Investigation On Progressive Collapse Mechanisms And Enhancement Of Robustness Of Steel Frame Structures

The structure progressive collapse is usually caused by initial structural member damage under the extreme load such as earthquake,fire,blast and vehicle impact that lead to lost bearing capacity and cause a structure collapse that is disproportionate to the initial damage.The progressive collapse can cause a serious accident,but the probability of the accident which can lead to extreme load is quite rare.So people used to neglect the importance of progressive collapse.The progressive collapse was attached due to a series of disasters.Because of the advantages of high strength,light weight,low carbon,environmental friendly and reusable,steel structure buildings have been paid more and more attention and promoted by the country.Many important large-scale buildings are designed with steel structure.Steel frame structure is a common design scheme in steel structure design.With the widespread use of steel frame structure,it is meaningful to prevent progressive collapse.In order to realize progressive collapse,we need to reveal collapse resisting mechanisms and dynamic responses of high-rise steel frame subjected to extreme load.The retrofit strategy was applied to steel frame to prevent progressive collapse.This paper will study the robustness of steel frame structure from the above contents.The main content of this work can summarized as follows:(1)A static push-down analysis was conducted experimentally using a one-story steel moment frame substructure.The behavior of steel moment frame under column loss scenario was investigated.The relationship between the load applied to the top of the stub column vs.the vertical displacement of the stub column and strain distributions along the control cross-sections was shown.The contributions of collapse resisting mechanisms including flexural action and catenary action to the robustness of the system as the increase of the vertical displacement of the center column were quantified.Dynamic responses of the test specimen were estimated using energy-based method.The dynamic increase factor was also estimated on the basis of the testing results.(2)A progressive collapse experiment of a one-story composite steel moment frame with slab was conducted through static push-down analysis.The robustness of the system was quantified.Dynamic responses of the test specimen were also estimated using energy-based method.The contributions of collapse resisting mechanisms including flexural action and catenary action to the robustness of the system as the increase of the vertical displacement of the center column were quantified through strains of column section.The influence of slab on the robustness of the system was clarified through tests results,which include the relationship between the load applied to the top of the stub column vs.the vertical displacement of the stub column and the contributions of collapse resisting mechanisms.(3)Two 3-D micro-models which include all components of steel frame with and without slab were created and validated by these test results.The internal force of reinforcement for slab and the influence of boundary constraint on the contributions of collapse resisting mechanisms including flexural action and catenary action to the robustness of steel moment frame were analyzed.(4)In order to find out whether the alternative load path method was safe enough,the progressive collapse test of a 10-storey steel frame structure which was impacted by vehicle was chosen to simulate,and compares the simulation results with the alternative load path method.In order to reduce the occurrence of progressive collapse accidents caused by vehicle impact,parameter analysis from factors such as vehicle mass,speed and impact angle were carried out.According to the simulated failure mode of the structure,two retrofit strategies,aiming at increasing the collapse resistance of simple connections by adding seat angles and steel plates with long-slotted holes,were proposed in order to address the vulnerability of steel gravity frames under column loss scenarios.These two retrofit strategies were then implemented to retrofit the gravity system of a 10-story,seismically designed steel frame structure,which has been shown to be vulnerable to progressive collapse after an interior gravity column was forcibly removed or impacted by a heavy vehicle with high speed.Numerical simulations were performed using a 3-D micro-based model and the simulation results illustrated that progressive collapse of the structure with enhanced gravity systems was prevented under both scenarios.