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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



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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



Page 3

(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.



Page 8

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



Page 17

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.



Page 18

*, 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:



Page 19

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:


)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


)h2g.(H..Q T (7)



Page 20

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.



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.



Page 26

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



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.



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.



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



Page 30

- 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:



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



Page 32

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.



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.



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.



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&GTVT 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



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


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


Hnh 2.2 - d: Nn l phong ho



Page 37

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:



Page 38

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



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. .



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!

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