| The result of the fire disaster showed that the static mechanical behavior and seismic performance of the structure after fire has some reduction in different degree,generally,the structure could continue to use until the repair and strengthen work finished.Therefore,the research on how to evaluate the static mechanical behavior and seismic performance of non-collapsed structures and repair and strengthen scientifically has a significant meaning.Reinforced concrete-filled steel tubular(RCFST)columns,is a component that put longitudinal reinforcement in concrete-filled steel tubular column.Compared with traditional concrete filled steel tubular columns and reinforcement concrete columns,RCFST has the advantages in bearing capacity and ductility after fire.The repair and strengthen work has higher operability after fire.And the research on high-temperature damage and seismic performance of RCFST is the precondition to service continuously.Meanwhile,in order to reflect the propagation path of seismic wave,the effect of SSI has taken into consideration.As a result,the research of seismic performance can be more safety and more economic.Based on these,aimed at solving the absence of relevant achievements on RCFST short columns under axial load and considering SSI after fire,the article set up the research by the compose of theory analysis,numerical simulation and experiment.The main work has been introduced briefly below:(1)Open flame experiment in fire furnace of concrete-filled square steel tubular columnsA flame experiment which including a total of 14 SCFST short columns and under ISO 834 curve has been done.The processes of warming and cooling are the main points.Heating time is 30 min & 60 min.The key measuring points on steel tube,reinforce bar and concrete have been tested with the change of temperature,and formed the temperature variation curve which could explain the after-fire form of the components.The experimental results showed that the measuring points along with the longitudinal position have an consistent regular.The temperature field in axial direction has an uniform distribution.Different position in the component has temperature gradient phenomenon that reflect an obvious hysteresis on time.The nearer to the core of the section,the lower temperature measuring point get.In other words,the longer measuring points get to the highest temperature while take more time to get cool.(2)The static performance of reinforcement concrete filled square steel tubular short columns under axial load after fireIn order to study the effects of heating up time and reinforcement ratio on reinforced concrete filled steel tubular short columns under axial load on components' mechanical property after fire,a series of experiment which include 18 columns under ISO 834 have been set up to test the residual bearing capacity after fire.Based on the key points on steel tube,reinforcement and concrete,load-displacement curve after fire,load-strain curve of steel tube after fire and residual bearing capacity,the failure mode has been obtained.The test results have showed that the temperature of concrete has an obvious hysteresis effect,the residual bearing capacity decrease with the heating up time increase while increase with the reinforcement ratio increase,all the components end up in shear failure which proving the kind of components have good ductility and deformation ability.(3)Analysis and building the mechanical property numerical model of RCFST columnThe study has set up the theory analysis model to research reinforce concrete filled square steel tubular columns under temperature field and axial load after fire which based on the determination of material's thermal parameter,constitutive relation,contact model,unit type,constraint type,and grid division form,etc.The results of analysis models and former experiments have been compared which proved the reliability of the numerical models.(4)Mechanical property parameter analysis of reinforcement concrete filled square steel tubular columns after fireBased on the element software ABAQUS,a model of RCFST after ISO 834 has been set up to analyze the mechanical properties.Since the model turned out to be correct,the paper take the using of element models to analyze the effect of heating up time,section dimension,strength of materials,steel ration and steel ratio on residual bearing capacity,axial compressive stiffness and ductility of RCFST after fire.Based on the simulation,the axial compressive stiffness decrease more than bearing capacity in ratio and a simplified calculation formula which considered the influence of residual bearing capacity,axial compressive stiffness and energy consumption could provide a reference for damage assessment of RCFST after fire.(5)Analysis and building the seismic performance numerical model which considered the effect of SSI of RCFST columnBased on the element software ABAQUS,the RCFST-RCB frame structure element model which considered the soil-structure interaction has been set up.The reasonableness of the model has been testified by the former experiments.The difference between frame structure after local fire and original structure on the dynamic characteristics of vibration mode and natural vibration period has been analyzed and compared.The structural horizontal displacement,displacement between the layers,interlayer displacement angle and inter laminar shear under frequently occurred earthquake are obtained by the method of elastic time-history analysis of structure and compared with the original structure to reflect how the fire effect on the structures' seismic performance on high temperature damage.The results have showed that the natural vibration period of frame structure is longer than the original structure.Their first three order vibration modal properties stay the same which means the first and second mode is the horizontal mode and the third mode is torsion vibration mode.The amplitude of displacement and time history curve of joints and the inter layer shear in structure after fire are quite much higher than original structure.The layer which exposed to local fire has a bigger interlayer displacement angle than the same layer in original structure. |
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