Performance Of Concrete-encased Cfst (Concrete-filled Steel Tube) Members Subjected To Laterally Impact Loads

Due to its good mechanical properties and construction performance,concrete-encased CFST(concrete-filled steel tube)members have been applied increasingly in engineering practice,such as bridge piers.Concrete-encased CFST piers might be subjected to impact loads during their service life.In the past,research on the working mechanism of concrete-encased CFST members subjected to impact loads has been limited.This dissertation thus investigates the working mechanism and design method of concrete-encased CFST members with two types of sections subjected to laterally low velocity impact loads.One is square or circular section which consists of a circular CFST component in the middle encased by a square or circular RC component(referred to as concrete-encased CFST member in the paper),the other is box section which consists of a RC box component and four circular CFST components encased in its corners.The main work and achievements are summarized below.1.Experimental study on the impact behaviour of concrete-encased CFST members was carried out.Failure modes of the members were studied and the characteristics of time-history curves of impact force and mid-span deflection were analyzed.A finite element analysis(FEA)model was established for mechanics property analysis of concrete-encased CFST members subjected to laterally low velocity impact.The FEA model considered the material strain rate effect,the element erosion criteria of concrete,the interactions between concrete and steel,as well as the coupling effect of static and dynamic load.With the help of the proposed FEA model,full-range mechanical analysis was performed.The failure process and failure modes were studied.The development process and characteristics of material stress distribution,contact stresses between the steel tube and concrete,as well as the load distribution between CFST and RC components were analyzed.2.Experimental study on the impact behaviour of concrete-encased CFST members with box sections was carried out.Test parameters include boundary conditions,thickness of the steel tube,axial load ratio and impact energy.Based on the test results,the failure mode of the specimens,the characteristics of time-history curves of impact force and mid-span deflection were analyzed,full-range mechanical analysis was performed.The influence of test parameters on the maximum impact force,maximum mid-span deflection and impact duration was investigated.3.A finite element analysis(FEA)model was established to study the mechanical behaviour of concrete-encased CFST members with box sections subjected to laterally low velocity impact.With the help of the proposed FEA model,full-range mechanical analysis was performed.The failure process and failure modes were studied.The development process and characteristics of material stress distribution,contact stresses between the steel tube and concrete,as well as the load distribution between CFST and RC components were analyzed.4.The influence of material strength,longitudinal bar ratio,steel ratio of CFST component,geometric dimension of CFST component and RC component,impact velocity,impact mass and axial load ratio on the impact resistance of the concrete-encased CFST members was analyzed.Based on parametric study,the simplified calculation methods of dynamic factor for the bending capacity for concrete-encased CFST members with square and box sections were suggested.Construction details of impact resistance design were also proposed.