UHPC梁受剪性能试验与抗剪承载力计算方法

为研究超高性能混凝土（Ultra-high Performance Concrete，UHPC）薄腹梁受剪性能和抗剪承载力计算方法，设计制作11片模型梁开展荷载试验，试验参数包括纤维率、纤维种类、配箍率、剪跨比和混凝土强度。分析了试验梁破坏形态、裂缝开展过程和主要因素对梁体受力响应影响规律。试验结果表明：UHPC梁的受力过程分为弯曲开裂前弹性阶段、"桥联作用"失效前和"桥联作用"失效后3个阶段。UHPC梁剪切破坏具备一定延性且有明显征兆，为半延性-半脆性破坏。由于纤维"桥联作用"，UHPC梁剪切开裂后呈多条剪切裂缝同时开展现象，破坏过程伴随着纤维持续从基体里拔出的"滋滋声"。此外，配置适量箍筋可使梁体破坏模式从脆性剪切破坏向更具延性的弯曲破坏转变。基于Rankine破坏准则，推导出剪压区混凝土简化强度准则；考虑T形截面翼缘的影响，提出腹板抗剪有效宽度计算方法；通过极限平衡法，得到考虑翼缘影响的混凝土抗剪贡献计算式。基于分项叠加思想，建立考虑混凝土、箍筋和纤维抗剪贡献的UHPC梁抗剪承载力理论计算式。该公式形式简单，物理意义明确，可以考虑纤维率、剪跨比和梁体尺寸等影响因素。用试验结果对提出的计算式进行验证，得到抗剪承载力理论计算值和试验值比值均值为0.94，标准差为0.17，计算结果表明提出的计算式可以较好地预测UHPC梁的抗剪承载力。

Experimental and Theoretical Investigations on Shear Strength of UHPC Beams

To investigate the shear behavior and shear strength calculation method of ultra-high performance concrete (UHPC) thin web beams, 11 beams were tested. The test parameters included fiber ratio, fiber type, stirrups ratio, shear span to depth ratio, and concrete strength. The failure mode, crack distribution, and effects of the main parameters on the structural responses were analyzed explicitly. The test results show that the structural response of UHPC beams can be divided into three stages, namely, the elastic stage before flexural cracking, the stage before failure of the fiber bridging effect, and the stage after failure of the fiber bridging effect. The shear failure of UHPC beams is a semi-ductile-semi-brittle failure with some ductility and an obvious signal. Multiple cracks propagate simultaneously after shear cracking due to the bridging effect of fibers, and an audible sizzling sound, corresponding to the fibers being pulled continuously from the matrix, can be heard during the failure process. In addition, moderate stirrups can change the failure mode from shear to a much more ductile flexural failure mode. The simplified failure criterion of the compression zone concrete was obtained based on Rankine's failure criteria, and the effective web width was proposed with consideration of the flange effect. The equation of the shear contribution of concrete was then derived based on the limit equilibrium method. A shear strength model for UHPC beams was proposed using the concept of the superposition method with consideration of the shear contribution of concrete, stirrups, and fibers. The proposed formula is simple in expression with clear physical meaning and takes several substantial factors into account, including fiber ratio, shear span to depth ratio, and beam geometry. Finally, the proposed formula was verified by the test results. The average value and the standard deviation of the ratio of the proposed to experimental results are 0.94 and 0.17, respectively. The result indicates that the proposed shear strength model is applicable to UHPC beams.