Research On Seismic Behavior Of Ultra-high Performance Concrete Filled High-strength Square Steel Tubular Columns

Ultra-high performance concrete(UHPC),as a new type of concrete material,has excellent properties such as high compressive strength,large elastic modulus,high tensile strength,and good durability,and has been widely used in the field of civil engineering.However,as the compressive strength of UHPC increases,its concrete matrix becomes more brittle.Using high-strength steel tubes as the external structure of UHPC,forming high-strength steel tube-UHPC columns,on one hand,improves the brittle failure of UHPC under external forces,and on the other hand,the support effect of UHPC can prevent steel tubes from becoming invalid due to premature buckling,fully exerting the performance of both.Currently,research on steel tube-UHPC mostly focuses on circular sections,and there are fewer reports on square steel tube-UHPC.Its seismic performance and working mechanism under low-cycle repeated loading are not yet clear,and the calculation method of horizontal bearing capacity prediction formula is not mature enough.The main research work and achievements are as follows:(1)By conducting quasi-static tests on square steel tube-UHPC columns subjected to low-cycle fatigue loading,the hysteresis curve was measured,seismic performance indicators were analyzed,experimental phenomena were described,and failure modes were analyzed.The results showed that the specimen mainly experienced elastic stage,elastoplastic stage,and failure stage.The characteristic points of each stage were analyzed,and it was found that the yield displacement of the high-strength square steel tube-UHPC column was delayed,and the interaction between steel tube and concrete was weaker than that of ordinary steel tube-concrete columns in the elastoplastic stage.In addition,the failure mode of the specimens was consistent,which was flexural failure on the loading side of the lower end of the column,and the range of bulging deformation was approximately 47 mm to 70 mm above the base beam.All specimens exhibited good seismic performance,and the brittleness problem of UHPC was well solved.(2)A finite element model of high-strength square steel tube ultra-high performance concrete columns was established using appropriate constitutive relationships through numerical simulation.The material defects of the component materials,assembly fixture positioning errors,and specimen fabrication deviations were comprehensively considered.The finite element calculation results were compared and analyzed with the experimental results.The results show that the finite element calculation results match well with the experimental results,which validates the accuracy of the finite element software.(3)Based on the results of the experimental study and finite element verification,the influence of axial compression ratio,slenderness ratio,and concrete strength grade on the bearing capacity of the specimens was analyzed by expanding the specimen parameters.The study shows that after the axial compression ratio exceeds 0.2,the peak bearing capacity of the specimen decreases continuously with the increase of the axial compression ratio,with small amplitudes of 2.22%,2.06%,1.50%,3.78%,2.32%,5.64%,and 2.47%,respectively.When the slenderness ratio is less than 38.48,the skeleton curve of the specimen continues to decline after the load reaches the peak value.However,when the slenderness ratio exceeds 38.48,the skeleton curve only has the elastic stage and the strengthening stage,indicating that the specimen changes from flexural failure to instability failure.Compared with the peak bearing capacity,the reduction amplitude of the specimen’s bearing capacity increases with the increase of slenderness ratio and ranges from 6.89% to 6.48%.(4)Based on the analysis of test and finite element results and in accordance with relevant national codes for steel reinforced concrete,a formula for calculating the horizontal bearing capacity of high-strength square steel tube ultra-high performance concrete columns was proposed.The predicted values from the formula were found to be in good agreement with the test values,verifying the applicability and reliability of the formula.This provides an important theoretical basis and experimental evidence for engineering design.