Tnh vng sn trong thc hnh thit k vi SAFE v8 & v12

Tnh vng sn trong thc hnh thit k vi SAFE v8 & v12

biAE&M Engineervo ngy 01 thng 3 2011 lc 12:56 sng

Trong thit k, vic tnh ton kim tra vng sn tr thnh yu cu cn thit m bo tnh kinh t i vi cc tnh hung sau:

- Chiu di nhp ln

- Ti trng ln, rt hay gp i vi cc sn nh dn dng (landscape tng 1, sn mi thit b c in nng)

Khi tnh ton vng cn ch :

- T hp ti trng theo TTGH th 2 (khng c h s vt ti)

- S xut hin ca vt nt trong btng khi chu lc, dn ti gim cng tit din v lm tng vng

- S lm vic di hn ca kt cu BTCT, cn xt ti cc yu t t bin v co ngt cng nh tc dng di hn ca cc loi ti trng. Theo TCXDVN 356-2005, vng ton phn f c tnh nh sau:

f = f1 - f2 + f3

trong :

+ f1: vng do tc dng ngn hn ca ton b ti trng

+ f2: vng do tc dng ngn hn ca ti trng di hn

+ f3: vng do tc dng di hn ca ti trng di hn

Vi kt cu sn lm vic theo hai phng, vic tnh vng ch tin trong thc hnh khi dng phng php PTHH c k n cc yu t trn khi tnh bin dng. Di y ti xin trnh by cch p dng SAFE 8.x v 12.x tnh ton vng t mt s kinh nghim thc hnh trong thit k.

Ti trng: tnh ton vng thng thng a vo cc trng hp ti sau:

- DEAD: ch k n trng lng bn thn (Self Weight Multiplier = 1)

- SDEAD: trng lng cc lp hon thin sn (Superimpose). Trong v d SDEAD=1500kgf/m2

- LIVE: hot ti tc dng ln sn. Theo TCVN 2737-1995, hot ti cng c thnh phn tc dng di hn, thng chim 20%-30% gi tr ca hot ti ton phn. thun tin, cc v d di y dng h s 0.3, LIVE=500kgf/m2

VD1: SAFE 8.8- K n tc dng ca vt nt: dng Normal and Crack Deflections Analysis vi ct thp phn b, trong v d l f16a300 2 phng. La chn Cubic cho Interpolation Options, chng trnh s tnh lp 3 ln.

- K n tc dng di hn: dng h s Long Term Deflection Multiplier l2, cng thc c th ly theo ACI 318. Trong v d tm ly l=2, tuy nhin trong SAFE vn khai bo bng 1 v nhn h s ny bn ngoi.

- Cc t hp tnh vng:

+ f1 = 1*DEAD+1*SDEAD+1*LIVE

+ f2 = 1*DEAD+1*SDEAD+0.3*LIVE

+ f3 = l*f2 = 2*f2

- Kt qu, ly vi vng max (n v m):

f1 = 0.051

f2 = 0.044

f3 = 2*f2 = 0.088

f = f1-f2+f3 =0.095VD2: SAFE 12.3.0- M hnh s dng cng cc c trng hnh hc, vt liu v ti trng

- K n tc dng ca vt nt: Cracking Analysis Options: Quick Tension Rebar Specification f16a300 2 phng. Phng php tnh cng sau khi nt Modulus of Rupture: Program Default.

- K n tc dng di hn: dng hai c trng l Creep Coefficient (CR) cho t bin v Shrinkage Strain (SH) cho co ngt.

C th tnh theo nhiu tiu chun, trong v d tnh theo Eurocode 2 vi cc iu kin: thi gian di hn, nhit v m mi trng theo iu kin Vit Nam.

Tnh ra: CR=1.7 v SH=0.

Cc t hp tnh vng: define trong Load Cases

+ f1, f2 nh trn, vi Analysis Type l Nonlinear (Cracked)

+ f3 nh f2, vi Analysis Type l Nonlinear (Longterm Crack); CR=1.7 v SH=0.0003

- Kt qu, ly vi vng max (n v m):

f1 = 0.067

f2 = 0.055

f3 = 0.081

f = f1-f2+f3 =0.093, kh ph hp vi tnh theo SAFE 8

VD3: Dng State Analysis trong SAFE 12

SAFE v12 mi b sung tnh nn phn tch theo State, c th trong phn Define Load cases/ Initial Conditions/ Continue from State at End of Nonlinear Case. Hiu nm na l cho php phn tch trng hp ti hin ti vi cc thng s bt u t mt trng hp khc coi l giai on chu ti trc (bn no dng Plaxis s quen vi kiu phn tch ny). Chc nng ny rt thun tin v ph hp vi thc t lm vic ca kt cu hn, c bit cho trng hp tnh vng.

C thm cc Load Cases nh di y vi Sh cho ngn hn v Lt cho di hn:

+ Sh1: 1*DEAD - Nonlinear (Crac ked) - Zero Initial Condition

+ Sh2: 1*SDEAD - Nonlinear (Crac ked) - Continue from State at End of Nonlinear Case Sh1

+ Sh3-1: 1*LIVE - Nonlinear (Crac ked) - Continue from State at End of Nonlinear Case Sh2

+ Sh3-2: 0.3*LIVE - Nonlinear (Crac ked) - Continue from State at End of Nonlinear Case Sh2

+ Lt1: 1*DEAD - Nonlinear (Longterm Crac ked) - Zero Initial Condition

+ Lt2: 1*SDEAD - Nonlinear (Longterm Crac ked) - Continue from State at End of Nonlinear Case Lt1

+ Lt3: 0.3*LIVE - Nonlinear (Longterm Crac ked) - Continue from State at End of Nonlinear Case Lt2

- Nh vy, cc t hp theo TCXDVN s l: f1 = Sh3-1, f2 = Sh3-2, f3 = Lt3

- Kt qu, ly vi vng max (n v m):

f1 = 0.0317

f2 = 0.0276

f3 = 0.0395

f = f1-f2+f3 =0.0436, nh hn nhiu so vi hai v d trn

C nhn ti thy kt qu ny ph hp hn, v vic phn tch theo giai on m phng chnh xc hn s lm vic thc t ca kt cu: m bo s thay i cng gy ra bi vt nt pht trin giai on trc c k n khi phn ti trng ph thm tc dng ti giai on sau.

Tham kho ngun: http://www.eng-tips.com/viewthread.cfm?qid=246507&page=1

Kim tra vng snRt xin li v thi gian va qua bn vic nn gi mi tip tc ch ny c..

Sau khi tnh c vng ton phn f nh trn, vic quan trng l phi kim tra v iu kin thng thng l f

- Sn cng-xn, L = 2 * vn cng-xn

- sn trn li ct ch nht lm vic theo hai phng vi chiu di nhp mi phng l L1, L2; L = min (L1,L2)

- Vi li ct bt k, L l khong cch gia 2 im k nhau trn sn c cong khi bin dng bng khng, khng nht thit phi l im gi (ct, vch). Cn vng f c xc nh l khong cch ln nht t ng ni 2 im n mt sn sau bin dng (xem hnh v, vi d k hiu thay cho f)

- Tng t nh vy, vn cng-xn c xc nh bng khong cc t im c cong bng khng pha trong sn n im bt k dc mp sn phn cng-xn (xem hnh v)

Gi tr gii hn trn cng ch l mt cn c kim tra, ngi k s khng nn ch cng nhc vi gi tr ny trong thit k. Tu tnh hung c th, khi f > [f], nn cn nhc c th chp nhn c trnh tng chiu dy sn gy tn km:

- Gi tr vng di hn l rt ln so vi tc thi v cng mang tnh d bo.

- Sn thit k m bo chu lc theo iu kin cng . iu kin v bin dng cn phi cn nhc cc yu t:

+ Thm m v cm gic tin nghi ca ngi s dng trn sn, gi tr L/250 c dng tham kho

+ Gii hn vng do yu cu trnh gy h hng cc vt liu hon thin trn v di sn (knh, vch)

+ vng gy tc ng xu n chc nng s dng ca sn (sn b cha)

- Khi c bin php bo v cc vt liu hon thin, c th chp nhn vng vt gi tr cho php nu trn. Nhng khng c vt qu sai s chp nhn c khi lp t cho cc vt liu hon thin. iu ny c bin quan trng vi nhng to nh vn phng c Faade l knh tm ln.

(Ngun tham kho: Dr. Bijan O Aalami - ADAPT Corporation)