Seismic Behavior And Theoretical Study Of Reinforced Concrete Special-shaped Columns Under Combined Loadings Including Compression,Bending,Shear And Torsion

As the special-shaped columns have the advantages of avoiding the appearance of column fluting,improving the interior decoration,increasing the use of room space and being light weight,etc.It is very popular among designers and owners because of its functional flexibility,so the special-shaped column frame structure is widely used in public buildings and residential buildings.However,due to the irregular cross-section of the special-shaped column,the position of mass center and shear center often do not coincide,which leads to inevitable warping and twisting of the special-shaped column under earthquake.In real structure,the column itself is often in the state of pressure and bending,and the special-shaped column structure may suffer combined loading including compression,bending,shears and torsion.The irregularity of the plane structure caused by the pursuit of aesthetics of the building will further increase the possible torsional deformation of the special-shaped column under the two-way seismic load.Therefore,the influence of torsion on the seismic performance of the special-shaped column structure cannot be ignored.Most of the researches on special-shaped columns are focused on the effects of bending,shear,or monotone compression-bending-shears-torsion,but the researches on the seismic performance of specialshaped columns under the combined action of bending,shear and torsion are rarely involved.Therefore,the seismic performance,numerical analysis model,damage model and calculation theory of special-shaped columns under the combined action of bending,shear and torsion are deeply studied in this paper,and the main research results are as follows:(1)In this paper,a pseudo-static test using an independently developed loading device was conducted on six reinforced concrete L-shaped columns under the combined action of compression-bending-shear-torsion.And seismic performance index for the L-shaped columns such as the crack development models,failure patterns,hysteresis curves,skeleton curves,bearing capacity characteristic parameter,ductility,stiffness degradation,strength degradation and energy dissipation capacity were obtained under different torsion-bending ratios and axial pressure ratios.It is found that with the increase of torsion-bending ratios,the specimens gradually show bending damage,bending torsional damage and torsional shear damage.When the ratio of torsion to bending increases from 0.17 to 0.36,the bending capacity and displacement ductility decreases by11.4% and 28.9% respectively,while the torsion bearing capacity and torsional ductility increases by 24.1% and 17.8% respectively.When the L-shaped columns were failed,the equivalent viscous damping coefficients of bending and torsion were between 0.20～0.28 and 0.16～0.23,respectively.In the experiment,the inter-layer displacement angle of the cracking and destruction of L-shaped columns can meet the requirements of the Code for Seismic Design of Buildings.(2)Numerical model of reinforced concrete L-shaped columns under compression,bending,shear and torsion had been established,which was verified based on experimental failure modes,hysteresis curves and energy dissipation capacity.On the basis of the numerical model,the influence of torsion-bending ratio,axial compression ratio,concrete strength,stirrups spacing,limb height to limb thickness ratio and other parameters to L-shaped columns were analyzed,which can enrich the calculation database of L-shaped column under the combined action of compression,bending,shear and torsion.(3)In comparison of 12 reinforced concrete columns(including 6 T-shaped columns and 6cross-shaped columns)studied by our research group in the same period under the action of compression-bending-shear-torsion,as the ratio of torsion to bending increases,the special-shaped columns all show the same failure patterns.Among the bending and torsion hysteresis curves of the special-shaped column,the hysteresis curve of the T-shaped column is most full,and the flexural bearing capacity is the highest under the same condition.For the ductility of shaped columns,the ductility of the cross-shaped column is higher than that of the T-shaped column and the L-shaped column.The elastic interlayer displacement angle and the elastic-plastic interlayer of the special-shaped column can meet the requirements of national regulations.(4)Based on the two-parameter seismic damage model,using experimental data of specialshaped columns from research group,considering the section characteristics of special-shaped columns,bringing in the ratio of torsion to bending and the distance between the shear center and the centroid,the combination coefficients can analysis the bending damage model and the torsion damage model under the combined action of compression-bending-shear-torsion are presented,and the corresponding modified models for reinforced concrete special-shaped columns are obtained.According to previous studies,a unified equivalent damage coefficient model considering the action of bending and torsion is proposed to reveal the overall damage development process of special-shaped column structures.(5)Based on the experimental data of special-shaped columns from research group,considering yield point,peak point,and failure points as key index,a three-fold line calculation model was established.According to modified damage models,cyclic degradation index was introduced to describe performance degradation of special-shaped columns such as yield load,peak load of each cycle,loading and unloading stiffness under cyclic reverse loading.Combined with the corresponding hysteretic rules,the load-displacement restoring force model and torsional angular restoring force model of special-shaped column based on damage effect are established.Compared with the experimental data,the validity of the restoring force model is verified.(6)Based on the theory of variable-angle space truss,by comparing the torsion design methods and calculation results of national standards,considering the weakening effect of bending moment and shear force on the torque under combined action,the resistance of reinforced concrete special-shaped columns under compression,bending,shear and torsion is proposed.Design formula for torsion capacity.According to the Mohr circle theory and the actual stress distribution on each side of the special-shaped column,the failure surface shape is quantitatively described;according to the oblique bending failure theory and the limit equilibrium theory,the return of the special-shaped column under compound torsion is established based on the failure surface shape.One formula...