承台冲切破坏的空间桁架传力机理研究

进行了5个对照组,总计8个1∶5四桩承台冲切破坏的模型试验与有限元分析,试件均按空间桁架理论设计,配筋形式包括集中于桩顶区域布置纵筋,布置腹部水平钢筋网与配置型钢桁架。研究结果表明:承台冲切破坏的传力体系为带有主要单向压应力区域的混凝土作压杆,桩顶区域间的纵筋作拉杆的空间桁架;以我国现行规范与ACI 318-08规范中的临界截面应力法为代表的上限解法在传力机理方面存在理论欠缺;当承台与桩筏筏板的破坏形式为冲切破坏时,可依据空间桁架传力机理,合理布筋,创造出性能优良的新型结构。给出了与试验结果吻合良好的承台与桩筏筏板的开裂荷载公式。

Spacial strut-and-tie model for punching load transfer mechanism analysis of pile-cap

Two way shear failure of four-pile caps was studied by testing and FEM analysis of eight 1∶5 scale model specimens in five compared groups.All specimens were designed according to spatial strut-and-tie model.The reinforcement included the bottom reinforcement concentrated in the vicinity of the pile top,the horizontal reinforcement grid in the middle part of the pile cap and section steel truss.According to the results,the load transfer mechanism under the two way shear failure of pile cap is formed by compression struts which are used to model the zones of concrete with primarily unidirectional compressive stresses,while the tension ties are supplied by the longitudinal reinforcement on the top of pile.The upper bound solution,per the Chinese code and the stress method of the critical section in the ACI 318-08 code,has theoretical shortage to describe the load transfer mechanism.Based on the spatial strut-and-tie load transfer mechanism,the pile cap and the pile-raft slab under two way shear failure can be designed by the rational reinforcement to bring about possible new structural configuration.The crack load formulae of pile cap and pile-raft slab are also derived and found in good agreement with test results.