Study On Impact Resistance Of Concrete-filled Steel Tubular Members Under Long-term Loading

Impact loading,as an accidental load,usually occurs during the natural employ phase of a buildings or structures.For example,the key parts of large public hub buildings is impacted by vehicles due to the terrorist attack,rolling stones impact the bridge piers when natural disasters such as mudslides occur,and the ship impact piers,etc.When the concrete-filled steel tubular(CFST)members are subjected to lateral impact loads,they often bear continuous axial loads at the same time.Therefore,the working condition for CFST members under both long-term loading and lateral impact loading may exist.Due to the effect of long-term loading on CFST members,the shrinkage,creep and stiffness degenerate will occur in the core concrete,which may cause the local buckling of outer steel tube because of a high local stress.Many experimental studies have shown that long-term loading has adverse effect on the static behaviors of CFST members.However,to the best of authors’ knowledge,there are very limited studies on the mechanical performances of CFST members considering the combined effects of long-term and impact loading.This paper presents an investigation of impact resistance of CFST members under long-term loading via using finite element analysis and theoretical analysis.The main contents are described as follows:(1)The appropriate constitutive models are adopted to model the material characteristic.Then numerical models of CFST members under long-term loading and impact loading,and after impacting were developed and verified,respectively.Comparing with typical test data,the simulated results from the numerical models show an acceptable accuracy.After verifying the accuracy of the numerical model at each stage,finite element model of CFST members under combined effects of long term and impact loading was established,and residual compressive bearing capacity of CFST members after impacting was also calculated.(2)Based on the reasonable finite element analysis model,the impact dynamic responses of the CFST members under the primary loading mode and the long-term loading are compared.After that,the impact force time history curves,displacement time history curves,change of impact force impulse,energy variation,mid-span section bending moment,stress state of outer steel tubular and core concrete,and residual compressive bearing capacity after impact were analyzed,respectively.The analysis results show that long-term loading has adverse effects on the impact resistance of CFST members.(3)Taking the mid-span ultimate deflection and the residual compressive bearing capacity coefficient after impacting as the impact performance evaluation index,the influence of different parameters on the impact performances of the CFST member under the primary loading mode and the long-term loading were analyzed.It shows that considering the effect of long-term loading will result in the increase of the ultimate deflection at middle span and reduction of the residual compressive bearing capacity.Increasing the steel ratio or reducing the long-term load ratio can improve the impact resistance of the CFST members under longterm loading.(4)Based on the calculation method of the static ultimate bending moment of CFST members,a simplified calculation method for the dynamic ultimate bending moment of the mid-span section of CFST members under the one-time loading mode is proposed by using the modified coefficient method.A good accuracy can be achieved by the comparison between the simplified calculation and numerical calculation results.In addition,the calculation method considering a coefficient reduction for the dynamic ultimate bending moment of the mid-span section of CFST members under long-term loading is proposed.