Studies On Collapse Behavior Of Steel Frame Structures Subjected To Fire

With the collapse of the World Trade Center towers in the "9.11" event,widespread concern is focused on the problem of structure progressive collapse under fire condition worldwide.Comparing with concrete structure,steel structure is much easier to cause damage under high temperature.Furthermore,once the damage in steel structure happens,it may lead a serious consequence.So it is significant to analyze the collapse behavior and failure process on steel structures subjected to fire.In order to minimize the fire damage to steel structure and reduce the personnel and property losses,researchers must have a strong understanding of the failure process of the steel structure under fire condition,and can make an accurate assessment of the possibility of structure damage as well as damage extent.For it is costly and wasted to carry out full-scale structure fire experiment,so fire testing limits to single element at present,including beam,column,slab,shear wall etc.In such a situation,it is particularly important to development numerical simulation and theoretical analysis methods on failure process analysis of the integrated structure exposed to fire.Focuses on the issue of steel frame structural responses at elevated temperature,this article carries out a series of research work,including numerical simulation and theoretical analysis.The main researches are shown as follows:(1)The two basic methods of tie forces and alternate path method for structure progressive collapse design are generalized through investigating the available literatures.The determination procedures of load and resistance factor,floor loads,and required tie strength in tie forces method,as well as rules of elements removal,loading cases,damage limitation,and acceptance criteria in alternate path method are both provided.Furthermore,the research achievements at home and abroad on thermal and mechanical property parameters of steel materials at elevated temperature are summarized.(2)Based on continuity theory,the internal force and deformation expressions of the steel frame shear-wall structure with temperature variation are proposed using transfer matrix technique.The expressions derived are programmed by MATLAB for easy applications.A model of 4 stories and 4 bays with steel frame and concrete shear wall is built.Fire response analysis is carried out for the model using the expressions derived in this research and finite element analysis.The two analysis results can authenticate each other.(3)Based on APM,the initial damage is induced by time history analysis functions.Then,both the structure dynamic responses after damage elements losing efficacy,and the full failure process of the remaining structure under fire secondly-disaster are provided.Meanwhile,the failure mode and safety evacuation time of the structure with damage in fire are also provided.(4)In order to investigate the influence of initial damage on steel frame structure fire behavior,both the failure processes of the structures with and without initial damage are analyzed under fire condition.In addition,the effects of the heating rate,the end and middle bay fire on structure fire resistance performance are studied by using different fire conditions and fire locations to the integral structure.(5)A calculation method for the internal force and deformation of the structure in fire from initial time of heating to element failure is developed.The influence of normal working stress level of the member in fire on its failure temperature is discussed.Furthermore,based on DCR acceptance criteria,a progressive collapse assessment method for the in-service steel frame structure under fire condition is proposed.The detailed assessment method is illustrated by examples.And the effect of beam span on the internal force and failure mode of the steel frame structure subjected to fire is investigated by using different beam spans.