板厚与基础高度按冲切或直剪计算的“分界荷载”值与规范的安全度

Safety of Punching Calculation in Codes and the Demarcation Load of Punching and Cutting Ravage for Calculating Thickness of Slab and Height of Foundation

通过公式推导 ,得到板厚及基础高度由冲切或直剪公式计算的“分界荷载”值 ,即当荷载小于“分界荷载”值时 ,板厚及基础高度由冲切计算公式控制 ,当荷载大于“分界荷载”值时由直剪计算公式控制 ;“分界荷载”值随着混凝土强度等级提高、柱截面 (板局部荷载边长 )尺寸增大、地基承载力特征值降低而增大。我国混凝土结构设计规范对板及基础仅有冲切计算公式 ,偏于不安全 ,建议增加直剪计算公式。对比分析不同版本的混凝土结构设计规范的冲切计算 ,安全度 (安全系数 )越来越低 ,容易引发直剪破坏事故 ,应引起重视。

The thickness of slab and height of foundation are controled by punching ravage when the load is less than “demarcation load”,or vise versa by cutting ravage. The demarcation load increases with the increasing of concrete intensity grade, section of column(length of part loads of slab) and the reducing of characteristic of sub-ground bearing capacity. There is only the formulae of punching ravage in the code. This paper suggests adding formulas of cutting ravage to the code. Contrasting calculation results of the 1974,1989,2002 editions of the codes, the safety of calculating of punching ravage is lower and lower, and the accident of cutting ravage would occur easily.