Performance Of Hexagonal Concrete-filled Steel Tubular (CSFT) Column And The Concrete-encased CSFT Column Base

The hexagonal CFST column has been utilized in practical structures due to its excellent mechanical and constructional performance,as well as the aesthetic performance and convienence for connection with other structural members.The application of CFST columns leads to requirement for stronger column bases.The concrete-encased column base can achieve both high strength and stiffness with good construction efficiency,which indicates its application potential as CFST column base.This dissertation thus investigates the mechanical performance of hexagonal CFST column and the seismic performance of the corresponding concrete-encased column base subjected to constant axial load and cyclic lateral load.The main work and achievements are summarized below.1.The static behavior of hexagonal CFST members subjected to axial compression,pure bending and combined tension and bending was studied.A series of experiments of hexagonal CFST members under load cases above were carried out.With the help of the proposed finite element analysis(FEA)model,mechanism analysis was conducted.The study showed the hexagonal CFST members achieved good ductility in different load cases.The failure mode and the confinement of the steel tube to the core concrete of the hexagonal CFST members were similar to those of rectangular CFST members.Based on parametric analysis,the design formulas for hexagonal CFST under load cases above were suggested.2.The seismic behavior of hexagonal CFST members under constant axial load and cyclic lateral load was studied.Experiments were carried out first.A finite element analysis(FEA)model for simulation of the seismic behavior was then established,where the cyclic constitutive models of steel and concrete were considered.Full range analysis was carried out.The stress condition and internal load distribution of the steel tube and the core concrete were studied.Based on the parametric analysis,design formulas for seismic design of hexagonal CFST were suggested.3.Seismic experiments of concrete-encased column base for hexagonal CFST subjected to constant axial load and cyclic lateral load were carried out.The parameters included height of the outer RC component,existence of shear studs between the CFST column and outer RC component,axial load level and size of the CFST column.All the specimens showed good ductility and energy dissipation capacity.Two failure modes were observed,i.e.,failure of the bottom section and failure of the CFST section.The load versus deformation relation,strain distribution,strength and stiffness degradation,ductility and energy dissipation ability of the specimens were studied,based on which the effects of parameters were discussed.4.A finite element analysis(FEA)model was established to study the seismic behavior of the concrete-encased column base for hexagonal CFST.The FEA model considered the modelling of base plate connection and modelling of crack in the bottom section,and it achieved good simulation results in both the load versus deformation relation and the loading and unloading behavior.With the help of the FEA model,the failure modes were discussed and the failure mechanism,load transfer mechanism and internal load distribution were studied.5.The design method for the concrete-encased column base for hexagonal CFST was suggested.Effects of the parameters of the CFST column,the base plate connection and the outer RC component,existence of shear studs and the axial load level on the seismic behavior of the column base were analyzed.Based on the failure modes and the load transfer mechanism,the design method of the concrete-encased column base for hexagonal CFST towards bending and shear force were suggested.Construction details were also proposed.