COLLAPSE BEHAVIOUR BEHAVIOUR OF STEEL BUILDING FRAMES AT ELEVATED TEMPERATURE

Abstract

Fire in. steel buildings is hazardous since the material properties like 'elastic modulus, yield stress etc, of struc-tural steel degrade considerably at elevated temperature and these are. accompanied by the effect of creep and large defor-mations., Specially, in multistoried buildings', to evacuate the stranded people in the upper stories as well as for fire fighting operations, the fire resistivity of the structural members should be normally one hour or more , The different national codes and by-laws of several advanced countries have made provisions in their design against this disaster., like any other calamity e.g. earthquake, wind etc s Contrary to other nations' codes, of practice, the Indian Standard Institution Codes are silent about the structural measures against fire„ The aim of the present study is to see the effects of fire in a building, on the deflection and collapse behaviour of the load-bearing, structures and to recommend some measures for the reprisal of fire effects„ Before analysing the effects of fire in an existing building, wide ragging literature review has been made to suitably improve analytical work . on this aspect. The algo-rithm of analysis includes a complex viscoplastic creep model,. geometric non.linearity due to large . deformations, deteriora-tion of material properties because of rise in temmrature, sensitivity of_ strain rate as stress changes and an implicit time marching scheme o The 'computer program works on incremental and iterative procedure to arrive at desired nonlinearity and provides the deformation- hystory of steel building frames under fire environments Using this program a four storied offioe-cum—residential building frame is analysed for fire conditions., The parametric study involves the determination of the effect of various load. combinations on the structural behaviour, the influence of location- of fire inside a buildjng=and lastly, the variation of fire resistivity with the alteration of sections of structural members, It has been found that for normal office/residential buildings the time required for failure under fire oonditior15-is less than one hour and increase in the sections also -does not help much in increasing this time' Thus the protective measures against fire, like insulation must be used in steel building frames,