Study On Bearing Capacity Of L-shaped Irregularly Concrete-filled Steel Tube Columns

Irregularly sectional concrete-filled steel tube columns have high bearing capacity and excellent seismic resistance performance,and can avoid exposure of column corners in residence building structures.In this paper we studied the mechanical performance of L-shaped irregularly concrete-filled steel tube columns（L-ICSTCs）,whose steel parts are composed of channel columns and I-columns,and are filled with concrete.L-ICSTC has the advantages of both concrete-filled steel tube columns and special-shaped columns,thus having vast application prospect in building structures.Firstly,a fiber model of L-ICSTC is built to investigate its sectional strength under different load conditions.Formula of sectional strength N_y under axial compression,correlation formula between bending moment in x direction m_x and bending moment in y direction m_y under pure bending moment,and correlation formula among axial compression ratio n,bending moment in x direction m_x and bending moment in y direction m_y under combined axial compression and bending moment load are put forward.Secondly,a finite element model of L-ICSTC with linear elastic constitutive relation is built to study its elastic buckling behavior under axial compressive load.The influence of column height and cross sectional shape on the elastic buckling load and mode is investigated.The comparison between elastic buckling load from FEA and the Euler load shows little difference.So the Euler load is treated as the elastic buckling load in this thesis,and formulas of elastic buckling load N_cr and regularized slenderness ratioλ_n are obtained.Then,the mechanical behavior of L-ICSTC under axial compression is investigated using a finite element model with elastoplastic constitutive relation.The stiffness,stress and deformation of a typical model during the whole loading process are analyzed.And we fitted the correlation formula between stability coefficientφand regularized slenderness ratioλ_n,which can be used to calculate the bearing capacity of L-ICSTC under axial compression.Next,combined axial compression and bending moment load is applied to the finite element model with elastoplastic constitutive relation.Results show that for an L-ICSTC model under certain axial compressive load,the ultimate bending moment is minimum or close to minimum when the direction of initial imperfection coincides with the direction of bending moment.So the direction of initial imperfection is set to be the same as the direction of the bending moment.The stiffness,stress and deformation of a typical model during the whole loading process are analyzed.And we fitted the correlation formula among axial compression ratio n,bending moment in x direction m_x and bending moment in y direction m_y,which can be used to calculate the bearing capacity of L-ICSTC under combined axial compression and bending moment load.Finally,an experiment of 4 specimens is designed and carried out to investigate the mechanical behavior of L-ICSTC under axial and eccentric compression.And we analyzed the influence of initial imperfection and load eccentricity on stiffness,bearing capacity,deformation and ductility of L-ICSTC.After that,finite element models with factual material properties and initial perfections are built to simulate the loading process,and the results turn out to coincide with experiment data,verifying the effectiveness of finite element models and proposed design formulas.