Progressive Collapse Analysis Of Frame Structures

Regular design has paid attention to the security design about structures with normal loads. However the abnormal loads have not been in the consideration. The phenomenon of progressive collapse is always triggered by the abnormal loads. Especially the 911 incident occurred, consequently more and more attentions about structural progressive collapse issues are drawn all over the world.Currently, the methods about the analysis of progressive collapse are complicated, which can not be applied in practice. An obstacle is set up in the front of structural designers. Based on the altenate load path, with plastic hinge model, the model of static progressive collapse analysis is presented. Through the comparision of elastic analysis of progressive collapse in GSA2003 code, the location of damage, the redistribution of internal force and the ultimate collapse zone can be obtained using the procedure presented in this paper.The regular design method is based on the premise of no local damage in structures, and so the resistance mechanism about the damaged frame is not considered. In this paper, a progressive collapse analysis for two planar steel frame structures about one and two column removal is carried out based on Pushdown method. Timoshenko beam element is used, which can consider the interactional effect of axial force and moment, to simulate the beam and column. In order to improve caculating efficiency, the model with mixed method of elastic-plastic element and elastic element is constructed. The results are shown that the resistance capacity of the damaged structure after the side column removal is lower than that after the middle column removal, and the second damage occurs firstly in the beam end away from the removed column.The local damage causes a complicated vibration phenomena. This vibration phenomena is different from the normal vibration response (for examaple: the earthquake response). With the mode-superposition method, the major contribution mode is investigated, and the result shows that the vertical natural mode is the major contribution to structural vibration after local damage. A model with initial deformation is constructed using mixed method, and a dynamic progressive collapse analysis is carried out with instantaneously unloading method. Studies were carried out to investigate the characteristic of vibration after local damage, and then the effect of failure time period, material characteristic, building span on the structural response were analyzed.The collapse evaluation is carried out for the single column removal and two columns removal, and the results are shown that the collapse can not occure after single column removal, however the collapse will occure after two columns removal.