KHOA HOC - CONG NGHE
Tinh toan dam be tong cot thep co mo lo nho
• ThS. CAP THI MAI HUONG - TrUdng Dqi hoe Giao thdng van tdi
TOM TAT: Hien nay, viec sd dung be tong cot thep CO 16 m6 da dupc dng dung rong rai d nuoc ta Tuy nhien, vigc tliiet ke chu yeu dpa theo l<inh nghiem ma cac tieu chuan tliieu su huong dan eg the Chinh vl v?y, can c6 nhung ly thuyet nghien cUu, tinh toan cu the ve van de nay Bai b l o trinh bay CO sd ly thuyet, cac phuong phap tinh toan dam be tong cot thep c6 16 m6 nh6 theo cac tieu chuan khac nhau, dong thoi khao sat cac yeu to nhu vi tri, kich thudc cua 16 md anh hudng tai sue khang cua dam be tong cot thep, tu do dua ra cac kien nghj cho viec lua chpn 16 md phu hpp nhat TL/ KHOA: Dam be tong, lo md trong dam.
ABSTRACT: Currently, the use of reinforced concrete with openings has been widely used in our country. However, the design is mainly based on experience that standards lack specific guidance.
Therefore, it is necessary to have specific research and calculation theories on this issue. The paper presentsthetheoretical basis, methods of calculating reinforced concrete beams with small openings according to different standards. At the same time surveying factors such as position, size of openings affect the resistance of reinforced concrete beams, from there make recommendations for choosing the most suitable open hole
KEYWORDS: Concrete beams, openings in beams
I.DATVANDE
Trong cdng nghe xiy dung hien nay ed nhieu ly do de can thiet md Id trong dam, nhU: Dudng dng, dng dan phia dudi dam,tao khdng gian sddung... Dam betdng cdt thep khi md Id se gay sU thay ddi dot ngdt mat eat ngang dam, SUtap trung dng sul't quanh 16 se lam gilm dp edng eua dam va tao ra cac vet ndt quanh 16. Can nghien edu Inh hudng cua Id khi thiet ke dam be tdng cdt thep, dac biet la sde khang eat cita dam kht 16 md gan goi tua.
Trong pham vi bai bao, tac gia dUa ra 3 phuong phap tinh toan eho dam cd Id md nhd. Dd II phuong phap truyen thdng (Tieu chuan ACI), phuong p h i p thuc nghiem AU va phuong phap dan deo, ket hop vdi vi du tinh t o l n de danh gia cu the 3 phuong phap tinh.
2. KHAI NllM CHUNG
2,1. Phan loai 16 md trong dam be tong cot thep Ld ngang trong dam cd nhieu hinh thdc khac nhau.
Cd the cd 16 trdn. vudng, ehd nhat, tam giac hinh thanh 60
hay cle hinh dang khic (Hinh 2.1b). Phan loai 16 md trong dam cd nhieu y kien khic nhau:
Theo tac g i l Mansur va Hasnat [2] cho rang 16 tr6n, vudng hoac gan vudng thl dupc coi la 16 nhd
Theo Somes v l Corley (1974) dUa ra phin loai 16 md nhu sau: Lo md dupe eoi la 16 nhd neu: Chieu eao h hoac dudng kinh nhd hon 0.25 chieu cao tiet dien dam v l be rdng Id khdng vUcrt qua ehieu cao 16 (Hinh 2. la).
^ — ^ " ~ni j>^'*=* 01°^
a) - Phan loai lo nho theo dieu kien hmh hoc
c) - Hinh dang lo Ion tren dam Hinh 2.1: Hinh dang iS md trong dam bi tdng cdt thep Tuy nhien, viee p h i n loai nay chi mang tinh chat tuong ddi. Khi xem xet dam ta nen dda vao sdc khang:
Ld md coi la 16 nhd neu khdng lam I n h hudng nhieu den sU lam viec cua dam.
2.2. Phan tich sU lam viec cDa dam be tong cot thep CO 16 md
2.2.1. Khi ddm be tdng cot thep chiu udn thuan tuy Gpi h^ la khoing each t d mep hi tdng chju nen den mep Id md.
Xet trudng hpp 16 nhd nam trong vijng n^n. Khi do, ehieu cao khdi dng suat nen khdng la a ma thanh h^.
Sdc khang udn «„ = i).m,'i',b[d,-'^)
Nhu vay, khi tinh t o l n vdi dam cd 16 md thudc viJng nen ta hoan toan ed the sd dung cac cong thdc eua dim dae nhung cd ke den su g i l m yeu cua 16 m d l a t nhi&n kht do, sdc khang uon cua dam se nhd hon so vdi ciia dam dac
a) - Dam co lomdd vung bi tdng chju nin
KHOA HOC - CONG NGHE . . So 04/2019
111
• i & - "T = A s f y b) - So do ung suat bien d^ng cua tiet diin ngang qua lomo
Hinh 2.2: Dam co iSm&thude vung nen 2.2.2. Phdn tich sUldm viee eda ddm khi chiu cdt Tai vi tri ed V Idn, M nhd: Tai day dng suit p h i p f nhd, ed the bd qua. Lfng suit keo nghieng mdt gde 45 dp v l dng suat nay bang dng suit eat v vdi gia trj Idn nhat tai true trung hda. Do dd, vet ndt cheo hau nhu nam gan true trung hda.
Su pha hoai theo tiet dien nghieng cd the theo hai dang:
-Vet ndt nghieng ehia dam thanh hai phan ndi vdi nhau bang vung be tdng chju nen va bang edt doc, cdt dai, cdt nghieng xuyen qua khe ndt. Hai phan dam n l y se xoay quanh vung nen, vung nen thu hep lai, cudi cung bi pha huy. Khi dd, cot thep dat gidi han ehiy hay bi keo tudt neo.
- Khi cdt thep eheo k h i nhieu va neo chat thi suquay cua hai phan dam bj can trd. Dam bi p h i hoat kht mien be tdng chju nen bi p h i vd do tac dung ehung eua luc cat va luc ep. Hai phan dam cd xu hudng trUOt len nhau v l tut xudng so vdi gdttUa.
a) - Dang phi ho$i kieu dam b) - Dang pha ho^i kieu khung Hinh 2.3: Cac dang phi hoai dam
3. P H A N TiCH, TfNH T O A N StfC K H A N G CAT D A M BE TONG COT TH^P C6 i d M d N H 6 6 G A N GOI
3.1. Cac phirong phap tinh toan 3.1.1. Phuong phdp truyen thong
Triet ly eCia phuong p h i p truyen thdng, md-men va lUccat dupe tinh t o l n ddc lap. Tai tiet dien ehiu t i e dung ddng thdi md-men va lUc cat thl thiet ke ehiu md-men trude. Cdt dpe ehiu lUe thu duac t d tinh toan sdc khang udn dupc dua vao khi tinh toan edt thep ngang ehiu lUe cat, bdi vi cdt thep nay gian tiep phan phdi lai sde khang eat. Tdng sdc khing cat trong dam gdm hai thanh phan la be tdng v l edt ngang.
* Phd hoai kieu ddm
Dang pha hoai nay. mat eat p h i boat la mat nghieng gdc 45" va dupc ti'nh toan nhU dam khdng ed 16 md.
- Sde khing c i t cCia be tdng:
Khi dam cd 16 md nhd, Mansur (1998) de xuat vi dien tieh be tdng bi thu hep, lam giam sde khang cat cua be tdng. Nen V^ tinh nhU sau:
tfinh 3.1: Mat eit tinh toin phi hoai kieu dam - Sde khang eat edt ngang:
Khi trong dam bo tri e l edt dai, edt xien:
A.fyM - do)
^ Vsv + Vsd - + A^fyaSina Mdt sd quy dtnh:
- Quy dinh khoang each tdi da cdt dai:
Khi 0.5(pV^ < V^ < 3(pV^ thl ^™. = T ^ '•°°'"'"
Khi 3(pV^ < V^ < 5pV^ thi ^-,.. - ; s soomm - Quy dinh ve dam bao dieu kien han ehe n g i n can p h i hoai cat do ehiy deo: V^ < [VJ^^_ = 5cpV^.
* Phd hoai kieu khung
Cae vet ndt n l y ehia dam t h i n h hat nda rieng biet.
Do dd, trong thiet ke, cac thanh phan dupc nghien edu dde lap. De hieu phuong p h i p nay, ta xet bteu do dam cdlomdsau:
a) - Dam va bleu do npl Ipc _ ^ ^ ^ ^ _ (V")i
L-t _ i _ i ^ L T T , I C =(N^)i - ,
b) - Thanh phan npi ipc t^i mat cit l-l qua tam lo Hinh 3.2: Mat eat tinh toin pha hoai kieu khung
M d - m e n u d n M t a i t i e t d i e n t i m 16 t a o ra n g a u lUe {HJ^ va (N Jj^ tai vung nen va keo chia elt bdi 16 md.
(/VJ, M„ -(WJi,
LUc eat V^ tach lam hat thanh phan (VJ^ va {WJ^. Hai thanh phan nay ty le vdi dien tieh mat eat ngang dam.
CK,). = K , - ( f J r
Vdi A|, A^ la dien tieh phan tren va dudi 16.
3.1.2. Phuang phdp gdn dung AU
Day la phuong p h i p ma Vien Kien true Nhat Ban
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KHOA HOC - C O N G NGHE _
(AU) -Ti&u chuan tinh toan ket cau be tdng dua ra 0998).
Theo phuong p h i p nay, mat e l t phS hoai nghi&ng gdc 45° di qua tam cua 16 md. Cdng thdc tinh V^ nhU sau:
11 i_i_LLl 1 ( n n M 1
.-\-^ H^-^)^
0,864VOI ;=- S 3
\ I I h§ sd chiSu cao l l m vi§c tinh t o l n tdi I n h hudng kich thudc ti^t di^n tdi lUc cSt,
f'w- Ty I? c6t th^p byng d i m dat trong khoing cIch d<fc 6/2 tinh t u tim 16 md;
K ' "7J".""' .vdi a l i gdc h9p bdi cdtxienva true dpc;
d - K h o i n g c I c h trpng tSm cot thep doc chiu keo v l n^n.
Trong cdng thdc tren, bi^u thdc dau ti^n la sdc khang e l t eua be tdng. Gi^ tri n l y se g i l m theo ty le vdi chieu cao cua 16 md. Bteu thdc t h d hai I I sdc k h i n g c3t cua cot thep bung dam. Nd duoc de xuat tinh b i n g ty le cot thep p„
nen cd the duoc thay b i n g bi^u thdc: (3 + 0,5p^fJ.
Hinh 3.3: Bdtncdt thip ehju eit quanh iSm6 3.1.3. Phuang phdp ddn deo
Su dich chuyen duac ehdng bdi mdt d i m b i n g sij" ket hop cua cO ch^ g i l n va vdm. Trong trudng hop mdt dam vdi mdt 16 md, rat khd de ph^t trien mdt cp che vdm va t l i trong dupe ehuyen tdi bdi hoat dpng cua dan.Tdhinh dudi eho thay hoat dpng cCra dan dUPc duy tri khi t l i trpng dupe truyen tdi thdng qua dd sau eua mdt dam vdi mpt 16 khoet, duac gia ed nhuhlnh. Mdt cP che dan tdn tai trong mdi eau kien thanh nd dUOc gia ed c l hai chieu dpc va ngang.
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' ' ' " ' M'MMrrrnii)n "i r ^ - i -
bj - Mo Hmh dan deo
Hinh 3.4: Hoat d^ng cua din trong dSm cd loitt&hinh ctiUnbii Gpi 4i; la gdc cua thanh ehiu nen phia tr^n vS phia dudi 16 md. Mui ten ngang ehi Cing suat lien Itet vh mil ten ddng ehi luc trong be tdng do luc cCia cot 3ai, phan khong duoc td dam chi viing ma khdng cd iing suat n^n xien. Lfng suat nen xien trong b^ tdng xung quanh 15 m6 se trd nen Idn hon khi vung nay trd len rang hon hoac 16 md to hon. Chieu cao llm vi^e d ^ eho co che dan fluoc xie dinh la:
C0S<ps Sptan<p^
Vdi s^ la khoang eleh giOfa hai thanh cot dai lien ke hai b§n 16 md.
Dai luong f^"""*"""*'^ la chieu eao theo phiiong ddng cua viing khdng td dam.
Cdt thep dai dUdc bd tri lien ke 16 md v6i khoSng each (^+e)co(^^ vdi e dupe tinh td tam 16 md den true hinh hpc cCia dam.
Cfng suat nen trong be tdng trong vung daoc to d a m : / n . = Pjyvit + C0t^4>s).
^ - Ty le cdt thep chiu cat dat Iten ke 16 md.
a) - So do bd tri cdt thep _f_ T f t T_LfJ.It _lT(r_Lr r . - L J — ' - J -
b) - Mo hinh dan deo
a) - Sodd bd tri cdt thip
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c) - Mo hinh dan gan Id ma Hinh 3.5: Tac dung din deo trong dam cd lo md
KHOA HOC-CONG NGHE. So 04/2019
Khi dng suat nenf^^ dat eudng dd nen tatinh nhUsau:
V I I yeu t d I n h hudng tdi cudng dp nen cua be tdng.
Sdc khang cat cua dam ed 16 md: V^ = bd^^pJ_^cot<j>^
Khi nhdng edt xien dupc gia eudng va dUpc neo nhU Hinh 3.6. Su phan bd lUc e l t trong cdt xien lay bang:
Trong do: 0^-G6c cua thanh cdt xien vdi true dam.
A^ - Dien tieh mat e l t ngang edt xien.
a) - Sa do cdt xien xung quanh lo md
f"]" t f
b) - Tac dijng dan deo Hinh 3.6: Cdt xien trong dam ed lomd
N e u t r o n g d a m c d ca c d t d a i v a c d t x i e n t h i s d e
• k h i n g c a t e u a c d t t h e p la: V^ = V^ -i- V^^
4 . V i D y T f N H T 0 A N
4 . 1 . Tinh toan dam be tong cot thep co Id mar nho T r o n g v i d u nay, m u e di'ch la l a m s a n g t d c l e b U d c
" t i n h t o l n d a m b e t d n g c d t t h e p e d 16 m d n h d va so s a n h eae p h u o n g p h a p t i n h v d i n h a u .
Sd l i e u t i n h t o a n : D a m g i l n d o n e d t i e t d i e n b x h la 3 0 0 X 7 0 0 m m , t a i t r o n g p h a n b d d e u 8 0 K N / m . C h i e u d l i d a m t i n h t o a n 8 m . L d m d h i n h t r d n v d i d u d n g ki'nh d ^ la 2 0 0 m m b d t r i e l e h g d i 8 0 0 m m . V a t l i e u : C U d n g d p n e n b e t d n g r = 3 0 M P a , G i d i h a n c h a y c d t d p c f^ = 4 2 0 M P a , C d t d a i f - 2 8 0 M P a .
I m - -
Hinh 4.1: Sa dd chit dim tinh toan
Yeu eau: Thiet ke edt thep gia cd ehiu e l t cho dam.
Tinh sdc k h i n g e l t eua tiet dien dam tai vi tri 16 md.
Bai giai:
4.1.1. Thiet ke theo phUdng phdp truyen thong - T h i e t ke u d n e h o t i e t d i ^ n g i d a d a m M.
6 4 0 K N m A ' 4 # 1 6 = 7 9 6 m m 2
A 6#25 = 3 0 6 0 m m ^
d 6 2 0 m m - N d i l u c d a m t a i t i e t d i e n 16 m d :
M^ = 2 3 0 . 4 K N . m V ^ - 2 5 6 K N - S d e k h a n g e a t e u a b e t d n g : K =l^b„(d- d.,) = u5icn - T h i e t k e e d t d a i k h u vUe e d I d m d d e n g a n n g d a s U p h a h o a i k i e u d a m .
V,cJn thiet 186 3 KN
Chon cot dai N"10
Chonn 6 thanh
'™.
155mm Chons 100mm
Sufc khdng eJtV„
167KN -Thiet ke edt dat khu vUc cd 16 md de ngan ngda sU p h i hoai kieu khung:
Do trudng hpp nay 16 md ed trpng tam 16 trung vdi true trpng tam dam hay A, - A^^ nen thanh tren va dudi nhu nhau.
' V T iv>),
128KN 200mm 548KN 95 8KN 50mm 40mm - B d t r i e d t x i e n :
Hinh 4.2: Bo tri eot thep - K i e m t r a lai sde k h a n g eat e u a d a m V = (p(V^ + V^) = (p(V^. + V^^ + \ J 0.85(1 is-H67+224.92) = 407KN > V u - 256 KN '
4.1.2. Kiem tra sdc khdng eat cda ddm tai tiet dien Id md theo cdc phuong phdp khdc
* Theo phuang phdpAU Bing tdng hpp ket q u i : K„
0.72 K 0 92
SiJc khang V^
201 KN P.
0.012 Sijrc k h i n g V,
2S7KN V = v + v
458KN ' Theo phuang phdp ddn deo
B a n g t d n g h o p k e t q u a :
0 0 0 1 1 , 321.4mm 214KN , 224.gKN , 4 3 8 9 K N T o n g h p p k e t q u i t i n h sdc k h i n g eat d a m e d Id m d t h e o 3 p h u o n g p h a p :
Ti^u cliuan ACI PhifOng phdp AU Philtfng phap dan d^o 115,0 201 0 0.0 V, (KN) 139 2
224 9 256 9 456 0
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KHOA HOC-CONG NGHE-
(AU) -Tieu ehuan tinh toan ket cau be tdng dua ra (1998).
Theo phuong phap nay, mat elt pha hoai nghieng goe 45° di qua tam cua 16 m d Cdng thdc tinh V„ nhU sau:
H I M I I M t t l 1 I T M
|o.oq2l(ufcp(/'/+17.7)
K^ la he sd ehieu cao lam viec tinh t o l n tdi anh hudng kieh thUdc tiet dien tdi lUe cat.
K„ - 0.82
P^-T^ le cdt thep bung dam dat trong khoing each doc d/2 tinh t d tim 16 md;
_^ ^ -^-("'•°'"'")^yt^l Q1^ g5c hop bdi edt xien va true dpc;
d^ - Khoing eleh trpng tam edt thep dpc chiu keo va nen.
t
Trong cong thdc tren, bieu thdc dau tien la sdc khang , cSt eua be tdng. Gia tn nay se g i l m theo ty le vdi ehieu cao"cda 16 md Bieu thde thd hai la sde khang eat cija cdt thep i' bung dam. Nd duoc de xuat tinh bing t^ le cdt thep p
nen cd the duoc thay bang bieu thdc: [3 -i- 0,5p^fJ.
11 1 l l l l l I " " t t l
7 . I - 1 ~ i 1111 II i m n r r i i i i i , . b) - M6 hinh dan deo
Hinh 3.4: Hoat ddng cda dan trong dam cd lo mdhinh ehi/nhit Gpi (fi^ la gdc cua thanh chiu nen phia tren vl phia dudi Id md. MiJi ten ngang chi Ung suat lien k^t v l mui ten ddng ehi lUe trong be tdng do lUc cija cdt dai, phan khdng dUoc td dam chi vung m l khdng ed dng suit nen xien. L/ng suat nen xien trong be tdng xung quanh 16 mft se trd nen Idn hon kht vung nay trd len rdng hon hoac 16 md to hon. Chieu eao lam viec d ^ eho eo che dan duoc xae dinh la:
do dtw = d„ : - - Sgtanips
cos(p^
Vdi s la khoang eleh gida hai thanh eot dai lien ke hai ben 16 md.
Oai luong !;^'*'^-"""^-^ la ehieu cao theo phuong ddng cua viing khdng td dam.
Cdt thep dai duoc bd tri lien ke 16 md vdi khoang each {^+e)cot(^^ vdi e dupe tinh t d tam 16 md den true hinh hpe eua dam.
Lfng suat nen trong be tdng trong vung dupc to dam: f™ = pjy^i^ + coc^*J.
p, -Ty le cdt thep chiu c i t dat lien ke 16 md.
Hinh 3.3: Bo tri edt thep ehiu cat quanh lomd 3.1.3. Phuang phdp ddn deo
Sudieh chuyen dUPe ehdng bdi mdt dam bang sU ket hop eua CP che gian va vdm. Trong trudng hop mpt dam vdi mdt 16 md, rat khd de phat trien mdt ep ehe vdm va t l i trpng dupc ehuyen tdt bdi hoat dpng eiia dan.Td hinh dUdi cho thay hoat ddng eua dan duoe duy tri khi tai trpng dupc truyen tdi thdng qua dd sau eiia mot dam vdi mpt Id khoet, dupe gia cd nhu hinh. Mot cp che dan tdn tai trong mot caii kien thanh nd dupe gia ed ea hat chieu dpc va ngang.
a) - So da bd tri cdt thep _.nTMI U_LLLL_I—
Tl 111 111 m [ b) - Mo hinh dan deo
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a) - So So bd tn'cdt thep c) - Mo hmh dan gan lo mo
Hinh 3.5: Tie dung dan deo trong dam cd lomd
