Experimental Study On Axial Compression Behavior Of Square Columns Confined By Circular Spiral Composite Stirrups

The circular composite spiral stirrup is composed of ordinary square stirrup in the outer ring and spiral stirrup in the inner ring;Although the ordinary square stirrup can effectively restrain the corner concrete,it has no obvious effect on the middle concrete,while the spiral stirrup can effectively restrain the middle core concrete,but it causes a great degree of waste on the corner concrete.Therefore,the reasonable configuration of circular composite spiral stirrup can theoretically improve the bearing capacity of concrete columns Deformation capacity and ductility;The bottom column often bears the largest axial or eccentric load in the building.In practical engineering,high-strength concrete and steel bars are often used to improve the bearing capacity of the bottom column.At this time,the use of composite stirrups can effectively save materials and costs,and greatly reduce the waste of resources,so it is of great significance.In this paper,six concrete columns are designed and tested under axial compression with the core of controlling the similarity of stirrup eigenvalues,stirrup spacing and concrete strength as variables.This paper proposes that the bearing capacity and ductility of concrete columns with circular composite spiral stirrups will be improved in different degrees compared with ordinary columns.The failure mode,crack developing speed and the failure phenomenon of steel bar before and after yielding were studied.The results show that the circular composite spiral stirrups can improve the mechanical properties of the columns,and the concrete strength and stirrup spacing are the key variables affecting the mechanical properties of the columns.The numerical improvement of the ultimate bearing capacity of different specimens is greater than the contribution to the bearing capacity brought by simply improving the concrete strength;The strength of concrete plays a leading role in ductility,and the effect of Stirrup on ultimate compressive strain is better than that of concrete strength.Through the nonlinear simulation analysis of seven designed models by ABAQUS,the results show that the whole concrete column presents "lantern like" deformation,the stress concentration appears at the corner,the larger plastic damage is always concentrated in the middle of the concrete,the overall restraint stress of spiral stirrup and ordinary stirrup is higher,and the concrete strength needs to match with the stirrup strength to make full use of each other,And when the stirrup eigenvalues are similar,the greater the concrete strength is,the greater the bearing capacity is.Based on the test and analysis,the axial bearing capacity formula of the column with indirect reinforcement is improved,and the calculation formula of the normal section bearing capacity of the column with circular composite spiral stirrups is proposed.The influence of the double stirrups on the improvement of the axial compressive strength of concrete is considered in the formula.The effectiveness of the proposed method is strengthened by comparing with the simulation value and the experimental value.