Study On The Anti-progressive Collapse Mechanism Of Space Steel Frame Structure And Dynamic Performance Under Impact Load

Frame structure was a common structural form,which might be subject to man-made destruction or accidental disasters during the service period,causing initial damage of local components and spreading to other components,causing collapse of a large area,and even leading to progressive collapse of the entire structure.The dissertation focused on steel frame structures.Firstly,the anti-progressive collapse mechanism was studied,and the multi-story frame was simplified to single-story substructure for anti-progressive collapse analysis,and the research results of single-story substructure were applied to multi-story frame structure;secondly,after the failure of key components,in addition to affecting the superstructure of the component,there would be falling structures impacting the substructure.Under the impact load,the substructure could resist the impact load,and it was also an important part of the anti-collapse study;moreover,the anti-collapse analysis often neglected the specific disaster,directly considering the structure resistance after component failure,it was necessary to further study the structure resistance in combination with specific disasters（such as automobile impact）.The main contents and conclusions are as follows:（1）The progressive collapse-resisting mechanism of the single-story steel frames was analyzed after middle and corner column loss.The resistance curve of the frame structure was obtained by solving the explicit expressions of the beam mechanism,the transition phase and the catenary mechanism,and by combining the concept of energy balance.The resistance curve could be used to estimate the maximum dynamic response after column loss.By comparing with other scholars’nonlinear dynamic analysis results,it shows that the simplified method is relatively accurate and simple to calculate.After the corner column loss,the single-story frame was unable to develop the catenary mechanism due to lack of sufficient axial restraint,and it could only rely on the beam mechanism to resist the unbalanced load.（2）The collapse-resisting mechanisms of multi-story structures were analyzed,besides the catenary mechanism and beam mechanism,it also showed Vierendeel mechanism.The moment of couple M₀ generated by the axial forces in each story participates in resisting progressive collapse.The contribution of Vierendeel mechanism to the whole frame structure was proposed for middle and corner columns loss,respectively.The case analysis showed that the contribution of Vierendeel mechanism to the collapse resistance was about 12%.Using the support system to strengthen the frame structure,it is found that the horizontal support system is more effective than the vertical support system to improve the structural collapse resistance.（3）For the impact analysis under two extreme conditions of complete plasticity and complete rigidity,combined with the resistance curve of the frame structure,the displacement of the beam after two extreme impacts was evaluated,and the accuracy of the method was verified by the existing drop-weight impact beam tests.By using the conservation of momentum and energy,the dynamic characteristics of structures under direct impact and rotational impact were analyzed.The results show that the energy has an important influence on the impact force and displacement.（4）The dynamical theory was used to analyze the excitation response of the structural system under vehicle impacting.The influence of different factors on vehicle impact force and dynamic response of components was analyzed.The results show that the impact position near the middle of the column is dangerous,and the axial compression ratio will not directly affect the impact process of the vehicle and the compression column,but indirectly affect the lateral displacement of the impact point through the second-order effect.A finite element model of existing steel frame structure was established,and the dynamic response of the structure under impact loads was analyzed.Compared with the results of the direct-column-removal method,it was found that the impact force not only destroyed the hit column,but also brought the initial stress and initial velocity to the surrounding member.The commonly used direct-column-removal method ignored the influence of adjacent components,and underestimated the dynamic response of the remaining structure.（5）The process of removing 4 columns of the Ohio Union building was simulated by direct-column-removal and impact-column-removal method,respectively.The results showed that after 4 columns were removed by direct-column-removal method,the structure was finally stabilized after a period of oscillation,while the structure collapsed after the fourth column was removed by impact-column-removal method.After the failed of each column,the beams on different stories showed different collapse-resisting mechanisms,and the Vierendeel mechanism worked together with other mechanisms to resist progressive collapse...