Behaviour Of Concrete Filled Steel Tubes With Square Cross-Section Under Repeated Impacts

Concrete-filled steel tubular(CFST)has excellent mechanical properties in static and dynamic loads,which can meet the requirements of modern structures with large span,high elevation difference and heavy load,so it is often used in high-rise buildings and bridges.During the service life of CFST columns,they may be repeatedly subjected to low-speed impacts,and relatively more than once,with a high probability of losing their bearing capacity due to multiple impacts,which poses a serious threat to people and property.Therefore,the dynamic response,residual bearing capacity and post-impact damage evaluation of CFST columns under repeated lateral impacts are analyzed to guide the impactresistant design of CFST structures.In this thesis,ABAQUS software is used to study the repeated lateral impacts of concrete-filled square steel tubular(SCFST)columns,as follows:(1)The finite element model of SCFST column subjected to repeated five lateral impacts is established,and it is compared and verified according to the existing drop hammer impact test of CFST column,residual bearing capacity test and repeated impact test of steel members.Among them,the Restart method of ABAQUS/Explicit is used to calculate the residual bearing capacity of damaged columns after impact,taking into account the strain rate during impact load and the influence of the contact relationship between steel and concrete.Including failure mode,impact force curve and displacement time curve,the test results are consistent with the finite element analysis results.The results show that the finite element model can accurately and effectively simulate the dynamic response of SCFST columns under multiple lateral impact loads.(2)In order to study the influence of the difference of dynamic mechanical constitutive models of different materials with different strengths,a finite element model of SCFST column under a single lateral impact load was established.The constructed model considers the influence of different steel dynamic constitutive models and concrete dynamic constitutive models,and carries out finite element analysis on the impact resistance of SCFST columns,respectively,and discusses and compares the applicability of the deduction models.(3)Using the established finite element model,the mechanical behaviour of the SCFST column under multiple lateral impact loads is analyzed.The analysis mainly includes the impact process of the beam,the development and distribution of internal forces,failure modes,section bending capacity and energy dissipation capacity,as well as the impact times of the SCFST column impact force and mid-span deflection.The results show that after five consecutive lateral impacts on the SCFST column,its impact force response is similar to that under a single lateral impact.The failure mode of the specimen mainly shows the overall bending failure,and the local buckling on both sides of the impact area becomes more and more obvious with the increase of the impact times.(4)The parameters affecting the dynamic response of SCFST columns subjected to multiple lateral impacts are analyzed.Two influencing parameters include high sensitivity parameters: steel strength,steel ratio,cross-sectional dimension,span length to section dimension ratio,impact energy,momentum,impact position and axial load level,and low sensitivity parameters: concrete strength,cross-sectional shape and drop hammer geometry.The influences of different impact parameters and self-material parameters on impact force,displacement,failure mode and dynamic flexural capacity are analyzed.Based on the results of parametric analysis,a simplified calculation method for calculating the dynamic flexural capacity of SCFST columns subjected to repeated lateral impact is proposed.The deviation between the simplified design method and the finite element results is within 20%.(5)The sensitive parameters which affect the residual bearing capacity of damaged SCFST columns after repeated lateral impact are analyzed.The parameters involved include steel strength,section steel ratio,cross-sectional dimension,span length to section dimension ratio,impact energy,momentum,impact position and axial load level,etc.The influence of each parameter on the residual bearing capacity is analyzed,and the residual bearing capacity of the damaged column is determined by regression analysis.The formula is a function including impact times and eight influencing parameters,and the proposed simplified formula is in good agreement with the numerical results.