Seismic Behavior Of CFDST Columns After Being Exposed To Fire

As a new type of composite specimen,concrete-filled double skin tube specimens perform well in capacity,flexural stiffness,seismic behavior,fire resistance,self-weight and it is relatively easy for construction.This type of specimens have been applied in some transmission towers and CFDST specimens are regarded to be potential in high-rise buildings,factories,bridges and off-shore structures.After the structures using CFDST specimens in seismic region get fired,it is necessary to assess their post-fire mechanical properties,especially anti-seismic property.The main research work of this paper is listed as follows:1.Temperature field experiment and post-fire cyclic experiment are carried out for twelve CFDST columns.Fire duration,axial load level,nominal steel ratio and hollow ratio are selected as the parameters in the experiments.Temperature versus time curves at different points on the cross-section are obtained.Through the lateral load versus lateral displacement curves,ductility,stiffness degradation,strength degradation and energy dissipation of specimens are analyzed to study the post-fire seismic behavior of CFDST columns.A simplified method based on the principle of superposition is suggested to calculate the initial flexural stiffness of CFDST columns after being exposed to fire;2.A heat transfer numerical model and a post-fire cyclic model considering the whole procedure of heating and cooling are established.Combined with the physical analysis of heat transfer process and test data obtained from temperature field experiment,a simplified model is supposed to consider the heat resistance between the outer tubes and sandwiched concrete.As for the post-fire cyclic model,a suggestion is given to calculate the damage parameter when the compressive strain level is large.FE models are verified by experiments and proved to be accurate generally.3.Based on the FE models established before,mechanism analysis and parameters are carried out.The influences of various parameters are analyzed.Based on the superposition principle,a simplified method to calculate the moment of post-fire CFDST columns at the peak lateral load is supposed in this paper.