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72

TNH TON CU KIN B TNG CT THP THEO TIU CHUN NHT BN

TS. Nguyn Nguyn HngNguyn GV Trng THTL3

1. T VN Chng trnh Tnh ton cu kin b

tng ct thp bng tiu chun Nht Bn lmt phn ni dung trong ti nghin cukhoa hc cp B Cng nghip 2001-2002 cta Nghin cu xy dng h thng phnmm thit k cng trnh thy in, bncnh cc chng trnh tnh ton n nh pv mi dc, chng trnh thit k tng chnt, chng trnh x l cc bn s trongcng tc thit k

Chng trnh Tnh ton cu kin btng ct thp bng tiu chun Nht Bn c chn bo co ti Hi ngh ng dngTon hc ton quc ln II ti H Ni, 23-25/12/2005 v c gii thiu ti Tp chng dng Ton hc, tp IV, s I nm 2008ca Hi Ton hc Vit Nam.

2. NI DUNGBi ton tnh v b tr ct thp cho cc

kt cu b tng ct thp chu un ko (hocnn) l bi ton rt ph bin trong thit kxy dng. y l bi ton phi th ng dnv ni chung thng chim kh nhiu th gikhi phi so snh la chn din tch mtct ngang thp sao cho c li v kinh t, cbit i vi cc hng mc cng trnh c khilng ln cc chi tit v cu kin chu lckhc nhau.

i vi cc kt cu c mt ct nganghnh trn, hnh vnh khuyn, mt ct chnht hoc hnh hp c thp chu lc b trtheo chu vi th vic tnh ton cng chn thp l mt vn phc tp v chac cp ti trong cc ti liu, qui phmtnh ton cho cc ngnh xy dng nc ta.Nhng bi ton ny c nghin cu rtrt thnh cng ti Nht Bn c hai phng

din l thuyt ln thc hnh. Da trn ccbiu thc l thuyt (kh phc tp v phngdin ton hc), ngi ta xy dng hthng cc biu quan h a phng nhmgip ngi thit k c th tra cu s dngsau khi thc hin cc tnh ton nhng thams cn thit.

Phng php trn y, c th ni, rttin li. Tuy nhin, n i hi ngi s dngphi thc hin mt phn tnh ton bng thcng xc nh cc tham s cn thit; mtkhc, trong nhiu trng hp bt buc phini suy. V vy, trn c s cc h phngtrnh ca l thuyt s c gii thiu diy, chng ti nghin cu xy dng mtchng trnh ng dng nhm gip cc k sc th la chn mt cch nhanh chng lngthp cn thit kt cu m bo tha mnyu cu v cng v cc yu cu v kinht.

ng dng ny cho php tnh thp choc cc cu kin BTCT c mt ct ngang hnhtrn, hnh vnh khuyn, mt ct ch nhthoc hnh hp c thp chu lc b tr theochu vi da trn Tiu chun tnh ton cc cukin BTCT hin ang lu hnh ti Nht Bnv c a vo b tiu chun thit k dn thy in Hm Thun - a Mi.

Khi s dng phn ng dng ny, ngithit k ch cn nhp cc d liu v ni lc(bao gm m men, lc dc, lc ct), kchthc mt ct ngang ca cu kin v chycc chng trnh tnh ton thng qua vic trli cc hp thoi xut hin trong qu trnhgiao tip vi mn hnh. Nh , s tit kimc mt thi lng ng k trong vic tnhton thit k cc cu kin BTCT.

a. S tnh ton & cc h phng trnhc bn

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73

ng vi mi loi cu kin c tit dinkhc nhau, ngi ta thit lp c mt hphng trnh c bn ring, v sau y chngti ch gii thiu cch tnh ton cho cu kinmt ct ch nht, ct thp b tr theo chu vi minh ha:

ng sut lm vic ca cu kin c xcnh bi cc cng thc:

ng sut nn trong b tng:

Cbd

Mc 2

'= (1)

ng sut ko nn trong ct thp:

Snbd

Ms 2

'= (2)

ng sut ct trong b tng:

Zbd

Qc = (3)

Vi M = M + Nu (4)

Biu thc xc nh cc h s C, S v Z[1]:

++++

=)12(

32)1)(1(2

31

2

2211

2 npknpkk

kC

k

kCS

= 1

++++++

+

+

=2222

122

3

1

)1(6

)21(2

)()1(3

1

)21)(1(1

1

npkkk

kknp

Z

Trong :

)1(2

)1)(1(2

1

1

12

++

+++=

npk

npk

H s k c rt ra t mt trong haiphng trnh sau:

Trng hp N0:

{

0)3312(3

)(2)1)(21(2)1(3

2

2112

3

=++

+++

npknpkk

Trng hp N=0:0)1(12)1(4 12 =+++++ npknpk

Trong :

( )211

211 2;; sss

s

ss AAAA

A

bd

Annp +===

uN

Mf

d

f

d

d +=== ;;;'

b. Phng php tnh ton phc v cng tc thit k cc cu

kin BTCT theo l thuyt c gii thiutrn y, Nht Bn ngi ta lp sn cc th quan h np~C, np~S, np~Z vi ccthng s nht nh, chng hn i vi cukin mt ct ch nht, l

d

d

A

A

d

f

s

s ','

, . Khi ng dng cc th

(5)

(6)

(7)

(8)

(9)

(11)

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74

c xy dng sn, trc ht cn xc nh

cc thng sd

f

d

d,

' v gi thit As, As

tnh ra np. Vi cc thng s k trn, cth dng th xc nh C, S v Z, sau tnh ra ng sut trong kt cu theo cc cngthc (1)~(3). So snh vi ng sut cho phpca b tng v ct thp, c th nh gi dintch thp chn ph hp hay cha.

Nhm nng cao hiu qu ca vic tnhton cc cu kin b tng ct thp da trnl thuyt nu, chng ti xy dng mtchng trnh ng dng bng ngn ngVisual Basic trn nn Microsoft Excel. Nidung ca chng trnh l xc nh h s kcho vic tnh ton cc h s C, S, Z theo cccng thc l thuyt phc tp c gii thiu phn trn bng cch gii gn ng mttrong hai phng trnh dng (9)~(10) tutheo tr s lc dc trong cu kin N=0 hayN0.

Vi cc bng tnh c lp sn trong ngdng, ngi dng c th tnh ton kim tracng cho cu kin vi

d

d

A

A

d

f

s

s ','

, v np bt k, trnh vic

phi ni suy khi tra cu th tm C, S, Zgia nhng gi tr nht nh ca cc thng s. Kt qu tnh ton ng sut trong cu kins c nh gi da trn c s so snh ving sut tng ng cho php ca b tng vct thp. Tu theo ni dung kt lun cthng bo trn mn hnh, ngi dng c thd dng nhn bit kt cu tha hay thiuthp, kch thc mt ct hp l hay khng;t , c hng chn li din tch thp hockch thc cu kin (nu c th) v chy licc chng trnh bng cch nhp chut vocc menu lnh (hoc button) tng ng.

3. KT LUNChng trnh tnh ton ct thp chu lc

cho cc cu kin b tng ct thp gii thiutrn y c xy dng trn mt c s rtcht ch v phng din l thuyt c hc l

cc h phng trnh ton c biu th quan hgia cc thnh phn ng sut vi mt lotcc tham s lin quan, c lp cho hu htcc loi tit din thng dng trong thc txy dng.

Vi vic ng dng chng trnh Tnhton cu kin b tng ct thp bng tiuchun Nht Bn, c th tnh ton c cccu kin BTCT mt ct ch nht hoc hnhhp c thp chu lc b tr theo chu vi l mtvn phc tp, cha c cp ti trongcc ti liu, qui phm tnh ton cho ccngnh xy dng nc ta.

Chng trnh tnh ton ny trn thc t c ng dng rng ri trong cng tctnh ton kim tra, sa i v thit k minhiu hng mc khc nhau thuc cng trnhthy in a Mi (cc tnh ton ny c tphp trong tp "Hydraulic and structuralcalculations during construction stage"),trong tnh ton thit k k thut Khu Qun lvn hnh & sa cha Nh my nhit in Mn, cc kt cu b tng ct thp Trung tmsa cha Nh my nhit it Ph Li, ktcu mt s hng mc cc cng trnh thyin ng Nai 3 v 4, v.v...

TI LIU THAM KHO[1].Uemae Yukitaka, Tamano Harumitsu,-Tnh ton kt cu b tng ct thp bngphng php th - Kindai Tosho Co., Ltd.,Tokyo, 1994 (ting Nht);

[2].Da Mi HP Scheme: "Hydraulic andstructural calculations during constructionstage", 1998;

[3]. Nguyn Nguyn Hng - Tnh ton n nhp, thit k tng chn t v cc cu kinBTCT trn Microsoft Excel, 2001 - tiNCKH cp B Cng nghip.