| Concrete Filled Steel Tube(CFST)has become very common in engineering.This type of specimen construction has the characteristics of high bearing capacity,good seismic performance,regular shape,strong adaptability of base,convenient construction,less environmental pollution and remarkable economic benefits.With the rapid growth of our national economy,it is widely used in high-rise buildings,Bridges,piers and transmission towers and other civil engineering structures in frequent earthquake areas,and has become one of the most important structural types to resist earthquake load.Among them,square concrete-filled steel tubular columns are widely applied in practical projects because of their regular shape and easy connection.However,the restraint effect of square steel tube outer wall on core concrete is much lower than that of circular steel tube outer wall,especially when they under high axial compression ratio,the tube wall is prone to buckling.Therefore,the ductility of square concrete filled steel tube column will be greatly reduced,which is significantly lower than that of circular concrete filled steel tube column.The application of concrete-filled square steel tube(CFST)columns in seismic structures with high intensity is severely limited.In order to improve the wall stability of concrete-filled square steel tube,high strength spiral stirrup can be set inside the concrete-filled square steel tube column to enhance its restraint effect on the core concrete and effectively improve the seismic performance of the member.For the sake of systematically study the seismic performance of reinforced concrete columns with high strength spiral stirrup and square steel pipe,low cycle and repeated tests were carried out on 9 columns with high strength spiral stirrup,3 columns with ordinary strength spiral stirrup and 3 columns with ordinary concrete square steel tube.The analysis was carried out from the test phenomenon,failure mode of the column,hysteretic curve,strain development of square steel pipe and spiral stirrup,and the comparison was made between each type of column to complete the parameter analysis of the test piece,and to investigate the influence of spiral stirrup spacing,spiral stirrup yield strength,axial compression ratio of the test piece and other parameters on the seismic performance.And compared with the theoretical ultimate bending moment bearing capacity of ordinary concrete-filled square steel tube column under the current norms,and then studied the role of the built-in spiral stirrup,to provide a basis for engineering applications,and completed the following work:(1)Pseudo-static tests were carried out on 15 specimens of columns.The specimens were brought to failure state by applying cyclic load.Based on two main parameters,spiral stirrule spacing and axial compression ratio,the final failure mode,hysteretic characteristics,skeleton curve,strain distribution of the specimens were studied in detail.The effect of high strength steel(HSS)spiral on the seismic performance of helical confined concrete filled steel tube(HSS-CFST)columns under high axial compression ratio is studied.(2)The experimental results indicated that(ⅰ)apparent outwardly local bucking of the steel tube and core concrete crushing were observed for all specimens in the plastic hinge regions at the ultimate state;(ⅱ)adding both the NSS and HSS spirals would beneficial in improving the hysteretic behavior and energy dissipation capacity of square CFST columns,but contributions to these improvements made by the HSS spirals were more significant than that of the NSS ones;(ⅲ)the increase of axial load ratio and internal steel spiral pitch would impair the load-carrying capability and ultimate deformation ability of square CFST columns;(ⅳ)ductility of the NSS-CFST and HSS-CFST columns could be improved as the increased of spiral volume ratio and the ductility enhancement was more pronounced in the HSS-CFST columns;and(ⅴ)the internal steel spirals were helpful in delaying the stiffness degradation rate of square CFST columns,particularly for those specimens under a high axial load ratio of 0.8. |
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