Chuong 1- Dap betong trong luc (mo)- Nguyen Chien.pdf

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p b tng v

b tng ct thp


10

Chng 1. p btng trng lc

Bin son: PGS.TS Nguyn Chin

1.1 Phn loi p v cc yu cu thit k I. Phn loi p

Trong chng ny trnh by cc kin thc v p btng trng lc trn nn . p trng lc l loi p c khi lng ln v c duy tr n nh nh trng lng bn thn p. C th phn loi p theo nhiu cch khc nhau.

1. Theo chiu cao p. Chiu cao p v loi nn l mt trong cc tiu chun dng phn cp p v cng trnh u mi.

Theo ti liu ca th gii, p thng phn thnh:

a. p cao: c chiu cao H 70m;

b. p cao trung bnh: 30m H < 70m.

c. p thp: H 30m.

Theo Ngh nh 209/2004/N-CP, cp ca p btng trn nn nh sau:

- p cp c bit : H > 150m.

- p cp I: 100m H < 150m.

- p cp II: 50m H < 100m.

- p cp III: 15m H < 50m. - p cp IV: H < 15m.

2. Theo kt cu mt ct ngang p, c cc loi:

a. p trng lc c (hnh 1 - 1a);

b. p trng lc khe rng (hnh 1 - 1b);

c. p trng lc c khot l ln st nn (hnh 1 - 1c);

d. p c neo vo nn (hnh 1 - 1d).

Hnh 1-1. Kt cu mt ct ngang p btng trng lc

(b) (c)(a)(d)


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a) p c; b) p c khe rng; c) p c khot l ln st nn; d) p c neo vo nn.

3. Theo chc nng ca p:

a. p trng lc khng trn: p c chc nng chn nc, khng cho nc trn qua (hnh 1- 1).

b. p trng lc trn nc: p c chc nng va chn dng nc, va cho trn nc qua. C th phn bit:

- p trn mt: trn t do hoc c ca van (hnh 1 - 2a).

- p c l x su: l x lng chng, hoc di y p (st nn), hnh 1 - 2b.

- p kt hp trn mt v x su (hnh 1 - 2c).

Hnh 1 - 2. Cc hnh thc p trng lc trn nc

a- Trn mt; b- X su; c-Kt hp trn mt + x su.

4. Theo dng b tr p trn mt bng

p btng thng l loi kt hp cc on p trn v khng trn trn cng mt tuyn. Tu theo iu kin a hnh, a cht v yu cu m rng din trn nc, c th b tr tuyn p theo cc dng sau:

a. p tuyn thng: khi a cht nn cho php, v khi chiu di tuyn b tr on trn nc.

b. p tuyn cong, s dng khi:

- a cht nn c ch yu, khng cho php b tr tuyn thng;

- Cn m rng din trn (b tr tuyn cong li v thng lu).

c)

a)

Ho

b)


12

II. Cc yu cu thit k p Khi thit k p btng trng lc, phi tun theo y cc yu cu v k thut v

kinh t, c bn nh sau:

1. p phi tho mn cc nhim v thit k t ra (dng nc, trn nc, li dng tng hp).

2. p phi m bo n nh trong mi iu kin thi cng, qun l khai thc v sa cha.

3. p phi bn, chng cc tc ng ph hoi ca ngoi lc, ti trng nhit, bin hnh nn v nh hng ca mi trng, m bo tui th theo quy nh.

4. B tr mt bng v kt cu p phi tho mn cc iu kin thi cng, qun l vn hnh, sa cha, m bo m quan.

5. p phi c tnh hin i, p dng cc cng ngh thit k, thi cng v qun l tin tin ph hp vi iu kin ti ch v xu hng pht trin ca a phng.

6. Gi thnh p phi hp l, ph hp vi nhim v ca n v vi cc iu kin ti ni xy dng.

1.2 B tr p btng trng lc trong cm u mi Trong cm cng trnh u mi thng c p dng, p trn v cc cng trnh khc

tho mn iu kin khai thc cng trnh v bo v mi trng (cng ly nc, nh my thu in, u thuyn hay cng trnh nng tu, ng th b, ng c i, cng trnh phc v du lch...).

Vi p btng trn nn , thng kt hp p dng v p trn trn cng mt tuyn. p trn thng b tr on lng sng trnh lm bin i qu nhiu n iu kin ni tip dng chy h lu so vi khi cha c p, cn phn p khng trn thng b tr 2 u tuyn, ni tip gip vi b.

Hnh 1.3. V d v b tr mt bng p btng trn nn .

1- p khng trn; 2- Trn mt; 3- X y; 4- Cng ly nc.

iu kin a cht ng mt vai tr quan trng trong vic b tr mt bng. Ni chung, khi b tr p trong cm cng trnh u mi cn tho mn cc iu kin sau y:

1 321

4


13

1. Chn tuyn p c a cht nn v 2 vai tt, trnh cc v tr nt gy hoc mm yu cc b, phi x l phc tp.

2. Khi cc tuyn c iu kin a cht nh nhau, nn chn tuyn p thng, ni lng sng thu hp gim khi lng cng trnh. Ch tr trng hp cn m rng din trn nc th mi lm tuyn p cong li ln thng lu.

Cng c th chn tuyn p gy khc khi phi n trnh cc vng c a cht yu cc b.

3. B tr p trn phi ph hp vi iu kin tho lu lng thi cng v phng php thi cng.

4. Khi trn cng mt tuyn c b tr nhiu hng mc khc nhau (p trn, nh my thu in, u thuyn...) cn phi phn tch chn v tr t thch hp cho tng hng mc gim nh nh hng ca vic tho l qua trn n s lm vic bnh thng ca cc hng mc cng trnh khc. Trong nhiu trng hp, cn b tr tng ngn cch di h lu tho mn yu cu ny.

5. Khi tho l thit k, cn huy ng n kh nng tho mt phn lu lng l qua cc cng trnh khc trong cm u mi nh nh my thu in, u thuyn, ng th b... Ngoi ra cng c th xem xt kh nng cho trn nc trn nh nh my thu in.

Lu lng cn x qua p trn c xc nh nh sau:

Qtr = Qth - aQ0, (1-1) trong :

Qth - lu lng cn tho, xc nh theo kt qu tnh ton iu tit l;

Q0 - tng kh nng tho qua cc cng trnh khc nh trm thy in, cng ly nc, u thuyn, ng th b...

a - h s li dng cc cng trnh khc tho l, c th ly a = 0,75 - 0,90 (xt n trng hp khng phi tt c cc t my thu in u lm vic, cc ca van x c th b s c ca van...).

6. Khi b tr mt bng p, cn nghin cu tng th bi ton ni tip dng chy ra h lu trong iu kin khai thc bnh thng v khi tho l, m bo iu kin khng xi l b v y lng dn h lu.

1.3. Mt ct p btng trng lc I. Cc yu cu khi tnh ton mt ct p

1. iu kin n nh:

p phi m bo iu kin n nh chng trt:

Kt Kcp, (1-2) trong :

Kt - h s an ton n nh chng trt (xem 1.4);


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Kcp - h s an ton n nh cho php, ph thuc vo cp ca p v t hp ti trng, xc nh theo tiu chun hin hnh.

Khi tnh p theo trng thi gii hn, tr s Kcp c th xc nh theo cng thc:

,mkn

K nccp = (1-3)

trong :

nc - h s t hp ti trng; nc = 1,0 vi t hp ti trng c bn; nc=0,9 vi t hp ti trng c bit v nc = 0,95 vi t hp ti trng thi cng, sa cha.

kn - h s tin cy, ph thuc vo cp cng trnh, tra theo tiu chun hin hnh.

m - h s iu kin lm vic. i vi p btng trng lc trn nn , tr s m ly nh sau:

+ Khi mt trt i qua cc khe nt trong nn: m = 1,0.

+ Khi mt trt i qua mt tip xc gia btng v hoc i trong nn c mt phn qua cc khe nt, mt phn qua nguyn khi: m = 0,95.

2. iu kin cng

- ng sut nn ln nht mp p khng c vt qu kh nng chu nn ca vt liu hoc ca nn:

N1 Rn, (1-4) trong : Rn - cng chu nn tnh ton ca vt liu hoc nn.

- Ti mp p, c bit mp thng lu khi h y nc khng cho php pht sinh ng sut ko:

N2 0, (1-5) trong : N2 - ng sut chnh nh nht ti bin thng lu p (ng sut nn mang du dng, cn ng sut ko mang du m).

Trong nhng iu kin nht nh, cho php pht sinh ng sut ko, nhng tr s tuyt i ca n khng c vt qu cng chu ko ca vt liu hay nn:

,R'N k2 < (1-6)

trong : Rk l cng chu ko tnh ton ca vt liu hay nn.

3. iu kin kinh t: mt ct p phi c din tch nh nht sau khi tho mn 2 iu kin trn.

4. iu kin s dng: mt ct p cn cn phi tho mn cc yu cu trong s dng, vn hnh nh cn c ng giao thng trn nh p, c ng hm trong thn p i li kim tra, sa cha, t cc thit b quan trc th nghim, b tr cc hnh lang thot nc... Ngoi ra, cn phi lu n vic to dng kin trc p ca cng trnh.

tho mn yu cu nu trn, khi thit k mt ct ngang p thng tin hnh theo 2 giai on:

1. Giai on xc nh mt ct c bn: da vo cc yu cu n nh, ng sut, kinh t tin hnh tnh ton chn mt ct c bn ca p.


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2. Giai on xc nh mt ct thc dng: theo cc yu cu v s dng nh giao thng, dn tho nc, kim tra, sa cha.... m b tr thm cc phn cu to nh p, cc ng ng tho, ly nc trong thn p, h thng ng hm v hnh lang trong thn p, b phn ni tip vi h lu ca p trn...

Sau khi tu chnh, thm bt cc b phn trn p, cn tin hnh tnh ton n nh v phn tch ng sut kim tra iu kin bn ca p.

II. Tnh ton mt ct c bn ca p 1. Hnh dng mt ct c bn

Mt ct c bn ca p btng trng lc c nhiu dng nh trn hnh 1-4.

Hnh 1-4. Cc dng mt ct c bn ca p btng trng lc

a. Mt ct ngang p c dng hnh tam gic c h s mi thng lu l m1, h lu l m2 (hnh 1-4a). y l dng c in nht ca mt ct p, n ph hp vi tnh hnh chu lc ca p (p lc nc x ngang cng c biu phn b dng tam gic). Vic chn m 1 0 nhm li dng thm mt phn trng lng nc ln mi thng lu lm tng thm n nh cho p.

b. Mt ct ngang p dng a gic (hnh 1-4 b, c, d, e): tu theo cc iu kin chu lc c th (cao trnh bn ct, mc nc h lu, p lc sng gi...) m c th s dng mt trong cc dng ny tng tnh hp l (tn dng ht kh nng chu lc ca vt liu ti mi mt ct, gim khi lng p...). Dng mt ct trn hnh 1-4b l kh hp l, c s dng nhiu nht, d b tr kt hp gia phn p trn v khng trn, thun tin cho vic b tr b t li chn rc v ca van ca ng tho nc di su.

2. Xc nh mt ct kinh t (mt ct c bn) ca p khng trn

a. Mt ct dng tam gic, h2 0: Vi cc p c chiu cao khng ln, c th s b xc nh nhanh mt ct kinh t theo cc biu lp sn tham kho trong quy phm Lin X CH123-60 (hnh 1-5). Cc biu ny c thit lp da trn cc gi thit sau:

- Mt ct p dng tam gic c h s mi thng lu l m1, h s mi h lu l m2;

hHm

1

(e)(d)

m1

(b)(a)

2m

m1

2m

2m

2m

m'2

(c)

m'm1

2


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- Mc nc thng lu ngang nh ca tam gic; chiu su nc thng lu l h1, chiu su nc h lu l h2;

- Trng lng ring ca btng: g1 = 2,4 x 104 N/m3; ca nc g = 1,0 x 104 N/m3.

- C xt n c p lc y ni v p lc sng.

Biu ny l kt qu tnh ton cho nhiu p vi cc tham s bin i l tr s h2/h1, ng sut nn ln nht khng ch mp thng lu N1, h s mi thng lu m1, ch tiu khng ct ca nn v h s an ton cho php fc/Kcp. Kt qu tnh ton s xc nh c h s mi h lu m2 hp l, tho mn cc iu kin n nh, cng v kinh t.

Hnh 1-5. Biu dng s b chn mt ct kinh t p dng tam gic.

Trong biu , tr s fc l ch tiu khng ct ca nn:

,ctgfc +j= (1-7)

trong tgj v c l cc c trng chng ct: tgj c xem nh h s ma st, cn c nh l lc dnh n v trn mt b ct; s - tr s ng sut nn trung bnh y p, khi xt cho 1 n v chiu di p (bi ton phng), ta c: s = SP/B, vi SP - tng lc ng, B - b rng y mt ct.

Tr s ng sut nn ln nht bin thng lu N1 c khng ch (c d tr an ton) bng cc tr s 1,1gh1 0,2gh1. Nu tr s thc t nm gia cc khong ny th c th dng php ni suy xc nh tr s (m1 + m2) cn thit.

s2p

s2p

s2p

s2p

s2p

s2p

s2p

s2p

s2p

s2p s2p

s2p

s2p

s2p

s2p

h1

h2

m=m +m1 2

c

cp


17

V d: mt p c h1 = 60 m; h2 = 6 m; ng sut nn ln nht mt thng lu khng ch khng ln hn 0,25gh1 (tc 15 x 10

4 N/m2), tr s tgj = 0,62; c = 20 x 104 N/m2; s = 70 x 104 N/m2; fc = tgj + c/s = 0,91; Kcp = 1,21; fc/Kcp = 0,75. Tra biu ng vi h2 = 0,1h1 c: m1 = 0; m1 + m2 = 0,69, tc m2 = 0,69.

b. Mt ct dng tam gic, h2 = 0

Trng hp ny, c th tnh trc tip b rng y p B theo chiu cao p h1 tng ng vi cc iu kin ng sut v n nh nh sau:

- iu kin cng : khng ch ng sut theo phng thng ng ti mp bin thng lu p khi h y nc sy = 0 s gii ra c:

B =

11

1

n)2n(n)1(

h

--+-, (1-8)

trong :

g1 - trng lng ring ca vt liu thn p;

g - trng lng ring ca nc;

a1 - h s ct nc thm cn li sau mn chng thm; n - t l gia chiu di hnh chiu bng ca mi p thng lu so vi ton b b rng

y p B.

Quan h gia n v h s mi thng lu m1 nh sau:

n = m1. Bh1 (1-9)

Trong thc t thng chn tr s m1 0 v, nu m1 < 0 th s kh khn trong vic b tr thi cng mt thng lu p v p chi v thng lu nh vy th s sinh ng sut ko mp h lu khi h cha tch nc (mi thi cng xong), hoc khi mc nc h rt xung thp nht (thi k khai thc).

Trong trng hp m1 = 0 (tc n = 0), ta c:

B =

11

1

h

-. (1-10)

Khi , h s mi h lu ca mt ct p l:

m2 =

11

1

-. (1-11)

- iu kin n nh. T vic khng ch h s an ton v n nh K = Kcp, rt ra c cng thc xc nh b rng y p theo iu kin n nh nh sau:


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B =

a-+

gg

11

c

1cp

nf

hK. (1-12)

trong : fc - h s khng ct ca mt tip xc gia p v nn, xc nh theo ti liu kho st a cht nn; tr s Kcp xc nh theo (1-3); cc k hiu khc nh nu trn.

Trong thit k, chn B theo tr s ln trong 2 kt qu tnh theo (1-8) v (1-12).

Vi nn c h s khng ct fc nh th tr s B theo iu kin n nh s ln hn nhiu so vi iu kin ng sut. Khi nn chn n thin ln nhm li dng trng lng khi nc trn mi tng n nh cho p. Tuy nhin, mi thng lu qu thoi s khng c li cho vic khng ch ng sut ko mp thng lu p. Tr s gii hn ca n tho mn ng thi 2 iu kin cng v n nh (cng thc 1-8 v 1-12) tm c t phng trnh sau:

13)(10f

K2

fK

nf

K22

fK

n

fK

1

12

2cp

112

2cp1

21

12

2cp

2

2cp12

2

2cp

-=

++

+-

+

+

+-

++

+

c. Mt ct dng a gic:

Cch tnh ton sau y ginh cho dng mt ct trn hnh 1-4b. Cn xc nh cc i lng x, m1, m2 sao cho mt ct p ng thi tho mn cc iu kin cng , n nh v c din tch nh nht. Vic tnh ton tin hnh theo cc bc sau:

- Gi thit mt lot cc tr s x, v d x = 0; 0,1; 0,2;...

-Vi mi tr s x, li gi thit nhiu tr s m1. Da vo yu cu n nh chng trt Kt = Kcp tnh c s m2 tng ng, tc lp c quan h m2 = f(m1) nh trn hnh 1-6a.

Hnh 1-6. Cc biu xc nh mt ct kinh t dng a gic

a) Cc ng quan h A ~ m1, m2 ~ m1, N1 ~ m1 ng vi mt gi tr x.

b) Biu chn tr s x ca mt ct kinh t.

OC

A

A a)

1mKt

x x

minA

Ab)


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- ng vi mi cp (m1, m2) ca bc trn, tin hnh tnh ton ng sut chnh mp bin p v lp quan h N1 = f(m1), xem hnh 1-6a.

- Cng vi mi cp m1, m2 , tnh c din tch A ca mt ct c bn v thit lp ng quan h A = f(m1); xem hnh 1-6a.

- Da vo cc ng quan h trn hnh 1-6a, chn c mt cp tr s m1, m2 (ng vi mt gi tr ca x) va tho mn yu cu n nh, cng v cho din tch mt ct p nh nht A (ng vi im C trn trc 0m1, hnh 1-6a).

- Vi cc tr s x khc cng lp li cc bc tng t. Cui cng lp c quan h A ~ x nh trn hnh 1-6b. T quan h ny xc c im Amin v tm c tr s x tng ng. Truy ngc li cc kt qu tnh cc bc trn, ta xc nh c cc tr s m1, m2 ca mt ct kinh t.

III. Xc nh mt ct thc t ca p trng lc 1. p khng trn

a. Cao trnh nh p: Vi p khng trn, cn m bo sng do gi t h cha khng vt qua nh, vi mt d tr cn thit. Cao trnh nh p xc nh theo cc cng thc sau y.

Z1 = MNDBT + Dh + hs + a; (1-14)

Z2 = MNLTK + Dh + hs + a; (1-15) trong :

MNDBT - mc nc dng bnh thng trong h;

MNLTK - mc nc l thit k;

Dh, Dh - dnh ca tm sng trn mc nc tnh ton, tnh tng ng vi gi ln nht Vp% v gi bnh qun ln nht Vbqmax.

hs,hs - cao ln nht ca nh sng khi gp vt chn thng ng, tnh tng ng vi Vp% v Vbqmax nh nu trn.

a, a - cc chiu cao d tr, ph thuc vo cp cng trnh, xc nh theo quy phm.

Cao trnh nh p c chn l tr s max (Z1, Z2). Ngoi ra, cn cn m bo mc nc l kim tra khng vt qu nh p, tc:

Z MNLKT. (1-16) b. Chiu rng nh p. Chiu rng mt nh p b xc nh theo iu kin thi cng,

theo yu cu s dng nh p lm ng cho ngi v xe c qua li trong thi k khai thc, v cho cc mc ch khc nu c. Ni chung mt p khng c nh hn 2 mt. Ngoi ra, hai bn mt nh p cn b tr h thng lan can phng h, cc ct n chiu sng v.v...

c. B tr h thng hnh lang trong thn p. H thng ny bao gm cc hnh lang dc v ngang thn p i li kim tra s lm vic ca cc ging tiu nc v trng thi btng thn p, t cc ng ng, cc thit b o kim tra, tin hnh phun xi mng vo cc khp ni, thi cng mn chng thm di nn, v tin hnh cc cng tc phc hi sa cha sau ny.


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Theo chiu cao p, cc tng hnh lang cn b tr cch nhau 15 - 20m. Cc tng c lin thng vi nhau bi hng lang chy vng theo mt nn, t b tri sang b phi. V nguyn tc, phi thit k hnh lang dc thp nht cao hn mc nc kit h lu m bo vic tho nc t chy. Trong trng hp mc nc kit h lu kh cao, phi t hnh lang thp hn th cn d kin vic bm thot nc ra.

Hnh 1-7. B tr h thng hnh lang trong thn p

a) Ct ngang p; b) Ct dc p; c) Cc dng mt ct hnh lang

Khong cch bt t mt ct thng lu p n mt thng lu ca hnh lang dc cn ly khng nh hn 2m v tho mn iu kin:

bt cpJh

,

trong :

h - ct nc tnh n cao trnh y hnh lang;

J - gradien ct nc cho php ca btng p.

i vi p trng lc thng ly Jcp = 20. iu ny khng p dng i vi p c lp chng thm mt chu p.

Kch thc ca cc hnh lang thng chn nh sau:

* i vi hnh lang tp trung nc, kim tra, b tr thit b o v cc loi ng ng:

- Chiu rng: bh 1,2 m;

- Chiu cao: hh 2,0 m. * i vi hnh lang pht va gn nn, kch thc ca n cn chn sao cho tho mn

iu kin t, vn hnh v di chuyn my khoan pht, thng chn nh sau:

- Chiu rng: bh 2 2,5 m;

c)

1 2

A

1

ll

a)

3 4

b)

h


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- Chiu cao: hh 3 3,5 m. Cc yu cu khc v cu to xem 1-7. 2. p trn nc

Mt ct thc t ca p trn cng c chn xut pht t mt ct c bn, c thm bt mt s phn cho ph hp vi iu kin tho nc v ni tip dng chy h lu.

a. nh p: nh p trn c th cu to theo dng khng chn khng hay c chn khng (u trn dng trn, elp). Loi mt ct c chn khng tuy c tng c h s lu lng ca trn, nhng lm vic khng n nh v d sinh chn ng cng nh pht sinh kh thc phn u trn n tr s chn khng vt qu mc cho php. V vy ch nn lm nh trn c chn khng khi c lun chng xc ng. i vi cc cng trnh quan trng, s lm vic ca p v mc chn khng nh p cn phi c kim tra qua th nghim m hnh.

Hnh 1-8. Cc dng mt ct p trn

a. p c u hnh elip (c chn khng); b) p trn dng khng chn khng; c) p khng chn khng c phn nh v pha trc.

Dng nh trn khng chn khng c ch lm vic n nh v thng c s dng nhiu hn (hnh 1-8b). Khi cn m rng on nm ngang trn nh b tr ca van v thit b, c th lm nh trn c phn nh v pha trc (hnh 1-8c).

To khng th nguyn ca mt trn khng chn khng c th xc nh theo cng thc WES (Phng th nghim cng trnh thu, Hip hi k s qun i Hoa K) [22]:

noo XK1Y = , (1-17)

trong : Yo = y/Ht ; Xo = x/Ht ;

Ht ct nc thit k ca p trn;

K v n cc hng s ph thuc vo nghing mi thng lu m1, ly nh sau:

m1 (mi thng lu) : 0 1/3 2/3 1,0

K : 2,000 1,936 1,939 1,873

n : 1,850 1,836 1,810 1,776 Vi p trn c mt thng lu thng ng, c th xc nh to mt trn Criger-phixrp (bng 1-1)

y y

R

a)

c)

R

b)

O x Ox


22

Bng 1-1. To khng th nguyn ca mt trn Criger - phixrp

Th t im

X0 Y0 Th t im

X0 Y0 Th t im

X0 Y0

1 0,0 0,126 14 1,3 0,475 27 2,6 2,122

2 0,1 0,036 15 1,4 0,564 28 2,7 2,289

3 0,2 0,007 16 1,5 0,661 29 2,8 2,462

4 0,3 0,000 17 1,6 0,764 30 2,9 2,640

5 0,4 0,006 18 1,7 0,873 31 3,0 2,824

6 0,5 0,027 19 1,8 0,987 32 3,1 3,013

7 0,6 0,060 20 1,9 1,108 33 3,2 3,207

8 0,7 0,100 21 2,0 1,235 34 3,3 3,405

9 0,8 0,146 22 2,1 1,369 35 3,4 3,609

10 0,9 0,198 23 2,2 1,508 36 3,5 3,818

11 1,0 0,256 24 2,3 1,653 37 3,6 4,031

12 1,1 0,321 25 2,4 1,894 38 3,7 4,249

13 1,2 0,394 26 2,5 1,960 39 3,8 4,471

b. Mi p: m bo iu kin n nh v ni tip lin tc vi phn p khng trn, mi thng v h lu ca mt p ct p trn u ly theo tr s m1 v m2 ca mt ct c bn tnh. Ti v tr chuyn tip t ng cong nh trn ( xc nh trn) sang ng thng mt mi h lu, ng thng ny phi tip tuyn vi ng cong nh trn trnh to ra im gy, tc trnh to ra ngun sinh chn khng, kh thc cc b trn mt trn.

c. Chn mi h lu: Tu theo hnh thc tiu nng c chn, chn mi h lu c th l b tiu nng (tiu nng y), mi phun (tiu nng phng xa), hay bc tht (tiu nng mt). Ni tip gia mt h lu trn v b phn tiu nng phi qua mt on chuyn tip, thng chn l cung trn bn knh R. Khi ni tip vi b tiu nng, tr s R chn nh sau:

R = (0,2 0,5)(P + Ht), trong :

P - chiu cao p tnh n cao trnh y b tiu nng;

Ht - ct nc trn nh trn.

Nu ni tip vi mi phun, bn knh R c th chn:

R = (6 10)hc, vi hc l su co hp trn mi phun.

1.4 n nh ca p btng trng lc trn nn I. Cc lc tc dng ln p btng trng lc.

1. Cc lc tc dng bao gm (hnh 1-19): - Trng lng p v cc thit b t trn : G;


23

- p lc thu tnh, thu ng t pha thng lu (T1, P1), h lu (T2, P2), trong T - thnh phn nm ngang; P - thnh phn thng ng.

- p lc thm (Wth) v y ni (Wn) t di y p;

- p lc sng Ts

- p lc bn ct t pha thng lu, thnh phn ngang (Eb) v ng (Pb).

- p lc gi Tg - Lc sinh ra do ng t (lc qun tnh ng t ca thn cng trnh, ca khi nc v

bn ct t hai pha ca p).

- Tc dng ca nhit trong thi k thi cng (do ta nhit khi btng ngng kt) v thi k khai thc (do dao ng ca nhit mi trng).

- nh hng ca bin hnh nn. Tr s cc lc v tc ng c xc nh theo cc ti liu chuyn mn. Ring p lc

thm y ngc xc nh theo ti liu s [17] nh trn hnh 1-9. Tr s ct nc p lc thm ti cc im c bit nh sau:

- Ti mp bin thng lu y p: Ht (ct nc thm tnh ton); - Ti trc mn chng thm: hm = am . Ht; - Ti hng l thot nc sau mn chng thm: ht = at.Ht. Tr s ca am v at xc nh theo cp cng trnh v t hp ti trng nh sau:

T hp ti trng c bn T hp ti trng c bit Cp cng trnh

am at am at

I 0,4 0,2 0,6 0,35

II 0,4 0,15 0,5 0,25

III V 0,3 0 0,4 0,15

Ghi ch: t hp ti trng c bit y ch ng vi trng hp thng lu l MNDBT, thit b chng thm v thot nc khng lm vic bnh thng.

Trng hp khng c hng l thot nc sau mn chng thm th biu p lc l ABD (hnh 1-9).

2. Cc t hp lc dng trong tnh ton (vic tnh ton c th theo tiu chun hin hnh).

a. T hp lc c bn: bao gm cc lc thng xuyn hoc nh k tc dng ln p, nh trng lng bn thn v cc thit b t trn p, p lc nc vi mc nc dng bnh thng, p lc sng, gi vi vn tc gi bnh thng (Vbqmax), lc thm v y ni, p lc bn ct.

b. T hp lc c bit: bao gm cc lc trong t hp c bn, cng thm hay thay th mt s lc xy ra trong trng hp c bit nh:

- Lc sinh ra khi c ng t;


24

- p lc thu tnh, p lc thm v p lc y ni khi c mc nc dng gia cng (thay th cho cc lc ny trong trng hp mc nc dng bnh thng);

- p lc thm khi thit b chng thm hoc thit b thot nc khng lm vic bnh thng (thay th cho p lc thm khi cc thit b ny lm vic bnh thng).

Hnh1-9. S cc lc tc dng ln p btng trng lc

Hnh 1-10.

a- S tnh n nh khi mt trt nghing v thng lu (cng thc 1-19)

b- X l chng trt khi nn p nghing v h lu

II. Tnh ton n nh ca p btng trn nn 1. Cc kh nng mt n nh. Vi p btng trng lc trn nn , cc kh nng

mt n nh c th xy ra l:

a. Trt theo mt mt no , c th l mt y p tip xc vi nn, hay mt phng i qua y ca cc chn khay (khi p c lm chn khay cm su vo nn). Trng hp nn phn lp th cn xt thm mt trt i qua cc mt phn lp, l ni cc c trng chng trt ca gim nh so vi mt trt qua nguyn khi. Khi thn p c cc v tr gim yu (khot l, mt ngang tip gip gia cc khi ,...) th cn xt mt trt i qua cc v tr ny.

Tu theo c im b tr cng trnh v cu to nn p m mt trt c th nm ngang hay nm nghing (nghing v pha thng lu hay h lu).

b. Lt theo trc nm ngang dc theo mp h lu ca mt mt ct no , thng l mt y p, hay mt ct m p b khot l, gim yu. Kh nng lt ch c th xy ra khi biu ng sut trn mt nm ngang tnh ton c gi tr m (trn mt phn mt tnh ton c ng sut ko).

c. Nn p b ph hoi khi tr s ng sut t p truyn xung vt qu sc chu ti ca nn. Trong trng hp ny, cn phi thay i hnh dng mt ct p, hoc tng b rng y p iu chnh li phn b ng sut di y p.

2. Tnh ton n nh v trt phng

p m bo n nh khi tho mn iu kin (1-2). Tr s h s an ton n nh trt xc nh nh sau:

H

h

b

SS

S b

S b

bS

S b

b


25

a. Vi mt trt nm ngang:

+j-=T

cFtg)WP(K t , (1-18)

trong :

tgj v c l cc c trng chng ct trn mt ph hoi ; F - din tch mt ph hoi ;

SP tng cc lc theo phng ng, tnh t mt trt tr ln ; W - tng p lc y ngc tc dng vo mt trt, bao gm lc thm v lc y ni thu tnh (nu c) ;

ST - tng i s cc lc nm ngang tc dng vo phn p tnh t mt trt tr ln.

Tr s ca tgj v c ph thuc vo c trng nn (khi mt trt nm trong nn), hay c trng ca vt liu b tng (khi mt trt ct qua khi b tng).

Trong thit k s b, vi mt trt trong b tng c th ly tgj = 1 (tng ng vi gc ma st bng 45o ), cn tr s c ly bng 5% ca cng khng nn ca vt liu.

i vi nn cng trnh cp I v II c cc iu kin a cht cng trnh n gin, trong thit k s b, gi tr tnh ton ca cc c trng ca tgjI, II v cI, II c th ly theo bng 1-2, trong ly cc h s Kj = 1,15 v Kc = 1,8 (Kj v Kc l cc h s an ton v vt liu p dng cho i lng gc ma st trong j v lc dnh n v c). Trong thit k k thut, cng nh khi xt mt trt i qua cc mt phn lp ca nn, tr s ca tgj v c cn phi xc nh bng th nghim. Tr s tnh ton ca tgj v c cn c xc nh trn c s x l thng k cc s liu th nghim (xem [15], ph lc 8).

Lu rng, cc gi tr th nghim ca 1 mu ch phn nh c trng cc b ca vt liu hay nn ti v tr ly mu, nn s lng mu th nghim phi ln v phi tun th cc quy nh theo tiu chun hin hnh [15].

b. Vi mt trt nghing v thng lu: h s an ton n nh trt c xc nh nh sau:

b-b

+j-b+b=sinPcosT

F.ctg).WsinTcosP(K t , (1-19)

trong : b l gc gia mt trt v mt nm ngang; cc k hiu khc nh nu trn.

c. Vi mt trt nghing v h lu: mt gc b, tng t ta c:

b+b

+j-b-b=

sinPcosTF.ctg).WsinTcosP(

K t , (1-20)

R rng vi mt trt nghing v pha thng lu s cho h s n nh v trt cao hn so vi mt trt nm ngang; cn trng hp mt trt nghing v pha h lu l bt li nht v mt n nh trt. V vy, ti cc v tr m nn p c th nghing v h lu th cn x l tng n nh chng trt bng cch lm cho mt tip gip gia p v nn thnh dng rng ca c cnh di nghing v thng lu (hnh 1-10b).


26

c I ,

CII

/Kc

(d

aN/c

m2 )

0,5

0,5

0,2

0,2

Ln

hn

20

tg j

I ,

tg j

II

......

......

......

......

......

......

......

......

......

......

......

......

......

......

......

......

....

Kj

0,55

0,55

0,45

0,15

c I ,

CII

/Kc

(d

aN/c

m2 )

1,0

1,0

0,5

0,3

2

n 20

tg j

I ,

tg j

II

......

......

......

......

......

......

......

......

......

......

......

......

......

......

......

......

....

Kj

0,7

0,7

0,65

0,5

c I ,

CII

/Kc

(d

aN/c

m2 )

1,5

1,5

1,0

0,5

n

nh

v

b

n c

c b

i

vi

cc

mt

v

mt

ph

ng

tr

t tr

ong

a

khi

the

o c

c kh

e n

t c

nh

t c

t v

t

c

st,

vi c

hiu

rn

g m

ing

khe

nt

(m

m)

Nh

hn

2

tg j

I ,

tg j

II

......

......

......

......

......

......

......

......

......

......

......

......

......

......

......

......

....

Kj

0,8

0,8

0,7

0,65

c I ,

CII

/Kc

(d

aN/c

m2 )

4,0

3,0

2,0

1,0

n

nh

v

b

n c

c b

i

vi

c

c m

t

v

mt

ph

ng

tip

xc

b

tng

-

; c

c m

t

tr

t tr

ong

a

khi

m

t

phn

th

eo v

t n

t,

mt

ph

n

tron

g kh

i

nguy

n.

tg j

I ,

tg j

II

......

......

......

......

......

......

......

......

......

......

......

......

......

......

......

......

....

Kj

0,95

0,85

0,75

0,7

CII

(d

aN/c

m2 )

40

25

15

3

Khi

tn

h

b

n c

c b

ca

nn

khi

m

t

tr

t kh

ng

trn

g v

i c

c kh

e n

t v

vi

mt

ti

p x

c b

tng

-

.

tg j

II

3,0

2,4

2,0

1,5

Bn

g 1-

2: G

i tr

tn

h to

n c

a c

c

c tr

ng

n

n tg

j I, I

I v

c I

, II

Lo

i

nn

c s

c c

hng

nn

tc

th

i m

t t

rc R

n >

500

daN

/cm

2 (d

ng

lin

khi

, ph

n k

hi

ln,

cc

kh

i d

ng p

hn

lp,

dn

g ph

in

t n

t n

, kh

ng b

ph

ong

ho)

c R

n >

500

daN

/cm

2 (d

ng

lin

khi

, ph

n kh

i

ln,

cc

kh

i d

ng p

hn

lp,

dn

g ph

in

nt

n v

a,

phon

g ho

y

u)

c R

n =

150

500

daN

/cm

2 (d

ng

lin

khi

, ph

n kh

i l

n, c

c k

hi d

ng

phn

lp,

phi

n n

t n

nhi

u)

c R

n =

50

500

daN

/cm

2 (p

hong

ho

yu

,

b

n nh,

t

t gy

)

na

cng

Rn

< 5

0 da

N/c

m2

(dn

g ph

in,

phi

n

mn

g, n

t n

trun

g b

nh v

m

nh)

Ghi

ch

: ch s

I

v I

I t

ng

ng

vi k

hi t

nh to

n th

eo tr

ng

thi

gi

i hn

1 v

2


27

3. Tnh ton n nh v lt

Ti mi mt ct tnh ton, phn p pha trn mt ct ny c th b lt quanh trc i qua mp bin h lu ca mt ct nu mp trn thng mt ct ny c xut hin ng sut ko. iu kin p khng b lt l:

cpl

cl KMM

, (1-21)

trong :

SMcl - tng mmen ca cc lc chng lt ly i vi trc nm ngang i qua mp h lu mt ct tnh ton;

SMl - tng m men ca cc lc gy trt i vi trc ni trn; Kcp - H s an ton cho php, xc nh theo (1-3).

Theo tiu chun ca Hoa K [23], khi xt mt mt ct tnh ton bt k, khng gy lt th hp ca tt c cc lc tc dng ln phn p pha trn mt ct ny phi nm trong khong 1/3 gia b rng mt ct khi xt t hp ti trng c bn, nm trong khong 1/2 gia b rng mt ct khi xt t hp ti trng bt thng v nm trong phm vi mt ct khi xt t hp ti trng cc hn.

III. Tnh ton bn ca nn p Vi mi t hp lc tnh ton, ng sut y mng p c xc nh theo cng thc nn

lch tm:

symax, min = WM

FP 0 , (1-22)

trong :

P - tng hnh chiu cc lc thng ng tc dng ln p tnh t mt y p tr ln (c xt c p lc thm v y ni);

M0 - tng mmen ca cc lc tc dng ln p tnh t mt y p tr ln, ly i vi tm mt y;

F - din tch mt ct y p;

W - mun chng un ca mt y p, theo phng un t thng v h lu.

i tng tnh ton c th l cho mt on p gia 2 khp ni, hoc mt mt di p (bi ton phng).

bn ca nn c m bo khi tho mn cc iu kin:

symax < Rn ; (1-23)

symin 0 ; (1-24) trong : Rn - cng khng nn tnh ton ca nn.


28

1.5. Tnh ton bn ca p btng trng lc I. Nhng vn chung 1. Ni dung tnh ton: tnh ton bn ca p bao gm phn tch trng thi ng sut

trong thn p kim tra iu kin bn trong cc trng hp lm vic khc nhau ca p, c th nh sau:

a. Tnh ton cc ng sut chnh ln nht v nh nht bin p kim tra cc iu kin bn trn bin (thng ng dng trong giai on thit k s b).

b. Xc nh trng thi ng sut trong thn p:

- Xc nh gi tr ng sut ti cc im khc nhau trong thn p;

- V ng ng ng sut chnh tin hnh phn vng s dng vt liu cho thch hp.

- V cc ng qu o ng sut chnh, t c th b tr cc mt khe thi cng, cc ng ng t trong thn p.

c. Tnh ton cc loi ng sut tp trung cc b xut hin quanh cc ng hm, l khot trong thn p v tnh ton b tr ct thp chu lc cho ring khu vc ny.

2. Cc trng hp tnh ton

Cn phi tnh ton vi cc trng hp lm vic khc nhau ca p, tc xt cc t hp lc c bn v t hp lc c bit khc nhau tc dng ln p. Trong thit k, cn da vo iu kin a hnh, a cht v c im cng trnh m ch ng t ra cc trng hp lm vic bt li ca p nh khi c ng t, khi c l c bit ln, khi c nhit thay i, c bin hnh nn...

3. Cc phng php tnh ton

Ty theo mc phc tp ca bi ton v yu cu chnh xc, c th p dng cc phng php khc nhau phn tch ng sut thn p nh:

a. Phng php tnh ng sut mt bin;

b. Phng php phn tch trng lc tnh ng sut trong thn p (phng php chia li).

c. Phng php l thuyt n hi.

d. Cc phng php s: c th l phng php sai phn hu hn, hay phn t hu hn. C s thit lp cc phng trnh tnh ton c th l l thuyt n hi hoc l mt gi thuyt no v c ch ph hu vt liu.

Sau y s trnh by cc phng php da trn l thuyt n hi, p dng cho bi ton phng.

II. Xc nh ng sut ti cc mp bin p Vic tnh ton tin hnh cho mt mt ct ngang bt k theo chiu cao (c th l mt ct

st nn, hay mt ct lng chng p).

1. ng sut php trn mt phng nm ngang: - S dng cng thc nn lch tm ca l thuyt sc bn vt liu:


29

sy, sy = WM

FP 0 ; (1-25)

trong :

sy - ng sut php trn mt phng nm ngang. y v cc phn tip theo, k hiu 1 du phy l dnh cho bin thng lu; 2 du phy l dnh cho bin h lu;

P - tng hp cc lc thng ng k t mt tnh ton tr ln (trng hp mt tnh ton st nn th P phi k n c lc y ni do thm v p lc thu tnh);

F - din tch mt tnh ton, y F = B x 1, B l chiu di mt tnh ton t bin thng lu n bin h lu p;

W - mun chng un ca mt ct: W = 6Bx1 2

;

M0 - tng mmen ca cc lc tc dng k t mt ct tnh ton tr ln i vi im gia ca mt ct . Khi quy nh chiu dng ca M quay thun chiu kim ng h v trong mt ct, bin thng lu bn tri, bin h lu bn phi th trong cng thc (1-25), du - p dng cho sy v du + cho sy.

Hnh 1-11. S xc nh cc loi ng sut bin p

a) Cc ng sut trn mt phng ng v ngang; b) Cc ng sut chnh.

2. ng sut ct txy ti bin (sau y vit tt l t) - Ti bin thng lu: t = (g.y - s.y).m1 ; (1-26) - Ti bin h lu:

t = (g.y - s.y).m2 ; (1-27) trong :

g - trng lng ring ca nc; y - chiu su nc thng lu tnh n mt ct tnh ton;

y - chiu su nc h lu tnh n mt ct tnh ton;

sy, sy - theo kt qu xc nh mc trn;

a

aa

s ''s '

a

dx

dxdx

dy

g yoys' s ''y

t' t'

1

x x

s 't"

dx

t"t' ys''

2y

g yo2 2N '1

N "1

t"

y

a)

y

b)


30

m1, m2 - tng ng l h s mi p thng, h lu.

Vi k hiu a1, a2 nh trn hnh 1-11 th: m1 = tga1; m2 = tga2

3. ng sut php sx trn bin - Bin thng lu:

sx = g.y - t.m1 ; (1-28) - Bin h lu:

sx = g.y + t.m2 ; (1-29) cc k hiu nh nu trn.

Khi h lu khng c nc th:

sx = t.m2 = sy.m22 (1-30)

4. ng sut chnh bin:

Hnh 1-12. S xc nh ng sut chnh bin

a) Bin thng lu; b) Bin h lu.

Da vo s lc ln phn t bin hnh 1-12, xc nh c:

- Ti bin thng lu:

N2 = g.y ; (1-31)

N1 = sy(1 + m12) - g.ym1

2 (1-32)

- Ti bin h lu:

N2 = gy; (1-33)

N1 = sy(1 + m22) - g.ym2

2 (1-34)

Khi h lu khng c nc ( y=0):

N2 = 0; N1 = sy(1 + m2

2) (1-35)

5. ng sut ct chnh:

ng sut ct chnh ti 1 im l tr s ng sut ct ln nht ti im . Theo l thuyt sc bn vt liu:

dl

t"dxt'dx

dxdx

s ' dxs" dx

dlds

1a

N' dl

2a

N"dl

a) xb)

O

dsN" dl

2


31

T = 2

21 NN - (1-36)

C th biu din quan h gia cc thnh phn ng sut ti mt im bng s vng Mo ng sut nh trn hnh 1-13.

Hnh 1-13. S vng Mo ng sut ti 1 im:

OE = sx; OF = sy ;ED = txy.

III. Xc nh ng sut trong thn p theo phng php chia li.

Theo phng php ny, mt ct p tnh ton c chia bng mt li trc giao c cc mt li theo phng x l Dx, theo phng y l Dy. Mc chnh xc t c ph thuc vo nh ca Dx, Dy. Gi tr ca cc thnh phn ng sut c xc nh ti cc im nt trn li.

Cc cng thc trong phn ny c lp vi h trc quy c nh sau: trc ox hng t h lu ln thng lu; trc oy hng t trn xung di. Cc tr s ng sut trn bin c xc nh mc trn.

1. ng sut php sy:

-==

+=

)/B"'(b;"abx;a

yyy

y (1-37)

2. ng sut ct t:

t+t+=

t+t+-=

=++=

2"'1

"'1

1

2111

)/B33B/Q6(c

)/B42Q/B(6b;"a

;xcxba

(1-38)

3. ng sut php sx:

,dydc

.31d

;dydb

21

mc

c

;dyda

mb

b

;"a;xdxcxba

12

1

2

12

1

2

12

x2

32

2222x

=

+=

+=

=+++=

(1-39)

N1ECFN2

D

t xy

s

t

O

T


32

trong : a1, b1, c1 l cc tham s ca biu thc tnh t ti cc mt ct xc nh mc trn. C th tnh gn ng cc o hm trong cng thc (1-39) nh sau:

y

ccdydc;

ybb

dydb;

yaa

dyda *111

*111

*111 -=

-=

-= , (1-40)

trong : a1*, b1

*, c1* l cc tham s ca biu thc t ti mt ct ln cn pha trn mt ct

ang xt (2 mt ct cch nhau mt khong Dy).

i vi cc p thp, n gin tnh ton, c th coi sx phn b tuyn tnh trn mt ct ngang:

-==

+=

)/B"'(b"a

x;ba

xx2

x2

22x

(1-41)

4. ng sut chnh. Theo cc cng thc ca sc bn vt liu: a. Tr s:

N1 = 22

yxyx 4)(

21

2

+-++

(1-42)

N2 = 22

yxyx 4)(

21

2

+---

(1-43)

b. Phng: phng ca N1 v N2 vung gc vi nhau (xem hnh 1-13). Phng ca N1 (ng sut php ln nht) hp vi trc x (khi sx > sy) hoc vi trc y (khi sx < sy) mt gc a; gc a xc nh theo:

tg2a = - yx

2-

(1-44)

Gc a ly theo chiu ngc kim ng h khi tr s a tnh c l dng.

5. ng sut tip chnh:

T = 22yx 4)(21

+- (1-45) 6. Cc ng ng ng sut v qu o ng sut

a. ng ng ng sut: V ring ng ng ng sut php ln nht N1 v ng sut nh nht N2 theo nguyn tc v ng ng mc sau khi in tt c tr s ng sut tng ng ln cc nt ca li.

ng ng N1 dng kim tra kh nng chu nn v phn vng vt liu p. ng ng N2 cho php khoanh cc vng x l c bit khi c ng sut ko.


33

Hnh 1-14. Cc ng ng ng sut, qu o ng sut

a) ng ng N1; b) ng ng N2; c) Qu o ng sut chnh

b. Qu o ng sut

Biu qu o ng sut cho bit phng ca cc ng sut chnh ti mt im bt k ca mt ct. l mt h gm 2 h ng trc giao: h th nht biu th phng ca N1; h th hai - phng ca N2.

Cch v: - V mi tn ch phng ca N1, N2 ti tt c cc im nt ca li. - Dng ni suy v dn cc qu o ng sut: Nu qu o i qua im nt th n phi tip tuyn vi phng ca N1(hoc N2) ti nt

; Nu qu o i qua khong gia 2 nt k nhau th dng ni suy xc nh phng

ca N1 (hay N2) ti im ang xt.

IV. Tnh ton ng sut trong p theo phng php l thuyt n hi (LTH)

Khi tnh ton p btng trng lc, ta xt bi ton phng ca LTH. Ni chung mt ct p thng c dng tam gic, di thn p l mt nn bn v hn, do phn tch ng sut thn p c th a v dng bi ton hnh nm bn v hn. Mt cch gn ng, khi vt liu thn p v nn l gn ng cht, c th xem p nh mt hnh nm v hn tnh ton. Vic tnh ton c tin hnh ring cho tng loi ngoi lc, v cui cng dng php cng tc dng xc nh ni lc tng cng. Sau y l mt s s c th.

1. p c dng hnh nm v hn di tc dng ca p lc nc v trng lng bn thn.

Theo l thuyt n hi ng sut ti mt im bt k trong thn p c chiu cao v hn (hnh 1-15) c biu din di dng cc hm s tuyn tnh:

+=

+=s+=s

ybxaybxaybxa

33

22y

11x

(1-46)

Hnh 1-15.S tnh ng sut theo l thuyt n hi

Cc h s ca cc phng trnh ny c xc nh t vic phn tch ng sut ti cc bin p.

y

sx=m

y 1

sy

x

1a

t

x=m

y2

2a

O x

a)

2,5

2,0

1,5

MPa

0,5

1,00,4

0,6

MPa

0,2

0

b) c)


34

-=-=

--+

-++

=

-++

--+

=

--+

-+

=

--+

+-+

=

13

23

122122

21

22

212

21

12

21212

21122

21

12

21221

222

21

22

212

21

11

2221212

2112212

21

11

abb1a

);mm2m(m)m(m

)m(m)m(m

b

);2mm3(m)m(m

)m(m)m(m

a

);mm3mm2(m)m(m

mm2)m(m

b

)2mm(mmm)m(m

)m(mmm)m(m

a

(1-47)

y: m1 = tga1; m2 = tga2 .

ng sut chnh v phng ca chng cng c xc nh theo cc cng thc (1-42); (1-43); (1-44).

ng sut ti cc bin p c xc nh t cc biu thc (1-46), (1-47) vi x = - m1y v x = m2y.

+=+=

+-=+=+=+=

+-=+=

+=+=

+-=+=

y)bma(ybxa")ybma(ybxa'

)ybm(aybxa")ybma(ybxa'

)ybm(aybxa")ybma(ybxa'

32333

31333

12111x

11111x

22222y

21222y

(1-48)

ng sut chnh ti cc mp p c xc nh theo cng thc (1-42) v (1-43) nhng cc tr s sx, sy, t trong c thay th bi cc tr s bin p c tnh cc cng thc (1-48).

2. p c dng hnh nm v hn chu ti trng phn b u

Trong mt s trng hp p c th chu tc dng ca ti trng phn b u c phng lm vi mt p mt gc no (hnh 1-16).

Bi ton ny c M. Levy gii v cho cc kt qu ring theo thnh phn lc thng gc p1 v lc tip tuyn p2 tc dng ln mt p.

ng sut do thnh phn lc thng gc p1 tc dng ln mt im n(x,y) bng:


35

-+-=

-

+

---=

-

+

+--=s

B

1tg1

A

p

;B

mAtg1

tgp

;B

mAtg1

tgp

2

1

122

1y

222

1x

(1-49)

Hnh 1-16. S tnh ton ng sut khi mt p chu ti trng phn b

u

y A = 1 - m1m2; B = m1 + m2 - (a1+ a2)(1 - m1m2).

ng sut do thnh phn tip tuyn p2 gy ra bng:

a+-a

+-=

---=

+--=s

Btg1

mtgmCmp

;B

1DCp

;B

mmDCmp

22

212

2

2

2y

2122

2x

(1-50)

y: ;mm

m1)(C21

22

21 ++

+=

D = ( )2122 mmtg1tg +

+

--

3. p c dng hnh nm v hn chu tc dng ca lc tp trung nh

Bi ton ny c Mitchen gii trn h to c cc vi gc O t ti nh tam gic (hnh1-17).

Trc to cc Oc l ng phn gic ca gc nh tam gic. Gc gia trc to vung gc Oy v trc Oc

bng 21

(a2 - a1). Ta k hiu sr, sQ, l ng sut theo phng

hng tm v phng tip tuyn do lc R gy ra ti im D (x,y) trong thn p. Ta phn lc R thnh hai thnh phn: lc thng ng P v lc nm ngang Q.

Hnh 1-17. S tnh ton ng sut khi p chu lc tp trung v mmen t ti nh.

y

2a1a

y

xn

P

P1

P2

a

O X

O XO

y

a

q

a1a2

a -a 2

2 1

a +a 2

12

c

D

sr

R

MQ

P


36

ng sut do P gy ra bng:

c

BA)mm(tgx)tg1y(

P2 1121

22

2rP+--

+=s (1-51)

ng sut do lc Q gy ra bng:

1

1121

22

2rQ C)BA(tgmmx

)tg1y(Q2 +--

+=s (1-52)

y:

+-++-+=

++=+++=

,)m(m)m1)(m1()(C);m)(mmm1(B

);m1)(m1)((A

221

22

21

2211

21211

22

21211

(1-53)

trong a l gc gia tia OD v trc y. Trong h to vung gc cc ng sut ti D c xc nh nh sau:

+=

=+

=

=+

=s

.tg1

tg

;costg1

;sintg1

tg

r2

2r2

ry

2rr2

2

x

(1-54)

4. ng sut do mmen tp trung M nh gy ra

( )( ) ( )[ ]

( )( ) ( )[ ]

( )( ) ( )[ ]

+++---+

-=

++---+

-=

+++---

-=s

.B)tg1.(ymmtgmm13tgmmtg12tgM2

;.B)tg1.(y

tgmm212mmtg31tg2M2

;.B)tg1.(y

tgmm2tgmm22tgmmtg3M2

32221

22112

24

M

3222112

23

y,M

32221

321

212

2

x,M

(1-55)

y B = m1 + m2 - (a1 + a2)(1 - m1m2).

5. ng sut trong p btng do cc ti trng khc gy ra Ta c th tnh c ng sut trong p hnh nm di tc dng ca cc ti trng khc

nh bn ct, sng, lc qun tnh ng t v.v...

V d khi xt ti trng bn ct ta c th chuyn p lc bn ct Wbc ti nh ca tam gic. Sau khi chuyn ta c mt lc Wbc v mt mmen M = Wbc.h (h l khong cch t lc Wbc n nh tam gic). Sau ng dng cc cng thc (1-51) (1-55) tnh ng sut trong thn p.


37

6. M t phn b ng sut trong thn p

a. V cc ng ng ng sut

T nh mt ct p tam gic k hng lot cc tia v nh s th t cc tia nh sau: trc Oy c k hiu l O, cc tia pha phi ca Oy c k hiu l 1, 2, 3... cc tia pha tri Oy c k hiu l - 1, - 2, - 3,... (hnh 1-18).

T cc cng thc (1-46) ta c th vit:

=

+=+=

=

+=+=

=

+=+=

.yybyxa y) bx (a

.yybyxa y) bx (a

.y;ybyxa y) bx (a

*3333

*y2222y

*x1111x

(1-56)

Ti 1 im A (x,y) trn mt tia bt k th yx

= tgan do ***

x ,, y tss l cc hng s cho

mi tia.

Thay cc biu thc (1-56) vo cng thc (1-42) v (1-43) ta c th tnh c cc ng sut chnh trn tia ang xt:

N1,2 = .yN.y2)2()()( *

2,1

2*2*y

*x

*y

*x =

+++ (1-57)

T = .yT.y2

NN.y

2

)2()(*

*2

*1

2*2*y

*x

=-

=++

(1-58)

Ti mt tia bt k ta c th tnh c nmtgyx

= v cc tr s

***x ,, y tss ,

*21 ,T,NN** , sau s dng cc cng thc (1-

57), (1-58) v ng ng ng sut.

Hnh 1-18. S v

ng ng ng sut.

Th d v ng ng ng sut php chnh N1 c tr s bng F kg/cm2 (hnh 1-18).

Ta c: N = F.yN*1 =

y = *1N

F (1-59)

Tr s *1N cho cc tia thay vo cng thc (1-59) ta c th tnh c cc tung yn ca cc im ca ng ng ng sut F, ni cc im ta c ng ng ng sut N1 = F.

Cho cc tr s khc ca N1 v tng t ta c cc ng ng ng sut vi cc tr s khc nhau.

F Ynx

1

y

y

yy

y

y

n

x

-2

F

-1O

-2a

1 2

A

3

4F1

F

2

1a

-1

a2

1

O

y

3

4

X


38

Cc ng ng ng sut thng c v vi cc tr s ng sut 5, 10, 15, 20 v.v... kG/cm2.

Hnh 1-19 biu din cc ng ng ng sut php chnh, trong hnh 1-19a cho trng hp h y nc; hnh (1-19b) cho trng hp h khng c nc; hnh 1-19c l tng hp ca hai trng hp trn, n biu th s thay i tr s ng sut trong tng vng ca thn p. V d vng c gch cho l vng chu ng sut php chnh vi tr s 15 kG/cm2.

Hnh 1-19. Cc ng ng ng sut php chnh

a) Trng hp h y nc; b) H khng c nc; c) Tng hp ca 2 trng hp trn.

b. ng qu o ng sut chnh

Phng tc dng ca cc ng sut php chnh c xc nh bng gc nghing q gia chng vi trc Ox (hnh 1-20). xc nh gc q ta thay cc biu thc (1-56) vo cng thc (1-44) v c:

tgq = 2

)()()( 2*2*y*x

*y

*x +---

(1-60)

Trong cng thc du dng ng vi ng sut chnh th nht; du m cho ng sut chnh th hai.

Ta tin hnh v ng qu o ng sut php chnh nh sau:

T nh tam gic ta v cc tia 1, 2, 3,... da vo cng thc (1-60) ta c th tnh c tr s tgq cho tng tia. Sau dng phng php gii v ng qu o ng sut php chnh (hnh 1-20). Theo phng x ly mt on OO c chiu di n v, ta v cc on O1, O2, O3,... theo iu kin

O'O/1'O = tgq1; O'O/2'O = tgq2; O'O/3'O = tgq3,... trong cc tr s tgq ca tng tia tnh c theo cng thc (1-60). Phng ca cc on O1, O2, O3,... biu th phng ca ng sut php chnh trn cc tia 1, 2, 3,...

Th d v ng qu o ng sut php chnh N2. T mt im A no trn tia 1 ta k

on, 1O//AB , im B nm trn ng phn

Hnh 1-20. S v ng qu o ng sut php chnh

0,5MPa

1,5

1,0

0,5

MPa2,5

1,5

2,0

1,0

a)

0

b) c)

1 2 3 4 5 6 7 8 9

O

89

tg q

567

4

3

3

0

1


39

gic ca gc hp gia tia 1 v tia 2; t B k on 2O//BC , im C nm trn ng phn gic ca gc hp gia tia 2 v tia 3. Tip tc lm tng t ta xc nh c cc im D, E, F, G v ng cong qua cc im A B C D E F G ta c ng qu o ng sut N2.

v ng qu o ng sut php chnh N1 ta tin hnh nh sau: t im A trn tia 9

v on 9O'B'A ^ im B nm trn ng phn gic ca gc c to thnh gia tia 9 v tia 8; t B v on 8O'C'B ^ , im C nm trn ng phn gic ca gc c hp thnh gia tia 8 v tia 7, tng t ta v c ng qu o ng sut chnh N1 l ABCDEFDv h ng qu o ng sut chnh N1.

Cc ng qu o ng sut tip chnh hng theo gc 450 so vi cc ng qu o ng sut php chnh.

a) b) c)

Hnh 1 - 21. Cc h ng qu o ng sut

a. H ng qu o ng sut php chnh khi h y nc;

b. H ng qu o ng sut chnh khi h khng c nc;

c. H ng qu o ng sut tip chnh khi h y nc.

Da vo cc ng qu o ng sut chnh ta bit c tnh hnh phn b v phng tc dng ca cc ng sut chnh trong thn p, trn c s ta c th b tr ct thp, chn v tr cc hnh lang, khe ln v cc khe thi cng mt cch hp l.

V. Tnh ton ng sut trong p v nn cng lm vic nh mt h thng nht.

1. Lun im chung: Khi chiu cao ca nm l hu hn, p c mt ct tam gic t ln nn dng na mt phng v hn, mt tip gip gia p v nn xut hin s phn b li ng sut trn p v trn ton thn p do s cng bin dng ca p v nn gy ra.

ng sut y ph thuc nhiu vo tnh n hi ca p v nn, tc vo t s cc mun n hi (bin dng) E v En, cng nh vo h s Poatxng m v mn. Vic gii bi ton tip xc ny rt phc tp v kh khn.

Tnh ton gn ng ng sut tip xc xut pht t li gii cho dm hay bn trn nn n hi.

Theo phng php ca I.A.Constantinp, ngi ta cho rng nh hng ca vng tip xc ln ng sut trong thn p c lan truyn ln pha trn vo khong 0,2h tnh t nn,

trong h l chiu cao p. phn p nm bn trn vng ny, ng sut c th xc nh hon ton theo l thuyt nm v hn. Phn p c chiu cao 0,2h st nn c th tnh nh

90

90

90 90


40

dm gn trc tip vi nn n hi, chu tc dng ca trng lng bn thn v ti trng ngoi dng ng sut truyn t phn trn ti (ng sut ny c tnh theo bi ton nm v hn).

Theo phng php ca c hc kt cu phi tnh lp nhiu, khi lng tnh ton ln v tc chm. Ngoi ra trong trng hp nn khng ng nht hay thn p c cc vng vt liu khc nhau th vic x l cng thm phc tp, phi s dng cc gi thit n gin ho, kh t c chnh xc cn thit. V vy, hin nay cc phng php tnh nu trn t c dng; thay vo l xu hng p dng rng ri phng php phn t hu hn v cc chng trnh my tnh.

2. Tnh ton ng sut p v nn bng phng php phn t hu hn (PTHH)

Ngy nay phng php PTHH c ng dng rng ri trong tnh ton cng trnh v nn. Theo phng php ny p c chia thnh cc phn t nh, chng hn chng c dng tam gic i vi bi ton phng hay khi t din bi ton khng gian. Cc ti trng khi lng (trng lng bn thn, lc thm...) cng nh ti trng b mt (p lc nc, bn ct...) trn mi p, nn, sn v b u c bin i v mt h lc tp trung t cc im nt (nh) ca cc phn t.

Theo phng php PTHH, bi ton quy v vic gii mt h phng trnh i s xc nh quan h gia cc lc tc dng vo nt ca phn t v chuyn v ca cc im gc ca phn t. Quan h ny c biu din di dng ma trn:

},U{}K{}F{ = (1-61)

trong :

{F} - vct ct ca cc lc nt;

{K} - ma trn cng ca h;

{U} - vct ct chuyn v cc im nt ca h.

Khi gii bi ton phng, mi nt xt cc lc (v chuyn v) theo 2 trc (x, y ), tng s phng trnh l 2n (n - s nt trong h). i vi bi ton khng gian (3 bin x, y, z), s phng trnh l 3n.

Khi gii bi ton thc t, phi chia min tnh ton (bao gm p v phn nn di ) thnh mt s ln cc phn t, s c th t ti hng nghn. Li gii ch c th tm c vi s tr gip ca my tnh.

Phng php phn t hu hn l mt phng php tng hp c s dng gii cc bi ton khc nhau, c th xt c cc vng c mun n hi ca vt liu khc nhau, vt liu c tnh d hng, s c mt cc vng gim yu, cc khp ni, khe nt...

Trn hnh 1-22 l mt v d v li phn t ca bi ton khng gian (a) v bi ton phng (b) khi gii theo phng php PTHH.

Cc phn mm phn tch ng sut p v nn c s dng ph bin hin nay l: SAP 2000 (Cng ty my tnh v kt cu SCI - M), SIGMA/W (hng GEO-SLOPE- Canada) v phn mm KC2 ca GS Nguyn Vn L trng i hc Thu li,...


41

Hnh 1-22. S li chia phn t ca p v nn

a) Bi ton khng gian; b) Bi ton phng; 1 - p; 2 - Nn;

3 - Phn nn, b gn p.

VI. Tnh ton ng sut tp trung quanh ng hm trong thn p

ng hm trong thn p btng thng dng 2 dng mt ct l dng mt ct tiu chun (phn di l hnh ch nht, trn nh l mt na vng trn nh hnh 1-23a) v dng mt ct hnh elp, pha di tin i li, kim tra sa cha, thng tin hnh btng t II lm cho y ng hm bng phng.

Sau y l phng php tnh ton ng sut tp trung quanh ng hm tiu chun.

S tnh ton nh hnh 1-24, trong trng ngoi lc tc dng quanh ng hm ly bng cc tr s sy, sx v t trung tm

ng hm tnh ton theo cc ni dung phn trn khi p cha c ng hm.

Hnh 1-23. Cc dng mt ct ca ng hm

a) Mt ct tiu chun; b) Mt ct elp

Dng cc bng biu lp sn theo kt qu th nghim quang n hi cho dng ng hm tiu chun di tc dng ca trng ngoi lc n v l sy = 1; sx = 1 v txy = 1 tin hnh tnh ton ng sut tp trung cc b quanh dng ng hm ny (theo kt qu nghin cu th nghim quang n hi ca Cc khai hoang Hoa K).

10075100

2

1

a)

100

3

b)

1:0,75

200

a) b)


42

Hnh 1-24. S tnh ton ng sut quanh ng hm tiu chun

a) S ng hm trong p; b) S ngoi lc tc dng ln vng ln cn ng hm; c) Mt ct tiu chun ca ng hm.

Cc bc tnh ton nh sau:

a. Xc nh ng sut sx, sy, txy ti trung tm ng hm trong thn p dng lm trng ngoi lc tc dng quanh ng hm.

b. Xc nh cc mt ct cn tnh ton quanh ng hm (xem hnh 1-24c). Cn ch trng cc mt ct A, B, C, D, E, F trn nh ng hm v cc mt ct cc cnh gc ng hm G, K.

c. Tnh ton ng sut tp trung cc b sq, sr v trq ti cc im trn mt ct A, B, C... ny theo cng thc sau y:

t+s+s=t

t+s+s=s

t+s+s=s

q

q

xy3x2y1r

xy3x2y1r

xy3x2y1

CCC

BBB

AAA

; (1-62)

trong : A1, A2, A3, B1, B2, B3, C1, C2, C3 l cc h s ng sut ca cc loi ng sut sq, sr v trq sinh ra di tc dng ca ngoi lc n v sy = 1, sx = 1, txy = 1 tra

Hnh 1-25. ng sut tp trung quanh ng hm

c theo cc bng 1-3 n 1-11.

d. Chnh bin kt qu, v biu ng sut tp trung quanh ng hm, nh th d ghi trong hnh 1-25 v tnh ton b tr ct thp.

xssx

sytxy

sy

1,8 r

r r

r

0,7

r0,

7 r

A BC

D

E

F

G

H

I

J

K

LM

a) b) c)

_

+ +

_


43

Bng 1-3. H s ng sut hng vng A1 di tc dng ca ngoi lc sy = 1

Khong cch t

mp l n

im tnh

ton

Mt ct

0 0,2r 0,4r 0,8r 1,2r 1,6r

A B C D E F G H I J K L M

- 1,00 - 0,94 - 0,55 - 0,84 2,10 2,75 2,50 2,18 2,26 3,20 3,20 3,20

- 1,00

- 0,34 - 0,11 0,34 0,79 1,6 2 1,96 2,01 1,89 1,92 1,82 0,80

- 0,12 - 0,53

- 0,10 0,01 0,33 0,73 1,38 1,58 1,70 1,68 1,72 1,44 0,59

- 0,10 - 0,22

0,00 0,08 0,41 0,67 1,10 1,27 1,42 1,43 1,46 1,32 0,53

- 0,04 0

0,02 0,11 0,39 0,66 0,97 1,10 1,28 1,29 1,31 1,22 0,52 0,04 0,15

0,05 0,12 0,36 0,63 0,89 1,04 1,20 1,17 1,26 1,22 0,52 0,13 0,21

Bng 1-4. H s ng sut hng vng B1 di tc dng ca ngoi lc sy = 1

Khong cch t mp l n im tnh ton Mt ct

0 0,2r 0,4r 0,8r 1,2r 1,6r


0 0 0 0 0 0 0 0 0 0 0 0 0

0,02 0,14 0,10 0,14 0,11 0,00 0,14 0,09 0,09 0,26 0,92 0,72

- 0,03

0,13 0,27 0,20 0,23 0,17 0,01 0,19 0,11 0,14 0,24 0,73 0,78

0

0,13 0,27 0,20 0,23 0,17 0,01 0,19 0,11 0,14 0,24 0,73 0,78

0

0,59 0,59 0,50 0,38 0,24 0,04 0,18 0,17 0,18 0,12 0,64 0,92 0,37

0,71 0,67 0,59 0,41 0,25 0,06 0,14 0,19 0,18 0,10 0,61 1,00 0,47


44

Bng 1-5. H s ng sut hng vng C1 di tc dng ca ngoi lc sy = 1


0 0,2r 0,4r 0,8r 1,2r 1,6r


0 0 0 0 0 0 0 0 0 0 0 0 0

0 0,38 0, 51 0,36 0,28 0,24 0,22

0 0

0,05 - 0,72 - 0,15

0

0 0,40 0,55 0,60 0,42 0,32 0,18

0 0

0,04 - 0,70 - 0,27

0

0 0,36 0,56 0,66 0,56 0,37 0,07

0 0

0,04 - 0,68 - 0,35

0

0 0,32 0,57 0,62 0,59 0,38 0,02

0 0

0,04 - 0,66 - 0,27

0

0 0,29 0,58 0,58 0,54 0,39

0 0 0

0,04 - 0,64

0 0

Bng 1-6. H s ng sut hng vng A2 di tc dng ca ngoi lc sx = 1


0 0,2r 0,4r 0,8r 1,2r 1,6r


4,00 3,42 2,33 1,21

- 0,24 - 0,91 - 1,10 - 1,08 - 0,98 3,40 3,40 3,40 2,72

2,83 2,68 1,85 1,09 0,41 0,06

- 0,60 - 0,43 - 0,64 - 0,05 0,20 2,20 2,09

2,20 2,13 1,53 0,99 0,40 0,07

- 0,40 - 0,20 - 0,39 - 0,22 0,09 1,57 1,86

1,52 1,52 1,22 0,85 0,39 0,09

- 0,17 0,09

- 0,11 - 0,18 0,05 1,33 1,57

1,32 1,33 1,08 0,77 0,38 0,11

- 0,04 0,26

0 - 0,10 0,05 1,29 1,50

1,23 1,21 0,99 0,72 0,37 0,12 0,03 0,34 0,04 0,02 0,05 1,23 1,40


45

Bng 1-7. H s ng sut hng vng B2 di tc dng ca ngoi lc sx = 1


0 0,2r 0,4r 0,8r 1,2r 1,6r


0 0 0 0 0 0 0 0 0 0 0 0 0

0,38 0,43 0,43 0,23 0,23 0,19

- 0,06 - 0,08 - 0,11 1,35 1,10

- 0,40 0,20

0,51 0,52 0,55 0,31 0,32 0,28 0,12 0,00

- 0,01 1,13 0,92 0,05 0,27

0,48 0,42 0,50 0,37 0,45 0,46 0,27 0,16 0,23 0,95 0,79 0,16 0,25

0,32 0,34 0,40 0,41 0,53 0,61 0,41 0,33 0,44 0,89 0,70 0,10 0,14

0,23 0,29 0,31 0,43 0,59 0,68 0,54 0,4 0,61 0,87 0,66 0,07 0,10

Bng 1-8. H s ng sut hng vng C2 di tc dng ca ngoi lc sx = 1


0 0,2r 0,4r 0,8r 1,2r 1,6r


0 0 0 0 0 0 0 0 0 0 0 0 0

0 - 0,30 - 0,47 - 0,60 - 0,50 - 0,42 - 0,20 - 0,05 0,09 0,42 0,70 0,18

0

0 - 0,32 - 0,55 - 0,70 - 0,57 - 0,47 - 0,20 - 0,06 0,19 0,42 0,80 0,01

0

0 - 0,35 - 0,66 - 0,76 - 0,69 - 0,44 - 0,20 - 0,06 0,19 0,42 0,80 0,01

0

0 - 0,35 - 0,66 - 0,76 - 0,69 - 0,44 - 0,20 - 0,06 0,19 0,28 0,76 0,03

0

0 - 0,36 - 0,65 - 0,76 - 0,70 - 0,43 - 0,20 - 0,06 0,17 0,22 0,70 0,05

0


46

Bng 1-9. H s ng sut hng vng A3 di tc dng ca ngoi lc txy = 1

Khong cch t

mp l n

im tnh

ton

Mt ct

0 0,2r 0,4r 0,8r 1,2r 1,6r


0 - 3,27 - 5,18 - 6,12 - 5,72 - 3,77 - 2,43 - 0,45 1,04 7,0 7,0 7,0 0

0 - 1,51 - 2,20 - 2,65 - 3,18 - 2,17 - 1,40 0,10 0,45 1,95 2,68 1,50

0

0 - 1,10 - 1,74 - 2,17 - 2,49 - 1,67 - 1,08 0,26 0,50 1,29 2,20 1,04

0

0 - 0,78 - 1,27 - 1,62 - 1,61 - 1,11 - 0,44 0,33 0,52 0,71 1,67 1,56

0

0 - 0,68 - 1,07 - 1,29 - 1,13 - 0,75 - 0,14 0,28 0,45 0,42 1,36 1,26

0

0 - 0,65 - 0,91 - 1,03 - 0,95 - 0,47 - 0,02 0,23 0,34 0,29 1,16 0,07

0

Bng 1-10. H s ng sut hng vng B3 di tc dng ca ngoi lc txy = 1


0 0,2r 0,4r 0,8r 1,2r 1,6r


0 0 0 0 0 0 0 0 0 0 0 0 0

0 - 0,33 - 0,34 - 0,37 - 0,1 - 0,39 - 32

- 0,63 0,18 1,55 0,21 1,21

0

0 - 0,24 - 0,26 - 0,31 - 0,34 - 0,49 - 0,49 - 0,65 0,26 1,46 0,19 1,42

0

0 - 0,03 - 0,06 - 0,03 - 0,20 - 0,39 - 0,57 - 0,64 0,25 1,12

- 0,01 1,35

0

0 0,15 0,17 0,15

- 0,02 - 0,26 - 0,50 - 0,57 0,15 0,77

- 0,19 0,91

0

0 0,28 0,35 0,30 0,20

- 0,10 - 0,35 - 0,43 0,07 0,47

- 0,30 0,56

0


47

Bng 1-11. H s ng sut hng vng C3 di tc dng ca ngoi lc txy = 1


0 0,2r 0,4r 0,8r 1,2r 1,6r


0 0 0 0 0 0 0 0 0 0 0 0 0

- 0,20 - 1,15 - 0,74 0,06 0,25 0,79 0,53 0,30 0,45 0,84 0,14

- 0,91 - 0,72

- 0,38 - 1,20 - 0,77 - 0,06 0,37 0,90 0,79 0,54 0,64 0,91 0,15

- 1,08 - 0,82

- 0,74 - 1,24 - 0,77 - 0,14 0,49 1,00 1,06 0,92 0,88 0,92 0,11

- 1,15 - 0,88

- 1,06 - 1,21 - 0,70 - 0,11 0,51 1,02 1,26 1,21 1,05 0,92 0,06

- 1,15 - 0,92

- 1,38 - 1,14 - 0,59 - 0,08 0,51 1,00 1,34 1,37 1,17 0,91 0,06

- 1,12

1.6. X l nn ca p btng trng lc p trng lc cn phi c xy trn nn tt. Nn cn phi tho mn cc yu cu sau

y:

- sc chu ng p lc t thn p truyn xung v c kh nng chng trt.

- Rt t ln hoc khng xy ra hin tng nn b ln khng u.

- t thm nc v khng b xm thc. Khi thit k cn phi xem xt k cu to ca nn v phi c bin php x l thch ng

nn t c cc yu cu trn.

I. Cng tc dn nn Cn bc b lp phong ho trn mt p c t trc tip trn lp rn chn t

nt n.

tng kh nng chng trt ca p, trn mt nn cn o thnh cc cp nm nghing, cc cp ny cao dn v pha h lu vi gc nghing a so vi mt phng nm ngang. Gc a thng ly t 50 ~ 100.

Dc theo trc p, ti nhng ni cao trnh mt tt t ngt tri ln cao hay lm su xung th nn p cng cn o thnh tng bc. V tr bc phi n khp vi v tr cc khe ln bin dng.


48

hai bin thng lu v h lu mt tip xc gia p vi nn cn o thnh 2 chn khay tng thm kh nng n nh chng trt.

Trc khi btng, mt nn sau khi bc ht lp nt n, phong ho phi c nh sch bng bn chi thp, ra nc v cui cng c thi sch bng kh nn.

C th cho php dng mn n ph nn, nhng cn ch dng liu lng thch hp v ch phm vi n mn. Khi nn bc gn n cao trnh thit k, phi chm dt n ph mn v thay bng cc bin php th cng hoc na th cng khng nh hng xu n cu to ca nn.

Nu nn l dip thch st v dip thch macn pha ct, macn v cc loi t d b phong ho nng khi l ra ngoi khng kh th cng tc bc dn nn cn phi c tin hnh theo cc quy nh c bit da trn c s cc th nghim ti hin trng v trong phng th nghim ng thi cn phi ch che y cc on mt nn c x l trc, khi tin hnh btng.

Ti nhng vng vi, khi xy dng p cn phi kim tra pht hin kp thi cc hang ng x l trnh cho cng trnh b mt nc v bo m an ton cho p. Nu ti nn khu vc xy dng p gp phi hin tng cact th thng x l bng bin php:

- Bt kn cc hang ng chy v pha h lu hoc dn sang cc khu vc khc.

- i vi loi nn c hin tng cact mi pht trin nhanh, c th lm mng chn n tn vng khng b ho tan, ng thi ko di lp ph phng thm v pha thng lu, gim tc ho tan ca nc thm.

Khi di mt nn c xen lp xu hoc vng ai xu, nm su (hnh 1-26) nu o b th va tn cng, tn vt liu, mt nhiu thi gian, lc c th dng mt trong hai bin php:

- Trn lp xu, lm thm mt lp m bng btng ct thp dng phng hoc dng vm (hnh 1-26a) truyn lc t thn p xung hai bn thnh tt nn.

- o thnh chn khay, b i mt phn xu, sau btng bt kn vng ai xu to thnh mt nt btng trn vnh ai (hnh 1-26b) ri tip tc btng thn p trn nt btng ny.

Hnh 1-26. Gia c vng ai xu nn

a) Lp m kiu vm; b) Nt bt phng

B

h

b)a)


49

II. Pht va gia c nn p trng lc Sau khi bc b lp phong ho trn mt nn, nu thy nn b nt n nhiu cn tin

hnh khoan v phun va xi mng gia c tng s lin kt gia v nn v tng mun n hi ca nn, nh vy s lm cho nn p b bin dng t, v cng chu nn, chng ct ca nn cng c nng ln. Cc l khoan phun va gia c trn mt bng c th b tr theo tng ch nht hoc so le xen k nh dng hoa mai. Chiu su pht va gia c cn phi da vo tnh cht nt n v su nt n ca quyt nh, thng t 5 - 15m,

khong cch gia cc l khoan phun thng t 1,5 - 3m. p lc khi pht va gia c thng ly bng 2 ~ 4 kG/cm2. Mun dng p lc ln hn cn phi lm mt lp m bng btng trc, sau mi tin hnh khoan pht va khng lm cho k nt ca pht trin. Phm vi pht va gia c ch cn tin hnh gn thng lu v h lu p. Phm vi pht va gia c thng lu v h lu p khng c ly nh hn 1/3 chiu rng p v cn ly vt ra ngoi mp thng lu (phn pha trc ca khoan pht su to mn chng thm) v h lu p. Ngoi ra vng nn c np gy vn nt cng cn c tng cng pht va gia c. Vic tnh ton, thit k, thi cng khoan pht theo tiu chun hin hnh.

III. Pht va to mng chng thm nn p v hai b 1. Nhn xt chung:

Tu theo tc dng, mng chng thm c th chia ra lm 2 loi:

- Mng dng chng thm

- Mng va dng chng thm, va c tc dng cng c mt phn nn.

Loi th nht c thi cng bng cc phng php phun nha ng (bitum), phun dung dch xi mng hoc phun hn hp nha ng v xi mng.

Loi th hai c thi cng theo cc phng php phun xi mng v silict ho.

2. Mng xi mng chng thm di nn p v hai bn b

Cc mng xi mng chng thm di nn p c tin hnh khoan phun di chn rng thng lu (hoc cng st mp thng lu cng tt) gim lu lng thm v gim p lc y ngc di y cng trnh.

Ti 2 bn b p v dc theo b h cha khi cn thit cng phi khoan phun xi mng chng thm, khng cho nc thm v h lu v sang khu vc cha nc khc.

Va xi mng dng phun to mng chng thm ch c ng dng di cc iu kin sau y:

- Chiu rng k nt ca nn t 0,1mm tr ln th xi mng mi c th chui vo cc khe nt.

- Lu tc ca dng nc ngm khng c vt qu 600m/ngy m. Vi nhng vn tc ln hn cn phi tin hnh th nghim trc mi p dng phun xi mng.

- Nc ngm khng c tc dng xm thc i vi xi mng. Trng hp nc ngm c tc dng xm thc, cn phi dng loi xi mng c bit, khng b xm thc.

Cng tc phun va xi mng to mng chng thm cn c tin hnh v kt thc trc khi h cha nc, nu khng di tc dng ca ct nc p lc cao pha thng lu, vic


50

thi cng mng chn s gp kh khn, nh hng rt ln n cht lng v gi thnh mng chng thm.

Xc nh chiu su phun va v khong cch gia cc l khoan

Nu tng khng thm nc nm khng su lm th cn khoan, phun mng chng thm xung tn tng khng thm. Nu tng khng thm nc rt su hoc khi khng c ti liu v v tr tng khng thm th mng chng thm cn lm su xung lp c hp th nc n v w 0,01 l/pht.m2 (tng ng vi h s thm K 2.10-5 cm/s), vi cc p c ct nc thp th tr s w c th ly ln hn.

Ngoi ra khi xc nh chiu su mng chn cn cn phi xt cc mt khc nh hiu qu ca mng chn lm gim p lc y ngc ca nc (thng qua h s a1 tc h s biu th ct nc thm cn li sau mng chn) do c th gim bt khi lng phn thn p, tc l phi so snh gia gi thnh thn p vi gi thnh mng chn ng vi mt s chiu su khoan phun va khc nhau sao cho c li nht v kinh t (i vi ton b cng trnh).

Khi chn chiu su mng chn cng cn phi da vo phng tin thit b v kh nng thi cng ca a phng.

hp th nc n v w ca l khoan l hp th nc tng ng vi ct nc 1m v 1 mt chiu su l khoan, xc nh theo cng thc:

w = H.lQ

, (1-63)

trong :

w - hp th nc n v ca l khoan (l/pht.m2) Q - hp th nc ton phn thc t ca l khoan khi th nghim p nc (l/pht);

H ct nc p lc khi tin hnh th nghim p nc (m);

l - chiu su l khoan trong khong th nghim (m).

hp th nc n v cho php [w] quyt nh phm vi khoan, tc chiu su mng chn chng thm ph thuc vo ct nc trc p H.

H < 30m [w] = 0,05 l/ph.m2

H = 30 ~ 70m [w] = 0,03 l/ph.m2

H > 70m [w] = 0,01 l/ph.m2

Khi tnh ton s b c th ly chiu su phun xi mng bng khong (0,4 0,6)H, trong H l ct nc trc p. Bng 1-12 thng k mt s t liu v mng chng thm ca mt s p trng lc trn th gii tham kho khi thit k.

Khong cch gia cc l khoan khng nhng ph thuc vo nt n ca v ct nc thm m cn ph thuc vo thit b, phng php thi cng v cht lng loi xi mng c dng phun. Khong cch gia cc l khoan nn xc nh da trn c s th nghim phun xi mng vo nn p. Khi tnh ton s b c th da vo biu trong hnh 1-27 v hnh 1-28 xc nh khong cch ny. Trong biu hnh 1-27 dng khi khe nt ca nn quan st hin trng thy tng i u v nh. Cn biu hnh 1-28 dng khi khe nt ln v u. K hiu trong hnh:


51

Hnh 1-27. th xc nh khong cch gia cc l khoan trong trng hp nn c khe nt nh

a - khong cch gia cc l khoan tnh bng mt;

w - hp th nc n v (l/pht.m2) H - ct nc thm (m)

Cch xc nh khong cch a tt nht l dng phng php tng dn l khoan trong khi thi cng, ngha l gia cc l tin hnh khoan phun xi mng xong, ngi ta khoan thm cc l trung gian cho n khi no thy th nghim p nc cho kt qu w t ng yu cu chng thm.

Hnh 1-28. th xc nh

khong cch gia cc l khoan trong trng hp nn c khe nt ln.

Xc nh lng xi mng: lng xi mng cn phun to mng chng thm nn ph thuc vo mc nt n ca nn v c quan h mt thit vi tr s w, tc hp th nc n v. Khi tnh ton s b, c th xc nh lng xi mng dng trung bnh cho 1m l khoan vi mc chnh xc dng cho thit k, thi cng theo th trong hnh 1-29 lp ra theo ti liu phun xi mng nn ca mt s p trng lc trn th gii.

V p lc phun va to mng chng thm kh ln nn ti u mng chn tc nh l khoan cn lm chn khay hoc mt lp m btng khng cho k nt ca nn pht trin. Chiu su chn khay hoc chiu dy lp

Hnh 1-29. th xc nh lng xi mng (kg) trn 1 mt l khoan

m ny ph thuc vo p lc c dng khi phun xi mng.

1.60

30.001

20.01

2750.1

5 73

1.00

0.60.

a(cm)

1.20

1.40

2w(l/ph.m )101.0

5 72 3 5 72 3

2.60

1.80

2.00

2.20

2.40

3.20

2.80

3.00

3.40a(cm)

1030

20

H=

100m

5070

30,001

3 520,01

27

1,0

2,0

3,0

4,0

5,0

a(m)

H=

100m

30

(l /ph.m )270,1

5 7 32 5 31,0

210,0

w75

1020

7050

0,0010,0001

0

20

7532 32

160

240

10,01,00,10,01

(l /ph.m )2

275 753 7532 532 w7

480

320

400


51

Bng 1-12. T liu v mng chng thm ca mt s p trng lc trn th gii

S TT Tn p Chiu cao p (m)

a cht nn p Chiu su l khoan (m)

T l gia chiu su l khoan vi chiu cao p %

p lc phun va (kg/cm2)

1 Gren ixen (Thu S) 284 hoa cng 150 52,9 50

2 Bun (M) 228 Sa thch 91,5 ~ 15 0 40,1 ~ 65,8 52,7 ~ 70,3

3 Branj Canino (M) 225 hoa cng 100 44,4

4 Bhara (n ) 207 Sa thch c kp dip thch st 45,7 ~ 76,2 22,1 ~ 36,8

5 St (M) 183,5 cm thch, vi 38 ~ 60 20,6 ~ 32,6 38,7

6 Kara (Iran) 172 vi, nn nt n nhiu 90 52,3

7 Gren Culo (M) 167,6 hoa cng 91 ~ 152,4 54,3 ~ 91 35,2

8 Fontana (M) 143,3 thch anh rn chc 46 32,1

9 Sambuco (Thy S) 130 vi 80 61,5

10 Pievedi Cador () 110 Sa thch c kp mt lp dip thch 35 31,8

11 Saran (Php) 109,8 hoa cng 34,5 31,4

12 Geniciat (Php) 103,7 vi 40 38,7

13 Fren (M) 97,6 Phin thch 30 31,1

14 Hioazo (M) 93 Phin thch kt tinh; phin thch anh 24 25,9

15 Buctaminxkaia (Lin X) 93,3 gabr 30 32,2


52

3. Chng thm bng cch phun nha ng vo nn p

Mng chng thm c to bng cch phun nha ng, c tc dng lm gim bt lng nc thm qua nn p v qua ch ni tip vi 2 b. ng thi cng lm gim bt p lc y ngc di y cng trnh v gim bt c tc dng xi ngm ho hc (xm thc) ca nc ngm i vi nn. Bng cch phun dung dch nha ng nng c th bt kn cc khe nt ca v to ra mng chng thm hu hiu dng trong trng hp nc ngm c cha nhiu cht khong hoc chy vi lu tc ln hoc khi nn c cc hp cht kim (trng hp b thch cao ho, hoc vi c kh nng to cact v.v....)

- Bn knh tc dng ca nha ng: khi tnh ton s b cho nn , c th tnh bn knh tc dng ca nha ng theo bng 1-13.

Bng 1-13. Bn knh tc dng phun nha ng

B rng ca khe nt (mm)

p lc trung bnh khi phun nha

ng (at)

p lc ln nht khi phun nha ng (at)

Kch thc bn knh tc dng khi phun nha

ng (m)

40 ~ 60

10

3

1,5

8 ~ 10

15 8 ~ 15

20 ~ 24

25

-

30

-

10

6 1,4 ~ 1,85

0,35 ~ 0,45

- Khong cch gia cc l khoan: theo ti liu ca B.P. Srybe c th s b xc nh khong cch gia cc l khoan theo bng 1-14 lp ra trn c s phn tch tng hp cc s liu th nghim v thi cng hin trng.

Bng 1-14. Khong cch (s b) gia cc l khoan phun nha ng

B rng khe nt ca nn (mm)

> 20 10 ~ 20 5 ~ 10 1,5 ~ 5 1 ~ 1,5 0,2 ~ 1

Khong cch a gia cc l khoan (m)

3 ~ 4 2 ~ 3 1,5 ~ 2 1 ~ 1,5 0,8 ~ 1 0,5 ~ 0,8

Ch : khi phun nha ng vo khng ng cht, c nt n thay i v c p lc thu tnh thay i theo chiu su, th khong cch gia cc l khoan cn phi xc nh theo tr s khe nt b nht.

- Lng nha ng: tng khi lng nha ng cn thit phun y cc khe nt c th s b xc nh theo cng thc:

x = 100

K.p.V (m3), (1-64)

trong : x - lng nha ng (m3);

V - th tch (m3);

p - t l th tch cc khe nt (%);

K - h s tn tht, bng khong 1,3 ~ 1,5


53

Theo ti liu ca B.P. Srybe, lng nha ng cho 1m di l khoan bng 200kg i vi c khe nt rng n 10mm, c hp th nc n v w = 0,005 ~ 0,008 l/ph.m2.

Phun nha ng lnh tc l phun dung dch nc v bt nha ng trng thi ngui, khng cn phi t nng l khoan. nht ca nc nha ng y nu c ch to tt th ch ln hn nht ca nc t 1,4 n 4 ln, do c th ngm vo tn cc l v khe rt nh trong .

Nc nha ng dng phun vo cc l v khe nt rt nh phi c kch thc nh hn 2 ~ 3mm. Hm lng nha ng trong nc khng c ln qu 25 ~ 50%, v nu hm lng nha ng ln hn 50% th nht ca nc nha ng tng rt nhanh.

4. Mng chng thm bng va xi mng + bentnite. Nhm ci thin tnh cht ca va pht to mng, trong thi gian gn y, loi vt liu

va xi mng + bentnite + ph gia chm ng cng c p dng khi x l chng thm cho cc loi nn. i vi nn c kch thc k nt nh, loi va ximng + bentnite c linh ng cao c th len li c vo cc k nt ny, lm tng hiu qu chng thm ca mn. nc ta, tng ho chng thm bentnite c p dng thnh cng khi x l chng thm cho thn v nn mt s p t (Du Ting, Dng ng,...).

i vi nn ca p btng, kinh nghim s dng loi vt liu ny cha c nhiu, nn khi p dng cn xem xt k cc yu t kinh t k thut, c tham kho ti liu nc ngoi v th dn tng bc (khoan pht v khoan kim tra) tm ra cc ch tiu k thut hp l (thnh phn va, c ly h khoan, p lc pht,...)

1.7 Mt s Cu to chi tit ca p btng trng lc trn nn

1. Thot nc cho nn v thn p

H thng thot nc thn p trng lc gm nhiu ng ng t sau lp btng chng thm mt thng lu. Khong cch gia cc ng thot nc ny thng t 1,5 ~

3,0m; ng knh ng d = 15 ~ 30cm.

Cc ng thot nc trong thn p thng dng ng btng c c nhiu l hoc bng ng btng c bng si, sn, t ct nc d ngm qua, chiu di mi on ng t 1 ~ 1,5m; cc ng ny nn t theo phng thng ng hoc thnh mt ng thng nghing v u tp trung vo cc ng hm t trong thn p (hnh 1-30).

Khong cch t ng ng n mp thng lu p khng c nh hn tr s (0,08 ~ 0,1)y,

Hnh 1-30. B tr ng ng thot nc trong thn p.

1. ng btng c c l thot nc;

2. l thot nc nn; 3. ng hm tp trung nc

trong y l su nc tnh t mt nc thng lu n v tr ang xt, nu khng nc thm c th ph hoi phn btng mt thng lu p.

1 3

15cm

15

1

2

3

3

1

3

1 1


54

thot nc v tng hiu qu gim p lc thm di nn tc gim p lc y ngc di y p, thng dng bin php khoan cc l xung nn tp trung nc thm di nn li v dn (hoc bm) v pha h lu. Cc l khoan thot nc nn t sau mng chng thm v cch mng chng thm t 2 ~ 3m v pha h lu, ng knh cc l khoan ny khng nn ly nh hn 15 ~ 25cm, khong cch gia cc l khoan t 1,5 ~ 3,0m v chiu su ca l khoan thng ly bng 1/2 ~ 2/3 chiu su mng chng thm.

Cc l thot nc nn u tp trung vo cc ng hm tp trung nc t ngay trn mt nn hoc vo ng hm va dng khoan phun xi mng chng thm va l ng hm tp trung nc t cao hn mt nn mt cht (hnh 1-31).

Hnh 1-31. H thng thot nc di nn p

2. ng hm trong thn p trng lc:

Xem 1.3.III

3. Cu to cc khp ni ca p trng lc

Cc khp ni ngang ca cc khe ln vnh cu trn thn p cn phi bo m an ton khi cc on p b bin dng do ln khng u hoc do nhit bin i ng thi phi bo m khng cho nc thm qua.

Tr cc khe ln kt hp lm khe nhit vnh cu trong p, tt c cc khe dc, khe ngang phn chia p thnh cc khi nh trong qu trnh thi cng v khc phc ng sut nhit trong giai on thi cng, trc khi p lm vic u phi x l phun va lin kt lp khe.

Khong cch gia cc khp ni ngang (khe ln) thng ly khong 15 ~ 20m, tr s ny c th thay i tu theo tnh hnh a cht thc t ti ni xy dng p.

Trong cc khp ni ngang cn t cc thit b chng thm (xem hnh 1-32 1-34), sau b phn chng thm cn b tr cc ging quan st kim tra s lm vic ca khp ni ngang. Thit b chng thm ng t cch mt thng lu t 2 4m cn ging quan st kim tra t cch thit b chng thm t 2 3m v pha h lu p.

Cu to ca cc khp ni ngang c trnh by trn hnh 1-33 v 1-34. Trong hnh 1-33 l cc loi khp ni bitum dng trong cng trnh thu li. Nhng ng kim loi t dc theo


55

sut chiu di khp ni c ng knh bng khong 50mm dn hi nng t chy nha ng (bitum).

Hnh 1-32. S b tr cu to ca mt khp ni ngang, n v: cm

1. nt btng u khe ngang mt thng lu; 2. tm ng chng thm dy 1,8mm; 3. tm btng c sn; 4. ging nha ng (bitum) chng thm c kch thc 20 x 20cm; 5. in cc

t nng nha ng; 6. mt khe ngang; 7. ging quan st, kim tra kch thc 80 x 90 cm.

Cc kiu khp ni trong hnh 1-34 c lm bng cc tm kim loi chng r (ng, thp khng r, nhm...). Tm kim loi chng thm thng rng 300 ~ 350mm, khng ly nh hn 200 ~ 250 mm, dy 0,8 ~ 2mm. Cc kiu khp ni trong hnh 1-34 ch to n gin lm vic tt nhng kh sa cha, khi phc v tn nhiu kim loi chng r.

Hnh 1-33. Cc kiu khp ni chng thm bng bi tum

Hnh 1-34. Cc kiu khp ni chng thm bng cc tm kim loi

Tt nht l kt hp dng c hai loi khp ni ny li dng tt c cc u im ca 2 loi. Khp ni kim loi thch hp vi trng thi bin dng ln, ging nha ng (bi tum) d sa cha, dng hn hp s lm cho hiu qu chng thm v tnh bn vng ca khp ni ngang c ci thin hn.

Cc ging kim tra thng lm theo hnh vung, kch thc 0,7 x 0,7 ~ 1 x 1m hoc c th ln hn. Cng c th dng hnh trn c ng knh nh hn 1m.

300

200

20

150

200

20

300350

w

150

1

52

5050

36

4 2

80 x 90

7


56

Cc tm kim loi chng thm v ging nha ng cn phi cm su vo nn 1 on 30 ~ 50cm v phi lm cao hn mc nc ln nht thng lu. Ti nh p trn, cn c cc tm np y bo v b phn ny ca khp ni (hnh 1-35).

Hnh 1-35. Cu to b phn bo v ca ging nha ng trn mt p trn

1. mt trn; 2. tm np kim loi; 3. in cc; 4. tm st y; 5. ging nha ng.

4. Lp ph mt ngoi ca p

Mt ngoi ca p thng c ph bo v chng thm, ngn chn cc nh hng xm thc ho hc c hi ca nc i vi btng thn p. Vi iu kin kh hu Vit Nam, cc p bng btng ch cn c bit ch phng chng thm mi thng lu.

Bin php thch hp:

- Lm lp btng cht lng c bit tt ph mt thng lu bng phng php chn khng.

- Trt 1 lp va xi mng chng thm dy t 2 ~ 3cm mt thng lu;

- Qut bi tum ln mt thng lu.

i vi mng p, trnh mng thm qu nhiu v trnh tc dng n mn ca nc cn phi s dng cc loi btng khng thm, khng b n mn hoc kt hp vi cc bin php khc.

5. Cu to nh p

Chiu rng nh p cn tho mn cc yu cu i li, kim tra qu trnh s dng v trong khi thi cng. Khi khng c yu cu v giao thng, chiu rng nh p nh nht khng nn ly nh hn 2m.

nh p phi c cao an ton so vi mc nc ln nht trc p v khng sng trn qua. Khi cn thit gim bt chiu cao p, c th lm tng chn sng pha thng lu. Cao trnh nh tng chn sng cng cn phi c cao an ton nh trn.

nh p cn lm hi nghing v 2 pha thng, h lu d tho nc ma trn nh p v phi lm rnh tp trung v tho nc ma v thng lu hoc h lu p.

Khi nh p c dng vo mc ch giao thng, cc kch thc v cu to nh p cn tho mn cc yu cu v giao thng.

Hai bn nh p cn b tr h thng lan can bo v, n chiu sng v phi m bo yu cu v m quan cng trnh.

3

5

4

1

2


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1.8 Vt liu xy dng p btng trng lc p trng lc c th lm bng btng hoc xy, nhng vt liu btng c s

dng rng ri nht.

I. Cc yu cu i vi vt liu lm p Vt liu lm p l btng thu cng cn tho mn cc yu cu sau:

1. cng chu lc

Cng chu lc c biu th qua mc btng. Mc c xc nh theo cng nn tui 28 ngy, tnh bng MPa (N/mm2). Khi tnh ton, vi trng hp p bt u lm vic tui di ngy hn th c th xc nh cng btng tui tng ng theo cng thc tnh i:

Rt = Kt.R28 , (1-65)

trong : Rt , R28 cng btng tui t ngy v 28 ngy ;

Kt h s quy i c xc nh s b theo bng 1-15

Bng 1-15. H s quy i cng nn ca btng theo tui

Tui btng, ngy 3 7 14 21 28 60 90 180

Kt 0,50 0,70 0,83 0,92 1,00 1,10 1,15 1,20

Cng chu ko khi un (cng un) c xc nh bng th nghim. Khi dng cc vt liu thng thng, tng quan gia cng nn v cng un c th tham kho bng 1-16.

Bng 1-16. Tng quan gia cng nn v cng un

Cng nn (MPa) 15 20 25 30 35 40 50

Cng un (MPa) 2,5 3,0 3,5 4,0 4,5 5,0 5,5

Trong thit k, cn da vo kt qu phn tch ng sut thn p v c im cng tc ca tng vng la chn mc btng cho ph hp.

2. cht v t thm nc

Cc bin php c bn nng cao kh nng chng thm ca btng tc m bo tnh ng nht v lin khi ca btng l:

- Hn ch t l N/X trong thnh phn btng bng cch tng lng dng xi mng; tng hm lng tng i ca ct trong hn hp ct liu.

- Trnh xy ra phn tng trong vn chuyn v btng, lm tt cng tc m cht khi lng btng khi .

- S dng cc cht ph gia ho do, cho php gim lng nc trn btng v nng cao n nh chng thm phn tng.

- Bo dng tt khi btng sau khi .

Theo tnh chng thm, ngi ta chia btng thnh cc loi mc nh sau: B-2, B-4, B-6, B-8, B-12.


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i vi btng thu cng vng thng xuyn ngp nc v vng mc nc bin i, mc chng thm c xc nh theo t s gia ct nc tc dng ln nht v b dy kt cu, hoc b dy lp bn ngoi ca kt cu (tc gradien ct nc Jh) nh trn bng 1-17.

Bng 1-17: Chn mc chng thm ca btng theo tr s Jh.

Mc: B-4 B-6 B-8 B-12

Jh < 5 5 10 10 - 12 12

3. Tnh chu nc

Mi trng nc c th gy tc ng n mn btng. Dng v mc n mn cn c xc nh trong qu trnh iu tra kho st bng cch phn tch thnh phn ho hc ca nc v kh nng n mn btng, t ra cc bin php phng chng thch hp. Gii php c bn nht gia tng bn ca btng chng tc ng xm thc ca nc lm tng cht ca khi btng bng cc bin php nh nu mc trn.

4. Bin dng nhit v m ca btng

S to nhit ca btng khi ngng kt ph thuc vo loi xi mng v hm lng ca n trong btng. V vy hn ch s to nhit khi btng ngng kt khi cn thit phi s dng cc loi xi mng t to nhit, thit k thnh phn btng vi lng dng xi mng t nht, hoc s dng ph gia l cc cht c hot tnh b mt.

Khi kch thc ca khi btng ln m vic p dng cc bin php trn vn cha h nhit cho khi btng khi ngng kt th buc phi p dng cc bin php c bit nh lun chuyn nc lnh qua thn ca khi btng, hoc l chia khi ln ra thnh cc khi nh hn.

- Bin dng co, n ca btng khi ngng kt. Theo tiu chun v cc yu cu k thut i vi btng thu cng, co ngt ca mu btng tui 28 ngy khi ngng kt trong khng kh c nhit 180C v m tng i 60% khng c vt qu 0,3 mm/m v tui 180 ngy th khng c vt qu 0,7 mm/m.

n di khi btng ngng kt trong nc khng c vt qu 0,1mm/m tui 28 ngy v 0,3mm/m tui 180 ngy.

5. bn chng mi mn v chng kh thc ca btng

Mt trn nc ca p btng chu tc ng mi mn khi dng chy mang nhiu bn ct. Mt trn c dng chy lu tc cao cng c th b ph hoi do kh thc khi trn mt c cc g gh cc b vt qu mc cho php.

nng cao bn chng kh thc v chng mi mn, cn p dng btng c c v bn cao (mc 350) vi ct liu nh v trung bnh, v vi g gh b mt l b nht.


59

II. Phn vng thn p

Hnh 1-36. Phn vng vt liu thn p a. p khng trn; b. p trn

Thn p btng thng c phn ra cc vng nh sau (hnh 1-36):

I. Vng ngoi ca p v cc b phn ca n chu tc ng ca kh quyn, nhng khng ngp nc;

II. Vng ngoi ca p nm trong phm vi dao ng ca mc nc thng, h lu, cng nh cc phn ca p b ngp nc tng thi k nh phn trn nc.

III. Vng ngoi v vng tip gip vi nn thng xuyn ngp di mc nc.

IV. Vng trong thn p, gii hn bi vng I, II, III.

Btng ca cc vng trong p phi t c cc yu cu sau:

Bng 1-18. Yu cu i vi btng cc vng

Vng p STT Yu cu i vi btng

I II III IV

1 bn chu nn x x x x

2 bn chu ko x x x

3 khng ngm nc x x

4 gin di gii hn x x x x

5 bn chng xm thc ca nc x x

6 bn chng mi mn v kh thc khi dng chy c V 15 m/s

x

7 to nhit khi btng ninh kt x x x x

Chiu dy ca vng ngoi ca p ph thuc vo loi p, ct nc tc dng, iu kin kh hu v kch thc cc b phn ca p. Thng khng ch d 2m.

Thng thng trong thit k p khng dng qu 4 loi mc btng trnh gy phc tp trong vic b tr thi cng. Trng hp p s dng qu 4 loi mc btng cn c lun chng xc ng.

III

MNCII

IIII

II

IV

MNC

MNHL

IV

II

a)b)


60

III. Vt liu ch to btng 1. Xi mng. Xi mng l mt trong nhng vt liu ch yu nht ch to btng thn

p. Vic gim lng dng xi mng trong xy dng p va c ngha kinh t va gim c lng nhit to ra khi btng ngng kt, do tng c tc thi cng v hn ch hnh thnh khe nt do nhit trong thi k thi cng. Cc bin php tit kim xi mng l:

- Phn vng p p dng cc loi btng c s hiu thch hp. Vic phn vng v la chn s hiu btng l da vo kt qu phn tch ng sut thn p (cc ng ng ng sut) v nhng ch dn v phn vng nu mc trn.

- Trit li dng cng ca btng thi k cui. Nu c cn c xc nh c l cng trnh ch bt u lm vic sau mt thi gian di sau khi btng th c th ly cng btng tui 60 ngy, 90 ngy tnh ton lng xi mng cn dng. Theo ti liu th nghim th cng btng sau 180 ngy tng 1,13 1,65 ln so vi cng sau 28 ngy.

- S dng ct liu c ng knh ln cho vng c btng khi ln

- Trn thm vt liu thay th mt phn xi mng.

Cc loi vt liu c th dng pha trn l: tro ho sn, t ialmic, vi v.v...T l thch hp nht gia s hiu xi mng v s hiu btng l 2,0 2,5 ln. Nu vt qu 2,5 ln th nn trn thm vt liu khc, khi c th s nh hng n cng btng. V vy loi vt liu pha trn v hm lng hp l cn phi xc nh bng th nghim.

- Trn thuc ho do vo btng. Thuc ho do c tc dng lm cho va btng linh ng, d , ko di c thi gian ninh kt ban u. Nh dng thuc ho do, c th gim c mt phn lng nc, nh tng c cht v tnh chng thm ca btng. Loi v liu lng dng thuc ho do cng cn c xc nh bng th nghim.

2. Ct liu btng a. Ct liu th (dm, si). Ch tiu quan trng ca ct liu th l thnh phn, cng

v cp phi ht. V thnh phn, cn trnh dng cc loi phn phin, c cha cc hot cht, d b xm thc. bn ca dng xay thnh dm btng cn phi vt qu 2,5 3 ln cng btng khi s dng vi; con s tng ng khi s dng cc loi cacbont l 2 - 2,5 ln.

Kch thc ht ca btng khng nn vt qu 120 mm. m bo tnh khng i v thnh phn ht ca btng, khi sn xut dm, ngh phn thnh cc nhm ht c cc kch c nh sau:

- Khi Dmax = 80mm, phn thnh cc c 2-10, 10-20, 20-40 v 40-80 mm;

- Khi Dmax = 120 mm, phn thnh cc c 5-10, 10-20, 20-40, 40-80 v 80-120mm.

b. Ct liu mn (ct)

ch to btng thu cng, ngi ta s dng ct thin nhin. Ct cn phi sch v bao gm cc ht cng khng cha cc tp cht hu c v ht bi. Thnh phn ht ca ct cn phi nm trong gii hn cc ng cong cp phi cho php. Theo ln ca ht, ct c phn thnh 3 nhm: ht th vi mun ln 3,5 2,8; ht trung vi mun ln 2,8 2,2 v ht mn tng ng vi 2,2 1,8. Vic s dng ct ht nh hn cn phi c lun chng.


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Khi s dng ct liu th, cng nh ct liu mn c xay t cc m , tuyt i khng c dng loi c cha silic v nh hnh, bi v yu t ny khi tc dng vi xi mng gy ra phn ng ho hc lm trng n th tch ca btng, lm cho btng nt n khng th khc phc c.

1.9. Cc loi p btng trng lc ci tin I. Lun im chung p btng trng lc khi ln c nhiu u im nh: n gin v kt cu v iu kin

xy dng; cho php tho lu lng l ln trong thi gian thi cng cng nh khai thc; lm vic an ton vi mi iu kin kh hu, mi chiu cao..

Bn cnh , p btng khi ln cng c nhng nhc im, l:

- C th tch ln, s dng nhiu vt liu, trong c vt liu xi mng, dn n gi thnh cao so vi cc kiu

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