高强混凝土方柱不同强度网格箍筋约束效果

Behavior of high-strength concrete square columns confined by grid stirrups with different strengths

GB 50010—2010《混凝土结构设计规范》中给出了螺旋箍筋和焊接环式箍筋作约束箍筋的约束混凝土圆柱轴心受压承载力计算公式,认为柱达到受压承载力时约束箍筋一定屈服,而事实上只有体积配箍率超过某个限值时箍筋才能屈服,同时规范未涉及网格箍筋约束混凝土方柱。为考察网格箍筋约束混凝土方柱达到轴心受压承载力时箍筋的拉应力水平,完成了混凝土轴心抗压强度为50.0~68.0 MPa,箍筋分别采用HRB400、HRB500、HRB600、PC800、PC1270、抗拉强度标准值为1570 MPa钢丝的42根网格箍筋约束高强混凝土方柱的轴心受压试验。试验结果表明:在约束混凝土柱达到轴心受压承载力时,存在箍筋不屈服的现象,混凝土轴心抗压强度、体积配箍率和箍筋间距对箍筋应力水平的发挥有较大影响。基于峰值受压荷载下箍筋未屈服的试验数据,建立了峰值受压荷载下箍筋实际拉应变计算公式,提出了峰值受压荷载下箍筋能够屈服的体积配箍率下限值计算方法。通过考虑箍筋实际拉应力对约束混凝土峰值压应力和压应变的影响,建立了网格箍筋约束高强混凝土受压应力-应变关系全曲线方程,为网格箍筋约束混凝土方柱的工程应用及计算方法的完善提供了参考。

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,PC1270,and steel wire with standard tensile strength of 1570 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.