Study On Equivalent Single Degree Of Freedom Method For The Dynamic Response Of Concrete-Filled Steel Tubular Columns Under Impact Loading

Concrete-filled steel tubular(CFST)structure is widely used in engineering projects due to its advantages such as high bearing capacity,excellent seismic performance and superior fire resistance.Concrete-filled steel tubular columns,which are used as load-bearing members,will cause significant casualties and property losses if subjected to lateral impact loading.In this paper,the LS-DYNA finite element software is used to study the impact resistance of concrete filled steel tubular columns under axial compression.Then the dynamic displacement of concrete filled steel tubular columns is calculated based on the equivalent single degree of freedom theory.The research contents are as follows:According to the experiments results,the reliability of LS-DYNA software is precisely verified.So it can be used to do more impact analysis.Then two kinds of finite element model of concrete filled-steel tube members,with different steel tube wall thickness,are established,which are named A and B.The results show that when the impact velocity is low,in a certain range of axial compression ratio,axial force is applied properly can improve the impact resistance performance of concrete filled steel tubular members.Beyond this range,members may be unstable damaged due to the P-? effect;when the impact velocity is too high,even low axial compression will sharply weaken the impact resistance performance of concrete filled steel tubelar.The simulation results show that the impact force time history curve of concrete filled steel tubular columns under axial compressionis generally divided into two forms-with a platform and without platform.In order to be conveniently used by equivalent single degree of freedom method,the actual complex impact force time history curve is simplified into rectangular pulse load and triangular pulse load according to the principle of impulse equivalence.And the error of the SDOF method is small in certain cases.The differential equations of motion for the equivalent SDOF system of steel tube concrete columns subjected to axial compression are derived.The displacement response under rectangular pulse load and triangular pulse load is obtained.The displacement response of various simulation cases are calculated by using the equivalent SDOF method.Comparison of the maximum displacements shows that when the impact velocity is low,the impact force history can generally be simplified into rectangular pulse load and the error of maximum displacement between the simulation results and SDOF method results is small;And when the impact velocity is large,the time history curve of the impact force is usually simplified into a triangular pulse load,the error of this result is greater than that of the rectangular pulse;And the error will be reduced when the axial compression increases in a certain range;And the error will be reduced when increasing the thickness of the steel tube.