Mechanical Behavior And Load-Bearing Capacity Calculation Of Concrete Columns Confined With Stirrups

The load-bearing capacity and deformability of concrete columns can be improved by the lateral confinement of stirrups.It is important for the load-bearing capacity calculation of concrete columns by determining the lateral stress of stirrups at peak load.In general,the great volumetric ratio of stirrups is needed to ensure that stirrups can reach the yield strength at peak load when concrete columns with high compressive strength of unconfined concrete and the high yield strength of stirrups.When stirrups do not reach the yield strength at peak load,the lateral confinement of stirrups and the load-bearing capacity of confined concrete columns will be overestimated if the yield strength of stirrups is used to calculate lateral confining stress of stirrups.In the process of concrete columns confined with stirrups with low confinement degree,the stirrups may be fractured in the descending branch of the load-deformation curves of specimens,causing the decrease of the load-bearing capacity and axial deformability of confined concrete.It is necessary to explore the mechanical behavior of concrete columns confined with stirrups by the axial stressstrain relationships of confined concrete with the effects consideration of stirrups types,confinement degree,unconfined concrete compressive strength,and stirrups fracture.Thus,it is of great theoretical significance and practical value to study on the mechanical behavior and load-bearing capacity calculation of concrete columns confined with stirrups.The dissertation mainly carries out the following work:(1)156 circular concrete columns confined with spiral stirrups were tested under axial compression and corresponding finite element analysis were carried out,meanwhile,141 square concrete columns confined with mesh stirrups were tested under axial compression and corresponding finite element analysis were carried out.The test parameters included the the compressive strength of concrete(25MPa～155.45 MPa),the yield strength of stirrups(for hot-rolled steel bar: 360MPa～1215 MPa,for medium strength prestressed steel wire: 818 MPa～1190 MPa),and the volumetric ratio of stirrups(0.9%～2.5%).The test results indicated that when the compressive strength of unconfined concrete was kept at the same value,with the increase of the confinement index,the deformability of concrete columns confined with stirrups was improved,the lateral expansion deformation of concrete was increased,and the stirrups tended to reach yield strength at peak load.When confinement index was kept at the same value,with the increase of the compressive strength of unconfined concrete,stirrups could not reach yield strength easily.When confinement index and the compressive strength of unconfined concrete were at the same value,the stirrups of the confined concrete column with medium strength prestressed steel wire as the stirrups were easy to fracture after the peak load.When the compressive strength of unconfined concrete was kept at the same value,with the increase of confinement index,the stirrups were easily to be fractured after peak load.When the compressive strength of unconfined concrete and the yield strength of stirrups were kept at the same values,the peak stress and peak strain of confined concrete were increased with the increase of the volumetric ratio of stirrups.When the yield strength and the volumetric ratio of stirrups were kept at the same values,the increased range of the peak stress and peak strain of confined concrete was decreased with the improvement of the compressive strength of unconfined concrete.When stirrups yielded at peak load,the peak stress and peak strain of confined concrete were increased with the improvement of the yield strength of stirrups in the same condition of the compressive strength of unconfined concrete and the volumetric ratio of stirrups.When stirrups did not yield at peak load,the effects of the stirrups on the load-bearing capacity of columns was not sufficient,but the effects of stirrups made the descending branch of the compressive stress-strain curves of confined concrete more gentle.(2)When stirrups did not yield at peak load,the lateral strain of stirrups at peak load was increased with the increase of the volumetric ratio of stirrups,while the lateral strain of stirrups at peak load was decreased with the improvement of the compressive strength of unconfined concrete.Take(effective confinement coefficient×stirrups volumetric ratio × stirrups elastic modulus÷unconfined concrete compressive strength)and(unconfined concrete compressive strength÷stirrups elastic modulus)as independent variables,the calculation formulas of the tensile strain and tensile stress for hot-rolled stirrups and medium strain stirrups at peak load were established,respectively.Let the tensile strength of stirrups equal to its yield strength(nominal yield strength),the formula for calculating the minimum volumetric ratio of stirrups required for the hot-rolled stirrups confined concrete column at peak load and the medium strong stirrups without obvious yield point was obtained.(3)The stirrups may be fractured in the descending branch of the compressive stress-strain curves of concrete columns confined with stirrups under axial compression.Take(the ratio between effective lateral confinement and the compressive strength of unconfined concrete)and the compressive strength of unconfined concrete as independent variables,the under surfaces to determined whether stirrups fractured for hot rolled steel bar and medium strength steel wire were obtained,respectively.Take the compressive strength of unconfined concrete and effective lateral confinement as independent variables,the fitting formulas of the axial stress and axial strain of confined concrete when stirrups fractured were obtained.(4)Take the compressive strength and the effective lateral confinement as independent variables,the fitting formula of the peak stress of confined concrete was established.Similarly,the fitting formulas for the peak strain of confined concrete,the axial strain of confined concrete when the stress dropped to the 85% peak stress of confined concrete,and the axial strain of confined concrete when the stress dropped to the 50% peak stress of confined concrete were obtained.The ultimate strain of confined concrete was the minimum value between the axial strain of confined concrete when the stress dropped to the 50% peak stress of confined concrete and the axial strain of confined concrete when stirrups fractured.The equation for the axial stress-strain curves of confined concrete with the confinement index between0.33%～24.22% was obtained by considering the effects of the compressive strength of unconfined concrete,the type of stirrups,the confinement degree and the fracture of stirrups.