In­tro­duc­tion

NBM&CW - - SEISMIC RESISTANT DESIGN -

The use of Cold-Formed Steel sys­tem is sig­nif­i­cantly in­creas­ing in the coun­tries where tra­di­tional struc­tural so­lu­tions have al­ways dom­i­nated the con­struc­tion sec­tor. This is due to rec­og­nized tech­ni­cal, struc­tural and eco­nomic com­pet­i­tive­ness of such sys­tems. The CFS mem­bers ob­tained by cold rolling are pro­duced by press­ing or bend­ing steel sheaths with thick­ness rang­ing be­tween 0.40.7mm. These mem­bers pro­vide sev­eral ad­van­tages such as light­ness of struc­tural sys­tems, high qual­ity of end prod­ucts, flex­i­bil­ity due to wide va­ri­ety of shapes and sec­tional di­men­sions, short ex­e­cu­tion pe­riod and min­i­miza­tion of en­vi­ron­men­tal im­pacts.

In steel struc­tures lo­cated in seis­mic area, fac­tors like pre­ma­ture lo­cal buck­ling and low out of plain stiff­ness are re­garded as main struc­tural de­fi­cien­cies. The use of Cold-Formed Steel struc­tural sys­tems in such area im­proves the en­ergy dis­si­pa­tion prop­erty of the struc­ture which ac­counts for the seis­mic re­sis­tance. These sec­tions have a high po­ten­tial for pre­ma­ture fail­ures be­cause of their thin walled na­ture. To

re­duce the chances of fail­ures am­ple duc­til­ity should be im­parted to the sec­tions.

Ad­van­tages of Cold Rolled Steel

• Some of the ad­van­tages of cold rolled sec­tions as com­pared to the hot rolled coun­ter­parts are as fol­lows:

• Cross-sec­tional shapes are formed to close tol­er­ances and these can be con­sis­tently re­peated for as long as re­quired

• Cold rolling can be em­ployed to pro­duce any de­sired shape to any de­sired length

• Pre-gal­va­nized or pre-coated met­als can be formed so that high re­sis­tance to cor­ro­sion, be­sides an at­trac­tive sur­face fin­ish can be achieved.

• All con­ven­tional con­nec­tion meth­ods i.e., riv­et­ing, bolt­ing, weld­ing, etc can be em­ployed.

• High strength to weight ra­tio can be achieved in cold rolled sec­tions.

• They are usu­ally light in weight, easy to trans­port and erect.

Case Stud­ies Ex­per­i­men­tal work on CFS el­e­ments for earth­quake re­silient mo­ment frame build­ings.

Ex­per­i­men­tal in­ves­ti­ga­tion on the use of thin walled cold-formed sec­tions as en­ergy dis­si­pa­tive el­e­ments for earth­quake re­sis­tant mo­ment frame for multi-storey build­ings were car­ried out. In multi-story mo­ment frames use of CFS of­ten led to pre­ma­ture lo­cal buck­ling and low out of plane stiff­ness. To over­come these de­fi­cien­cies the fol­low­ing ex­per­i­men­tal in­ves­ti­ga­tion was car­ried out. The tests were per­formed on 6 bolted beams to col­umn con­nec­tions with two dif­fer­ent beam thick­nesses 3mm and 4mm re­spec­tively. The test in­volved step-bystep de­vel­op­ment curved flange sec­tions and beams with dif­fer­ent types out of plane stiff­en­ers in the con­nec­tion re­gion; the three con­fig­u­ra­tions of the stiff­ener con­nec­tions be­ing: with­out stiff­ener, min­i­mum stiff­ener and op­ti­mum stiff­ener. The test spec­i­mens were sub­jected to cyclic load­ing through hinge con­nec­tions at the beam end. From the re­sults ob­tained through the ex­per­i­ment it as con­cluded that:

1. The pre­ma­ture buck­ling due to dis­con­ti­nu­ity in the flange con­nec­tion can be de­layed by us­ing com­bi­na­tion of ver­ti­cal or hor­i­zon­tal out of plane stiff­en­ers.

2. Duc­til­ity fac­tor was in­creased by 75%, mo­ment strength by 35% and en­ergy dis­si­pa­tion by 240%.

Dam­age Con­trol sys­tems

In or­der to con­trol rock­ing be­hav­iour of cold formed steel in multi-storey shear walls an in­no­va­tive dam­age con­trol sys­tem was pro­posed. The sys­tem in­volved hold-down equipped with a fuse func­tion and was placed on the foun­da­tion of multi-storey shear walls. This HDF (hold-down fuse) re­duces the dam­age to both struc­tural and non-struc­tural mem­bers in the cold-formed steel build­ings.

To un­der­stand the ba­sic be­hav­iour of pro­posed con­trol­ling rack­ing sys­tems in an earth­quake, shake ta­ble tests were per­formed us­ing one storey, one span, real

size spec­i­mens. The spec­i­men con­sisted of steel sheet shear wall with a height and width of 3000 and 6000mm re­spec­tively and steel brac­ings with height and width of 3000 and 1500mm re­spec­tively.

Two steel shear walls were set par­al­lel to the shake di­rec­tion. Fig­ure shows the con­trolled-rock­ing sys­tem with multi-storey shear walls for cold-formed fuse frames. In this sys­tem, a fuse made of a steel plate which was placed be­tween the rack­ing frames and the foun­da­tion. It was con­nected to a chan­nel of the up­per frame and an an­chor bolt sup­ported by the foun­da­tion. In con­ven­tional cold-formed steel struc­tures a frame is rigidly con­nected to the foun­da­tion by steel hold-downs. There­fore the build­ing dis­si­pates the earth­quake en­ergy mainly through plas­tic de­for­ma­tion of shear walls or brac­ing mem­bers in up­per frames. Whereas, the newly pro­posed HDF’s dis­si­pate most of the seis­mic en­ergy and as a re­sult mem­ber and walls within the frame sus­tain less dam­age.

Shake ta­ble tests of a cold-formed steel shear panel

Sev­eral in­ves­ti­ga­tors have tested CFS shear walls for static cyclic load­ings and the re­sults were eval­u­ated. But, there is very lim­ited lit­er­a­ture avail­able for the per­for­mance of CFS shear walls un­der dy­namic load­ings. To ac­count for this, a shake ta­ble test of a full scale two storey sin­gle bay struc­ture was con­ducted. They were separated by 3.9m cen­tre to cen­tre in the out of plane di­rec­tion.

The shear panel was 2.8m wide and the storey height was 3m. Both the storeys were iden­ti­cal but the bot­tom storey was sub­jected to greater loads. The ex­te­rior col­umns were built-up by chan­nel sec­tions and were welded to the steel an­chors and bolted to the slab.

The slab was a heavy re­in­forced one, 200mm thick and 4.40mm2 in area weigh­ing 93kN. This slab acted as a rigid di­aphragm. The spec­i­men de­sign was done con­sid­er­ing the lo­ca­tion of Seat­tle, Washington us­ing the data guide­lines from TI 809-07 (III). The seis­mic base shear for the shake ta­ble was es­ti­mated to be 96.1kN per frame. How­ever the frames were de­signed for a base shear of 96.4kN each. Shake ta­ble spec­i­men was in­stru­mented with ac­celerom­e­ters, dis­place­ment gauges and strain gauges.

Through the test re­sults it was seen that the thin steel straps used a cross brac­ing in CFS build­ings are very tough and duc­tile. After the lo­cal buck­ling, the CFS built-up ex­te­rior col­umns per­formed well. The col­umns con­tri­bu­tion to the shear ca­pac­ity of struc­ture was min­i­mum but de­pend­able through­out earth-quake sim­u­la­tion. When the brac­ings failed to pro­vide strength and stiff­ness to the earth­quake re­sponses, the col­umns pro­vided en­ergy dis­si­pa­tion.

The CFS struc­ture pro­vided to be a very af­fec­tive and de­pend­able struc­tural sys­tem for seis­mic loads and pro­vide the brac­ing mem­ber is pre­vented from frac­ture.

Seis­mic be­hav­iour of sheathed CFS shear walls cladded by gyp­sum and fiber ce­ment boards.

Ex­per­i­ments are car­ried out to in­ves­ti­gate the seis­mic re­sponse of steel sheathed CFS shear walls us­ing gyp­sum and fi­bre ce­ment board claddings. The ex­per­i­ment was car­ried out on six such spec­i­mens of var­i­ous con­fig­u­ra­tions and was tested un­der cyclic load­ing. With the in­tro­duc­tion of claddings, it im­posed ad­di­tional forces on the mem­bers in the load path to­wards the foun­da­tion which re­sulted in the in­creases of shear strength of walls. Also use of sin­gle and dou­ble sided steel sheath­ing on CFS shear walls in­creased their shear strength and en­ergy dis­si­pa­tion ca­pac­ity. It was con­cluded that the shear strength and se­cant stiff­ness of CFS steel sheathed walls can be in­creased by 30% to 32% for sin­gle sided and 61% to 80% for dou­ble sided clad­didng.

Con­clu­sions

Use of CFS prod­ucts are im­por­tant and ef­fec­tive in the con­struc­tions car­ried out es­pe­cially in seis­mic ar­eas; light­weight struc­tural sys­tems, high qual­ity end prod­ucts, flex­i­bil­ity, short ex­e­cu­tion pe­riod, min­i­mum im­pacts on the en­vi­ron­ment are some of the im­por­tant prop­er­ties of CFS sys­tems which make them suit­able for seis­mic ar­eas.

CFS sys­tems are un­der re­search and some of the re­search works dis­cussed above yield the fol­low­ing con­clu­sions:

• With the use of curved flange beams in earth­quake re­sis­tant mo­ment frame multi-story build­ings re­sulted in the in­crease of duc­til­ity (upto75%) and mo­ment ca­pac­ity (upto35%) and also

hys­teretic en­ergy dis­si­pa­tion ca­pac­ity (upto240%).

• By pro­vid­ing sin­gle and dou­ble sided gyp­sum and fiber ce­ment cladding CFS steel sheathed walls, the shear strength and en­ergy dis­si­pa­tion ca­pac­ity can be in­creased by

31% and 32% for sin­gle sided cladding and up to 80% and 67% for dou­ble sided cladding

• With the use of ver­ti­cal/hor­i­zon­tal out of plane stiff­en­ers the pre­ma­ture buck­ling due to dis­con­ti­nu­ity in the flange is de­layed.

Ref­er­ences

1] R. Re­ta­males, R. Davies, G. Mosqueda, and A. Fil­i­a­trault. ASCE (2013). “Ex­per­i­men­tal Seis­mic Fragility of ColdFormed Steel Framed Gyp­sum Par­ti­tion Walls”. 2] Bagheri, S. Mi­hail, et.al.“Novel Coldformed Steel El­e­ments for Seis­mic Ap­pli­ca­tions”. Twenty-First In­ter­na­tional Spe­cialty Con­fer­ence on Cold-Formed Steel Struc­tures St. Louis, Mis­souri, USA, Oc­to­ber 24 & 25, 2012.

3] T. N. Dao, and W. John, V.D. Lindt. “Seis­mic Per­for­mance of an In­no­va­tive Light-Frame Cold-Formed Steel Frame for Midrise Con­struc­tion”. ASCE (2013). 4] B.W.Schafer, Johns Hop­kins Univer­sity Bon­nie Man­ley, Amer­i­can Iron and Steel In­sti­tute. “Ad­vanc­ing Cold-Formed Steel Earth­quake En­gi­neer­ing”. 10th U.S na­tional con­fer­ence on earth­quake en­gi­neer­ing. (2014)

5] B.W. Schafer “Cold-formed steel

struc­tures around the world” (2011). 6] B.W. Schafer “Cold-formed steel struc­tures: spe­cial is­sue”. Jour­nal of Struc­tural En­gi­neer­ing ASCE April 2006. 7] H. Moghimi, and R. G. Driver. “Economical Steel Plate Shear Walls for Low-Seis­mic Re­gions”. ASCE (2013) 8] M .Lecce and K. J. Rasmussen. “Dis­tor­tional buck­ling of cold – formed stain­less steel sec­tions: Ex­per­i­men­tal in­ves­ti­ga­tion”. ASCE (2012).

9] Y.B. Kwin, H.S. Chung, and G.D. Kim. “Ex­per­i­ments on cold – formed steel con­nec­tions and por­tal frames. ASCE (2006).

10] S, Mo­hebbi, S.R. Mirghaderi, et.al. “Ex­per­i­ments on Seis­mic Be­hav­iour of Steel Sheathed Cold – Formed Steel Shear Walls Cladded by Gyp­sum and Fi­bre Ce­ment Boards”. El­se­vier jour­nal (2016).

11] N. De­lat­tle “Fail­ure of Cold -Formed Steel Beams Dur­ing Con­crete Place­ment.” ASCE (2005).

12] G.D. Corte, L. Fior­ino, and R. Lan­dolfo. “Seis­mic Be­hav­iour of Sheathed Cold – Formed Struc­tures: Nu­mer­i­cal Study”. ASCE (2006).

13] A.Sato and C.M.Uang. “Seis­mic Per­for­mance Fac­tors for Cold-Formed Steel Spe­cial Bolted Mo­ment Frames.” ASCE (2010).

14] R. Par­nell, B.W. Davis and L. Xu. “Vi­bra­tion Per­for­mance of Light­weight Cold-Formed Steel Floors”. ASCE (2010).

Fig­ure 1: a) Dif­fer­ent flange shape lay­outs; b) 3-D sim­u­lated view of curved flanged beams.

Fig­ure 2: Ex­per­i­men­tal set-up.

Fig­ure 3: Hold-down steel fuse

Fig­ure 4: a) Dif­fer­ent flange shape lay­outs; b) 3-D sim­u­lated view of curved flanged beams

Fig­ure 6: Plan cross-sec­tions of the spec­i­mens

Fig­ure 7: Fram­ing de­tails and screw ar­range­ment of the test spec­i­men

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