E LASTO-PLASTIC APPROACH TO ANALYSIS AND DESIGN OF MULTISTOREYED FRAME OF STEEL

Abstract

The problem of an acute shortage of space dui to the need of more cesaercial houses, greater influx of population into the big cities, undezirabiiitl of diverting agricultural lands for ha-bitation and the need for greater open space and greenery, has found its solution in vertical expansion of the cities by build-ing up a series of rultistoreyed buildings both for commercial as well as residential purposes. Besides,, tall buiidin;,,,s have also been advocated due to variety of other reasons such as ; difficulty of shifting industries to new cities; the demand for rehabilit-*-tiny the slum dwellers in the se area, and the revulsion of the urban duellers to being uprooted and transfix to rural or newly developed areas. In designing tall rigid traces the main problem is to find a method which is sufficiently accurate and at the sake time avoids heavy design costs. The difficulty is increased by the fact that the designer Rust steer a course between speed of design on the one hand and loss of accuracy on the other. The best aethod of design is one which combines in it all the three basic requirements, that is,, it should be ; (1) a true representative of the, actual behaviour of the structure. (it) rapid and convenient In its applicatign. (iii) most economical. U 1voaaly opo 4.p thovo oiffiot Cao 4ioti tly difforont nothol o of dooicn and+►.,o~.~o ,a of Piaid fc`.r oo thoq aro $ (a) T1 oco bapCt on naziaun aflo ablo cafo otrosooa in tho t3 vario c pcboro of the otr+ eturog ouch otro00oo both uihhn t olriotic ono. (b) ih..oao bocci on the ultinato load carrying, capacity of the otructuroo The tuo 03to;ottou diffoi in their basic approach to the pvablcn. & ho ibr.or i.o0 laotie nothodcg is older and Mora oonoor-vativoa bt in baood on the aowuz►p ton that otablo oqutlibrii ozictt bet con in orncl and o Ctory-A fbrcoo c trithin optic limit any of ;o in loadins conditions door not produce die proportiorato 8neroaoo of atrooe or olaotic distortion of frcoo £ho d©airX to band on the anUyaic of atr0000a in ncboro and pL'ovi dank a acetion of edogmato atron3th to rociot the forcoa. Min Method aoounoa that if local collapso of ncbora can be provontod thootrcaeturo to a " 'lo to aatoo The ooco6d 17)o which han coio to voguo recently finds PC prSnoipal 0pplfc3tg~n at proaont to the dooirn of ota aioally tndotoiinato Or dart frcmoa atsaetuaroa of mild otool which cavey loc1 by i~' 0 of the reciatanco of their ncnboro to bonding action*xfii.^ox ~lc~l r a otmeturo may fail in a valor of uaya. It may poach itio ,fib of uaof lnona through inotability fattjao4 or ogooaaivo 6~ e ti0ae 0 Altornativoly, if none of thono codoe of ?011'Mre occi' bh'O nbrncturo B will continuo to curry load beyond the oleatio 1i~~ f $ It ronchos ito zltinuto load bhcoi h plastic defbrx4tions aM then collapses. Most of the indeterminate struc•• tural steel building frames and similar structures fall into this category. Plastic analysis provides a rational method for basing the design on this most typical mode of failure. In other words it bases the design on the •axioum load that the structure can support. Elastic analysis of all but the simplest of strictures is eam. plicated. There are so many secondary effects that deserve atten. tion while working within the elastic range. Simplifying assumptions Lust be made to the point where the relation between a practical solution and a rigorous Zlastic Theory is quite obscure, But it has been shown that at ultimate load the deformations of the stru-cture become so large that the defoaations due to the secondary effects are comparatively negligible and hence simplifying asstmp• tions are not necessary. It would be seen that the two theories look upon a structure in a different way even though the resultant design way be more or less the same. Structures designed on ultimate load theory however take the advantae of the ductility of steal so that when the stresses are higher than the elastic limit the structure can still carry more load because of the two facts, that is# 1) Distribution of stresses at a section, from the highest stressed fibres to the understress.d fibres, 2) Redisttibution of moments due to the formation of plastic hinges.