| Progressive collapse refers to the complex dynamic process of buildings’ largescale collapse which results from a single or sevreal elements’ sudden failure.It has a low probability of occurrence,but once happens,will lead to severe consequences.It is of great meaning to prevent buildings,especially sturctures of great importance,from progressive collapse.At present,although many countries have promulgated design codes for resisting progressive collapse,but most of these codes only give design suggestions and regulations for preventing progressive collapse of new structures,provisions for retrofitting strategies to improve existing structures’ progressive collapse resistance can seldom be found in these codes.A lot of researches have indicated that steel gravity framed structures(SGFS)have a high risk of suffering progressive collapse when subjected to sudden failure of a moddle column.Therefore,it is very necessary to retrofit this kind of structures to ensure they have adequate robustness.To solve the problem,several retrofitting strategies will be proposed in this study,and their effectiveness and feasibility is studied systematically.Firstly,the detailed finite element model of a three-column-two-beam SGFS is established,it contains beams,columns,shear connections and composite slabs.Then its accurancy is validated systematically in mutiple levels ranging from the material model to the shear connection and the composite frames,to ensure that it can accurately simulates SGFS’s overall reponses and local failure.Then,the collapse resistant mechanisms of SGFS under middle column removal scenario is studied simulationally,based on that,six retrofitting strategies,which can be sorted as two categories,are proposed.The first category is called flexible methods,including seat angle method,slot-bolted cover plate method,and top-and-seat angle method.The second is named rigid methods,including seat stiffened angle method,top-and-seat stiffened method and wleded cover plate method.Effect of each method is investigated simulationally and the progressive collapse resisting mechanisms of it is specified.In addition,parameter analysis of angle thickness in single angle methods and size of slotted in the second flexible method is conducted.The results show that flexible methods improve SGFS’s ultimate collapse resistance effectively,and rigid methods can obviously decrease the vertical displacement of middle joint when SGFS suffers to middle column removal.Finally,feasibility of each method is discussed with the goal of retrofitting,difficulty of construction,and cost taking into consideration,and design suggestions are given based on different fortification targets. |
