The Chinese code GB 50010—2010 Code for Design of Concrete Structures gives a formula to calculate the axial compression capacity of confined concrete columns by using the yield strength of spiral and welded ring stirrups

and it assumes that the stirrups have yielded under the peak compressive stress of confined concrete. In fact

the stirrups can yield only when the volume stirrup ratio exceeds a certain limit. Meanwhile

the code has not codified the case for square concrete columns confined by grid stirrups. To explore the tensile stress level in grid stirrups under peak loading

we conducted axial compression tests on 42 high-strength concrete square columns confined by grid stirrups. The axial compressive strength of the unconfined concrete ranged between 50.0 MPa and 68.0 MPa

and the stirrups were made of HRB400

HRB500

HRB600

PC800

PC1 270

and steel wire with standard tensile strength of 1 570 MPa. Results show that majority of the stirrups did not yield under the peak compressive stress of the confined concrete. The axial compressive strength of concrete

volume stirrup ratio

and hoop spacing had great impact on the stress level of stirrups. On the basis of the test data of unyielding stirrups under peak compressive load

a formula for calculating the actual tensile strain of stirrups under peak compressive load was established

and a method of calculating the lower limit value of volume stirrup ratio for stirrups capable of yielding under peak compressive load was proposed. By considering the effect of the actual tensile stress of stirrups on peak compressive stress and strain of the confined concrete

the full curve equation for the compressive stress-strain relationship of grid stirrups confined high-strength concrete was established

which provided reference for the engineering application and calculation of square concrete columns confined by grid stirrups.