柱下九桩厚承台承载性能试验研究

Experimental study on the load-carrying properties of nine-pile thick caps under a column

为了进一步研究在多排多列桩情况下桩基厚承台的承载性能,共制作了4个缩尺比例为1∶10的九桩钢筋混凝土承台模型试件并进行了破坏性加载试验。对荷载与位移(挠度)关系、承台侧表面混凝土应变特征及承台内部应力进行了系统地分析,并将试验所得破坏荷载与目前常用设计方法进行比较分析,揭示了9桩厚承台及一般多排多列桩厚承台符合空间桁架模型的受力特点,即配置在桩顶范围内的钢筋充当拉杆,桩顶至柱下区域的混凝土作为斜压杆。加载初期,承台处于受弯状态,混凝土裂缝通常在弯矩较大区域的承台底部最先出现,并随荷载增加沿水平方向及竖向发展。随着裂缝的出现,底部受拉区混凝土逐渐退出工作,当加荷值达到破坏荷载的50%以上后,钢筋拉杆作用增强,桩径范围内各根钢筋应力沿长度方向的变化很小,在设计中可认为近似相等,空间桁架体系逐步形成,距桩中心连线0.75倍桩径范围内的钢筋能充分发挥拉杆作用。

The objective of this investigation is to study the load-carrying properties of thick caps with multiple piles. Four 1：10 scale reinforced concrete nine-pile caps were cast and tested to failure. The correlation between load and displacement （deflection）, the concrete strains of the lateral surface of the pile caps and the internal stresses were analyzed. Furthermore, the failure loads of these specimens were predicted by using some of the present design methods, and comparative analysis with the experimental failure loads conducted. The analytical results revealed that the load- transferring mechanism can be approximated by a strut-and-tie model, with the zones of concrete with primarily unidirectional compressive stresses modeled by compression struts and the principal reinforcement modeled by tension ties. At the early loading stage, the pile cap was in bending, and cracks appeared in the region with large bending moment and developed along horizontal and vertical directions with increasing loads. When the load reached up to 50% of the failure load, the reinforcements played the role fully. As far as the same reinforcement was concerned, strains at various locations tend to be of a uniform value, which can be taken as approximately equal values in design. The space truss model was formed gradually, with the tie function of the reinforcement arranged within a range of 0.75 times of the pile diameter brought into play.