bÀi giẢng nỀn mÓng mỐ trỤ cẦu

8/10/2019 BI GING NN MNG M TR CU

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TRNG I HC XY DNG MIN TRUNG

KHOA CU NG

BMN CU & CNG TRNH NGM

--------------------

ON HU SM

BI GING MN HC

NN MNG V MTRCU

Chuyn ngnh : XY DNG CU NG

(DNH CHO SINH VIN HCAO NG)(LU HNH NI B)

TP. TUY HA, thng 12 nm 2011


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MC LC

CHNG 1: KHI NIM CHUNG VNN MNG V MTRCU ................1

1.1. Khi nim chung vnn, mng ...........................................................................1

1.1.1. Khi nim cbn vnn, mng...........................................................................1

a. Nn cng trnh.....................................................................................................1

b. Mng cng trnh..................................................................................................2

1.1.2. Phn loi nn, mng.............................................................................................2

a. Phn loi nn .......................................................................................................2

b. Phn loi mng....................................................................................................6

1.2. Khi nim chung vmtrcu ...........................................................................9

1.2.1. Khi nim chung vmtrcu ...........................................................................9

1.2.2. Phn loi mtrcu...........................................................................................10

a. Phn loi theo cng dc cu.........................................................................11

b. Phn loi theo hthng kt cu nhp ................................................................13

c. Phn loi theo vt liu xy dng.......................................................................15

d. Phn loi theo phng php xy dng..............................................................16

e. Phn loi theo hnh thc cu to ca mtr .....................................................16

f. Phn loi theo yu cu sdng.........................................................................18

1.2.3. Vt liu xy dng mtrcu.............................................................................18

1.2.4. Xc nh cc kch thc cbn ca mtrcu ...............................................19

a. Cao nh mng .............................................................................................19

b. Cao nh mtr............................................................................................20

c. Kch thc mmtrtrn mt bng ................................................................21

d. Xc nh mt scc kch thc khc ...............................................................23


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CHNG 2: TNH TON THIT KMNG NNG..............................................24

2.1. Cc khi nim cbn vmng nng.................................................................24

2.1.1. Khi nim...........................................................................................................242.1.2. Cu to ...............................................................................................................28

2.2. Thit kmng nng trn nn thin nhin ..........................................................32

2.2.1. Trnh tthit k..................................................................................................32

2.2.2. Xc nh sbkch thc y mng ................................................................34

a. Trng hp mng chu ti ng tm ................................................................36

b. Trng hp mng chu ti lch tm..................................................................392.2.3. Tnh ton nn theo trng thi gii hn (TTGH).................................................45

2.2.4. Tnh ton cng bn thn mng ...................................................................46

a. Xc nh chiu cao b mng .............................................................................46

b. Tnh ton ct thp cho b mng........................................................................49

CHNG 3: TNH TON THIT KMNG CC .................................................57

3.1. Khi nim, phn loi mng cc .........................................................................57

3.1.1. Khi nim mng cc ..........................................................................................57

3.1.2. Phn loi cc, mng cc ....................................................................................58

a. Phn loi cc .....................................................................................................58

b. Phn loi mng cc ...........................................................................................59

3.1.3. Cu to cc v bcc.........................................................................................60

a. Cu to cc........................................................................................................60

b. Cu to bcc ...................................................................................................63

3.2. Xc nh sc chu ti ca cc.............................................................................68

3.2.1. Khi nim...........................................................................................................68


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c. Xc nh quan hgia i v v, u, w...............................................................130

d. Xc nh cc chuyn vv, u, w ca i cc ....................................................131

CHNG 4: CU TO TRCU DM................................................................136

4.1. Cu to cc bphn ca trcu.......................................................................136

4.1.1. Mtr...............................................................................................................136

4.1.2. Thn tr ............................................................................................................138

4.1.3. Mng tr...........................................................................................................138

4.1.4. Lt mt mtrcu ...........................................................................................138

4.2. Cu to trcu ton khi .................................................................................139

4.2.1. Trnng ...........................................................................................................139

4.2.2. Trthn ct ......................................................................................................141

4.2.3. Trthn hp .....................................................................................................144

CHNG 5: CU TO MCU DM .................................................................145

5.1. ngha, nhim vca mtrong cng trnh cu..............................................145

5.2. Cu to mton khi .......................................................................................154

5.2.1. Cu to mch U ............................................................................................154

5.2.2. Cu to m vi .................................................................................................157

5.3. Mcu 4 khp .................................................................................................160

CHNG 6: TNH TON MTRCU DM ....................................................162

6.1. Khi nim chung ..............................................................................................162

6.1.1. Khi nim chung vthit ktheo hsti trng v sc khng (LRFD).........162

6.1.2. Cc ti trng tc dng ln cng trnh cu ........................................................164

6.1.3. Cc trng thi gii hn, hsti trng v thp ti trng..............................165

6.2. Xc nh mt sti trng tnh ton mtrcu...........................................168


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6.2.1. Ti trng thng xuyn....................................................................................168

6.2.2. Ti trng tc thi..............................................................................................169


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b. Mng cng trnh

Mng cng trnh l mt bphn thuc kt cu phn di ca cng trnh, n lin

kt trc tip vi kt cu phn trn ca cng trnh, tip nhn ton b ti trng tbn

trn (v c thni l cbn hng mng) v truyn xung cho nn cng trnh gnh .

1.1.2. Phn loi nn, mng

a. Phn loi nn

Nn thng c phn bit ra nn thin nhin v nn nhn to.

Nn thin nhin: l nn gm cc lp t c kt cu tnhin c to thnh tlch

shnh thnh cc lp a cht, c khnng chu c ti trng cng trnh theo thit k

m khng cn sdng cc bin php kthut ci thin khnng chu lc ca nn.

Nn nhn to: khi nn c cc lp t ngay st bn di mng khng khnngchu lc vi kt cu tnhin ca n, xy dng cng trnh nhng vtr ny ta cn

phi p dng cc bin php kthut nhm nng cao khnng chu lc ca cc lp t

hoc thay cc lp t bng cc lp t tt hn. Nhng nn nhvy c gi l

nn nhn to.

Cc bin php kthut sdng trong nn nhn to:

Ci to kt cu khung ht t nhm gia tng sc chu ti v gim bin dng ln

ca nn t: m vt liu ri (m ct, m , ).

Gia ti trc.

Gia ti trc kt hp bin php tng tc thot nc.

Ct vt liu ri (ct ct, ct , ).

Ct t trn vi hoc trn xi mng.

Phng php in thm.

Pht va xi mng hoc vt liu lin kt vo vng nn chu lc.


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Hnh 1.2. Gia ti trc kt hp bc thm thot nc ng v thot nc ngang.

Hnh 1.3. Thi cng ct vt liu ri (ct ).


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Hnh 1.4. Thi cng ct t trn xi mng.

Hnh 1.5. Pht vt liu lin kt gia cng nn.

Tng cng cc vt liu chu ko cho nn t (t c ct): Sdng si hoc vi a kthut.

Sdng thanh hoc va kthut.

Sdng thanh neo.


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Hnh 1.6. ng dng vi a kthut trong xy dng nn ng.

Hnh 1.7. ng dng li a kthut trong xy dng nn ng.

Hnh 1.8. Sdng thanh neo tng cng n nh ca nn ng trn sn dc.


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b. Phn loi mng

C nhiu cch phn loi mng khc nhau:

Phn loi theo vt liu xy dng mng: mng gch, hc, b tng, b tng ct

thp, thp, g.

Phn loi theo cng ca mng: mng cng, mng mm (mng chu un). Phn loi theo phng php thi cng: mng ton khi, bn lp ghp, lp

ghp.

Phn loi theo c tnh chu ti: mng chu ti trng tnh, mng chu ti trng

ng (thng gp l mng my).

Phn loi theo su chn mng: mng nng, mng su, mng na su.

Thng thng khi ni ti vic phn loi mng, ta thng quan tm n cch phn

loi theo su chn mng. Hin nay, csphn loi mng theo cch ny cthng nht hn ctrong ngnh xy dng l khi tnh sc chu ti ca t nn th thnh

phn ma st gia thnh mng vi t nn c xem xt nhthno. Theo cch phn

loi ny, mng c chia thnh mng nng, mng su v mng na su.

Mng nng: l phn mrng ca y cng trnh nhm c c mt din tch tip

xc thch hp t nn c thgnh chu c p lc y mng, loi mng ny khng

xt n lc ma st gia thnh mng vi t xung quanh khi tnh sc chu ti ca nn

t. Mng nng thng c chia thnh mng n, mng phi hp, mng bng, mng

b.

Mng su: khi chiu su chn mng ln hn chiu su ti hn Dc, tsu ny

sc chu ti ca t nn y mng khng tng theo chiu su na m t gi tr

khng i v thnh phn ma st gia thnh mng vi t xung quanh c xt n khi

tnh sc chu ti ca nn t. Mt sloi mng su nhmng tr, mng cc.

Mng trgm cc trln chn su gnh cc cng trnh cu, cng, gin khoan

ngoi bin, Nu trgm cc t rng v phng php thi cng hvo lng t tng

on trc thc hin ging nh thi cng ging th mng c gi l mng ging

chm. Khi mng ging chm t qu su hoc a cht xung quanh khng cho php ht

nc o v xy, ngi ta phi bt mt trn cc ging to thnh mt bung kn, sau

dng hi p bm vo bung ny y nc ra m tin hnh o t hging, lc

ny mng c gi l mng ging chm hi p.


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Mng ccc cu to bi cc thanh c kch thc b hn tr, gi l cc hay c.

Loi ny rt a dng nhcc g, cc thp, cc b tng ct thp, cc ng thp nhi b

tng, Cc b tng ct thp c loi ch to sn (tit din vung, trn, ng c hoc

khng c thp dng lc) v loi nhi b tng vo trong l to trc trong lng t

(cc khoan nhi, cc barrette tng barrette)Mng na su: khi chiu su chn mng nh hn chiu su ti hn Dc, nhng

khng phi l mng nng nh: mng cc ngn, mng trngn v mt smng ging

chm.

Hnh 1.9. Mng n. Hnh 1.10. Mng bng.

Hnh 1.11. Mng b.


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Hnh 1.12. Mng cc chto sn Hnh 1.13. Mng ging chm.

v mng cc khoan nhi.

Hnh 1.14. Thi cng tng barrette.


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1.2. Khi nim chung vmtrcu

1.2.1. Khi nim chung vmtrcu

Hnh 1.15. Mv tr. 1-Mcu. 2-Trcu.M trcu l mt bphn quan trng trong cng trnh cu, c chc nng kt

cu nhp v truyn ton bti trng xung t nn.

Vmt kinh t, mtrcu chim mt tlng ktrong tng vn u txy dng

cng trnh.

Mtrcu l cng trnh thuc kt cu phn di. Ngoi trcc cu trn cn, a s

mtrcu nm trong vng m t, dbxm thc, xi l, bo mn. Vic xy dng,

thay i, sa cha rt kh khn nn khi thit kcn ch sao cho ph hp vi a

hnh, a cht, cc iu kin kthut khc v don trc spht trin ti trng.

V vy mtrcu phi m bo nhng yu cu vkinh t, kthut v khai thc.

m bo yu cu vkinh t, k thut ngha l m trcu sdng vt liu mt cch

hp l, cc kch thc cbn c chn sao cho c trsnhnht m vn m bo v

cng , cng, n nh, khng bxi l, ln, st. m bo yu cu vkhai thc

bao gm m bo thot nc thun li di cu, m bo tnh mquan, khng cn tr

lu thng di cu trong cu vt, chng bo mn bmt mtr


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Hnh 1.16. Mt cng trnh cu trong thc t.

Mcul mt bphn thuc kt cu phn di ca cng trnh cu, c btr

hai u cu, ngoi chu ti trng tkt cu nhp truyn xung cn lm nhim vca

mt tng chn t, chu p lc ngang ca t p, m bo n nh ca nn ng

u cu.

M cn l b phn chuyn tip, m bo xe chy m thun tng vo cu.

Ngoi ra mcn l cng trnh iu chnh dng chy v m bo chng xi lbsng

i vi cc cu vt sng.

Ti trng ngang truyn vo mchyu theo phng dc cu v gi tr ti trng

ngang theo hai hng ca phng dc cu thng khc nhau nhiu nn cu to ca m

theo phng dc thng khng i xng.

Trcul mt bphn thuc kt cu phn di ca cng trnh cu, c vtr giahai nhp knhau, phn chia cc nhp cu, chu ti trng tkt cu nhp truyn xung.

i vi cu vt sng, trcu c xy dng trong phm vi dng chy nn tit

din ngang phi c dng hp l vthy ng hc thot nc tt, bn ngoi trc

lp vc bit chng xm thc. Ngoi ra trcu cng phi c thit khp l

m bo thng thuyn i vi cu c thit kt thng thuyn.

i vi cc cu vt v cu cn, hnh dng trphi m bo mquan v khng

cn trlu thng di cu.Trcu lm vic theo hai phng: dc cu v ngang cu, v thng c cu to i

xng theo chai phng.

1.2.2. Phn loi mtrcu

C nhiu cch phn loi mtrcu:


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Phn loi theo cng dc cu.

Phn loi theo hthng kt cu nhp.

Phn loi theo vt liu xy dng.

Phn loi theo phng php xy dng.

Phn loi theo hnh thc cu to ca mtr. Phn loi theo yu cu sdng.

a. Phn loi theo cng dc cu

Theo cng dc cu c thphn thnh: mtrcng v mtrdo.

Mtrcngl loi c kch thc ln, cng ln. Thng gp trong hu ht cc

cng trnh cu. Khi chu lc, bin dng ca mtrtng i nhc thbqua. Mtr

c khnng chu c ton bti trng ngang theo phng dc cu tnhp truyn n

v ti trng ngang do p lc t gy ra. Hnh 1.16 thhin mt dng mtrcng.Mtrdol loi c kch thc thanh mnh, cng nh. Trn mmtrchc

gi cnh hoc khng cn gi. Khi chu lc ngang theo phng dc cu, ton bkt

cu nhp v trs lm vic nhmt khung v ti trng ngang s truyn cho cc tr

theo tlcng ca chng. Lc ny cu lm vic nhmt khung nhiu nhp, do

gim lc ngang tc dng ln tr v kch thc tr khi tnh ton thit k s gim i

nhiu. Tuy nhin, mtrdo chu x va km nn cn lu khi chn loi mtrny.

Trdo thng c dng trct, trcc hoc tng mng (hnh 1.17). Trdo thng

p dng cho cc cu nhp nhv c chiu cao khng ln lm (thng l cu dn).

Hnh 1.17. Trdo. a-Trcc. b-Trtng mng. c-Trct.

1-Dm m(x mhay mtr). 2-Cc. 3-Ct. 4-Mng. 5-Thn tr.


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Hnh 1.18. Mtrdo dng cc vi kt cu nhp l cc dm gin n lin kt vi mmtrbng cc gi ta cnh (lin kt cht).

Hnh 1.19. Trdo dng cc thp v g.

Hnh 1.20. Trdo dng tng mng. Hnh 1.21. Trdo dng ct.

Lin bin Lin gia


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b. Phn loi theo hthng kt cu nhp

Theo hthng kt cu nhp c thphn thnh: mtrkhng chu lc y v mtr

chu lc y tkt cu nhp.

M tr khng chu lc y t kt cu nhp thng c cu to tng i n

gin: mtrcu dm, mtrcu khung. Mtrchu lc y tkt cu nhpl hthng chu lc y ngang ln nn c

cu to nng n, thit kphc tp v khng c khnng lp ghp: mtrcu

vm, mtrcu treo.

Mtrcu dm: di tc dng ca ti trng thng ng trn kt cu nhp, chc

p lc thng ng truyn xung gi ca mtrcu.

Hnh 1.22. Mtrcu dm.

M tr cu khung: ging cu dm nhng tr lin kt ngm vi kt cu nhp to

thnh nt khung nn trong thn trxut hin moment ln.

M M

Tr


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Hnh 1.23. Trcu khung.

Mtrcu vm: di tc dng ca ti trng thng ng trn kt cu nhp, mtr

cu chu lc y ngang ln ti chn vm.

Hnh 1.24. Mtrcu vm.


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Mtrcu treo: di tc dng ca ti trng thng ng trn kt cu nhp, mcu

chu lc nhxin rt ln.

Hnh 1.25. Mtrcu treo dy vng.

c. Phn loi theo vt liu xy dng

M trcu c thc xy dng bng xy, b tng, b tng ct thp ( ti

ch, bn lp ghp, lp ghp). Mt strng hp nhtrcu cn c thc xy dng

bng thp, g. Cc thp cu treo cng c thc xy dng bng thp.

Hnh 1.26. Mtrcu bng xy.


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trng hp cn m bo tm nhn v khng cn trgiao thng di cu (cu cn, cu

vt, cu cho) c thp dng trmt ct.

Hnh 1.28. Trdng ct.

M tr c mng ring: mng tr c th l mng nng, mng cc, mng ging

chm

Mtrc cu to lin vi mng: kt cu mng khng tch bit r vi phn thnmtr: mtrdng cc, ct ng.

M tr c cu to thng trn bnh : trc tim cu vung gc vi dng chy

hoc hng lu thng.

Mtrc cu to xin trn bnh : trc tim cu xin gc vi dng chy hoc

hng lu thng. m bo yu cu thot nc v lu thng tt nht, trn mt bng

chiu di trnn btr song song dc theo dng chy, chiu ngang trnn hp ti mc

ti a (hnh 1.29a). Loi ny thng p dng cho cu c xin ln. i vi cu quang, cn c thbtr mtrtrn mt bng theo hnh bc thang (hnh 1.29b), kt cu

nhp vn t thng. Loi ny thng km mquan v cn trtm nhn nn t c p

dng.


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Hnh 1.29. Smtrtrong cu xin.

1-Tim ng. 2-Tim cu. 3-Tim cc ct tr. 4- k gi.

f. Phn loi theo yu cu sdng

Theo yu cu sdng c cc loi m trcu ng t, m trcu ng st,

hoc kt hp cng t v ng st.

1.2.3. Vt liu xy dng mtrcu

i vi b tng v ct thp, p dng theo tiu chun thit kcu 22TCN-272-05.

xy: xy mtrcu l cc loi tnhin, cht lng tt, khng bnt n,

phong ha, c cng ln hn 600 kg/cm2(60 MPa), kch thc nhnht ca

hc l 25 cm. Nhng trbng b tng hc, c cng ln hn 400 kg/cm2(

40 MPa), lng khng ln hn 20% khi lng b tng ton b.Va dng trong cc tr lp ghp hoc trong cc tr xy yu cu phi c cng

ti thiu l 100 kg/cm2(10 MPa).


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1.2.4. Xc nh cc kch thc cbn ca mtrcu

Hnh dng mtrcu v cc kch thc cbn ca chng c xc nh xut pht

tiu kin thy vn, a cht, chiu cao cu, chiu di nhp v hng lot cc yu t

khc.

Tuy nhin, mt vi kch thc cbn c xc nh theo cc yu cu cu to vkhai thc nhcao nh mng mtr, cao nh mtr, kch thc mmtrtrn

mt bng, chiu dy ti thiu ca cc bphn, kch thc cc g, bc.

a. Cao nh mng

Cao nh mng c quyt nh xut pht tiu kin lm vic ca m tr

trong qu trnh khai thc, tiu kin xy dng v nhng l do kinh t.

Trn nhng vng kh cn, phn bi sng, cu vt, cu cn, cu qua thung lng,

cao nh mng khng phthuc loi mtrv thng t cao ti mt t, tr

cc loi mvi.

i vi cc tr cu qua sng c mng trn nn thin nhin, mng cc b thp,

mng su, cao nh mng thng t di mc nc thp nht (MNTN) t0,5 m

n 0,7 m. Ti cc nhp thng thuyn, cao nh mng phi m bo tu b qua

li khng va vo mp nh mng.

Hnh 1.30. Cao nh mng trong mng cc.

1-Bmng. 2-Cc. 3-Thn tr.

Vi nhng mtrmng cc bcao, btrc tht cao bt k(hnh 1.30).

Khi t b tr cao hn MNTN th xy dng tr d dng, khi lng thn tr gim

nhng khng m bo yu cu mquan. Nu cc c chiu di ln, gim moment


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un v chiu di tdo chu nn ca cc, nn t bcc su hp l, va c li v

mt chu lc, va thun tin cho vic thi cng.

b. Cao nh mtr

Cao nh mtrc quyt nh xut pht tyu cu sau: y dm cng nh

nh mtrphi cao hn mc nc cao nht tnh ton (MNCN) ti thiu l 0,5 m. Vtr y kt cu nhp c xc nh tchiu cao tnh khng di cu i vi cu vt,

cu cn hoc tchiu cao tnh khng thng thuyn vi nhng nhp thng thuyn v c

cy tri. Cao y kt cu nhp cao hn cao nh mtrmt trsbng chiu cao

gi cu (hnh 1.31).

Hnh 1.31. Xc nh cao nh tr.

a-nh trcao hn MNCN.

b-Chn cao nh trtheo mc nc thng thuyn (MNTT).

i vi nhng cu vt qua thung lng, khe su, nhng yu cu trn khng cn xtv chiu cao cu, chiu cao trc xc nh tcao tuyn ng qua cu.

Trong trng hp chung, cao nh trsly trsln nht trong hai cao sau:

MNCN + h

MNTT + htt hg

Trong :

MNCN Mc nc cao nht tnh ton.

MNTT Mc nc thng thuyn.h Khong cch nhnht tMNCN n nh tr, trn sng khng thng

thuyn h = 0,5 m.

htt Chiu cao nhnht cho php ca khthng thuyn.

hg Chiu cao gi cu.


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Lin quan n tnh khng thng thuyn, tiu chun thit kcu 22 TCN-272-05 ni

r: trkhi c chnh khc, khgii hn thng thuyn phi tun theo cc gi trcho

trong bng sau, ly tTCVN 5664-1992.

Khgii hn thng thuyn trn cc sng c thng thuyn

Khgii hn ti thiu trn mc nc cao c chu k20 nm (m)Theo chiu ngangCp ng sngCu qua sng Cu qua knh

Theo chiu thng ng(trn ton chiu rng)

I 80 50 10II 60 40 9III 50 30 7IV 40 25 6 (thch hp)

5 (ti thiu)V 25 20 3,5VI 15 10 2,5

Trn nhng vng kh cn, y kt cu nhp phi cao hn mt t ti thiu l 1m.

c. Kch thc mmtrtrn mt bng

Chiu rng nhnht (theo phng dc cu) v chiu di (theo phng ngang cu)

ca mmtrc xc nh nhhnh 1.32.

Hnh 1.32. Xc nh kch thc mmtr.

1-Kt cu nhp.

2-Tht gi.

3-Tm k gi ( k gi).

4-Mtr.

5-Tng nh.


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d. Xc nh mt scc kch thc khc

Chiu cao tng nh ca mh1c xc nh bng tng chiu cao xy dng ca

kt cu nhp (tnh ty dm ti mn cao phn xe chy), chiu cao gi cu v

chiu dy tm k gi (hnh 1.32).Chiu dy tng nh ca mnng (ti cao mm) c chn bng (0,50,6)h1

v bdy pha trn tng nh khng nhhn 0,5m.

Kch thc tit din thn trcu phthuc vo nhiu iu kin: hnh dng mtr,

chiu cao mtr, trsti trng, vt liu V vy ty trng hp cthsc xc

nh theo quy trnh hoc theo kinh nghim thit k.

Chiu dy x m ca cc tr cc, m tr v dm m ca cc loi m tr khc

khng c nhhn 0,4m m bo phn b ti trng tkt cu nhp n cc b

phn hoc cc khi xy thn tr. Chiu dy tng ca cc khi BTCT tit din hnh

hp rng khng c nhhn 15 cm (nu cc khi ny khng c lp y bng b

tng) v khng nhhn 1/15 chiu cao tit din khi khng c bn ngn ngang.

Chiu dy thnh trng khng nhhn cc trssau:

ng knh trong ca ng d (m) Chiu dy t (cm)0,4 80,6 10

1,23 1245 14

Theo kinh nghim thit k, chiu dy trnng b tng hoc b tng hc ti mt

ct nh mng khng nn nhhn 1/51/6 chiu cao. Chiu dy tng trc (tng

thn) ca cc mc tng cnh hoc thn mvi (ti mt ct nh mng) khng nn

nhhn 0,350,4 chiu cao t p. Chiu dy tng ca cc khi b tng rng khng

nn nhhn 2530 cm.

Tuy nhin trong cc trng hp ring, nu cng nghchto kt cu hon thin th

cc nghtrn c ththay i.


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24

CHNG 2: TNH TON THIT KMNG NNG

2.1. Cc khi nim cbn vmng nng

2.1.1. Khi nim

Mng nng l phn mrng ca y cng trnh nhm c c mt din tch tip

xc thch hp t nn c thgnh chu c p lc y mng.

Khi tnh sc chu ti ca nn t, thnh phn lc ma st v lc dnh gia thnh

mng vi t xung quanh c bqua.

Cng c nh ngha vmng nng da vo nhiu quan trc thc nghim kt hp

vi th nghim xuyn tnh CPT nhsau: mng nng l mng c tsDe/B < 1,5.

Hnh 2.1. Chiu su ngm mng tng ng.

Vi Del chiu su ngm mng tng ng (hnh 2.1), xc nh theo cng thc

sau:

0

1( )

D

e c

ce

D q z dzq

Trong :

qcel sc khng mi tng ng, xc nh nhsau:

31( )

3

D a

ce cc

D b

q q z dz a b

Vi qcc(z) l sc khng mi qc(z) san bng cc gi trln hn 1,3qcm

qcml gi trtrung bnh ca qc(z) trong khong t(D b) n (D+3a)

31( )

3

D a

cm c

D b

q q z dz a b


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Hnh 2.2. Stnh chiu su ngm mng tng ng.Trong cc cng thc trn: a = B/2 nu B>1m

a = 0,5m nu B


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p lc y mng (cng nhphn lc t nn) phthuc vo cc yu tsau:

cng ca mng.

Loi t nn (, t dnh, t) v trng thi ca chng.

Chiu su chn mng.

Thi gian ckt i vi t ht mn. Hnh dng v kch thc ca y mng.

Hnh 2.4. Cc dng biu phn lc t nn ln y mng.

i vi t thot nc nhanh, qu trnh thay i p lc y mng xy ra gn nh

tc khc. i vi t thot nc chm, qu trnh thay i p lc y mng phthuc

vo thi gian phn tn p lc nc lrng thng dtrong t nn.

Trong tnh ton nn mng thng c 2 cch chn biu p lc y mng (cng

nhphn lc nn):

Vi mng tuyt i cng: p lc y mng c xem l tuyn tnh. Vi mng chu un (mng mm): p lc y mng thng c githuyt l t

l vi chuyn v thng ng ca y mng (bin dng n hi theo phng

ng ca nn). Nn t di y mng nh vy thng c gi l nn

Winkler (hay nn n hi cc b) v t nn c c trng bng hcc l xo

n hi.

Mng cng Mng chu un Mng cng Mng chu un

a. t cng b. t dnh

Mng cng Mng chu un

c. t ct


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Mng nng c thc phn loi theo cc cch sau:

Theo hnh dng mng, c thchia thnh:

Mng n (mng n chu ti ng tm, chu ti lch tm, mng chn vt).

Mng phi hp (mng phi hp bi cc mng n).

Mng bng (mng bng n thun di tng, ct, mng bng giao thoa). Mng b (mng b di nhiu tng, nhiu ct ca mt phn hoc ton b

cng trnh, mng b dng bn, dng sn nm, dng hp).

Hnh 2.5. Cc dng mng n chu ti lch tm mt phng.

Theo vt liu xy dng mng, c thchia thnh:

Mng gch: thch hp cho cng trnh c ti trng nh, nm trn mc nc

ngm (MNN) v chchu ng sut nn.

Mng hc: thch hp cho cng trnh c ti trng trung bnh, c thnm trn

hoc di mc nc ngm v chchu ng sut nn.

Mng b tng: sdng trong cc mng chchu ng sut nn.

Mng b tng ct thp: c sdng phbin trong mi trng hp.

Theo phng php xy dng mng, c thchia thnh:

Mng ton khi (hay mng ti ch): c xy dng ngay ti v tr cn

xy dng cng trnh.

Mng lp ghp: gm cc cu kin c chto sn nh xng, bi c. Sau

vn chuyn cc cu kin ny n vtr xy dng cng trnh v lp ghp to

thnh mng.

y

N N

DfMxHy

yB

z

ey

L

x

B y

z

MxHy

B

MxHy

zx x

B B B

N

N

Hy Hy

LL

Df Df

yy y

ey

ey

N

Hy


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Mng bn lp ghp: l skt hp gia hai phng php trn.

Theo cng ca mng, c thchia thnh:

Mng cng: khi chu ti, bin dng ca mng rt nh, c thbqua trong qu

trnh tnh ton.

Mng mm (hay mng chu un): khi chu ti, mng bun cong, bin dng camng c xt n trong qu trnh tnh ton.

a scc mng n u l nhng mng cng, trong khi cc mng bng di

ct, mng b di cc tng, ct l nhng mng mm.

Theo c im chu ti ca mng, c thchia thnh:

Mng chyu chu ti thng ng, n nh ca mng phthuc vo ln ca

mng v sc chu ti cho php ca nn t.

Mng chyu chu ti ngang nhmng ca bk, tng chn t, mcu, p, tr qut gi, tr in n nh ca cc loi mng ny ph thuc vo

cng chng trt ca nn t.

2.1.2. Cu to

Mc ny strnh by chyu vcu to mng nng trong ngnh cu ng.

Khi thit kmng nng, trc ht ta phi xc nh c sbcc kch thc ca

mng.Hnh dng mng ty thuc vo iu kin a cht, thy vn, ti trng v cu to

ca cng trnh bn trn.

su t mng cn cvo mt ct a cht chn, phi t ln cc tng t c

cu to n nh v cng tnh ton ph hp, ngoi ra cn phi m bo khng blt

do hin tng xi mn lng sng.

i vi mtrcu, y mng phi c chn su di mt t sau khi xi lln

nht, ti thiu l:

h = h + k

Trong :

h su chn mng.

k Sai sc thxy ra trong khi tnh ton su xi l.

h su t mng trong t m bo sn nh ca mtr.


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Gi trk c thly t10% 20% su xi ltnh ton.

Gi trh ty thuc vo iu kin cng v n nh ca t nn, thng thng

h 2,5m.

i vi nh dn dng v mt scng trnh cng nghip c thchn mng su

h 0,5m.Khng nn t mng trn mt t hoc nn t mi p. Mng t trn mt t s

bnh hng ca nhiu yu tnhsxi mn ca nc ma trn mt t, sph hoi

ca cn trng cng nhnh hng trc tip ca thi tit. Mng t trn nn t mi

p thng c ln ln, ln khng u c thlm ph hoi cng trnh.

Mt trn ca mng nng thng t ngang mt t i vi nhng ni khng c

nc mt, chng hn nhmng ca mtrcu trn cn, trn bi sng. Nu mtrcu

t nhng ni c nc mt th mt trn ca mng thng ly thp hn mc ncthp nht (MNTN) t0,5 0,7m. nhng sng c thng thuyn, mt trn ca mng

cn ch n iu kin m bo khng va chm ca phng tin lu thng trn sng.

Hnh 2.6 thhin cu to mng nng ca trcu v mcu.

Hnh 2.6. Tr cu v

m cu trn mng

nng.

a-Trcu.b-Mcu.

1-Mt trn mng

2-y mng

3-Thn tr

4-Mtr

5-Thn m

6-Mm7-Tng cnh


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Gmng thng ly t0,2 1m i vi mtrcu v 0,1 0,5m i vi nh

dn dng v mt scng trnh cng nghip.

Kch thc ca y mng xc nh ty thuc vo cng ca t nn.

Nu mng c xy dng bng cc vt liu nhgch, xy, b tng v y mng

c mrng ra qu nhiu th di tc dng ca phn lc nn cc bc mng c thbgy (v cc vt liu ny c cng chu ko rt km so vi cng chu nn).

m bo cho cc bc mng khng bgy th gc phi nhhn mt trscho php

(ty thuc vo vt liu xy mng) trong mng chxut hin ng sut nn m khng

xut hin ng sut ko.

i vi mng nng ca nh dn dng v mt scng trnh cng nghip, c thly

theo bng sau:

Bng gi trgc

Mng xy dng bng cc loi vt liu khc nhau Gi trgc

- Mng hc bng va tam hp 23o

- Mng hc bng va xi mng 30o

- Mng b tng n hc 33o

- Mng b tng 40o

Gc c xc nh gn ng vi gc phn bng sut bn trong ca khi xy

mng.

Nu cu to ca mng c gc ln hn cc trsquy nh trn, do tc dng ca

phn lc nn, mng schu un v sinh ra ng sut ko y mng. Trong trng hp

ny, c thsdng mng b tng ct thp vi ct thp c btr y mng chu

ng sut ko do moment un tphn lc nn sinh ra.

Khi thit kmng, hnh dng y mng nn chn sao cho p lc y mng phn b

u.

Nu ti trng chc lc ng tc dng ng tm, y mng nn lm i xng. Nu

ti trng bao gm lc ng, lc ngang v moment th y mng nn cu to khng i

xng sao cho p lc y mng phn bu hn. Ni cch khc l lm sao cho hp lc

ca ti trng cng gn trng tm y mng cng tt.

Ni chung hp lc ca ti trng khng nm ngoi li mng.


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Hnh 2.7. Mng nng chu ti ng tm v lch tm.

a. Mng chchu tc dng ca lc ng ng tm, cu to mng i xng.

b. Mng chu tc dng ca lc ng, lc ngang v moment, cu to mng

khng i xng.

i vi cc cng trnh chu lc ngang ln, c thxy ra trng hp cng trnh btrt y mng v bph hoi. Khi xc nh hnh dng ca mng, nu thy tg< f

(vi f l hsma st gia y mng v t) tc l lc gy trt nhhn sc khng

trt y mng th y mng c tht nm ngang. Nu gc qu ln, hay gp i

vi mca cu vm hay tng chn t, c thlm mng nghing i mt gc no

(hnh 2.8a).

Hnh 2.8. y mng c cu to nghing v cu to bc.

a. y mng ca mcu vm c cu to nghing.

b. y mng c cu to thnh bc t trn tng c phn lp nm nghing.Khi mng mtrcu t trn tng , yu cu phi ph ht lp phong ha trn mt

v t mng vo lp cng ti thiu l 25cm. Trng hp tng di y mng c

cc phn lp nm nghing, c thcu to mng thnh cc bc nhhnh 2.8b

Khi thit kmng t khng su lm, m bo cho nn t chu c ti trng

bn trn phi mrng y mng ln hn gc . Trong trng hp ny thng sdng

NM

H

RN

pmin

pmax

ab


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mng b tng ct thp. Mng bng b tng ct thp c ththi cng theo phng php

ton khi, lp ghp, hoc bn lp ghp.

Hnh 2.9 thhin ct thp c btr trong mt sdng mng n bng b tng

ct thp thi cng theo phng php ton khi v bn lp ghp.

Hnh 2.9. Mt sdng mng n b tng ct thp ton khi v bn lp ghp.

2.2. Thit kmng nng trn nn thin nhin

2.2.1. Trnh tthit k

Trnh t thit k mng nng trn nn thin nhin ni chung cho cc ngnh xy

dng c ththc hin theo cc bc sau:

Xc nh cc ti trng tc dng ti nh mng.

nh gi a cht, thy vn, cc chtiu cl ca t nn.

Chn su chn mng.

Xc nh sbkch thc y mng. Tnh ton nn mng theo cc trng thi gii hn.

Trong ngnh xy dng cu ng, khi thit k theo tiu chun thit k cu

22TCN272-05 (da trn tiu chun thit kcu AASHTO xut bn nm 1998 ca M,

trn csphng php lun thit ktheo hsti trng v hssc khng LRFD),

trnh t c thhthng thnh cc bc nhthhin trong shnh 2.10.


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Hnh 2.10. Strnh tthit kmng nng cho cng trnh cu.

1. Btr sbvtng thcngtrnh cu (ST)

2. Xem xt cc iu kin a cht,thy vn c sn (GT)

3. Kho st hin trng (GT)

4. Xc nh su chn mng ttnh ton xi l(GT)

6. Xc nh cc chtiu cl cat nn tth nghim hin trng v

th nghim trong phng (GT)

5. Xc nh ti trng tc dng lnmng (ST)

7. Tnh ton sc khng danh nhtrng thi gii hn cng vtrng thi gii hn c bit (GT)

8. Tnh ton sc khng danh nh

trng thi gii hn sdng (GT)

9. Tnh ton sc khng trt v sckhng bng ca t trng thi

gii hn cng v trng thi giihn c bit (GT)

10. Kim tra n nh tng thdi

tc dng ca ti trng sdng (titrng khng c hs) (GT)

11. Xc nh kch thc mng trng thi gii hn sdng (ST)

12. Kim tra p lc y mng, lch tm v trt trng thi gii

hn cng (ST)

13. Kim tra p lc y mng,

lch tm v trt trng thi giihn c bit (ST)

14. Thit kkt cu mng sdngti trng c hs(ST)

ST = Structural (thuc vtnh ton kt cu mng)GT = Geotechnical (thuc vtnh ton nn)


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35

Lu :t trn mc nc ngm, dng dung trng tnhin. t di mc nc

ngm dng dung trng y ni.

Gi trcc hsiu kin lm vic

Hsm2i vi nh v cng trnh c skt cu cng vi tsgia chiu di

ca nh (cng trnh) hoc tng nnguyn vi chiu cao L/H bng:

Loi tHs

m1

4 1,5

t hn ln c cht nht l ct v

t ct khng kt phn v bi1,4 1,2 1,4

Ct mn:

- Kh v t m

- No nc

1,3

1,2

1,1

1,1

1,3

1,3

Ct bi:

- Kh v t m

- No nc

1,2

1,1

1,0

1,0

1,2

1,2

t hn ln c cht nht l st v

t st c st IL0,51,2 1,0 1,1

Nhtrn c st IL> 0,5 1,1 1,0 1,0

Ghi ch:

1. Skt cu cng l nhng nh v cng trnh m kt cu ca n c khnng c

bit chu ni lc thm gy ra bi bin dng ca nn.

2. i vi nh c skt cu mm th hsm2ly bng 1.

3. Khi tsL/H ca nh, cng trnh nm gia cc trsni trn th hsm2xc nh

bng ni suy.

Gi trhssc chu ti A, B, D

o

(rad)

0,25Acotg

2

B 1

cotg2

cotgDcotg

2

0 0.000 0.0000 1.0000 3.1416

2 0.035 0.0290 1.1159 3.3196

4 0.070 0.0614 1.2454 3.5100

6 0.105 0.0976 1.3903 3.7139


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o

(rad)

0,25A

cotg2

B 1cotg

2

cotgD

cotg2

8 0.140 0.1382 1.5527 3.9326

10 0.175 0.1837 1.7349 4.1677

12 0.209 0.2349 1.9397 4.420814 0.244 0.2926 2.1703 4.6940

16 0.279 0.3577 2.4307 4.9894

18 0.314 0.4313 2.7252 5.3095

20 0.349 0.5148 3.0591 5.6572

22 0.384 0.6097 3.4386 6.0358

24 0.419 0.7178 3.8713 6.4491

26 0.454 0.8415 4.3661 6.9016

28 0.489 0.9834 4.9338 7.3983

30 0.524 1.1468 5.5872 7.9453

32 0.559 1.3356 6.3424 8.5497

34 0.593 1.5547 7.2188 9.2198

36 0.628 1.8101 8.2403 9.9654

38 0.663 2.1092 9.4367 10.7985

40 0.698 2.4614 10.8455 11.7334

42 0.733 2.8785 12.5138 12.7874

a. Trng hp mng chu ti ng tm

Xt mt mng n c y l hnh chnht kch thc lb, chu tc dng ca ti

ng tm nhhnh 2.11.


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Hnh 2.11. Mng n (lb) chu ti ng tm.

Cc lc tc dng ln mng bao gm:

Ti trng cng trnh bn trn truyn xung

ti nh mng: tcoN

Trng lng bn thn mng: tcmN

Trng lng t p trn mng trong phm

vi kch thc mng: tcdN

Phn lc nn t tc dng ln y mng: tcp

Biu phn lc nn thc cht c dng ng cong, nhng i vi mng cng ta

c thxem l ng thng, v trong trng hp ti ng tm th biu c dng phn

bu.

iu kin cn bng tnh hc ca mng:

tc tc tc tco m dN N N p .F

t tc tcm d tb f G N N .D .F

tb - trng lng ring trung bnh ca mng v t p trn mng trong phm vi

kch thc mng, c thly 3tb 20 22kN / m .

fD - su chn mng.

tc tc tco o tb f p .F N G N .D .F

tctc o

tb f

Np .D

F vi

t ttc oo

NN

n (n = 1,15)

Kch thc mng c xc nh theo iu kin:

tc tcp R tc

tcotb f

N.D R

F

tco

tctb f

NF

R .D

l

Df

Notc

F = lb


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Trong biu thc trn ta thy F v Rtcu l nhng i lng c cha chiu rng

y mng b v trong Rtccn c cha su chn mng Df. Do vic xc nh din

tch F trn c thquy vxc nh chiu rng y mng b, sau xc nh chiu di

y mng ltheo tsk = l/bchn trc. V trnh t c ththc hin nhsau:

Chn trc mt su chn mng Df Chn trc tscc cnh ca y mng k = l/b1, ta sc: F =lb= k.b2

Biu thc trn c thc vit li nhsau:

tc2 o

* tcf tb f

Nkb

m(A b BD Dc ) .D

khi Rtctnh theo TCXD 45-70

* tc tcf tb f o

3 2kmA k m(BD Dc ) .D b N 0b

tc2 o

*1 2II f II II tb f

tc

Nkb

m m (Ab BD Dc ) .Dk

khi Rtctnh theo TCXD 45-78

* tc1 2 1 2II f II II tb f o

tc tc

3 2m m m mk A k (BD Dc ) .D Nb b 0k k

Gii bt phng trnh bc 3 trn ta c trschiu rng y mng b cn tm.

T xc nh c l= kb.

Bt phng trnh trn c thgii theo phng php ng dn tm gi trb nh

nht tha mn bt phng trnh .V d2.1:

Mt mng n c y hnh chnht chu ti ng tm Nott = 500 kN, su chn

mng Df= 1,5 m, MNN nm ngay ti y mng, nn t c: = 18 kN/m3, sat= 20

kN/m3, = 18o, c = 20kN/m2. Xc nh kch thc y mng, bit: m1= m2= ktc= 1;

n = 1,15; tb= 22 kN/m3,

Li gii

su chn mng c cho trc Df= 1,5 mChn k = l/b= 1,2

Chiu rng y mng c xc nh tiu kin sau:

* tc1 2 1 2II f II II tb f o

tc tc

3 2m m m mk A k (BD Dc ) . Nk

b D 0k

b


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Hnh 2.12. Mng n chu ti lch tm theo 2 phng (tm ct trng tm mng).

ey, ex lch tm ca hp lc ca ti trng trn mt bng y mng theo phng

y v phng x tng ng.

Mxtc moment un tiu chun quanh trc x ti trng tm y mng.

Mytc moment un tiu chun quanh trc y ti trng tm y mng.

Ntc tng ti trng thng ng tiu chun ti y mng.

Moxtc moment un tiu chun quanh trc x ti nh mng.

Moytc moment un tiu chun quanh trc y ti nh mng.

Hoxtc lc ngang tiu chun theo phng x ti nh mng.

Hoytc lc ngang tiu chun theo phng y ti nh mng.

Notc ti trng thng ng tiu chun ti nh mng do cng trnh bn trn truyn

xung.

G trng lng trung bnh ca mng v t p trn mng trong phm vi kch

thc mng.

Trong trng hp tm ct khng trng tm mng, chng hn nhhnh 2.13.

DfMox

tcHoy

tc

yb

z

ey

l

x

b

Notc

Hoytc

y

ex

Hoxtc

DfMoy

tcHox

tc

xl

z

Notc

ey= Mxtc/ Ntc

ex= Mytc/ Ntc

Vi Mxtc= Mox

tc Hoytc.h

Mytc= Moy

tc Hoxtc.h

Ntc= Notc+ G = No

tc+ tb.Df .F

=> ey= [Moxtc Hoy

tc.h] / [Notc+ tb.Df .F]

V ex= [Moytc Hox

tc.h] / [Notc+ tb.Df .F]

h h


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41

Hnh 2.13. Mng n chu ti lch tm theo 2 phng (tm ct khng trng tm mng)

Lu :

Trong cc cng thc xc nh eyv ex:

Cc thnh phn (Hoytc.h) v (No

tc.y) ly du (+) khi cc thnh phn ny l nhng

moment cng chiu vi moment Moxtc, ly du () trong trng hp ngc li.

Cc thnh phn (Hoxtc.h) v (No

tc.x) ly du (+) khi cc thnh phn ny l nhng

moment cng chiu vi moment Moytc, ly du () trong trng hp ngc li.

i vi mng chu ti lch tm, khi xc nh sbkch thc y mng ta vn

thc hin tng tnhtrng hp mng chu ti ng tm. Sau , ta tng kch thc

y mng ln chu ti trng lch tm. Thng thng, kch thc y mng c gia

tng nhsau:

l+ 2ex hay l+ 2el vi ex= el

b + 2ey hay b + 2eb vi ey= eb

Sau , kim tra li cc gi trkch thc y mng va chn theo iu kin sau:

DfMox

tcHoy

tc

yb

z

ey

l

x

b

Notc

Hoytc

y

ex

Hoxtc

DfMoy

tcHox

tc

xl

z

Notc

ey= Mxtc/ Ntc

ex= Mytc/ Ntc

Vi Mxtc= Mox

tc Hoytc.h No

tc.y

Mytc= Moy

tc Hoxtc.h No

tc.x

Ntc= Notc+ G = No

tc+ tb.Df .F

=> ey= [Moxtc Hoy

tc.h Notc.y] / [No

tc+ tb.Df .F]

V ex= [Moytc Hox

tc.h Notc.x] / [No

tc+ tb.Df .F]

h h

x

y

y x


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42

tc tctb

tc tcmax

tcmin

p R

p 1,2R

p 0

vitc tc

tc max mintb

p pp

2

Mt scng trnh c cu chy, cn yu cu tc tcmin maxp 0,25p

Khi mng c dng bt k:

tctc tcytc x

maxmin y x

MN Mp

F W W

Khi mng c dng hnh chnht:

tctcmaxmin

6e 6eNp 1

F b

l

l

Vi tc tc tc tc tc tcy x x y bM N e N e ; M N e N e l

2 2

y x

b bW ; W

6 6

l l

Nu mng chlch tm theo phng x (phng cnh di l):tc

tcmaxmin

6eNp 1

F

l

l

Nu mng chlch tm theo phng y (phng cnh ngn b):tc

tc

maxmin

6eN

p 1F b

Trong thit kphi khng chsao cho tcminp 0 , tc l 6e ll hoc b 6e trong

trng hp mng chu ti lch tm theo 1 phng hoc tng ng vi iu kin rng

buc trong trng hp mng chu ti lch tm theo 2 phng.

V d2.2:

Cho mt mng n chu ti lch tm mt phng nhhnh v: Nott = 600kN, Mo

tt =

30 kN.m, Hott= 50kN, Df= 1,5 m, nn t c: = 18 kN/m

3, sat= 20 kN/m3, = 20o

(A = 0,515, B = 3,059, D = 5,657), c = 15 kN/m2,. Xc nh kch thc y mng,

bit: m1= m2= ktc= 1; n = 1,15; tb= 22 kN/m3.

Li gii


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44

Xc nh lch tm:

tt tto otc tc tc tc tc

y oy ox o ox tttc tc tc

oo tb f o tb f tb f

tcy

x tc

M H.hM M H .h M H .h n ne e

NN N D F N D FD F

n

30 50 .0,6M 52,1741,15 1,15e e 0,08m600N 648,45922.1,5.(1,6.2,4)1,15

l

l

Chn li chiu di y mng:

l= 2,4 + 2el= 2,4 + 2.0,08 = 2,56m Chn l= 2,6m

Kim tra p lc y mng theo iu kin:

tc tctb

tc tcmax

tcmin

p R

p 1,2Rp 0

vi

tc tc

tc max mintb p pp 2

Sc chu ti tiu chun ca nn t:

tc *1 2II II f II II

tc

tc 2

m mR R (Ab BD Dc )

k

R 1 0, 515.1, 6.10 3, 059.1,5.18 5, 657.15 175, 688 (kN / m )

Tnh ton v kim tra p lc y mng:

tctcmax

2 tc 2

tctcmin

2

tc tctc max mintb

6eN 648, 459 6.0,08p 1 1F 1,6.2,6 2,6

184,657 (kN / m ) 1,2.R 210,826(kN / m )

6eN 648, 459 6.0,08p 1 1

F 1,6.2,6 2,6

127,102(kN / m ) 0

p p 184,657 127,1p

2

l

l

l

l

2 tc 2

02

2

155,88(kN / m ) R 175,688(kN / m )

Vy vi kch thc y mng b = 1,6m v l= 2,6m th p lc y mng tha iu

kin kim tra.


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2.2.3. Tnh ton nn theo trng thi gii hn (TTGH)

Hin nay, khi tnh ton thit kcc cng trnh xy dng ni chung ngi ta thng

tnh ton theo trng thi gii hn.

Trng thi gii hn (pht biu theo quy trnh thit kcu cng theo trng thi gii

hn 22TCN18-79) l trng thi m kt cu hoc nn mng khng cn tha mnc yu cu v khai thc na do nh hng ca cc tc ng lc. Hoc theo tiu

chun thit kcu 22TCN272-05, trng thi gii hn l trng thi ng vi iu kin

m vt qua n th cu hoc cu kin ca cu ngng tha mn cc quy nh c

da vo thit k.

Theo quy trnh thit kcu cng theo trng thi gii hn 22TCN18-79, cng nh

cc tiu chun khc ca ngnh xy dng, khi tnh ton thit k cng trnh phi tin

hnh theo 3 TTGH sau:

Tnh ton theo TTGH 1: nhm m bo cho cng trnh khng bngng sdng

do khng cn khnng chu lc (vcng , n nh, chu mi) hoc

pht trin bin dng do ln.

Tnh ton theo TTGH 2: nhm m bo cho cng trnh khng pht sinh bin

dng chung qu ln nhdao ng, chuyn v, ln, gy kh khn cho vic s

dng bnh thng.

Tnh ton theo TTGH 3: nhm m bo bn chng nt cho cng trnh

trnh gy kh khn cho vic sdng bnh thng.

i vi nn, khi tnh ton thit k, chtnh theo TTGH 1 v TTGH 2.

Tnh ton nn theo TTGH 1 bao gm:

Kim ton p lc y mng.

Kim ton tnh n nh cng trnh, bao gm:

+n nh chng lt.

+n nh chng trt.

+n nh chng trt su.

Tnh ton nn theo TTGH 2 bao gm:kim tra ln n nh, ln lch v gc

nghing gii hn ca mng

Vphn ny c thxem thm sch Nn v Mng cng trnh cu ng (Bi Anh

nh, Nguyn SNgc).

Theo tiu chun thit kcu 22TCN272-05, cc TTGH c chia thnh:


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TTGH sdng.

TTGH mi v ph hoi gin.

TTGH cng (I, II, III).

TTGH c bit.

Trong tiu chun thit k cu 22TCN272-05, vic tnh ton nn v mng theoTTGH khng tch ri nhau. Cc TTGH lin quan, c sdng khi thit knn mng

bao gm:

Trng thi gii hn sdng: phi bao gm:

Ln (xem xt da trn tin cy v tnh kinh t).

Chuyn vngang

Sc chu ti c tnh dng p lc chu ti ginh

Trng thi gii hn cng : phi xt n: Sc khng , loi tp lc chu ti ginh.

Mt tip xc qu nhiu.

Trt ti y mng.

Mt ngang.

Mt n nh chung.

Khnng chu lc ca kt cu.

Mng phi c thit kvmt kch thc sao cho sc khng tnh ton khng nhhn tc ng ca ti trng tnh ton.

Trng thi gii hn c bit: thit knn mng theo TTGH c bit theo quy nh.

Vphn ny c thxem thm tiu chun thit kcu 22TCN272-05.

2.2.4. Tnh ton cng bn thn mng

Theo cch tnh thng thng trong cc ngnh xy dng, tnh ton cng bn

thn mng bao gm xc nh chiu cao bmng (chiu cao i) v tnh ton ct thpcho bmng.

a. Xc nh chiu cao bmng

Chiu cao h (hay chiu dy) bmng thng c ginh trc khi xc nh s

bkch thc y mng, sau c kim tra li theo iu kin xuyn thng. Ngha

l nu chiu cao bmng khng ln th bmng sbph hoi theo thp xuyn


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thng xut pht t chn ct v nghing mt gc bng gc cng ca vt liu so vi

phng thng ng (gc cng ca b tng bng 45o).

iu kin mng khng bxuyn thng:

PxtPcx

Trong :Pxt: lc gy xuyn thng.

Pcx: lc chng xuyn thng.

a.1. Trng hp mng n b tng ct thp chu ti ng tm

Hnh 2.14. Tnh ton xuyn thng mng n b tng ct thp chu ti ng tm.

Pxt= ptt. Sngoi y thp xuyn= p

tt. [l.b (lc+ 2h0).(bc+ 2h0)]

Pcx=0,75.Rk. Sxq thp xuyn= 0,75.Rk.[2.(lc+bc+ 2ho). ho]

Vi ptt phn lc nn tnh ton gy xuyn thng.

tttt oNp

F . y cn ch rng trng lng ca mng v t trn mng

khng lm cho mng b un v khng gy ra xuyn thng mng nn

khng kn.

ho= h a : chiu cao lm vic ca mng.

a : khong cch t trng tm ct thp chu ko y mng n thb

tng chu ko ngoi cng ca y mng.

Mt chng xuyn tnh ton

Nott

Nott

ptt

bc

h0

h0

b

45o h0

bc+ h0lc+ h0

Mt bxuyn thng

lc+ 2h0

lc

h0

h0

lc h0h0

l

lc

bc

y

x


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Rk: cng chu ko ca b tng mng.

a.2. Trng hp mng n b tng ct thp chu ti lch tm theo 1 phng

Trng hp mng chu ti lch tm theo phng cnh di l(hnh 2.15).

Chcn tnh ton cho mt thp xuyn bt li nht. Mt thp xuyn bt li nht lmt ng vi phn lc nn tnh ton ln nht pttmax

Hnh 2.15. Tnh ton xuyn thng mng n b tng ct thp chu ti lch tm theo 1

phng.

Pxt= {0,5.(b+bc+2h0).0,5.[l (lc+ 2h0)]} .0,5.[ptt

max+ p1tt]

Pcx= 0,75. Rk. S1 mt bn thp xuyn = 0,75. Rk.[(bc+h0)h0]

Trong :

pttmaxv ptt

minxc nh theo cng thc sau:

ttttytt o

maxmin y

MN

p F W

Vi Mytt= Moy

tt Hoxtt.h

2

y

bW

6

l

p1tt: phn lc nn ti chn thp xuyn ng vi mt thp xuyn bt li nht, c

thxc nh tpttmaxv ptt

mintheo hthc lng trong tam gic ng dng.

y

x

Mt chng xuyn tnh ton

Nott

pttmax

bc

h0

h0

b

45o h0

bc+ h0lc+ h0

Mt bxuyn thng

h0

lc h0h0

l

lc

bc

Mott

Hott

Nott

Mott

Hott

pttmin

p1tt pttmax

pttminp1

tt


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c ott tt tt tt

1 min max min

0,5 2hp p p p

l l

l

Trng hp mng chu ti lch tm theo phng cnh ngn b, ta c thtnh ton

tng tnhtrn bng cch o cc k tb v lcho nhau:Pxt= {0,5.(l+lc+2h0).0,5.[b (bc+ 2h0)]} .0,5.[p

ttmax+ p1

tt]

Pcx= 0,75. Rk. S1 mt bn thp xuyn = 0,75. Rk.[(lc+h0)h0]

Trong :

pttmaxv ptt

minxc nh theo cng thc sau:

tt tttt o xmaxmin x

N Mp

F W

Vi Mxtt= Mox

tt Hoytt.h

2

x

bW

6

l

p1tt: phn lc nn ti chn thp xuyn ng vi mt thp xuyn bt li nht, xc

nh tng tnhtrn nhng pttmaxv ptt

minly theo biu phn lc nn tnh

ton theo phng lch tm tng ng (phng cnh ngn b).

c ott tt tt tt

1 min max min

0,5 b b 2hp p p p

b

b. Tnh ton ct thp cho bmng

Xem bmng nhmt dm consol c ngm ti mp chn ct.

b.1. Trng hp mng n b tng ct thp chu ti ng tm

Hnh 2.16. Tnh ton ct thp trong mng n b tng ct thp chu ti ng tm.

(l lc)/2

ptt

b

Nott

hMb

b

b

l

l lbc

lc

b

b

l


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Moment tnh ton ti mt ngm theo phng b:

2

c c ctt ttb

bM p .b. . p .

2 4 8

l l l l l l

Moment tnh ton ti mt ngm theo phng l:

2

c c ctt ttb b b b b bM p . . . p .2 4 8

lll

Din tch ct thp cn thit cho phng b v lc thtnh gn ng theo cng thc

sau:

b ba(b) a( )

a 0 a 0 a 0 a 0

M M M MF ; F

R h 0,9.R h R h 0,9.R h

l l

l

b.2. Trng hp mng n b tng ct thp chu ti lch tm theo 1 phng

Trng hp mng chu ti lch tm theo phng cnh di l(hnh 2.17).

Hnh 2.17. Tnh ton ct thp trong mng n b tng ct thp chu ti lch tm theo 1

phng.


c c c ctt tt ttb 2 max 2

2 2 2

c c ctt tt tt tt tt2 max 2 max 2

1 2M p .b. . . p p .b. . .

2 4 2 2 3 2b b bp . p p . 2p p

8 12 24

l l l l l l l l

l l l l l l

Vi ctt tt tt tt

2 min max min

0,5p p p p

l l

l


(l lc)/2

pttmax

b

Nott

hMb

b

b

l

l lbc

lc

b

b

l

Hott Mo

tt

pttminp2

tt


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2

c c ctt tttb tb

b b b b b bM p . . . p .

2 4 8

l

ll

Vitt tt

tt max mintb

p pp

2

Din tch ct thp cn thit cho phng b v ltnh tng tnhtrng hp mngchu ti ng tm.

Trng hp mng chu ti lch tm theo phng cnh ngn b, ta c thtnh ton

tng tnhtrn bng cch o cc k tb v lcho nhau:


c c c ctt tt tt2 max 2

2 2 2c c ctt tt tt tt tt

2 max 2 max 2

b b b b b b b b1 2M p . . . . p p . . . .

2 4 2 2 3 2

b b b b b bp . p p . 2p p

8 12 24

ll l

l l l

Vi ctt tt tt tt

2 min max min

0,5 b bp p p p

b


2

c c ctt ttb tb tb

bM p .b. . p .

2 4 8

l l l l l l

Vi ptt

tbxc nh tng tnhtrn nhng ptt

maxv ptt

minly theo biu phn lcnn tnh ton theo phng lch tm tng ng (phng cnh ngn b).

Din tch ct thp cn thit cho phng b v ltnh tng tnhtrng hp mng

chu ti ng tm.

V d2.3:

Xc nh chiu cao mng h hp l mng tha mn iu kin xuyn thng trong

v d2.1. Tnh ton v btr ct thp cho mng ng vi chiu cao h va chn. Bit: a

= 5 cm; kch thc ct lcx bc= 30 cm x 20 cm; b tng mng mc 200 c Rn= 90

kG/cm2; Rk= 7,5 kG/cm2; thp trong mng c Ra= 2300 kG/cm

2.

(1 kG/cm2102kN/m2)

Li gii

Trong v d2.1, ta xc nh c kch thc y mng: l= 2m v b = 1,6m.

Gish = 0,5 mho= h a = 0,5 0,05 = 0,45 m.


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Chn 614 c tng din tch Fa= 9,23 (cm2) btr thnh 14a340 theo phng

cnh di l

V d2.4:

Kim tra iu kin xuyn thng trong v d2.3. Tnh ton v b tr ct thp chomng. Bit: a = 5 cm; kch thc ct lcx bc= 30 cm x 20 cm; b tng mng mc 200

c Rn= 90 kG/cm2; Rk= 7,5 kG/cm

2; thp trong mng c Ra= 2300 kG/cm2.

(1 kG/cm2102kN/m2)

Li gii

Trong v d2.3, ta xc nh c kch thc y mng: l= 2,6m v b = 1,6m.

Chiu cao mng h = 0,6 m ho= h a = 0,6 0,05 = 0,55 m.

Phn lc nn tnh ton:

ttttytt 2o

maxy

MN 600 60p 177,509 kN / m

F W 2,6.1,6 1,803

ttttytt 2o

miny

MN 600 60p 110,953 kN / m

F W 2,6.1,6 1,803

Vi Mytt= Moy

tt+ Hoxtt.h = Mo

tt+ Hott.h = 30 + 50.0,6 = 60 (kN.m)

2 2

3y

b 1,6.2, 6W 1,803 m

6 6

l

c ott tt tt tt1 min max min

2

0,5. 2hp p p p .

0,5. 2,6 0,3 2.0,55110, 953 177, 509 110, 953 .

2,6

162,15 kN / m

l l

l

ctt tt tt tt2 min max min

2

0,5p p p p

0,5. 2,6 0,3110, 953 177, 509 110, 953 .2,6

148,071 kN / m

l l

l

Lc gy xuyn thng:

Pxt= {0,5.(b+bc+2h0).0,5.[l (lc+ 2h0)]} .0,5.[ptt

max+ p1tt]

Pxt= {0,5.(1,6 + 0,2 + 2.0,55).0,5.[2,6 (0,3 + 2.0,55)]}.0,5.[177,509 + 162,15]


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Pxt= 147,752 (kN)

Lc chng xuyn thng:

Pcx= 0,75. Rk. S1 mt bn thp xuyn = 0,75. Rk.[(bc+h0)h0]

Pcx= 0,75. 7,5.102.[(0,2+0,55).0,55] = 232,031 (kN)

Pxt< Pcx. Vy iu kin chng xuyn thng c tha mn.


2

ctt ttb max 2

2

bM 2p p

24

1,6. 2,6 0,32.177,509 148,071 177,423(kN.m)

24

l l


2 2tt ttc ctt max min

tb

2

b b b bp pM p . .

8 2 8

2,6 1,6 0,2177,509 110,953. 91,875(kN.m)

2 8

l

l l

Din tch ct thp cn thit cho phng b v l:

4 2 2b ba(b) 2a 0 a 0

M M 177,423F 15,6.10 m 15,6 cm

R h 0,9.R h 0,9.2300.10 .0,55

Chn 816 c tng din tch Fa = 16,08 (cm2) b tr thnh 16a200 theo

phng cnh ngn b

4 2 2a( ) 2a 0 a 0

M M 91,875F 8,07.10 m 8,07 cm

R h 0,9.R h 0,9.2300.10 .0,55

l l

l

Chn 614 c tng din tch Fa= 9,23 (cm2) btr thnh 14a400 theo phng

cnh di l

(l lc)/2

pttmax

b

Nott

hMb

b

b

l

l lbc

lc

b

b

l

Hott Mo

tt

pttmin

p2tt


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Lu : Trong mi trng hp, khng b tr khong cch gia cc ct thp dc

trong cng mt lp vt qu 400mm.

Theo tiu chun thit kcu 22TCN272-05, c thxc nh din tch ct thp nh

sau:

Xt mt cu kin chu un c tit din hnh chnht, btr ct thp n. Di tcdng ca moment un ngoi lc M, khng xt slm vic ca b tng vng chu ko,

ta c sng sut trn tit din nhhnh 2.18.

Hnh 2.18. Xc nh din tch ct thp n cho tit din chu un hnh chnht.

Bit b, h, fc, MXc nh As

Trong :

h chiu cao ca tit din.

b chiu rng ca tit din.ds khong cch tthchu nn ngoi cng n trng tm ct thp chu ko

(gi l chiu cao lm vic ca tit din).

as khong cch tthchu ko ngoi cng n trng tm ct thp chu ko.

a chiu cao vng nn quy i.

fc cng chu nn ca b tng.

fy gii hn chy ca ct thp.

As din tch ct thp chu ko.Tsng sut trn tit din ca cu kin (hnh 2.18), ta c:

Phng trnh cn bng moment trn tit din ly i vi trng tm ct thp chu

ko:

c s

aM 0,85.f .a.b d

2

As

as

h

b

ds

a

0,85fc

As.fy

(ds a/2)

MTTH


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56

u us s s s

c c

2M M2Ma d d d d ; M

0,85.f .b .0,85.f .b

Vi Mu moment do ti trng tc dng gy ra TTGH cng .

hssc khng (hay cn gi l hsgim bn)

Phng trnh cn bng lc trn mt ct ngang:

c s y0,85.f .a.b A f

cs

y

0,85.f .a.bA

f


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CHNG 3: TNH TON THIT KMNG CC

3.1. Khi nim, phn loi mng cc

3.1.1. Khi nim mng cc

Mng cc l mt loi mng su, khi tnh sc chu ti theo t nn c kn thnh

phn ma st v lc dnh gia thnh mng (thnh cc) vi t xung quanh v c chiu

su chn mng kh ln so vi brng mng.

Da vo nhiu quan trc thc nghim iu kin lm vic ca mng su kt hp vi

kt quth nghim xuyn tnh CPT, mng su c nh ngha l mng c tsgia

chiu su ngm mng tng ng trong t v brng mng l De/B > 5.

Vi mng c 1,5 < De/B < 5, mng c xem l mng na su.

Hnh 3.1. Mng cc

Mng cc gm 2 bphn chnh l i cc (hay bcc) v hcc.

i cc: lin kt cc cc li vi nhau, tip nhn ti trng tcng trnh bn trn vtruyn xung cho cc cc.

Hcc: c lin kt bi i cc, tip nhn ti trng ti v truyn cho nn t

xung quanh cc v bn di mi cc. Mi cc lun c thit kcm vo cc lp a

cht tt gnh ti trng cng trnh bn trn cng vi thnh phn sc chu ti do

ma st v lc dnh xung quanh cc.


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nhln u cc, hoc nu xi mnh hn na, chcn trng lng bn thn ca cc v

ba cng chtt xung, c bit l giai on u khi hcc.

Cc xonc chto c cc cnh xon bng thp u cc, thng l vng xon

lin tc c ng knh ln hn ng knh thn cc. Cc c hnh thit bxoay

thn cc lm cho cc xuyn vo t nn.Cc nhic sdng phbin hin nay, l loi cc c b tng ti chvo

cc lkhoan hoc ho c khoan (o sn) ngay ti vtr cn btr cc.

Theo c im chu ti ca cc, c thphn thnh: cc chu mi, cc ma st.

Cc chu mikhi phn ln ti trng c truyn qua mi cc vo lp a cht tt

mi cc. Cc chu mi cn c gi l cc chng.

Cc ma stkhi cc khng ta n lp a cht tt mi cc, ti trng c phn

bphn ln qua thn cc nh lc ma st v lc dnh vi t xung quanh cc v ch

mt phn nhti trng truyn vo t nn mi cc. Cc ma st cn c gi l cc

treo.

Cc cng c thc phn thnh cc ch yu chu ti trng ng, cc ch yu

chu ti trng ngang.

i khi cc c phn chia thnh cc ng, cc xin.

Ngoi ra, cc cn c phn loi theo kch thc thnh cc c ng knh nh,

ng knh ln.

Cc khng mrng y v cc mrng y.

Cng cn c khuynh hng phn chia cc thnh cc c chi (cc h bng

phng php ng hoc p) v cc khng c chi (cc nhi).

b. Phn loi mng cc

Mng cc c thc phn loi da theo vic phn loi cc hoc c thc phn

loi thnh: mng cc i thp, mng cc i cao, mng cc i n, i kp, i bng,

i b. Trong , ta thng quan tm hn c l mng cc i thp v mng cc i

cao.

Mng cc i thpc i cc nm di mt t tnhin, ti trng ngang tc dng

ln i cc hon ton do cc lp t ty i trln tip nhn, hcc chyu ch

chu nn.


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Mng cc i cao thng c i cc nm trn mt t tnhin, hcc va chu

nn va chu un.

3.1.3. Cu to cc v bcc

a. Cu to cc

a.1. Cc g

Cc gthng c lm bng cc loi gthng, gtthit (glim, hng sc, tu,

) v cc loi gny thng, thn cy tng i u, cng tng i cao. Cc g

c th lm bng cc thn cy nguyn dng hoc cc thanh g x. Chiu di cc g

nguyn (khng ni, khng ghp) thng thng l 5-8 m, ng knh khong 20-30 cm.

nh cc gphi c bo vbng ai thp (hnh 3.2c) khi ng khi bnt n.

Mi cc gc vt nhn v bt thp khi ng c ddng v khng bte (hnh3.2a, b). Khi cc gcn c chiu di ln th c thni cc cy gli vi nhau (hnh

3.2d). Khi cn c tit din ln th c thghp nhiu cy gli vi nhau (hnh 3.2e).

Hnh 3.2. Cu to cc g

a. Mi cc vt 4 cnh; b. Cc bt mi thp; c. ai thp nh cc;

d. Mi ni; e. Cc ghp; f. Cc gdn.


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a.2. Cc b tng ct thp

Cc b tng ct thp c nhiu loi tit din khc nhau: trn, vung, chnht, ch

T, chI, tam gic, a gic, vnh khn, vung c lrng. (Hnh 3.3)

Hnh 3.3. Cc dng tit din cc b tng ct thp

Loi cc tit din vung c sdng rng ri hn cv n c u im chyu l

chto n gin, c thchto ngay ti cng trng. Kch thc tit din ca loi cc

ny thng l 2020cm, 2525cm, 3030cm, 3535cm, 4040cm. gim bt trng

lng ca cc (khi khng c ba nng ng cc), c thdng loi cc vung c

lrng trn gia. Chiu di thng dng ca cc b tng ct thp l 4-25m.

V kh khn trong qu trnh vn chuyn v iu kin hn chvgi ba nn phnln cc c chto thnh tng on ri ni li vi nhau trong qu trnh hcc. Chiu

di mi on cc thng khong 6-8m. Hnh 3.4 thhin cu to ca cc b tng ct

thp tit din vung thng c sdng hin nay.

on cc mi.

on cc gia.

Hnh 3.4. Cu to cc b tng ct thp thng tit din vung.

D

L

D

L

Ct thp dc

Ct thp ai

1-1,5D150

1000 Mc cu, 16

6 a100

1000

6 a10020,1m

Li thp ucc, 6 a50


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Hnh 3.5. Cu to chi tit u cc, mi cc v mi ni cc.

Khi ng cc, ng sut cc bpht sinh nh cc, do phm vi nh cc phi

c tng cng thm bng cc li ct thp. Mi cc phi c btr thanh thp gia

cng chu lc khng xuyn khi hcc vo cc lp a cht cng. Ct thp ai

hai u btr dy hn vi bc 5-10cm so vi on gia vi bc 15-20cm. Ct thp

ai c thdng ai nhnh hoc xon c. Ct thp dc thng gm 4 hoc 8 thanh c

= 16-32mm, ct thp ai dng = 6-8mm. Lp b tng bo vca cc c chiu dy

ti thiu l 3cm. B tng dng lm cc phi c mc thp nht l 300 i vi ngnh cu

ng, thy li, 150 i vi ngnh xy dng dn dng v cng nghip. Trong mi cc

(hay mi on cc) phi btr mc cu cc trong qu trnh vn chuyn v lp dng

cc ln gi ba.

a.3. Cc thp

Cc thp dng trong cc cng trnh chu ti trng ngang ln. Cc thp c u im

l chu c ti trng ln do cng tnh ton ca vt liu cao, v vy thng dng

A-A

Hp ni cc

AA

Mi thp Mi hn

u cc

Ni cc


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khi cc chu moment ln. Tuy nhin, thp l mt loi vt liu c gi thnh cao nn

c cn nhc khi sdng cho cng trnh.

Cc thp cng c sdng rng ri lm vng vy thi cng cc loi mng, cc

cc ny c dng thanh cc vn, c chiu di thng 8-22m. Hnh 3.6 thhin mt s

dng tit din ngang ca cc vn thp.

Hnh 3.6. Mt sdng tit din ngang ca cc vn thp.

b. Cu to bcc

Bcc c tc dng lin kt cc u cc thnh mt khi cng tham gia chu ti

trng ca cng trnh bn trn truyn xung. Bcc thng c chto bng b tng

hoc b tng ct thp.Bcc i vi m trcu thng c chiu dy 1-3m. Mt trn ca bc xc

nh ty vo mt bng ca kt cu bn trn. Kch thc y bphthuc vo slng

cc khi thit k.

Bcc mtrcu thng dng b tng mc ti thiu l 150. Nu y bc m

rng ln th m bo cho bchu c lc do cc hng cc ngoi tc dng ln phi


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btr thm ct thp trong b tng y bchu moment un. Cc phi c neo

cht vo b(khng kphn b tng bt y). Thng yu cu i vi cc loi cc nh,

u cc phi cm trong bt nht 2 ln ng knh hay brng ca cc; nu cc c

ng knh ln hn 60cm th chn su ca u cc trong bti thiu l 1,2m.

tng cng s lin kt gia cc v b thng i vi nhng cc b tng ctthp tit din c, ngi ta thng phn ct thp chca u cc t20 -40, cn

phn b tng u cc ngp vo trong b t

nht 10cm (khng kphn b tng bt y).

i vi cc rng nh cc ng b tng ct

thp, cc thp, thng cho vo u cc

mt khung ct thp, u nhng ct thp ca

khung ny nm trong b1-2,5m.

Ct thp trong bthng dng cc thanh

c ng knh khng nh hn 20-25mm.

Khong cch gia cc thanh thng t 10-

20cm. Lp b tng bo vct thp ti thiu

l 5cm.

Nu phn lc do u cc truyn ln b

tng b(khng klc dnh ca mt bn cc)

vt qu cng tnh ton chu p ca b

tng th pha trn u cc ngi ta t

nhng li thp gia cng to bi cc thanh

ng knh khng nhhn 12mm, mt li

1010cm n 1515cm. u ca nhng cc

ng cnh mp b nn tng cng bng

nhng vng ai neo. (Hnh 3.7)

Cc btr trong btt nht l lm sao cho ni lc trong cc cc xp xbng nhau.Nhng nhvy thng lm cho cng vic tnh ton phc tp thm ng thi khi thi

cng cng kh khn hn. Thng cc c btr thnh tng dy theo hnh chnht

hay vung hoc hnh hoa mai trn mt bng. (Hnh 3.8)

Khong cch gia tim cc cc y b khng nhhn 1,5 ln ng knh cc,

khong cch tmp cc ngoi cng n mp bphi ln hn 25cm.

Hnh 3.7. Cu to ct thp bcc.1-Ct thp y b; 2-Li thptrn u cc; 3-Ct thp chong;4-B tng bt y.


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66

Khong cch gia tim cc chn cc ty vo loi cc. i vi nhng cc thng

thng khong cch ny khng nhhn 3d, nhng i vi cc c mrng hoc cc

xon, tnh khng gia cc bu hay cnh xon nh nht l 1m. Vi cc cc nmn,

khong cch tim ca cc chn cc phi ln hn 1,6 ln ng knh ca bu.

Khi b tr cc nghing so vi phng thng ng, khong cch gia tim cc ucc c gim nhso vi chn cc cho php rt nhkch thc b, gim khi lng

vt liu.Tuy nhin, vic thi cng hcc nghing vo t nn kh khn hn cc ng.

i vi cc nghing, su ng cc ti thiu khng c nhhn 4m.

Khi thit kmng cc, cc c b tr lm sao cho ti trng truyn vo cc ch

yu l lc dc trc, cn lc ngang chxut hin vi gi trnhnht c th. Trong cc

cng trnh chu ti trng ngang ln ni chung nn b tr cc nghing chuyn mt

phn lc ngang ngoi lc thnh ni lc dc trc trong cc.

Hnh 3.9a thhin mng cc di tng chn chu cc lc chyu l trng lng

bn thn tng v p lc t sau tng. Chiu nghing ca cc nn b tr thin v

hng ca hp lc tc dng bn trn. i vi cc cng trnh chu lc ngang ln m li

lun thay i chiu (nh trcu), thng phi b tr cc nghing theo 2 chiu (hnh

3.9b).

Hnh 3.9. Cng trnh trn cc nghing.

a. Tng chn t; b. Trcu.


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3.2. Xc nh sc chu ti ca cc

3.2.1. Khi nim

Mt cc nm c lp trong t nn (cc n) v khi nm trong nhm cc th c kh

nng chu ti trng khc nhau. Tuy nhin, cho n nay khi thit kcc loi mng cc,

ngi ta thng githit rng sc chu ti ca mi cc trong nhm cc bng sc chu

ti ca cc n. V vy, trong phn ny s trnh by cc phng php xc nh sc

chu ti ca cc n.

Hnh 3.10. Mng cc. a. Cc n b. Nhm cc

Sc chu ti ca cc l mt i lng rt quan trng c sdng trong sut qu

trnh thit kmng cc. Vic xc nh chnh xc i lng ny l cc kquan trng vn nh hng n san ton v gi thnh ca cng trnh.

Mng cc c thbph hoi do mt trong hai nguyn nhn sau y:

Bn thn cng vt liu lm cc bph hoi.

t nn khng sc chu ng.

Do , khi thit kphi xc nh sc chu ti ca cc vchai mt: sc chu ti

ca cc theo cng vt liu cc v sc chu ti ca cc theo cng t nn. Tr

snhnht trong hai trs sc sdng tnh ton mng cc. Khi thit kphi chn kch thc ca cc sao cho hai trssc chu ti ca cc khng chnh

lch nhau nhiu qu m bo iu kin kinh t. Trong tt cmi trng hp, khng

c chn kch thc cc m sc chu ti tnh ton ca n xc nh theo cng vt

liu cc li nhhn sc chu ti ca cc xc nh theo cng ca t nn.

a) b)


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r bn knh cc trn hay cnh cc vung.

d brng ca tit din chnht.

l0 chiu di tnh ton ca cc, l0= vl

l chiu di thc ca on cc tnh tu cc n im ngm trong t, hoc l

c chn l chiu dy ca lp t yu m cc i qua.v hsphthuc lin kt 2 u cc, ly nhsau

v = 2

u cc ngm trong i

v mi cc nm trong

t mm

v = 0,7

u cc ngm trong i

v mi cc ta ln t

cng hoc

v = 0,5

u cc ngm trong i

v mi cc ngm trong

Khi xt n shin din ca t bn long xung quanh cc, M. Jacobson ngh

nh hng un dc nhsau:

= L/r 50 70 85 105 120 140

1 0,8 0,588 0,41 0,31 0,23

L chiu di cc; r bn knh hoc cnh cc.

Theo quy phm xy dng Vit Nam 21-86, sc chu ti nn dc trc ca cc theo

vt liu c xc nh theo cng thc sau:

Qvl= k m Rgh

k = 0,7 hsng nht.

m = 1 hsiu kin lm vic.

Rgh sc chu ti nn gii hn ca vt liu lm cc.

Vi cc b tng ct thp chto sn:

Qvl= k m (RnAb+ RaAa)

Sc chu ko cng ca cc b tng ct thp theo vt liu khi cc lm vic chu nh:

Qvl,nh= k m RaAa


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Theo chtiu chc ca t nn, cn gi l phng php tnh.

Theo chtiu trng thi, cn gi l phng php thng k.

Theo th nghim nn tnh cc ti hin trng.

Theo th nghim ng cho cc cc hvo t bng ba ng.

a. Xc nh sc chu ti dc trc ca cc theo ch tiu c hc ca t nn(phng php tnh)

a.1. Sc khng mi ca t mi cc Qp

a.1.1. Phng php Terzaghi

Cng thc xc nh sc khng mi ca cc da trn cscc cng thc sc chu

ti ca mng nng, vi strt ca t di mi cc tng tnhstrt ca

t di mng nng.

2

p p p p c f q pQ A q R 1,3cN D N 0,6 R N

(Cho cc trn bn knh Rp)

2p p p c f q pQ A q D 1,3cN D N 0,4 B N

(Cho cc vung cnh Bp)

Trong : Nc, Nq, Nl cc hssc chu ti ca Terzaghi c thit lp cho mng

nng tit din trn hoc vung, xc nh nhsau:

2 3 /4 /2 tg

q2

c q

p

2

eN

2cos4 2

N N 1 cot g

K1N 1 tg

2 cos

Vi Kp hsp lc bng ca t tc ng ln mt nghing ca nm nn

cht di y mng.

Hoc Nc, Nq, Nc thc tra tbiu hnh 3.11 hay bng tra bn di.


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Hnh 3.11. Biu cc hssc chu ti Nc, Nq, N

Bng cc hssc chu ti Nc, Nq, Nca Terzaghi

o (rad) Nq Nc N o (rad) Nq Nc N

0 0.000 1.000 5.712 0 26 0.454 14.210 27.085

1 0.017 1.105 5.997 27 0.471 15.896 29.236

2 0.035 1.220 6.300 28 0.489 17.808 31.612

3 0.052 1.347 6.624 29 0.506 19.981 34.242

4 0.070 1.487 6.968 30 0.524 22.456 37.162 19.7

5 0.087 1.642 7.337 0.5 31 0.541 25.282 40.411

6 0.105 1.812 7.730 32 0.559 28.517 44.036

7 0.122 2.001 8.151 33 0.576 32.230 48.090

8 0.140 2.209 8.602 34 0.593 36.504 52.637

9 0.157 2.439 9.086 35 0.611 41.440 57.754 42.4

10 0.175 2.694 9.605 1.2 36 0.628 47.156 63.528

11 0.192 2.975 10.163 37 0.646 53.799 70.067

12 0.209 3.288 10.763 38 0.663 61.546 77.495

13 0.227 3.634 11.410 39 0.681 70.614 85.966

14 0.244 4.019 12.108 40 0.698 81.271 95.663 100.4

15 0.262 4.446 12.861 2.5 41 0.716 93.846 106.807

16 0.279 4.922 13.676 42 0.733 108.750 119.669

17 0.297 5.451 14.559 43 0.750 126.498 134.580

18 0.314 6.042 15.517 44 0.768 147.736 151.950

19 0.332 6.701 16.558 45 0.785 173.285 172.285 297.5

20 0.349 7.439 17.690 5 46 0.803 204.191 196.219

21 0.367 8.264 18.925 47 0.820 241.800 224.549

22 0.384 9.190 20.272 48 0.838 287.855 258.285 780.1

23 0.401 10.231 21.746 49 0.855 344.636 298.718

24 0.419 11.401 23.361 50 0.873 415.146 347.509 1153.2

25 0.436 12.720 25.135 9.7

o

NcNqN

Nc, Nq, N


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Nc, Nq, Nl cc hssc chu ti ca Vesic, xc nh theo cng thc sau:

tg 2q

c q

q

N e tg4 2

N N 1 cot g

N 2 N 1 tg

Hoc Nc, Nq, Nc thc tra tbng c lp sn.

Bng cc hssc chu ti Nc, Nq, Nca Vesic

o (rad) Nq Nc N o (rad) Nq Nc N

0 0.000 1.000 5.142 0.000 24 0.419 9.603 19.324 9.442

1 0.017 1.094 5.379 0.073 25 0.436 10.662 20.721 10.876

2 0.035 1.197 5.632 0.153 26 0.454 11.854 22.254 12.539

3 0.052 1.309 5.900 0.242 27 0.471 13.199 23.942 14.470

4 0.070 1.433 6.185 0.340 28 0.489 14.720 25.803 16.7175 0.087 1.568 6.489 0.449 29 0.506 16.443 27.860 19.338

6 0.105 1.716 6.813 0.571 30 0.524 18.401 30.140 22.402

7 0.122 1.879 7.158 0.707 31 0.541 20.631 32.671 25.994

8 0.140 2.058 7.527 0.860 32 0.559 23.177 35.490 30.215

9 0.157 2.255 7.922 1.031 33 0.576 26.092 38.638 35.188

10 0.175 2.471 8.345 1.224 34 0.593 29.440 42.164 41.064

11 0.192 2.710 8.798 1.442 35 0.611 33.296 46.124 48.029

12 0.209 2.974 9.285 1.689 36 0.628 37.752 50.585 56.311

13 0.227 3.264 9.807 1.969 37 0.646 42.920 55.630 66.192

14 0.244 3.586 10.370 2.287 38 0.663 48.933 61.352 78.024

15 0.262 3.941 10.977 2.648 39 0.681 55.957 67.867 92.246

16 0.279 4.335 11.631 3.060 40 0.698 64.195 75.313 109.41117 0.297 4.772 12.338 3.529 41 0.716 73.897 83.858 130.214

18 0.314 5.258 13.104 4.066 42 0.733 85.374 93.706 155.542

19 0.332 5.798 13.934 4.681 43 0.750 99.014 105.107 186.530

20 0.349 6.399 14.835 5.386 44 0.768 115.308 118.369 224.634

21 0.367 7.071 15.815 6.196 45 0.785 134.874 133.874 271.748

22 0.384 7.821 16.883 7.128 46 0.803 158.502 152.098 330.338

23 0.401 8.661 18.049 8.202 47 0.820 187.206 173.640 403.652

a.2. Sc khng bn ca t xung quanh cc Qs

Thnh phn Qsxc nh bng cch tch phn sc chng ct n vfstrn ton b

mt tip xc ca cc v t xung quanh cc, sc chng ct ny xc nh theo cng

thc ca Coulomb:

fs= ca+ h tga= ca+ Ksv tga

ca- lc dnh bm gia cc v t.

a- gc ma st gia cc v t.


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h - ng sut php tuyn hu hiuti mt bn ca cc.

h= Ksv = Ks z ; Ks- hsp lc ngang

Hsp lc ngang Ksc thxc nh theo cc khuynh hng sau:

Khuynh hng 1:

Xem t nn l vt liu n hi, ta c:

sK 1

;

- hsnn hng.

- hsPoisson (hsnhng).

Khuynh hng 2:

HsKsc chn theo hsp lc ngang trng thi tnh K0

Ks= K0= 1 sinKhi cc c hvo nn t ckt trc th:

sK (1 sin ) OCR

OCR - hsckt trc.

Khuynh hng 3:

Khi ng hoc p cc vo nn t, thtch cc chim chca t v t dn t

gn n trng thi cn bng bng (Kstin dn n Kp). Bowles nghKsl gi

trtrung bnh ca K0, Ka, Kptheo cng thc sau:a w 0 p

s ww

K F K K K ; F 1

2 F

Thc to c, hsKsthay i theo chiu su, theo bin dng thtch v cht

ca t xung quanh cc. u cc KsKp (Kp xc nh theo Rankine), mi cc

KsK0

Trong tnh ton thc tc thly theo bng sau.


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Gi trKs(theo B. J. Das)

Cc khoan nhi

Ks= K0= 1 sin

Cc ng, c thtch t bchim chnh

Ks= K0 (gii hn di)Ks= 1,4.K0 (gii hn trn)

Cc ng, c thtch t bchim chln

Ks= K0 (gii hn di)

Ks= 1,8.K0 (gii hn trn)

Gi trKstheo ENPC (kt qunghin cu ca Broms)

Loi cc a Ks(ct cht trung bnh) K s(ct cht)Cc thp

Cc b tng

Cc nhi

Cc g

20o

3/4

3/4

2/3

0,5

1

0,5

1,5

1

2

0,5

4

Ngoi ra, cn c cc phng php xc nh sc khng bn ca cc theo t nn

nhsau:

a.2.1. Phng php

Tomlinson nghthm vo thnh phn lc dnh mt hs trong cng thc xc

nh sc chng ct n vgia cc v t.

fs= ca+ h tga= ca+ Ksv tga

Hs c thly theo cc bng sau:

Bng gi trtheo Vin Du ha ca M(API)Sc chng ct khng thot nc cu(kPa) Hs

< 25 1

25 75 1 0,5

(cc gi trtrung gian c ni suy)

> 75 0,5


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Bng gi tr(theo Tomlinson)

Loi t L/D Hs

Ct cht hoc st cng < 20

> 20

1,25

cu< 75: = 1,25

cu= 75 180: = 1,25 0,4St mm, bi v t dnh cng 8 20

> 20

0,4

cu= 0 25: = 1,25 0,7

cu> 25: = 0,7

St cng 8 20

> 20

0,4

cu= 0 30: = 1,25 1

cu= 30 80: = 1

cu= 80 130: = 1 0,4

cu> 130: = 0,4

Bng gi tr(theo Peck, 1974)

Sc chng ct khng thot nc cu(kPa) Hs

0 1

50 0,95

100 0,8

150 0,65

200 0,6

250 0,55

300 0,5

Sladen (1992) nghcng thc tnh hscho t ct mn bo ha ncnh

sau:

v1

u

Cs

Vi C1= 0,40,5 cho cc nhi.

C1= 0,5 cho cc ng.

su= cu- sc chng ct khng thot nc.


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a.2.2. Phng php

Sc chng ct n vgia cc v t c dng:

fs= Ksv tga= v

vi = KstgaKhi c ti ngoi phn bu rng khp trn mt t:

fs= (v + s )

vi s- ng sut phthm do ti ngoi phn bu khp trn mt t.

Trong cng thc trn = 0,25 0,4 (khi Ks= K0), hoc theo Bhushan (1982),

xc nh nhsau:

= Kstga= 0,18 + 0,0065 Dr

hoc Ks= 0,5 + 0,008 Drvi Dr- cht tng i ca ct.

a.2.3. Phng php

Focht v Vijavergiya nghsc chng ct n vgia cc v t stc dng:

fs= (v + 2cu)

vi (v + 2cu) - p lc t nm ngang bng.

-thay i theo chiu su ng cc, xc nh theo biu hnh 3.15.

Hnh 3.15. Biu xc nh gi trhs


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Ngoi cc phng php xc nh sc khng bn ca t xung quanh cc trn, c

thkn mt scc phng php khc nh:

Phng php Coyle - Castillo

Phng php tng qut Kulhawy

Phng php th nghim hin trng: t kt qu th nghim nn p ngang(Mnard), th nghim xuyn tnh CPT.

b. Xc nh sc chu ti dc trc ca cc theo chtiu trng thi (chtiu vt l)

ca t nn (phng php thng k)

Sc chu ti cho php ca cc xc nh theo cng thc sau:

tca

QQ

k ; k =1,4 1,75

Qtc- sc chu ti tiu chun ca cc.

b.1. Sc chu ti tiu chun ca cc chng

Sc chu ti tiu chun ca cc chng (khi nn t c Eo> 50 MPa) c xc nh

theo cng thc sau:

Qtc= Apqp

Khi cc ta ln , t ht ln v st cng:

qp= 20 MPa

Khi cc ngm trong :

tcpn 3

pd 3

q hq 1,5

k d

Vi tcnq - cng chu nn trung bnh ca trng thi no nc.

kd= 1,4

h3- su chn cc trong .

d3- ng knh cc ngm trong .

Khi cc ng ta ln :

tcn

pd

qq

k


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b.2. Sc chu ti tiu chun ca cc ma st

i vi cc ng c cnh cc t250 800mm:

Sc chu nn ca cc:

n

tc R p p f si i

i 1

Q m q A u m f

l

mR, mf: hsiu kin lm vic ca t mi cc v mt bn cc

Sc chu nhca cc:

n

tc f si ii 1

Q mu m f

l

m - hsiu kin lm vic chu nh.

m = 0,6 khi cc hvo t < 4m

m = 0,8 khi cc hvo t > 4m

Bng gi trhsmRv mf

Hsiu kin lm vic ca tPhng php hcc

Di mi cc mR mt bn cc mf

1- Hcc c v cc rng bng c

bt u bng ba hi, ba diesel1 1

2- Rung v p cc vo:

a) t ct cht va:

- Ht th v ht va

- Ht mn

- Ht bi

b) t st c st IL= 0,5

- ct

- st

- St

c) t st c st IL< 0

1,2

1,2

1

0,9

0,8

0,7

1

1

1

1

0,9

0,9

0,9

1

Cc hsmRv mfca t c st trong khong 0 0,5 xc nh bng cch ni

suy.

Sc chu ti n vca t mi cc qpxc nh theo bng sau.


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(Trong bng trn, cc gi trtrong ngoc c sdng cho t st)

Sc chng ct n vgia cc v t xung quanh cc fsxc nh theo bng sau.


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86

(Cc gi trfsca ct cht tng thm 30%. Khi xc nh fsnn chia cc lp t c

chiu dy khng qu 2m)

i vi cc nhi:

Sc chu nn ca cc:n

tc R p p f si ii 1

Q m m q A u m f

l

m - hsiu kin lm vic.

m = 0,8 (t st c bo ha Sr> 0,85).

m = 1 (cho cc trng hp cn li).

Hsiu kin lm vic ca t di mi cc mRxc nh theo bng sau

Hsiu kin lm vic ca t mt bn cc mfxc nh theo bng sau

qp- sc khng mi n vca t di mi cc nhi v cc ng, c tnh nh

sau:

Vi t si ln ct v ct:

qp= 0,75(dp Ak0+ * L Bk

0): cc nhi, cc ng ly nhn.

qp= (dp Ak0+ * L Bk

0): cc ng ginguyn nhn


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c. Mt sphng php thng dng khc v tnh sc chu ti dc trc ca cc

theo t nn

Xc nh sc chu ti dc trc ca cc theo chtiu cng ca t nn.

Xc nh sc chu ti dc trc ca cc theo kt qu th nghim xuyn (CPT,

SPT).

V d3.1:

Cho cc BTCT cnh 30x30cm, di 18m gm 2 on cc 9m ni li, cc c ng

vo tng t c cu to a cht nhsau:

Vi Ks= 1,4Ko, a= , ca= c

1. Xc nh SCT cc hn ca cc theo chtiu vt l Qtc.

2. Xc nh SCT cc hn ca cc theo ch tiu c hc Qu (vi Qs tnh theo

Coulomb v Qptnh theo Terzaghi).

3. Xc nh SCT cc hn ca cc theo ch tiu c hc Qu (vi Qs tnh theoCoulomb v Qptnh theo Meyerhof).


Coulomb v Qptnh theo Vesic).

(Lu : 1 T/m2 = 10 kN/m2)

Li gii

MNNLp 1: Bn st nho, IL > 1,

c = 5 kN/m2, = 50,

= 15kN/m3, sat= 16kN/m3

Lp 2: St pha ct, do mm, IL= 0,52,

c = 20 kN/m2, = 160,

sat= 20kN/m3


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1. Xc nh SCT cc hn ca cc theo chtiu vt l Qtc.

Ta c tc R p p f si iQ m q A u m f l

- Hsiu kin lm vic ca t mi cc v mt bn cc (tra bng):

mR = mf = 1 (v l cc ng)- Din tch tit din cc: Ap= 0,3 . 0,3 = 0,09 m

2

- Chu vi tit din cc: u = 4 . 0,3 = 1,2 m

- Sc khng mi n vca t mi cc qp (tra bng):

qp= 160,4 T/m2= 1604 kN/m2

(st ca t mi cc l IL= 0,52, tra bng tng ng vi su mi

cc z = 20m v ni suy gia cc gi trst IL= 0,5 v IL= 0,6 ta xc nh

c sc khng mi n vqp)

- Sc chng ct n vgia mt bn thn cc v t xung quanh tng ng vi cc

lp t fsi (tra bng):

fs1= 0,6 T/m2= 6 kN/m2, tng ng on cc l1= 6m v su trung bnh ca

lp t trong on ny l z = 5m

fs2= 2,62 T/m2= 26,2 kN/m2 , tng ng on cc l2= 12m v su trung

bnh ca lp t trong on ny l z = 14m

tc R p p f si i p p 1 1 2 2

Q m q A u m f q A u f f

1604.0, 09 1, 2. 6.6 26, 2.12 564,84 kN

l l l


Coulomb v Qptnh theo Terzaghi).

Ta c u s pQ Q Q

- Tnh sc khng bn Qs theo Coulomb:

s si iQ u f l Vi ' 's a h a a s v af c t g c K t g

s 0K 1,4.K 1,4.(1 sin )

's a v af c 1,4.(1 sin ). t g


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Coulomb v Qptnh theo Meyerhof).

Ta c u s pQ Q Q

- Tnh sc khng bn Qs theo Coulomb (nhtrn):

sQ 992, 04(kN) - Tnh sc khng mi Qp theo Meyerhof:

p p p c qQ A q A cN q N

T= 16otra biu b crL / D 3,5

Xem lp bn st nho l lp t yu, ta c chiu di cc cm trong lp t tt

(st pha ct) l Lb= 12m bL / D 12 / 0,3 40

V b b cr

L / D 40 L / D 3,5 Ncv N

qly gi trti a khi tra biu .

T= 16otra biu theo Meyerhof c qN 20, N 6

Vi v 1 1 2q .4 .4 .12

215.4 (16 10).4 (20 10).12 204(kN / m )

p p p p c qQ A q A cN q N 0,09 20.20 204.6 146,16(kN)

Sc chu ti cc hn ca cc Qu:

u s p

Q Q Q 992,04 146,16 1138,2(kN)


Coulomb v Qptnh theo Vesic).

Ta c u s pQ Q Q

- Tnh sc khng bn Qs theo Coulomb (nhtrn):

sQ 992, 04(kN)

- Tnh sc khng mi Qp theo Vesic:p p p p c v qQ A q A (cN N DN )

T= 16otra bng theo Vesic c qN 11,631, N 4,335, N 3,060

Vi v 1 1 2q .4 .4 .12

215.4 (16 10).4 (20 10).12 204(kN / m )


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's a v af .c (1 sin ) t g

Chia cc thnh cc on tng ng vi vtr mt phn cch ca cc lp t v v

tr ca mc nc ngm.

Xc nh sc khng bn n vtheo phng php cho tng on cc chia, vi

cc i lng trong cng thc c ly ti vtr gia mi on.on 1: (l1= 4m, su z = 26m)

's1 a v a

0 0 2

f .c (1 sin ) t g

1.5 (1 sin 5 ).24. 6,92t g5 (kN / m )

Vi ' 2v 1 sat1 w.4 ( ) (16 10).4 24(kN / m )

= 1 (tra bng theo Peck 1974 ng vi cu= 5 kN/m2)

on 2: (l2= 16m, su z = 622m)

's2 a v a

2

f .c (1 sin ) t g

0,65.150 0 (kN / m )97,5

Vi = 0,65 (tra bng theo Peck 1974 ng vi cu= 150 kN/m2)

s si i si iQ u f D f 3,14.1. 6,92.4 97,5.16 4985,3(kN) l l

- Tnh sc khng mi Qp theo Meyerhof (trong trng hp t st bo ha):

2 2

p p p c u p u p u

D 3,14.1Q q A N c A 9c A 9c . 9.150. 1059,75(kN)

4 4

Sc chu ti cc hn ca cc Qu:

u s pQ Q Q 4985,3 1059,75 6045,05(kN)

Bi tp

Bi 3.1: Cho cc BTCT cnh 30x30cm, di 12m, cc c ng vo tng t cng

c lc dnh khng thot nc cu = 200 kN/m2. Tnh Qu ca cc vi Qs tnh theo

phng php ..

Bi 3.2: Mt cc BTCT cnh 30x30cm, di 16m c ng qua nn t ri c =

350, eo= 0,85, emax= 0,95, emin= 0,75, sat= 20 kN/m3. MNN ngay ti mt t. Tnh Qu

ca cc vi Qstnh theo phng php .

Bi 3.3: Mt cc ng thp c D = 40cm, di 20m c hvo st mm c cu= 70

kN/m2, sat = 20 kN/m3, MNN ngay ti mt t. Tnh Qu ca cc vi Qs tnh theo

phng php .


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3.3. Thit kmng cc i thp

Khi thit kmng cc i thp, phi thc hin cc nhim vsau y:

1. Chn loi, kch thc ca i cc v ca cc.

2. Xc nh sc chu ti tnh ton ca cc ng vi kch thc chn v iu kin

a cht cho.3. Sbxc nh slng cc cn dng.

4. Btr cc trn mt bng v mt ng.

Thc hin xong nhim vth4 tc l ta ra c mt phng n mng cc

i thp c y kch thc chi tit vcc v cch btr chng.

5. Tnh ton kim tra phng n va ra xem c tha mn cc yu cu cn

thit hay khng.

Vic tnh ton kim tra mng cc c tin hnh theo 3 TTGH sau:

a. Tnh ton mng cc theo TTGH 1: bao gm vic kim tra ti trng tc dng

ln cc, kim tra sc chu ti ca nn t mi cc.

b. Tnh ton mng cc theo TTGH 2 (vbin dng): bao gm vic kim tra

ln v chuyn vngang ca mng cc.

c. Tnh ton mng cc theo TTGH 3 (hnh thnh khe nt): bao gm vic tnh

ton cc chu lc trong qu trnh vn chuyn v lp dng cc ln gi, tnh

ton i cc.

Nu mt trong cc yu cu kim tra khng tha mn th phi thay i li kch

thc ca mng cc, s lng cc v tin hnh tnh ton li tu cho ti khi tha

mn tt ccc yu cu.

Qu trnh thit k chi tit mt mng cc i thp trong thc t c thc tin

hnh theo cc bc tng ng vi cc mc sau.

3.3.1. Kim tra iu kin mng cc lm vic dng i thp

Khi tnh ton mng cc i thp, ti trng ngang tc dng ln i cc c xem

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