g i n bdng 45°, tuc Id hinh chieu eua tilt dign nghidng len phaang ndm ngang bang ehieu cao lam vige eCia d i m d. Vdi EC2, gid tri gdc nghidng ndy doge thay doi t d 22° eho den 45°, phy thupc vdo dieu kign khdc nhau cCia d i m . Vigc gid thilt goe nghieng cua v l l ndt hay
ACI (dan vj theo psl, ldp, in): V^
chju ddng thdi u l n vd cat. Ddi vdi tieu ehuin TCVN, vige gia thiet gdc nghieng nay taang doang vdi vide xdc dinh chilu ddi eua hinh c h i l u tiet dign nghieng nguy hiem nhlt CQ. S y khde bigt ea ban gida TCVN vdi hai tieu ehuin tren la goe nghieng 9 trong TCVN d y g c xde cfinh thong qua vigc eye tieu hda hdm kha nang chju edt eua dam tren cae till dign nghieng khac nhau, edn hai tieu chuan edn lai thi thong qua ttii nghigm de de x u l l gid trj gdc nghidng ndy. Khi tinh toan kiem tra hogc t h i l l kd eau kign ehju uon-cat, ba tieu chuan deu ehia chiing thanh hai trydng hgp: dam BTCT c6 vd khdng c6 c l t thdp dai ehju c l t . 3.1 Dam BTCT khong cd cot thep dai chju cat
Kha nang chju cat cua d i m phy thugc hodn todn kha nang chju eat cOa bd tong vung nen, vdo lye clt lien k i t gida hai mgt eua vet ndt va vao lye chit ciia c l t thep dge nha da trinh bdy d cdc phan d trdn. 06i vdi ACI vd EC2, thi tong cgng cua ba thdnh phin trdn doge quy doi thdnh kha nang ehju cat cua be tdng V,.
(ACI) va VRd,c {EC2). Gid trj ndy doge xdc djnh theo cdng thde (16) va (17).
(15)
EC2 (dgn vj theo MPa, N, mm): F, trong do: fc hay fcn Id cadng dp chju ndn dge trang eua be tong; d la chieu eao lam vigc cCia tilt dign, b„ la chieu rgng nhd nhat eua ddm; p =p, = Asi/(bwd) < 0,02 Id hdm logng cua cot thep chju kdo;
Asi la dign tieh tiet dign ngang cua c6l thep chju kdo;
Mu vd Vu Id md men u l n va Iyc c l t tde dyng tgi till dign dang xet; A = 1+ I < 2,0 la hg so thye nghigm; YC la hg so an todn cOa vat Hgu be tdng thudng lay bang 1,5.
\0,l6yfZ + l'i^^\bJ'^0,ny[/Xd<0,29yfZbJ bj[{0,Wy^)k{l00pjJ"~^>bj[{0,035)lc'"f^'] (16)
dan vj theo MPa, N, mm: Q ^'Pb4{^ + 'Pn)Kbf'l
Dieu kign k i l m tra hoac thiet ke kha nang ehju c^t cho tilt dign theo ACI vd EC2 Id:
ACI (dan vj theo psi, kip. In): V,, < (pV^ (17) EC2 (dan vj theo MPa, N, mm): V^ < V^^^ (18) Irong do ^ Id hd s l gidm dp ben, lay bang 0.86;
Vu, Vsd la Iyc cat tdc dyng tai tiet dign dang xet.
Doi vdi TCVN, cae thdnh phan lao ndn khd nang chju cat eua be tdng khong d y g c trinh bay cy the trong tidu chuan, cdng thde k i l m tra khd ndng chju eat tren tilt dif n nghieng nguy hiem nhlt Id:
^%A^+%)RM_
vi'i^'^O + ' P j ^ A ^
(19)trong do: (pM = 1 cho be long nang; <pn = 0 doi vdi dam; epos = 0,6 doi vdi be tdng ngng; Q Id Iyc edt tdc dung tgi tiet dign dd; R^ Id cadng dp chju kdo tinh toan cCia be tong; b la chieu rpng va ho la chieu cao Idm vige.
T d cac cong thde (16) den (20), ta nhan thay rang ed nhang sy khdc bigt nhat djnh giya cdc tieu c h u i n ve vigc tinh loan kiem tra khd nang chiu edt cua dam BTCT khdng cd cot thdp dai.
68
- Theo ACI va EC2, kha ndng chju cat cua be tong phy thupc vao ba thanh phan (1) kha nang chju cat cua be tong vCing ndn; (2) Iyc dinh k i t giya cdc vgt ligu giya hai mgt cua v l t ndt vd (3) kha nang chju cat eua cot thep dpc hay cdn ggi la Iyc chot. Trong khi d6, cdc thanh phan tgo ndn kha nang chju eat eCia be long khong d y g c trinh bdy ky trong TCVN.
- Theo ACI vd EC2, khd ndng chju cat cOa be tong '.
ty' Ig vdi can bdc hai hoac bac ba c o d n g dO chju ndn T9P chi KHCN Xiy dyng - s o 3/2014
QUYCHUAN - TI§U CHUAN
dge tmng cua bg tdng, trong khi do theo TCVN, gid trj ndy ty Ig vdi codng dg ehju kdo tinh toan eua bd long (bang codng dg chju kdo dac tnj'ng ehia cho hg so an todn eua vgt ligu). Hg s6 an todn cua vgt ligu doge dya vdo hg s6 an todn long t h i trong ACI (thdng qua
^ d eong thde (18)), edn trong EC2 hg s6 an todn ndy doge daa vdo ngay t d khi linh codng dp chju cat cua be tdng (thdng qua yc d cdng thde (17)).
- Ca ACI vd EC2 ddu tinh d i n dnh hodng cOa col thdp dpc d i n khd nang ehju cat cCia be tdng thdng qua hg s6 k vd hdm logng c l t thdp dpc trong ede cdng thde (16) hoge (17). Trong khi do, TCVN khong d l cgp d i n v i n d l nay.
- ACI cho phdp tinh d i n sy ket hgp gida mo men vd lye eat Mu vd Vu khi xae djnh kha nang chju cat cOa bd tdng (cong thde (16), trong khi dd EC2 vd TCVN khdng de cdp den logng tdc ndy. Tuy nhien, ACI cung daa ra gidi hgn dodi eho khd ndng ehju cat eua bd tdng ma khdng tfnh d i n gid trj Mu vd Vu (gid tq x I p xi theo cong thde (16)).
3.2.Dim BTCT c6 cot thep dai thing dung chju cat Khdc bigt Idn nhat gida cdc tidu chuan khi tinh todn col thdp dai eho dam Id ACI vd TCVN dya trdn cdc eong thde thye nghigm, trong khi dd, EC2 dya trdn ea cde cdng thde thye nghigm vd md hinh tinh todn ly thuylt gidn toang doang vdi gdc nghieng eua v l l ndt thay doi d l xde djnh ngi loc trong cdc thdnh phan chju Iyc c l t .
3.2.1. Theo ACI
Gdc nghidng cua v l t ndi do cat gdy ra trong d i m nghidng vdi tn,ic d i m khodng 45°, thye c h i t ACI l l y mgt cdeh g i n diing rdng hinh chieu cua v l t ndt gdy ra trong d i m do c l t Idn tn,jc hodnh b i n g vdi chilu eao Idm vigc cda t i l t didn. Thye nghidm eho thay r i n g , gid thilt ndy Id r i t thidn ve an todn trong hau h i t cdc tnfdng hgp d i m Bernoulli [5,6]. Dam thiet ke chju edt phdi thoa man d i l u kign: f „ <<^V„ (20). vdi V^ Id khd
nang chju cat ciHa tiet dign can tfnh c l t thdp dai, Vn = Vc + Vs. tdc la bdng kha nang ehju edt cCia bd long vd eot thep dai epng Igl; Vu Id long Iyc cat tinh todn tai t i l t dign dang xdt. Vdi gid thiet vet ndt g i n 45° nho tren va ehi xdt c6l thep dai thing gdc vdi trye dam, dieu kign cadng do (20) doge viet lai nho sau:
K ^^K , M/,^
(21)Kha nang chju eat cua bd long V^ vdn dwiyc tinh todn nho d cdng thde (16) vdi dieu kign hgn e h i Id Vud < Mu, de dam bdo rdng v l t ndt xien se doge hinh thanh, v d ^ < 0 , 7 M P f l , de the hign kha nang chju eat toi da ciia be tdng eh! dgt d i n 8 MPa trong mpi tradng hgp.
Ngodi ra de dam bao rang be tdng khdng bj ep v d tren ddi nghieng theo v l t ndt, dng s u i t cat trung binh X V i—r tren tiet dign phai thda man v = —^^ < 0 , 0 7 y / . (22).
Neu dieu kign (22) khong doge thda mdn, chung fa phdi tang kich thydc tilt di§n hoac tang cydng dO chju ndn dgc tryng cua bd tdng. Vide thiet ke chju cat theo ACI khd dan gian, thong thydng chiing ta se tinh khd ndng chju cat cOa be tong, gid t h i l l dudng kfnh col thdp dai theo cot thdp dgc trong dam roi I d do xdc djnh khodng edeh cot thdp dai phy thugc vdo gid trj loc cat tinh todn theo phoang trinh (22).
3.2.2.Theo TCVN 5574:2012
Dieu kign k i l m tra dau tien khi t h i l l ke cot thdp dai eho d i m BTCT ehju c l t Id kiem tra kha nang ehju lye tren ddi nghieng gida cdc v l t ndi:
Q<0,3<p„^<p^^R^bh„i23)\ trong dd (fwi = 1 + 5aM„< 1,3 vdi a - Es/Eb, fiw = AsJ(bs); tpbi = 1 - 0,1 Rt, cho be long ndng, Rt Id cudng dg ehju ndn tinh todn eua bd long, theo MPa; ho Id ehieu eao Idm vlec eiJa tiet dign.
Neu d i l u kign (23) khdng doge dam bdo, chung ta phai tdng kfch Ihode tiet dign hogc tang e l p dp b i n eua be tdng chiu ndn. D i l u kign kiem tra cudng dg d l i vdi dam chju cat Id Q<Q,+ Q„ (24). Qb Id kha nang chju edt cua bd tdng, xdc djnh theo cdng thde:
(25)
trong dd: ^2-"2- vdi bd tdng n^ng; ^ Id h§ s6 xdt d i n dnh hydng cua cdnh chju ndn trong tilt di$n chd T vd chd I, nd b i n g 0 vdi t i l t di$n c h d nhgt; (pn = 0 doi vdi dam vl Iyc dgc nho; RM Id c y d n g dO chju kdo tfnh todn cua bd tdng. Q^ = " - e l d kha nang chju c l t
cua cac cot thdp dai md m^t edt nghieng cua vet ndt eat qua, vdi Rg^ md codng dp chju cat tinh todn cua cot thdp dai vd Asw Id di^n tfch tilt dl^n ngang ciia 1 dai. Tieu chuan TCVN khdng de cdp ldi gdc nghieng ciia vet ndt nay md dya tren vigc coc tieu hda ve phdi cua phygng trinh (24), de t d do di tim tilt dign
T^p chi KHCN Xiy dyng - s6 3/2014
QUY CHUAN - TI^U CHUAN
nghieng nguy h i l m nhat CQ. D I dang nhgn doge m2{^+9.+<P/)KbhoS .
R^A^ - I d vigc xdc djnh dQsv/dc = 0 Do do khd nang chiu cat toi thieu eua eot thep dai trdn tilt dign nghieng nguy hiem nhlt Id:
e„=:?=4».,^ (26) s
Thay gid trj Qs„ t d phaang trinh (26), gid trj Co dd tim dage d tren va cdc hg s6 cho be tong ngng, cdng-thdc kilm tra hoac thilt ke c l t thdp dai se trd thdnh:
Q<JsR,,h^b "" ' " (27) [11]. Phaang trinh (27) dung d l t h i l l ke cot thdp dai chju Iyc eat.
3.2.3. TheoEC2
Vdi edeh tiep can hodn todn khdc, EC2 d y a tren mo hinh gidn do vdi phoang phdp gdc nghidng cOa phan be long ehju nen thay d l i d l thiet k l cot thdp dai chju edt eho dam BTCT. De xde ifinh eae phyang trinh linh todn, tde dpng ciia mgt dam BTCT khi ehju eat doge dgi dign bdi mgt hg gidn do taang doang nho tren hinh 6. Be tdng tdc dyng n h d thanh chju nen d dinh va cdc thanh chju ndn xien nghidng mgt goe e so vol trye hoanh. Thanh d ddy gidn do la col thep kdo nam ngang vd cdc thanh c l t thdp dai se tao thdnh cac thdnh phan thang ddng chju kdo cOa gidn.
Hinh 6. Sg-phan bi cOa cic Iyc dt ngi iyc trong dam vai e6t dai dung [7]
Can nhin mgnh rdng, mo hinh gidn do t h i hidn a. Dai be tdng chiu nen va gdc nghieng 6 dng x d ehju cat i l l ed cdc Iyc cat se doge chju bdi
cde cot thep dai thang ddng md khdng eo sy tham gia Lye cdi tac dyng Idn tiet dign phai d y g c han ehe cua be long.' Gdc nghieng 6 tang vdi dp Idn eua Iyc sao cho cdc dng suat ndn qud Idn khdng xay ra trong eat Idn nhlt trong dam vd vl vgy vdi ede lye nen trong thanh gidn chju nen dan den s y pha hogi cila be long cdc thanh gidn xidn. EC2 quy djnh rang gdc nghieng chju nen. Vi vay Iyc e l l t h i l t k l Idn n h l t VRd.n,ax di^oc ndy thay doi t d 22 d i n 45 . Trong hau h i t cdc trudng
hgp ehju tdi trgng phdn b l deu thi goe nghieng ndy Id 22° nhang doi vdi cdc tai trgng tap trung Idn nd cd t h i idn hdn d l chong Igi vide dp v d be tdng ciia thdnh phan xien ndy. Vide phdn tfch ngi Iyc theo mo hinh gidn ndy se dygc thye hign theo each thde sau:
hgn c h i bdi cadng dp chju nen ciia thanh gidn xien vd thdnh p h i n ddng cCia no trong mo hinh gidn trdn.
Dya vao hinh 6, di$n tfch t i l t didn ngang hi^u qud cOa bd tong tde dyng nha mOt thanh gidn xien chju ndn d y g c xae djnh bdi bwXZCosG vd dng suat chju nen - Gid thiet cadng dg chju ndn ciia thanh gidn xidn thilt ke ciia be long fed = fck / YC = U /I-5. Ta co:
be tdng vd goe nghieng 9 eua no; Codng dg l6i da cua thanh dang chju ndn = Ong suit
• '.h^'^K * ° ' " . . ' ° * ^^^^ ' ^ ' ' ' ^ * ^ ^ ' ' ^ ^ ^ ^ ^ ' ' ^ ° "•^^'^ ^^ ' * " " h i t X dien tfch till di^n ngang = (fcv /1.5) cac thanh gian thang dang; s^ y a \CK . ; - Tinh t o i n c6t thep chju keo phu thSm A,, cho " (l'">'^<^°='*) " ^ * * ' " ^ PhS" d i n g cQa nd = [ ( U /1,5) thanh gian o phia du-ii. " (b,xzcose)] x sine.
Vay ^«,.,„ =(/„6.zcosesin9)/l,5= / . t i . z l,5(cote + tane)
Theo EC2 [8], phtiong trinh nay diip'c thay doi b4i viec thSm vao he so v , k l ai cu'6'ng dO b8 tong bi giSm xuong khi niM do cat, ngn F-, / , | t . Z V |
l,5(cot9 + tane) . V4i v, = 0,6 (1 - fck/250) va thay the 2 = 0,9d ta co:
T9P chi KHCN XSy dimg - sS 3/2014
QUY CHUAN - Tieu CHUAN
^0,36b^d{l-f^,/250)fa '"•"'" (cote + tan0) Qk dam bdo thanh gidn xien chju nen khdng bj dp vd, ta c i n k i l m tra d i l u kign Vgd < VRd,niax (29). vdi Veo Id gia trj lye eat ngoai lye tgi till dign can tinh eot thdp dai. Ollu kign ndy can phdi k i l m tra eho gid tri loc edt Idn nhat tren s u i t ehieu ddi dam, thong thudng tai vj trf mep g i i tya, Vsr, nen VEF < VRd.rnax-
Nho da de cap d trdn, EC2 quy dinh gdc nghidng e thay d l i t d 22° den 45°.
- Vdi e = 22° (thadng phu hgp vdi tai trpng phan bo deu):
T d phygng trinh (29) ta cd:
(30)
(28) Neu VR(,,max(22) ^ Vsf thi mpt gia trj Idn hgn ciia 9 ndn dage lya chpn sao cho ddi be tdng ehju ndn xidn cd gid tri thdnh phan ddng cdn bdng vdi VEd-
- Vdi 9 = 45° (gid trj Idn nhat ciia 9 cho phdp bdi EC2):
T d phoang trinh (29) ta ed:
^ * / , ^ ( « ) = 0 . 1 8 b „ ( ; ( l - / ^ / 2 5 0 ) ^ ( 3 2 ) . Day Id gid Iri can tren ciia codng dg thanh gidn xien chju ndn trong md hinh gian toang daang. N l u VRd.max[22) ^ Vsf thi ta phai lang kich thadc tilt dign hoac tdng codng dp chju nen dac trang eua be tong.
„=0,124b^rf(l-X,/250)Xt - Vdi 0 nam gida 22 vd 45 :
Gid tli can thiet ciia 9 cd the xdc djnh t d vide cdn bdng VRO y ^y , 0 , 3 6 M ( l - X . / 2 5 0 ) ^
" ^'•"'°" (cot^-Htan^) (28) nhy sau:
dygc:
VI (cot^-l-tan5'
VEd vd viec xdc djnh 9 I d phoang trinh
= sm5cos0=O.5sin{20)ndn ta ed
= 0,5 sin" <45''hay ^ = 0,5sin" ^45" (31)
\Q,mj{\-f^J25Q)f^
b. Cot thip dai thing ddng
Nho da de cgp, tat cd Iyc cat se doge khdng lai bdi cae c l t thep dai ma khong ed s y ddng gop ndo t d kha ndng chju edt cOa ban thdn be tdng. S d dyng cdc phygng phdp ciia mgt edt ta cd t h i thay tgi mgt cat X-X nho tren hinh 6, ngi lye trong cdc cot thdp dai ddng (Vwd) phai can b i n g vdi lye tdc dyng (VEJ). tde Id:
y..=y^ =s^A„ J-i^J-j^^,i,%if^A„
Neu t i t ed cdc c6t thdp dai durfc dat vdi m^t khodng edeh s, s l lygng dai trdn dogn ehieu ddi di qua khe ndi zeolO Id (zcot9)/s ; do dd Iyc c i t trong m6i cot thdp dai se bj gidm mgt cdeh l y g n g dng va dage xde djnh
2 cot Q A V
bdi F , = V^. = 0,87 LA^ . Neu thay the z = 0.9 d, ta nit ra dage: - ^ = ^ (32) s s 0,78df^^cotd
khoang cdeh roi di xdc djnh kha nang ehju loc edt Id gid trj nhd hon trong hai phoang trinh (29) vd (35) sau do so sdnh vdi gia trj Vgd-
c. Lye dQC tic dyng phy them trong cot thep chju uon Khi s d dyng phygng phdp gidn do d l tfnh todn col thdp dai, ta phai k l them mpt Iyc dgc tdc dyng Idn col thdp ehju kdo gdy ra bdi Vsd. Lye keo dgc AFM ' doge gdy ra bgl cac thdnh phan ngang de cdn bang vdi lye nen trong thanh gidn ndn nghidng. Giai Igi cae Iyc ngang trong mat c l t Y-Y nhy trdn hinh 6, thdnh p h i n Iyc dgc trong thanh gidn ehju ndn Id: Lye dgc = (VEd/sine) X cos9 = Vga xcot9. Neu gia sd rdng, mgt nda Iyc nay d y g c chju bdi cot thdp trong vung keo Phogng trinh (34) dung de bo trf eot thep dai vd
se phy IhuOc vdo gid trj gdc nghidng 9 s d dyng trong qua b-inh tinh toan. Doi vdi hau h i t cae taro-ng hgp vdi cdc ldi trgng phdn b6 deu gdc nghieng 9 se bang 22° vd cot9 = 2,5. N l u khdng thi gid tq 9 se doge xde djnh I d phygng trinh (33). Phogng trinh (34) cd the doge sap xep Igi de xdc ^ n h khd ndng chju Iyc ciia d t thdp dai VR(|,S d l i vdi mOl hinh dgng c l t thdp dai nhat cnnh AsJs.
Ta c6 V^^ =^0,7Sdf^ c o t ^ (33) Thdng thodng, vigc t h i l l ke c6l thep dai thodng
dygc thye hign theo phygng phdp t h d vd sal, ed nghTa Id chung la lya chpn hinh dgng cot thdp dai,
T^p chi KHCN Xay dung - so 3/2014 71
QUY CHUAN - TI^U C H U A N
cCia dam vd dieac xdc djnh theo: AFw = 0,5 VEd xcol9 (34).
De chju doge Iyc dpc phu them ndy, tgi b i t ky tilt dign ndo, ta can phdi cung cap them mgt logng col thep dpc bo sung eho cot thep dpc dage thiet ke chju uon thong thodng. Trong thoc t l , vigc tang chilu dai c l t thep cua c l t thep dpc ehju keo phia dodi thadng dam bdo dieu kign ydu eau neu d tren.
3.2.4. Nhan xdt
Sy khde nhau gida vigc thiet ke eot thep dai thing ddng trong cde tieu chuan co t h i dage long hgp nha sau:
- ACI vd TCVN ehii yeu dya tren sy Idm viec thye nghigm d l hinh thdnh cong thde tinh todn edt thdp dai. Cd hai tidu chuan ndy deu ke den sy ddng gop ciia be tong vdo khd nang chju cat tong the ciJa dam.
Trong khi dd EC2 dya trdn cd thye nghigm Idn mo hinh gian do diing de xde djnh ngi loc trong ede thdnh phan eiia d i m . EC2 khong ke den sy dong gdp ciia bd tdng tdi kha nang chiu edt tong the eCia dam khi eo eot thep dai;
- Toang t y nho tradng hgp dam be tdng eot thdp khong eo eot thep dai, vigc thilt ke cot thep dai cua ACI dya trdn thye nghigm vdi v l t ndt nghieng vdi trye hodnh mgt gdc khodng 45°; TCVN thi dya tren tilt- dign nghidng nguy hiem nhat, thdng qua md hinh dai so; EC2 dya trdn thye nghigm vd md hinh gidn do dan gian vdi goe nghieng thay doi t d 22° den 45°, phy IhuOc vdo tdi trgng vd dgng lien k i t cua d i m ;
- Khi t h i l l k l e l l thdp dai TCVN khdng k l d i n sy tygng tdc gida md men uon vd Iyc edt vd s y ed mgt ciia cot thdp dpc; ACI va EC2 thi ke den nd khi tinh todn khd ndng ehju lye eat eua bd tdng;
- Vol cdc eong thde Igp dygc. vige tinh todn cot thdp dai ciia cd ba tieu ehuan deu khd giong nhau, hodc dya tren phoang phap thd va sai hogc dya trdn vigc gia thiet dudng kinh col thep dai, so nhanh cua nd roi di tfnh toan khoang each can thiet de nd chju du Iyc cat ngogi lye tren tiet dign nghieng;
- Vdi vigc sd dyng mo hinh gian do, EC2 cho chiing la h i l u ro han nguyen nhdn tai sao can kdo ddi col thdp dpc chju keo d phia dodi cua c l u kign, vdi mye dich Id dam bdo kha ndng chju Iyc phy thdm trong cot thdp dge ehju keo do Iyc edt gdy ra.
4. K i t lugn
Trong bai bao nay, dng x d cua dam BTCT ehju uon va eat da d y g c trinh bdy dya trdn tong hgp mpt so tdi lieu da xuat ban. Ben canh do, vige t h i l l ke edt
eho d i m BTCT theo ba tieu c h u i n TCVN, ACI yd EC2 eijng da doge gidi thigu. Thdng qua dng x d ciia dim BTCT khi chju uon, eat va vige so sanh ba tieu chuan ndy chiing ta eo the nit ra mpt so k i t lugn sau:
Dam BTCT ed the bj phd hogi trdn tilt dign nghieng do sy k i t hgp gida uon va cat. Vet ndt xidn cd gdc nghidng thay doi, tuy thugc vdo loai tdi trpng tae dyng vd vao lidn k i t cua dam. Ci'ng x d cua dim phy thuge vdo khd nang ch|U ndn trdn till dign nghieng vd vdo kha nang chju cat cua d i m tren tilt dign nghieng do, bao gom b i n thanh phan chinh (1) khd nang chju cat cua vdng be tong chaa bj ndi (Vd);
(2) khd nang chju c l t do lien k i t eac thdnh phan d p p h l i gida hai mat ciia vet not (Viy); (3) kha nang ehju edt ciia cot thep dgc (Vd); (4) khd ndng chju cat cua cot thdp dai (VRd,s).
Tidu ehuan EC2 bo qua s y ddng gdp eiia bd long eho khd nang chju edt cua dam nhyng goe nghieng ciia vet ndt thay d l i tiiy thugc vdo tdi trgng. Ngygc Igl, ACI va TCVN c6 ke den s y ddng gdp ciia bd tdng khi chju edt nhong gdc nghidng eua vet ndt c6 djnh.
Vlec dp dyng ACI la de dang nhat vi cdc dilu khodn tfnh todn rd rang va dan gian. EC2 Id tidu ehuin the hign ro nhat s o Idm vige eua dam khi chju uon vd eat.
T A I L I E U THAM K H A O
1. Lfi NGQC H C N G , "Si>c bin vgt ligu", Nha xuat bin khoa hgc ky thuat, 2006.
2. TCVN 5574:2012, "Kit c l u be tdng vd bd tdng c6 thSp - Tieu chuin tiiiet ke", Nha xuit ban Xiy dyng, 201Z 3. ACI 318 - 08, "Building Code Requirements for
Structural concrete (ACI 318 - 08) and Commentary", American Concrete Institute, 2008.
A. Eurocode2. EN1992-1-1, "Design of concrete slnjctures- Part 1-1: General mles and mies for buildings", 2004.
5. R. PARK and T. PAUIAY, "Reinforced Concrete Structures", A WSey-tnterscience Publication, Jofm Wiey and Sons, NewYork, London, Sydney and Toronto, 1974.
6. ATHUR [,. NILSON, DAVID DARWIN, CHARLES W/.DOLAN, "Design of Concrete Stmctures". Mc Graw Hill Higher Education, 14"'edition, 2010.
7. R. S. NARAYANAN and A. BEEBY, "Deagner's Guide to EN1992-1-1 and EN1992-1-2 Eurocode 2: IDesign of concrete stmctures. General mles and mles for buiMings and structural fire deagn" Thomas Tegord Publication, 2005.
8. PHAN QUANG MINH. NGO THt PHONG, NGUYEN DlNH C O N G , "Kit c l u be tdng c l t thdp - Phdn Clu kign co bdn", Nhi xuat bin khoa hgc ky thu$t. 2006.
Ngay nhan bai su-a: 2/9/2014.
72 T^p chi KHCN Xay dyng - s6 3/2014
