Axial Compression Performance And Bearing Capacity Calculation Of Spiral Stiffened Thin-Walled Square Steel Tube Concrete Composite Column

To improve the engineering suitability of steel pipe concrete members,thin-walled steel pipe concrete was introduced,and its economic advantages were significantly improved.However,the thin-walled steel pipe causes its local buckling problem to be more prominent.To address this problem,this paper proposes a new type of restraint-spiral stiffening ribs,spirally distributed along the outer wall of the steel pipe,to form a spatial restraint system that can limit buckling and lateral deformation.In this paper,based on the spiral rib restraint concrete axial compression test,finite element parametric analysis and theoretical study of axial compression load capacity of stacked members are carried out,and the following conclusions are obtained:(1)Axial compression damage mode of ordinary thin-walled square steel pipe concrete column: core concrete crushing,steel pipe outer drum buckling,spiral rib restraint steel pipe concrete axial compression column damage mode: outer concrete extension cracking along the spiral rib,spiral rib between the steel pipe local buckling,the degree of buckling is significantly improved compared with ordinary members.(2)Spiral ribs restrain the axial compression performance of stacked columns,which is greatly affected by the working condition of the concrete outside them.The single spiral rib and spiral rib-vertical plate(welded)restraint forms are ineffective in limiting concrete cracking,thus weakening the stiffness and load-bearing capacity of the member;the spatial restraint system composed of spiral rib and reinforcement can delay the flexure of thin-walled steel pipe,limit the overall lateral deformation,and ensure that all parts of the member work together,which improves the load-bearing capacity and ductility by 18% and 8%,respectively,compared with ordinary members.(3)The spiral rib-steel bar constraint member’s axial compression performance is superior.The stress mechanism is as follows: the stiffeners divide the concrete and the steel tube into several spiral strip structures.The axial load of the composite column is converted into the tangential shear force and normal pressure of the spiral rib by the angle of the spiral rib.Under compression shear,the concrete cracks obliquely,and the steel tube buckles slightly.(4)The finite element parameter analysis concludes that the composite member is greatly affected by the pitch.Reducing the pitch,the steel pipe is enhanced by the constraint effect of the spiral rib,and the stress behavior tends to the ring stiffener.The larger the pitch,the higher the contribution to the vertical stiffness of the member,and its stress behavior develops towards the vertical stiffener.Changing the width-thickness ratio of the spiral rib and the diameter of the steel bar has little effect on the component’s performance.The design suggestions are put forward: The width-thickness ratio of the spiral rib is 11～15,the width-thickness ratio of steel pipe is 165～220,the pitch is 1～1.2 column height,the diameter of the steel bar is 1/10 steel bar spacing(spacing is not more than 1/3 pipe diameter).(5)The buckling load-carrying capacity of the steel plate under the constraint of the spiral steel rib bar is solved based on the energy method.Based on the test and simulation results,computational methods for the load-carrying capacity of outsourcing concrete and core concrete considering the strength reduction coefficient α of external concrete and the enhancement coefficient η of core concrete material are proposed,respectively.Finally,based on the superposition principle,the calculation formula of elastic buckling bearing capacity of steel pipe concrete axial compression column under spiral rib-vertical reinforcement restraint is proposed.The research results can be used as a reference for the engineering application of the new composite component.