Elasto-plastic Behavior And Modeling Of Square Steel-concrete Hybrid Stub Columns Under Axial Compression

The seismic capacity of composite structures depends largely upon the strength and ductility of columns.Due to high strength and reliable ductility,square steel-concrete hybrid tubular columns have been widely used as the gravity-sustaining component in composite structures.When the steel-concrete hybrid columns are vibrated by unexpectedly strong earthquakes,strength degradation after the peak resistance,which leads to further deformation concentration and causes the overall collapse of the structure,is difficult to be avoided due to disadvantages such as local buckling of steel plates.To accurately evaluate the overall seismic behavior of steel-concrete hybrid columns under major earthquakes,the stress-strain models to trace compressive behavior up to large strain are indispensable.The hollow square steel tube(HSST)columns,the square concrete columns jacketed by the bolted square steel tube(BSST)and concrete filled square steel tubular(CFST)columns are selected is this paper.The HSST stub columns was studied to provide experimental and analytic information what are indispensable to the modeling of the compressive stress-strain model for CFST stub columns.The experiments on these three types of stub columns under monotonic axial compression and analysis were conducted and the primary conclusions obtained are listed below:1.Based on previous tests on sixty-eight HSST stub columns under monotonic axial compression,a stress-strain model which focus on the post-local buckling behavior is developed to define the relationship between axial compressive stress and strain for HSST stub columns.The proposed model has a significant advantage over previous models in that it can trace effects of both the residual stress induced during the manufacturing process and the local buckling of steel plate on the compressive stress-strain behavior of HSST stub columns.Another feature of the proposed model is that it can be used to the stub columns made of very thin steel plates.In order to further verify reliability and accuracy of the proposed model,ten more HSST stub columns were tested under monotonic axial compression.Comparisons between the experimental results and the calculated ones indicate that the proposed model can not only predict the local buckling strength and the corresponding strain very well,but also trace the compressive stress-strain behavior of HSST stub columns up to large strain with relatively high accuracy.2.A new type of jacketing method for concrete is proposed.In this jacking method,the lateral confinement is provided by bolting two pieces of fabricated L-shape or channel steel plates.Twenty-one square concrete stub columns jacketed by the proposed bolted square steel tube(BSST)were fabricated and tested under monotonic axial compression to verify the effectiveness of this new jacketing method.The test results indicate that the BSST canprovide confinement to improve ductility of the core concrete as effectively as the welded square steel tube(WSST)while the the peak stress was about 8% lower on average than that of the concrete WSST.3.Based on the principle of virtual work,a stress-strain model has been developed to evaluate the axial behavior of the concrete jacketed by the proposed BSST as well as by WSST.Comparisons between the experimental and calculated results exhibit that the developed stress-strain model can predict the axial compressive behavior well up to a large strain.4.Twenty-four CFST stub columns were tested under monotonic axial compression.A loaddisplacement curve of CFST is established by the superposition method on the basis of analytical experimental results.The calculated results show good agreement with the experimental results obtained in previous studies as well as in this paper.