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HNG DN THIT K CNG TRONG N TT NGHIP May 14, 2014
T u n g K e n - 0 9 7 4 2 7 7 9 8 6 t u a n t u n g v u @ g m a i l . c o m
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HNG DN THIT K CNG
TRONG N TT NGHIP
Ti liu c tng hp t nhiu ngun ti liu khc nhau v mt cht xu hiu
bit ca bn thn nhm gip cc bn sinh vin trong qu trnh lm n tt
nghip. Mnh hc H GTVT nn hng dn cng theo trnh t n ca
trng GT. Nu c nhm ln hi vng cc bn iu chnh v sa i gip. Hi
vng l khng
Trc khi vo thit k, cn tm hiu qua cht v cng c th hnh dung ra cu to
ca cng. Hiu c cu to cng th bc vo thit k s d dng hn rt nhiu.
Hn na hiu cu to ca cng th vic tr li cc cu hi tt nghip cng kh ngon.
I KHI NIM V CNG
- Cng l mt cng trnh nhn to kh ph bin trn mt tuyn ng. Tc dng ch
yu ca cng l dng thot nc ca cc dng chy thng xuyn hay nh k
chy qua pha di nn p, ngoi ra cng cn lm ng chui dn sinh.
- Khu cng l chiu rng ln nht ca tit din thot nc. Trng hp cng c
nhiu l th khu c tnh bng tng s khu ca mi l.
- S lng cc cng trnh thot nc trn tuyn ph thuc vo iu kin a hnh,
kh hu trong cng chim 80%-:-90% s lng cc cng trnh thot nc.
Vit Nam i vi ng cp IV min ni trung bnh 1km ng c 4-:-9 ci cng.
Gi thnh xy dng cng thng chim 10%-:-20% gi thnh ton b tuyn.
- Cng khc cu nh ch l nc chy trong cng khng nhng ch c ch
chy khng p m cn c loi chy c p hoc bn p v chiu cao t p trn
nh cng (k c chiu dy kt cu o ng) ti thiu l 0,5m (i vi ng t)
cn i vi ng st ti thiu l 1,0m.
- Khi gi thnh xy dng cng v cu nh nhau th vic la chn dng cng c
nhng u im sau:
+) Cng khng lm thay i cc iu kin chuyn ng ca xe t chy trn
ng khi qua v tr cng; khng hn ch mt ng v l ng; khng
yu cu thay i loi kt cu mt ng trn cng.
+) Vic b tr cu trn ng cong (cong bng hoc cong ng) hay trn ng
dc thng gy nn s phc tp v kt cu; song i vi cng c th b tr
HNG DN THIT K CNG TRONG N TT NGHIP May 14, 2014
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mt cch d dng vi bt k mt t hp no ca biu v trc dc m vn
khng gy nn s phc tp ca kt cu.
+) Do nm su di nn ng nn s tng ti trng ca on xe t nh hng
n cng. V vy khi nng cp ng (tng cp ti trng) t khi phi tng
cng cng (nht l khi chiu cao t p trn nh cng 2m).
+) Ngi ta ch dng cu khi m cng khng th m bo thot ht nc chy
qua ng.
II PHN LOI CNG:
D ri, phn ny cc bn qu r hoc google search cng ra thi. Phn loi
theo vt liu lm cng, phn loi theo hnh dng, phn loi theo ch chy, phn
loi theo s l cng, Ni chung phn loi theo kiu g l do ngi ta t. Chi tit
th cc bn t tm hiu.
III CU TO CNG.
Khi bit cu to cng cc bn c th v v b tr tt cng trn bn v tt nghip,
s hiu cc thng s cn chn, b tr ct thp.
Cu to mt cng bao gm 3 b phn c bn nh sau:
u cng
Thn cng
Mng cng
Hnh 1- 1. Cc thnh phn cu to c bn ca cng
1- t cng ; 2- tng u cng ; 3- tng cnh cng ; 4- gia c sn cng
5- mng thn cng ; 6- mng u cng ; 7- khe ni t cng ; 8- t p trn cng
HNG DN THIT K CNG TRONG N TT NGHIP May 14, 2014
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(a). u cng
- Tc dng:
+) iu tit dng nc chy vo v chy ra khi cng;
+) Gi n nh cho mi dc nn p hai u cng;
+) Gi n nh cho cng khng b dch chuyn dc.
- Cc b phn c bn ca u cng gm c:
+) Tng u, tng cnh c xy bng hc, gch nung hoc b tng
M150. Mt ngoi cng v phn tip gip gia tng u vi nn t trt lp
va xi mng M100 dy 1cm.
+) Sn cng v gia c thng, h lu cng.
- Cc dng u cng: C nhiu kiu khc nhau nhng ph bin gm c cc kiu
sau y:
+) Kiu hnh lang:
c im: c hai tng ko di song song vi tim cng, c un cong
hai u ngoi v c chiu cao khng i.
u im: ci thin tt iu kin dng chy, tn tht thu lc nh mt
khc do c hai tng ko di song song nn bc nc du tin trc
cng b y li v nm hon ton ngoi u cng m khng ri vo
trong thn cng.
Nhc im: tn vt liu v thi cng tng i phc tp.
+) Kiu tng cnh cho:
c im: L dng ci tin ca kiu hnh lang, c hai tng cnh c
t m rng u ra pha ngoi v chiu cao thay i dn, ph hp vi
dc ca mi dc nn ng. Gc m tt nht ca tng cnh so vi tim
cng khong 300 i vi tng cnh thng lu v t 120-:-150 i vi
tng cnh h lu. Trng hp lu tc h lu khng ln lm th dng
chung gc m 300 cho c 2 pha.
u im: ci thin tt diu kin dng chy, tn tht thu lc nh v cu
to n gin d thi cng. V vy y l loi c s dng rt ph bin.
Nhc im: bc nc u tin trc cng khng nm hon ton
ngoi u cng m ri mt phn vo trong thn cng. khc phc v
tng kh nng thot nc cho cng c th tn cao on thn cng k vi
u cng thng lu.
Trong n chng ta hay s dng kiu tng cnh cho.
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+) Kiu 1/4 nn:
c im: gm tng u v hai phn 1/4 hnh nn. Kiu ny c c
im tng t kiu tng cnh cho.
u im: so vi kiu tng cnh cho th tit kim c b tng hai
tng cnh m thay bng hai khi t 1/4 nn c mt ngoi lt .
Nhc im: vic xy dng hai khi 1/4 nn tng i phc tp.
+) Kiu tng u v kiu c o:
u im: cu to n gin, d thi cng v tn t vt liu.
Nhc im: khng ci thin iu kin dng chy ca dng nc qua
cng dn n tn tht thu lc ln.
+) Kiu hnh loa:
u im: c hnh dng ph hp vi dng chy, m bo iu kin ca
dng nc qua cng l tt nht, gim sc cn thu lc.
Nhc im: thi cng tng i phc tp.
(b). Thn cng
- L b phn ch yu ca cng cho nc thot qua di nn ng v chu ton
b ti trng ca t xung quanh v ca on xe tc dng ln n.
- Ti trng tc dng khng phn b u theo chiu dc cng: phn gia cng c
tr s ln nht sau gim dn v hai pha u cng, do nn t di cng
thng b ln khng u dn n cng d b un dc hoc b nt v. Do
ngi ta thng chia thn cng thnh cc on, gia cc on b tr mt khe
phng ln bng cc vt liu n hi nh ay tm bitum, matit bitum,... c
nht y v kn cc khe trnh cho nc khng b thm xung nn t.
- i vi cc loi cng trn thi cng lp ghp ngi ta thng c cng thnh
cc on nh c chiu di 1 t l 1m, cn i vi cc loi cng thi cng
lin ti cng trng (cng hp) ngi ta thng chia ra lm nhiu on, mi
on c chiu di thng 3-:-5m.
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Hnh 1- 2. S ti tng tc dng dc theo thn cng
1- p lc tc dng do hot ti ; 2- p lc tc dng do tnh ti
(c). Mng cng
- C tc dng truyn v phn chia p lc ca ti trng xung nn t v gi n
nh cho cng theo phng thng ng. Trong mt s trng hp mng hai
u cng cn c tc dng gi n nh dc cng, khng cho cng b tri v
chng thm nc vo nn t di mng cng.
- Mng cng c cu to tu thuc vo iu kin a cht v yu cu k thut ca
cng trnh. Thng thng nn mng ca cng c chia lm 3 loi bao gm:
+) Loi I: mng cng t trn nn t thin nhin. Loi mng ny p dng i
vi loi t nn l si cui, ct cht, st cng c cng > 2,5kg/m2. Cao
t cng trn mc nc ngm ti thiu l 0,3m.
+) Loi II: mng cng l mt lp m dm trn ct. Loi mng ny p dng
i vi nn l phong ho hoc lp t st, ct ht nh, nn t khng
thot nc.
+) Loi III: mng c xy bng c cng 40kg/m2 tr ln hoc gch
mc M100 xy bng va xi mng mc M100, lm bng b tng hoc BTCT
lp ghp. Loi mng ny c p dng i vi tt c cc loi t st, t ct
c cng tnh ton ln hn ng sut tnh ton di t mng.
- Cn c vo c im cu to ca cc loi mng cng m ngi ta li chia ra
lm loi l mng mm v mng cng. Mng cng l loi mng cng c t
trn nn t nhin hay mng xy, b tng, b tng n hc, hoc
BTCT.
1
2
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- Khi xy dng cc cng c mt ct ngang ln trong nn t yu hoc nn t
p cao hoc cng c dc ln nn dng kiu mng cng dng khi trnh
cho cng khng b bin dng cc b do mng b ln khng u.
(d). t p trn cng
- bo v thn cng v lp sn phng nc th sau khi xy xong cng phi
p ngay t trn cc on cng, t p trn cng dng nh loi t p nn.
Khi p phi chia thnh tng lp dy 15-:-20cm.
Trong n tt nghip, chng ta hay s dng cu trn c D=1-2m. Trong hng dn
ny mnh s hng dn cc bn thit k cng trn. Cng hp cng tng t, ch khc
v mt ct thi, cn cc thng s khc nh mng cng v u cng th tng t.
CC BC TNH TON V THIT K CNG
TRONG PHM VI N TT NGHIP
- B1: Tnh din tch lu vc (F) trn bnh 1:10 000
Cc bn ting hnh khoanh lu vc trn bnh , cch khoanh nh sau : Xc nh v
tr ng t thy cng s i qua, lu vc s l ton b sn i xung quanh ng t
thy ny m nc s v. Nh vy ranh gii khoanh s l cc ng phn thy bao
ng t thy m ta nh thit k cng qua. Vic khoanh lu vc ch mang tnh cht
gn ng ch ko th chun 100%.
B2: Xc nh lu lng tnh ton (Qtt):
Cng thc tnh ny cc bn hc trong mn thy vn ri, cch d nht
l t cng thc Q cn tnh di, cc bn i ln lt xc nh tng thng
s mt, ch yu l tra bng nn khng c vn g kh khn.
Quy trnh thit k cu nh, cng ca B Giao thng vn ti quy nh tnh lu
lng ln nht (lu lng thit k) Qtk:
Qtt = Ap**Hp*l*F (m3/s)
Hp lng ma ngy (ng vi tn sut thit k);
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Lu lng ma ngy Hp ng vi tn sut thit k p% xc nh theo ti liu o
c ca trm kh tng thy vn gn nht hoc tham kho ti liu thng k (xem ph
lc 1).
tnh ton khu cng cn xc nh c lu lng nc dn v theo tn
sut thit k p (ph thuc vo cp hng ng):
Vtk > 80km/h p = 2%
Vtk 80km/h p = 4%
F din tch lu vc ( tnh trn B1)
h s dng chy l xc nh theo bng, ph thuc vo c trng lp t mt
ca lu vc, lng ma ngy Hp v din tch lu vc F (xem bng 2.8);
Ap m un tng i ca dng chy ln nht (khi Q = 1)
Ap = qp/(*Hp)
qp m un dng chy.
Ap xc nh theo bng 2.9 ph thuc vo:
c trng a mo thy vn ca lng sng L;
Thi gian tp trung dng chy trn sn dc s;
Phn khu ma ro (vng ma).
1 1 1
3 4 41 1
1000
( )L
P
L
m i F H
il dc lng sng, sui, dng chy ();
L chiu di lng chnh (km);
ml h s nhm ca lng sng, sui, dng chy (bng 2.10).
Thi gian tp trung dng chy trn sn dc s xc nh theo bng 2.11 ph
thuc vo vng ma v c trng a mo ca sn dc S.
0,6
0,3 0,4( )s
S
s s p
B
m i H
is dc sn lu vc ();
bs chiu di bnh qun ca sn lu vc, m
ms h s nhm ca sn dc (tra bng 2.12)
Vi nhng lu vc c din tch ln hn 10 km2 trong cc min a l khc nhau,
khi thiu ti liu c th ly thi gian nc chy trn sn dc s gn ng nh sau:
vng rng gi vi lu vc khng c m ly: 60 100 pht, vi lu vc
c m ly: 150 pht.
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Vng rng non: 40 60 pht;
Vng ng c v ng c kh cn: 20 40 pht;
Vng cn ct: 10 15 pht;
Vng ni: 10 60 pht.
Vi a hnh kh li lm hoc t ln t , ly gii hn di, vi sn dc
bng phng ly gii hn trn.
l h s iu tit lu lng ln nht do ao h (xem bng 2.13)
Khi ao h, m ly vng trung lu hoc ri u trn lu vc th l ly theo tr
s bnh qun ca hai ct.
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Vy l cc bn tnh xong Qtt .
B3: LA CHN CU TO CNG.
Sau khi tnh c Qtt tip theo l xc nh xem s dng cng hay cu nh?
Nu l cng s dng loi cng g? Khu nh th no? Ch chy ra sao.
- C s la chn :
Nguyn tc:
- S dng vt liu a phng; trc ht tranh th dng , tit kim xi mng
st thp;
- p dng rng ri thit k mu v thit k lp ghp t mc ch xy dng
nhanh. Trong phm vi mt tuyn ng nn c gng dng t kiu loi cng
v cng khu tp trung ch to hng lot, n gin bt iu kin xy
lp.
- Trong nhng iu kin sau y nn dng cng:
+) Khi cng c kh nng thot ht lu lng nc tnh ton.
+) Chiu cao t p trn cng khng b hn ch.
Ngoi nhng iu kin trn mi dng cu nh.
- tin cy v khai thc ca cng ln hn cu nh, v cng c th cho thot
qua lu lng ln hn lu lng tnh ton ca bn thn cng. Do vy khi l
ln, cng c kh nng bn vng hn cu nh.
- Khi lu lng tnh ton ln m thng lu khng c iu kin tch nc th
phi xt kh nng chuyn sang lm cu nh.
- Khi tuyn qua sng sui ni khc ngot c bi hoc vng chy, bn
chy hay tch nghim trng th nn dng cu nh, v nu dng cng d
tc, kh no vt.
- Ni khe su nn p rt cao, vic dng cng hay cu cn phi so snh cc ch
tiu kinh t k thut m quyt nh. Nn p cao c iu kin nn mng tt
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th kinh ph thi cng nn p thng t hn cu. Thng nn p cao 25-:-
30m th 1m cu gn bng 1m nn p.
- Vng m ly gim ng sut y mng, tt nht l dng cu, khng nn
dng cng.
- Khi thit k thot nc trn sn ni, nu dng cu nh v cng m gi
thnh khng chnh lch nhau bao nhiu th nn dng cu. c bit l vng
nn o sn ni, lu lng nc ln thi khi nn dng cu nh v pha
thng lu lm dc nc hay cc cng trnh ph tr khc.
- Khi tuyn ng qua vng h cha nc hay qua cc dng nhnh th tt nht
l dng cu, trng hp lu lng nh th mi xt n dng cng.
- nhng on quy hoch thu li, h thng thot nc ca thnh ph hay
cc nh my, cng trng, hm m,... phi tng tng lin h vi cc n v
hu quan, khi b tr cu cng cn phi xt tng hp, phi kim tra nh hng
nc dng khng nn thu hp khu , nn phn tn, khng nn st nhp.
- Vng t yu, nu t cng d b ln mnh lm chm cng, d b ng nc,
ng bn bo dng khng tin, v vy nn lm cu nh.
Tm li trong cng vic cng nh n th:
Khi lu lng dng nc tnh ton di 15m3/s th nn dng cng trn
BTCT s c gi thnh r hn cu vi bt k chiu cao nn p no. Khi chiu
cao nn p ti thiu khng m bo th nn dng cu nh hay cng bn ni.
Khi lu lng nc trn 20m3/s, nu chiu cao nn p tho mn yu cu ti
thiu th trong nhiu trng hp dng cng r hn cu. Khi chiu cao nn p
ln hn 5-:-6m khi lng m cu tng nhanh th trng hp ny dng cng
vm xy dng hay b tng r hn cu.
Coi nh Qtt < 15m3/s (thng l th). Ta la chn cng thit k l cng trn BTCT.
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*, Xc nh khu cng:
- Cng c 3 ch chy: c p, bn p v khng p.
(a). Cng lm vic ch chy t do (khng p):
- Kh nng thot nc ca cng c tnh theo cng thc:
)h2g.(H..V.Q CCCC (1)
Trong :
hC: chiu su nc ti mt ct thu hp ca vo ca cng (m).
g
V0,730,9hh
2C
KC
hK: chiu su phn gii l chiu su dng chy ng vi ch nc
chy phn gii.
C : din tch mt ct t thu hp ca dng chy trong cng c tnh
vi chiu su thu hp hC; (m2)
VC: lu tc mt ct c chiu su hC (m/s)
: h s lu tc ; = 0,85 i vi cc loi u cng thng thng
ngoi tr hnh loa.
H: chiu su nc dng trc cng c tnh theo cng thc:
2
2C
C2g.
VhH
(Phng trnh Becnuli) (2)
Thay = 0,85 ; gV
0,730,9hh2C
KC vo (2) ta c:
C
2C h2
g
V43,1H
(3)
- Thay (3) vo (1) ta c:
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2gH..85,0Q C (4)
- i vi cng mt ct hnh ch nht th C = b.hC = 0,5.b.H do :
3/23 1,33.b.H2gH.0,425.bQ
(b). Cng lm vic ch chy bn p:
- Kh nng thot nc ca cng c tnh theo cng thc:
)h2g.(H...V.Q CCC (5)
Trong :
H: chiu su nc dng trc cng
hC = 0,6 . hT
hT: chiu cao lng cng
: din tch mt ct ngang cng
H s lu tc = 0,85 i vi cc loi u cng thng thng; =
0,95 i vi u cng hnh loa.
H s co hp: = 0,6 i vi cng hnh ch nht ; = 0,65 i vi
cng trn.
- Thay vo (1) ta c:
)h6,02g.(H..51,0Q T (6)
(c). Cng lm vic ch chy c p
- Kh nng thot nc ca cng c tnh theo cng thc:
)h2g.(H..Q T (7)
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Trong :
hT: chiu cao lng cng
: din tch mt ct ngang cng
Trong n tt nghip, ta chn ch chy ca cng l khng p, tnh Q i
vi trng hp ch chy khng p.
nhanh v n gin ngi ta lp sn cc bng tra v kh nng thot nc ca
cng cho tng loi cng vi cc khu khc nhau. Tham kho thm ti phn ph
lc sch "Thit k ng t tp 3 - Cng trnh vt sng" ca NXB Gio dc).
Khi lm n tt nghip, cc bn dng cch tra bng ph lc sch.
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Cc bn nn download hoc xem trong sch cho r tra. Pha trn mnh c copy
bng tra cho cc bn.
Cc bn la chn sao cho vi khu la chn th nng lc thot nc Q ca
cng ln hn Qtt lc u l ok.
V d nh sau:
Hy chn phng n khu cng trn kiu tng cnh cho thot lu lng
nc Q = 9 m3/s, chiu cao nn p l H; ch chy l khng p hoc c p.
- Chiu cao nn p ti thiu trn cng c tnh theo cng thc:
aHminn1
v a'HHminn2
Trong : : khu thot nc ca cng trn.
: chiu dy nh cng.
a: chiu cao t p ti thiu trn nh cng; a = 0,5m.
a': chiu cao ti thiu ca nn p so vi mc nc dng trc cng; a' = 0,5m
khi chy t do v a' = 1,0m khi chy bn p v c p.
H: chiu cao nc dng trc cng.
HNG DN THIT K CNG TRONG N TT NGHIP May 14, 2014
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Page 25
Phng
n
Khu
cng
(m)
Chiu
dy
(m)
S l
cng,
n
Lu
lng
qua
mi
l
(m3/s)
Ch
chy
Nc
dng
trc
cng
H (m)
Lu
tc
trong
cng
V
(m/s)
minn1
H minn2
H
1 1,50 0,15 2 4,50 Bn p 2,01 3,0 2,15 3,01
2 1,50 0,15 3 3,00 Khng
p 1,47 3,1 2,15 1,97
3 1,25 0,13 2 4,50 Bn p 3,24 5,8 1,88 4,24
4 1,25 0,13 3 3,00 Bn p 1,89 3,8 1,88 2,89
5 1,25 0,13 4 2,25 Khng
p 1,33 3,0 1,88 1,83
6 1,00 0,12 3 3,00 Bn p 3,16 6,0 1,62 4,16
7 1,00 0,12 4 2,25 Bn p 2,07 4,5 1,62 3,07
- T kt qu trn ta thy nu chiu cao t p H khng hn ch tc l:
H max (minn1
H ; minn2
H )
th ta c th chn phng cc phng n (1) ; (3) v (6) l nhng phng n c s
l cng t; trong phng n (1) v (3) th nn chn phng n (3) v c khu b
hn. Vic cn li l so snh gia phng n (3) vi (6) v gi thnh xy dng
la chn phng n xy dng c gi thnh r hn.
Xong. Vy l cc bn xong 3 bc t tnh ton thy vn n la chn c
khu cng ph hp thot nc.
Tip theo s l phn b tr cng.
HNG DN THIT K CNG TRONG N TT NGHIP May 14, 2014
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B TR CNG
BA NGUYN TC QUAN TRNG TRONG VIC B TR CNG:
3 KHNG: - KHNG TREO
- KHNG VI
- KHNG T TRN T MN.
- B tr cng trn bnh :
- Trong nhiu trng hp phi dch sui t cng trc giao vi ng. Nh
m chiu di cng s ngn i v cng t ni kh ro to iu kin
thun li cho vic xy dng. Tuy nhin nu khi lng o mng dn nc
qu ln th cho giao xin gc.
a- Cng t vung gc vi tim ng
b- Ci sui ct vung gc vi ng
HNG DN THIT K CNG TRONG N TT NGHIP May 14, 2014
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Page 27
- i vi trng hp cng giao cho vi ng phi gii quyt hnh dng u
cng cho ph hp. C 2 cch sau y:
+) Nu lu lng nc nh, khu cng trn nh hn 1m hoc khi dng
u cng lp ghp th dng u cng dng bnh thng c p thm phn
t ph u cng.
+) Nu lu lng nc ln th c th dng kiu tng u song song vi tim
ng hay vung gc vi tim cng.
- B tr cng trn ct hnh ct dc:
- i vi vng a hnh bng phng, cao nn p khng hn ch th nn t
lng cng trng vi cao lng sui t nhin tc l cao v dc lng
cng trng vi cao v dc lng sui t nhin. Nu do iu kin thu
vn, iu kin kt cu m phi h cao lng cng th cn phi ch :
y lng dn nc pha h lu c thot c nc hay khng. Nu dc
h lu qu nh th dn n ng, bi lng.
- Khi y sui c dc ln th phi xt tng hp v kiu loi cng, loi
mng, iu kin a cht theo nguyn tc kinh t.
- i vi a hnh dc ln, khi lu lng nc nh (Q < 1m3/s) m chiu cao
nn p khng hn ch, th c th t cng trn nn p c d dc bng
dc phn gii v cao u cng hng lu trng vi cao sui t nhin
hoc t cng trn nn na o, na p.
- Khi lu lng nc ln (Q > 1m3/s), ni a hnh dc ln, vn tc nc chy
ln nn d gy ra xi mn lng dn, c th ph hoi cng trnh cu cng
thot nc. V vy lng dn thng, h lu phi c bin php gia c c bit
m bo s lm vic an ton ca cu cng. Bin php thng dng l xy
dng cc dc nc, bc nc, ging tiu nng.
- Khi dc lng sui t 0,2%-:-2% th nn t cng dc phn gii,
thng v h lu cng t dc nc loi thng thng c tit din khng i
hoc dc nc c nhm cao. on qu gia dc nc v cng l dc
nc c tit din thay i (gc m 150). Ch vo cng b tr mt on bng,
khng dc nhm lm gim sc chy.
HNG DN THIT K CNG TRONG N TT NGHIP May 14, 2014
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Page 28
- Khi dc lng sui > 2% v lu tc on cui dc nc qu ln, khi
mi dng bc nc v ging tiu nng.
- Khi dc lng sui qu ln c th dng bin php t ging tiu nng
u cng thng lu. Bin php ny khng p dng trong trng hp dng
chy mang nhiu ph sa, c rc, d lm bn ging v gy tc cng.
- Khi dc lng sui qu ln m xt thy nu t cng dc phn gii th
khi lng tng nhiu th c th t cng theo dc ln. Khi phi c
bin php c bit gi cho cng khng b tri; mng cng phi lm bng
xy hay b tng; y mng phi to bc, tng u cng phi tng kch
thc c tc dng nh tng chn.
- Khi b tr cng c dc (>5%) phi tun theo cc quy nh sau y:
+) Cng vm v cng bn, thn cng b tr kiu bc thang; chiu cao bc ln
nht khng qa 0,7m nhng mp trn nh cng vm on sau khng
c thp hn mp di nh vm ca on trc trnh tt t vo
trong cng v thun tin cho vic bo dng.
+) i vi cng trn, thn cng b tr thng xin; mng cng phi lm bng
b tng hay xy v lm kiu bc thang; cng c th b tr thn cng
kiu bc thang ni dc khng ln lm nhng chiu cao bc khng ln
hn 3/4 chiu dy cng.
+) Khi b tr thn cng kiu bc thang, chiu di mi bc thn cng hay
mng cng i vi xy hay b tng khng nh hn 2m v i vi
BTCT khng nh hn 1m.
Trong phm v n th vic tha mn cc iu kin trn l kh. Cc bn c
hiu ch khng cn qu ch tm vo gy rc ri cho n ca mnh v cc
s liu n cho khng xem xt tt c cc yu t trn.
HNG DN THIT K CNG TRONG N TT NGHIP May 14, 2014
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Page 29
- Xc nh chiu di cng
a. Trng hp chiu cao nn p 6m
c
c
b
H
h1
M1 L1 L2 M2
b
h2
1 2
HvT
H
mt ct dc cng
cv
b
H
b1 2
HvT
H
mt ct dc cng
cc
ct
i l
Bm2
Bgc2
Bl2
b1
imigcilcv
ct
Bm2
Bgc2
Bl2
b2
im igc
HNG DN THIT K CNG TRONG N TT NGHIP May 14, 2014
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- Chiu di cng c tnh theo cng thc: L= L1 + L2
Vi:
c1V1
1m.i1
c)hm(HbL
(01)
c2V2
2m.i1
c)hm(HbL
(02)
Trong :
L1: chiu di thn cng thng lu tnh t tim ng.
L2: chiu di thn cng h lu tnh t tim ng.
b1; b2: na b rng nn ng pha thng lu v h lu.
m: h s mi dc ca nn p.
H: chiu cao nn p ti tim ng tnh t y lng cng ti vai
ng.
ic: dc lng cng.
h1; h2: chiu cao ca cng thng lu v h lu.
c: chiu dy tng u cng.
- Thng thng m = 1,5 ; b1= b2 = b ; h1 = h2 = h
.i5,11ch)(H5,1b
L1
(03)
.i5,11ch)(H5,1b
L2
(04)
b. Trng hp chiu cao nn p > 6m
- Khi chiu cao nn p ln (H > 6m) th phi lm mi dc c 2 dc (m1 =
1,5 v m2 = 1,75) th cng thc tnh chiu di cng vn c p dng nh
(01) v ( 02) nhng thay:
HNG DN THIT K CNG TRONG N TT NGHIP May 14, 2014
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Page 31
b1 = b'1 ; b2 = b'2 ; m = m2 trong :
b'1 = b1 + m1.H1
b'2 = b2 + m1.H1
m1 ; m2: h s mi dc ca mi dc phn trn v phn di.
H1: chiu cao ca mi dc ti a tng ng vi mi dc phn trn.
Thng thng H1 = 6m ; b'1 = b1 + 9 ; b'2 = b2 + 9
- Do :
.i75,1 1c)h(H75,19b
L' 111
(05)
.i75,11c)h(H75,19b
L' 222
(06)
L'1
b' b'1 2
1 2
H1
B(mt) / 2 B(mt) / 2
c
c
b
Hh1
M'1 L'2 M'2
b
B(l)
B(nn)
B(l)
h2
HNG DN THIT K CNG TRONG N TT NGHIP May 14, 2014
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c. B tr cc t cng
- i vi cng lp ghp hoc cng bn lp ghp (cng trn BTCT, cng bn
BTCT,...) th chiu di thn cng phi c b tr vi s chn cc t cng
hay vi s chn ca tm bn (k c khe ni).
- i vi mng mm, chiu rng khe ni gia cc t cng l 1cm. Gi n l s
t cng, s mi ni s l: n-1 . Chiu di cng c xc nh nh sau:
0,011)(nnL
Khi bit chiu di cng th s t cng c tnh nh sau:
1,010,01L
n
- i vi mng cng: chiu rng khe ni gia cc t cng l 1cm v c
khong 3 t cng th b tr 1 khe phng ln rng 3cm.
- Thng chiu di mi t cng l 1m.
Vic xc nh c chiu di kia khi cc bn p c cng ln trc ngang,
p nh sau:
- Xc nh dc cng.
-K ng thng vi dc bng dc cng
- a ng thng ny ln trc ngang sao cho n gn vi ng t nhin nht
nhng phi tha mn cng ko b treo (khng cao qu nc ko vo cng c) , khng
b vi (ming cng vi di t). Vic p ng ny ty thuc vo trc ngang thit
k. Nn mnh ko th ni mt cch chnh xc v trong nhiu trng hp ng
thng ny c b tr khc, min sao khi cc bn p cng thy hp l l ok. Cao
ng thng ny chnh l cao y cng.
- K mt ng thng khc song song vi ng thng kia,sao cho khong cch
gia 2 ng thng ny chnh l khu D ca cng. Ct taluy ti 2 im. Gii
hn 2 im ny s lm phm v b tr cc t cng.
HNG DN THIT K CNG TRONG N TT NGHIP May 14, 2014
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Page 33
- Tip l phn chia cc t cng, da trn gii hn ca 2 im ny, b tr cc
t cng th no th pha trn mnh ni ri.
- Sau khi b tr cc t cng xong th tin ny b tr mng cng, tng cch cng,
tng thn cng, sn gia c.
* Cc bn da vo cu to di y ca cng trn la chn cu to cc b
phn ca cng cho hp l:
(a) u cng:
u cng c dng ph bin nht l kiu tng cnh cho hoc kiu 1/4 nn.
- u cng c th xy ti ch hoc lp ghp bng cc tm BTCT c sn.
- Nu u cng xy bng khi xy th mc va xy khng thp hn M30,
cng R > 2000 kG/cm2.
- Nu u cng lm bng b tng ti ch th mc b tng khng thp hn
M150.
- Nu u cng c c sn th mc b tng khng thp hn M200.
(b) Thn cng:
c phn thnh cc t vi chiu di 1m, trng hp c bit c th l 0,75m
hoc 2m.
+) Gia cc t cng l cc khe ni c tc dng phng ln v phng s co
dn ca b tng khi nhit thay i. Thng th c hai loi khe ni:
phng v khng phng. Khe ni phng c p dng cho trng hp khu
di 2m v p lc nc khng ln. Khe ni khng phng c p
dng trong trng hp khu cng ln hn 2m, v cng chy c p.
+) Khe ni c lp y bng vt liu cch nc n hi t hai pha ngoi
vo trong. Pha ngoi khe ni c ph y bng hai lp giy du rng
25cm dn xen k gia 3 lp bitum nng. Pha trong chiu su 3cm trt
va xi mng gi khng cho vt liu cch nc n hi b chy rt khi
nhiu cao.
HNG DN THIT K CNG TRONG N TT NGHIP May 14, 2014
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Page 34
+) B mt ngoi ca cc t cng c qut 2 lp bitum nng. Ch trng
hp c bit mi lm lp cch nc ny sau khi h chnh cc t cng.
+) Khi cn thot lu lng nc ln c th s dng cng hai, ba hoc thm
ch bn l cng. Trng hp ny khong gia cc cng c lp y bi
vt liu cng loi vi mng m. C ly ti thiu gia cc ng cng l 20-
:-26cm m bo chn cht c lp vt liu nu. Mt trn phi to
c dc v hai pha 3-:-4% v ph bng lp t st do dy 15-:-
20cm.
+) tng kh nng thot nc ca cng c th tn cao on t cng gip
vi u cng thng lu kiu hnh nn ct.
+) Cc t cng thng c ch to sn xng. B tng ch to t cng
thng dng mc M200, ct thp thng dng loi CT3, CT5,.... Chiu
dy t cng ph thuc vo cp ti trng (loi xe), chiu cao t p,
khu cng v thng c gi tr 8-:-15cm hoc 20cm.
* B tr ct thp cng:
Ct thp chu lc ca cng c b tr theo hai kiu: Kiu mt vng elp hoc
kiu hai vng trn ng tm:
- Kiu mt vng elp c u im l ph hp vi s ni lc ca kt cu, ct
thp c t vng chu ko v vy tit kim c ct thp. Nhc im l kh
ch to, khi ch to phi nh du v tr ct thp khi xy dng t t cng cho
ph hp.
- Kiu vng trn tn ct thp hn kiu elp song vic ch to d dng hn.
+)Theo chiu dc t cng cc ct thp chu lc ny c th b tr thnh tng
vng ring bit hay b tr theo kiu hnh l xo dy xon:
Vic b tr ct thp thnh tng vng ring bit c nhc im ln l s
mi ni ct thp nhiu ( mi vng 1 mi ni) nn tn tht ng sut trong ct thp
nhiu, mt khc khng tn dng c chiu di ca cc thanh thp, d gy lng ph
ct thp.
HNG DN THIT K CNG TRONG N TT NGHIP May 14, 2014
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Page 35
Kiu b tr ct thp theo kiu hnh l xo dy xon khc phc c nhng
nhc im trn ca kiu b tr thnh tng vng ring bit. V vy kiu b tr ny
c p dng ph bin.
+)Ct thp chu lc c lin kt vi nhau v gi ng v tr bi cc ct
thp dc v ct thp ai. Ct thp dc cn c tc dng chng li lc ct v un
trong thi cng.
+)Cc t cng trong cc thit k nh hnh thng c hai loi: loi thng
thng v loi gia cng, loi gia cng ch khc loi thng thng ch lng
ct thp nhiu hn. ng cng gia cng thng c p dng khi cng t trn
nn cng .
Do lm theo nh hnh nn cc bn c th download b cng trn nh hnh 533-
01-01 do vin KHCN>VT thit k v. trong c sn cc bn v b tr ct
thp.
1200
CT THP TRONG
1-D10-02-SPRING
48-D6-04
45
100
40
50
15
MT CT II-II
1000
100
CT THP NGOI
1-D10-02-SPRING
100
CHI TIT A
1000
OU
TE
R D
IA =
114
8IN
NE
R D
IA =
105
4
Lmin = 10004x200=800
4x200=800
900
300
32-D6-03
HN
1002828
44100
75
32-D6-04
100
25
CHI TIT A
4x200=8001000
100
2844
75
28
MT CT I-I
1-D10-01-SPRING
1-D10-02-SPRING
100 25
1-D
10-0
1-S
PR
ING
1-D
10-0
2-S
PR
ING
4x200=800
900100
300
Vt liu trt mi ni
HNG DN THIT K CNG TRONG N TT NGHIP May 14, 2014
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Page 36
c, Mng cng:
Mng cng trn BTCT c th bng cc khi xy, b tng, b tng ct thp
lp ghp hoc trong trng hp t nn tng i cht c th dng lp m
bng dm, si, ct,...
La chn mng cng
.
80.75
24 31 149 89 124 109 1079/121.00
30 38 190 130 151 150 13010/131.25
35 45 230 170 180 190 15612/151.50
Kch thc (cm) S: cho cc kiu mng
c
Khu cng d
m l c I II III IV-A IV-B
17 23 114 54 91 74 78
mng cng kiu IV-B
120
S
d
2d 2d
p t xung quanh cng
Lp chn bng b tng thng
Hnh 2.2 - e: Nn l khng phong ho
mng cng kiu II
120
S
30
d
2d 2d
mng cng kiu I
120
d
2d 2d
S
p t xung quanh cng
p t xung quanh cng
Chn bng t ti ch
m bng cp phi dm hoc ct th
Hnh 2.2 - a: Nn l si cui cht, ct th hoc va
Hnh 2.2 - b: Nn l t st, st pha ct, ct mn ni kh ro
mng cng kiu III
120
d
2d 2d
p t xung quanh cng
hc xy va xi mng M150
Hnh 2.2 - c : Nn l t st, st pha ct, ct mn ni m t hoc
.30
30
l
m bng cp phi dm hoc ct th
hoc ct pha st ni kh ro hoc m t
mng cng kiu IV-A
120
S
20
d
2d 2d
p t xung quanh cng
m bng cp phi dm hoc ct th
Hnh 2.2 - d: Nn l phong ho
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Nh vy cc bn hiu b tr cng cng nh cu to cc b phn cng v b tr
ct thp t cng, trn kia cng trnh by v gii thch mt vi cu hi hay b
hi trong n tt nghip nh:
- B tr mt lp ct thp cng c ko? Nu b tr th b tr nh th no?
- Qut bitum mt ngoi cng c tc dng g?
Tip theo s l phn Tnh xi v xc nh hnh thc gia c h lu cng trnh:
HNG DN THIT K CNG TRONG N TT NGHIP May 14, 2014
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TNH XI V GIA C H LU CNG TRNH
- Nguyn nhn:
S xi l lng dn sau cng trnh do hai nguyn nhn ch yu l:
+) Lu tc dng chy ch ra khi cng trnh ln hn lu tc trong cng
(n 1,5 ln) v c th t ti 5-:-6m/s; trong khi lu tc cho php khng
xi l i vi nn t t nhin ca lng dn ch l 0,7 -:- 1m/s.
+) Dng chy sau khi ra khi cng thng c dng chy xit c lu tc v
ng nng rt ln.
- ngn nga s xi l lng dn h lu cn phi c bin php tng cng
bo v trn mt on di nht nh. Vic la chn hnh thc v chiu di gia
c l tu theo iu kin khng xi l cc b hay cho php xi phi m bo
an ton ca bn thn on gia c cng nh cng trnh.
- Vic xy dng on gia c phng rt di pha h lu cng trnh l khng
hp l v mt kinh t. V vy ngi ta ch cn gia c trn mt on ngn v
kt thc bng mi nghin bo v m cho php xi l . Khi bc nhy
thu lc s b y dn v t on gia c v ch b xi nhng do chiu su
mi nghing ng k nn s xi l khng nh hng n on gia c.
(a). Kch thc on gia c:
- quyt nh chiu su mi nghing bo v cn phi tnh ton chiu su xi
l sau on gia c tu thuc vo dng chy khi ra khi cng.
+) Chy t do: chiu su xi l ph thuc vo chiu di on gia c v chiu
cao nc dng trc cng. Theo kt qu nghin cu ca Anry v kt
qu rt ra t nghin cu nh sau:
g/c2,5.lb
h.2H
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Page 39
Trong :
: chiu su xi l on gia c phng.
H: chiu su nc dng trc cng trnh.
lg/c: chiu di on gia c phng tnh t ch dng chy t cng trnh
bt u b khuych tn.
h: khu cng.
Bng 3-6
Chiu su xi l tng i sau on gia c
b
tg.lg/c
H
b
tg.lg/c
H
0 1,55 4 0,59
1 0,98 5 0,54
2 0,78 8 0,45
3 0,65 10 0,40
Ghi ch: l gc khuyt tn ca dng chy sau khi chy ra khi cng
- u cng kiu tng cnh cho vi gc m th = ; nhng khng ln
hn 450 ; u cng kiu tng u thng ng, kiu c o th = 450 ; kiu
1/4 nn th = 300.
- Qua bng 3-6 ta thy chiu di on gia c phng khng nn ln hn 3 ln
khu cng, bi v khi lg/c > 3 ln khu cng th chiu su xi l on
gia c khng gim c bao nhiu m din tch gia c li tng ln rt nhiu.
- Chiu rng on gia c lng dn sau cng trnh c quyt nh xut pht t
kch thc ca h xi t nhin dng chu Xtrphid vi chiu di trc lc =
18. ; v chiu rng Bc = 12. .
HNG DN THIT K CNG TRONG N TT NGHIP May 14, 2014
T u n g K e n - 0 9 7 4 2 7 7 9 8 6 t u a n t u n g v u @ g m a i l . c o m
Page 40
- Thng chiu rng gia c ly bng 4-:-6 ln khu cng. (Cc bn c
th la chn lun gi tr ny , phn trn c hiu thi)
(b). Hnh thc gia c:
- Thng bng lt khan, lt c trt va hay lt tm b tng c sn hoc
b tng lin khi. Vic la chn hnh thc gia c ph thuc vo lu tc
dng chy sau cng trnh v iu kin cung cp vt liu, kh nng xy
dng,...
- Mi nghing bo v cui on gia c thng c t su hn h xi t nht
0,5m.
hm + 0,5
- gim bt khi lng gia c, c th p ng bin php rt ngn hn na
chiu di gia c bng cch tng nhm lng dn ngay sau cng trnh bng
cc tng hay tng nhm, cho php xi su gia c v to vo h xi.
- Mi dc nn p h lu trc cng c gia c ti chiu cao ca tng u.
- Gia c thng lu cng: chiu di gia c c ly bng 1/4 chiu di gia c
h lu. Kiu gia c c tnh ti lu tc bng lu tc trong cng v gia c
mi dc u cng c thit k n chiu cao bng: H + 0,5m.
- Ch cc l phn cc bn cn ch la chn thng s cho hp l.
Nh vy n y l cc bn hon thnh xong vic thit k cng. Phn tnh ton
kt cu cng nh phn thi cng cng th trong n tt nghip khng yu cu nn
mnh khng trnh by. Bn no cn th c th lin h mail mnh, mnh c th gi
cho cc bn mt vi l thuyt v vn ny. hiu hn v khng phi tng
tng cc bn c th search hnh nh cng trn trn mng rt nhiu, hoc i ra thc
t xem 1 ci cng l cch tip cn nhanh nht.
Chc cc bn lm n thnh cng!