Research On Ultimate Bearing Capability Of Super Rectangular Concrete-filled Steel Tubular Columns With Stiffening Plate Subjected To Axial Compression

The application of rectangular concrete-filled steel tubular(CFST)columns is increasingly widespread in practice due to their merits such as convenience in steel tube manufactured,simple joint construction,ease to meet with beams and requirement of architectural plane,large inter of cross-section to resist bending moment,compared with circular CFST columns.However,confinement effect of rectangular concrete core is relatively weak and decreases with the inceasing size of cross section since the confinement mainly exists at the corners,which becomes weak beyond the region of corner.Meanwhile,the steel plate of each side is subjected to a state of longitudinal compression and lateral compression so that it is prude to local buckling which results in failure.Thus,rectangular CFST column is inferior to circular CFST columns on aspects of bearing capacity and ductility.Aiming at such shortcomings of rectangular CFST column,the super rectangular CFST column with stiffening plate is suggested to improve the capability of rectangular CFST columns based on the background of a super high-rise building.An extensive study on mechanical behavior of rectangular CFST columns with stiffening plate subjected to axial compression is conducted,including the following main aspects.(1)Twenty-one models with or without welding longitudinal stiffening plates on the inside surfaces of the steel tubes were simulated under axial compression to investigate the behavior of super rectangular CFST stub columns.The main parameters varied were width-thickness ratio of the steel tube,thickness and number of the stiffening plate.The load-deformation relationship curves of rectangular CFST stub columns subjected to axial compression were analyzed using the finite element software ABAQUS.On the basis of the experimentally verified reliability of the finite element model,mechanical behavior of the rectangular CFST stub columns without stiffening plates,with one stiffening plate and with two stiffening plates were analyzed respectively and compared in terms of stress versus strain relationship curves,longitudinal stress distributions of steel tube and core concrete,and their interaction.The analysis results show that using stiffening plates can result in the increase of maximum axial stress of core concrete and the tensile stress areas of steel tubes are decreased as a result of the welding stiffeners.The stability of steel tube is also improved so that the axial bearing capacity and the ductility of rectangular CFST stub columns are increased.The confinement effect of rectangular CFST stub columns with stiffening plate is mainly developed at steel tube corners and the places where stiffening plates were welded.And the confinement effect on steel tube corners can be enhanced with increased number of stiffening plates.(2)Calculation methods for the ultimate capacity of rectangular CFST columns with stiffening plate from foreign and domestic design criteria are compared and analyzed.The study results have shown that all results of codes are somewhat conserved.It is not suitable to adopt the above codes to design super rectangular CFST stub columns with stiffening plate.Based on considering the contribution of stiffening plates reasonably,the ultimate strength of concrete core is detemined by applying the concept of the effective lateral confining pressure and Twin Shear Unified Strength Theory.A method for calculating the axial bearing capacity of rectangular CFST stub columns with stiffening plates is proposed.The calculated values of the axial bearing capacity of CFST stub columns are in good agreement with the results of finite element analysis.(3)Assumed that the steel plate is subjected to spring constrain at the unloading edge,the formulae for calculating local buckling strength of the steel plate of rectangular CFST column under axial loading are deduced using energy approach.The analysis results were in good agreement with results of finite element analysis.A further analysis on the effect of stiffening plate on local buckling of steel plate shows that the stiffening plate can increase the strength of local buckling and improve the capacity of steel plate to resist local buckling.Proper thickness of stiffening plate and appropriate limitation for width-thickness ratio of steel tube are suggested for rectangular CFST column with stiffening plates under axial loading.