Thit k cng trnh chu tc ng ca ti trng ng t bng ETABS version 9

Tnh ton cng trnh chu ti trng ng t bng ETABS, bin son: Nguyn Cnh Ton

Thit k cng trnh chu tc ng ca ti trng ng t bng ETABS version 9

1. Cc khi nim cn lu :

1.1. Khi lng, m:

Theo nh lut Newton, lc F = m x a. Khi lng m ( F ( gia tc a = 1 m/s2.

1.2. Bc t do, DOF:

C hai loi sau: MDOF, h nhiu bc t do; SDOF, h mt bc t do.

1.3. Tn s dao ng t nhin v mode dao ng t nhin (Natural Frequency, dao ng m cng trnh c c do trng lng bn thn ca n, khng c s tc ng ca ti trng ngoi)

2. Cc bc thc hin:

2.1. Khai bo nguyn vt liu, Material properties:Btng: Khi lng ring, Mass per Unit volume : ( = ( / g = 2,5 / 10 = 0,25.

L do phi khai bo mc ny v ta c chu k dao ng T c tnh bng , do nu cng trnh khng c khi lng ring m, hoc gi tr m rt nh dn ti gi tr k rt ln ( gi tr T m ta c c rt b. Nh vy, ngi thit k d nhm ln cng trnh ca mnh t cng, v chu k T c c nm trong phm vi gii hn. Do vy, mc Mass per Unit volume bt buc phi khai bo khi tnh ton phn tch ng lc.

2.2. Khi lng tham gia tnh dao ng:

Khi lng , dm, ct, sn, vch ( ETABS t tnh ton;

Khi lng hon thin, khi lng tng, mt phn hot ti ( ngi thit k tnh ton.

Theo tiu chun M, phn khi lng tham gia tnh dao ng l TT + 0,5 HT ton phn. Theo tiu chun VN, gi tr tng ng l TT + 0,8 HT di hn. C hai cch tnh ny u cho gi tr tng ng. V d: Theo TCVN 2737:1995, ti trng ton phn tc dng ln sn theo bng tra l 200 daN/m2 , trong phn di hn l 130 daN/m2 phn ngn hn l 70 daN/m2. Theo TCVN, HTDH x 0,8 = 130 x 0,8 = 104 daN/m2; theo TC M, HTTP x 0,5 = 200 x 0,5 = 100 daN/m2. Nh vy, thng nht, ta p dng h s gim ti cho hot ti c nu trong TC 229:1999 l 50% cho cng trnh c k n ngi v trang thit b.

2.3. Khi lng tp trung M:

Phn tch phng: M = m x S

trong : m khi lng phn b, ;

S din tch sn.

Thng thng, gi tr q = 9 ~ 14 kN/m2 (1 T = 10 kN), thng chn q = 10 ~ 11 kN/m2

Sau gn khi lng tp trung M bng Joint Mass: Assign Joint/Point ( Additional Point Mass

Nhp tham s M vo Direction X,Y.

+ Cch khc cho phng php tnh ton khung phng: Ch cn tnh khi lng ca sn chia cho gia tc trng trng th c gi tr M, sau gn tng t nh trn.

Phn tch 3 chiu:

Ch : khng cn phi chia nh phn t shell bng Edit ( Mesh Areas hay Assign ( Shell/Area ( Area Object Mesh Options khi gii bi ton dao ng v sai s ch l 1% vi khi c chia nh cc phn t shell.

Khai bo khi lng: C cc cch sau:

Cch 1:

Tnh cc lp hon thin, vd: 1,2 kN/m2 Tnh cc lp tng, vd: 2,5 kN/m2 Xc nh hot ti, vd: 2 kN/m2 M = [q = 1,2 + 2,5 + 2 = 5,7 kN/m2] / g = 9,81

Sau gn ln shell bng Area Mass,

Cch 2: Khng cn nhp sn, ch cn nhp khung sau tnh ton ton b khi lng sn l bao nhiu, sau chn ht cc nt giao gia dm ct, ri gn (vi N = s nt trn sn) bng Joint Mass.

Cch 3:

Nhp m hnh.

Khai bo cc trng hp ti trng, theo hnh sau:

Gn ti trng tht s, cha cn nhp gi hay ti ng t trong giai on ny.

Phn TLBT, cn ch : Khi khai bo ngun khi lng Mass Source

Nu chn mc From Self and Specified Mass and Loads th mc Define Mass Multiplier for Loads nh hnh di y:

Nu chn mc From Loads th chn nh hnh di y:

2.4. Cng trnh cn bao nhiu mode l ?

Theo VN, s mode cn tnh l Smode sao cho fS < fgh < fS+1. Theo TCVN 229:1999 th vi kt cu BTCT: fgh = 1,3 Hz ( TS+1 < Tgh = 0,8 s < TS.

V d: Phn tch mode dao ng ca 1 cng trnh ta c bng sau:

3. Mode4. T (s)

5. 16. 1,6

7. 28. 1,0

9. 310. 0,9

11. 412. 0,82

13. 514. 0,75

Ta thy mode 4 c T = 0,82 s > Tgh = 0,8 s, mode 5 th T< Tgh nn s mode cn phn tch l 4 mode.

Theo M, s mode cn tnh l S sao cho y = m1 + m2 + + mS # 90% yg (xc nh bng cch xem file *.OUT)

Nh hnh trn, phn gch di mu l v tr m y # 90% yg nn s mode cn tnh l 5 mode.

(Tham kho bi vit ca GS Edward L. Wilson trong file E-MAN08.pdf phn Manuals ca phn mm Etabs).

2.6.Phng trnh dao ng xc nh nh th no?Phng trnh dao ng ca 3 dng dao ng tnh ton nhng khng k n s tham gia ca cc lp hon thin, trang thit b, trn cng trnh nn cho kt qu khng c chnh xc, nn t 1986, phng php tnh ny t c s dng. Hin nay, trn th gii s dng phng php Ritz tnh ton phng trnh dao ng. Phng php ny c k n s tham gia ca cc ti trng k trn khi tnh ton v phn tch dao ng cho cng trnh.

Cch xc nh phng trnh dao ng trn ETABS: Ta chn nt, nm trn nh v gia cng trnh, xut bng Display ( Show table ( Xem hnh bn di

hoc

2.7. Nhp ti trng ng t bng TIME HISTORY ANALYSIS

Phng trnh:

Cc bc gii nh sau:

Hm thi gian, nhp chu k (sn c). Theo , ta c cc hm n gin nh sin, cos:

Hoc hm ngu nhin (cc hm do cc my o cc trn ng t ghi li, trong ETABS c sn mt s hm c ui .TH trong th mc \ETABS\Time History Functions. C 2 dng file hm: dng file ch c gia tc, phi cho bit Time step ; dng file hm th 2 cho bit T, giy (s).

Click chn Browse mc Function File, dn ng dn ti th mc \ETABS\Time History Functions ta c bng sau:

Click view file bit c dng format file hm.

Hnh trn l dng file hm ch cho bit gia tc v = 0,020 sec. (ch n v gia tc l cm/s2, cn nhn h s 0,01 thnh m/s2 trong ETABS, s ni phn sau)

Hnh trn l dng file hm cho bit chu k v giy.

V d s dng file hm ALTADENA-1.TH phn tch.

Khai bo trng hp ng t

Define ( Time History Cases

To mt COMBO c DONGDAT xc nh Max Min ni lc cho phn t trong thi im xy ra ng t

COMBO ny c th tc dng vi cc TH ti ( (vd gi) tnh ton ni lc cho cng trnh.

T s cn

BTCT ( = 5%, Thp ( = 3%

Xem ni lc cho phn t trong khi din ra ng t:

Display ( Show Time History Traces

Sau khi click Define Functions ta c bng di y:

Click display bng Time History Display Definition ta c bng sau

2.9. Xc nh gia tc nh nh th no?Theo phng trnh ta thy c rng trong cch tnh s dng phng php tnh ch c cng k nhn vi chuyn v u nn khng th xc nh c gia tc. Do c th s dng phng php quy i nh sau: Gi s chuyn v c dng u = uosin(t ( u = (uo()cos(t ( u = -(uo(2)sin(t.

Ta xc nh c ymax (umax) trong ETABS, sau ( amax = (2 x umax

Vi chu k ring ( = 2(f = 2( / T.

TI LIU THAM KHO

[1] ETABS MANUALS.

[2] Ths. Nguyn Hu Anh Tun, Gio trnh Thc hnh phn tch v thit k kt cu mt s v d c bn bng ETABS, 2006, Tp. H Ch Minh.

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