可变荷载效应占高比重时荷载分项系数取值研究

针对现行可靠度设计统一标准给出的荷载分项系数值对应可变荷载效应比值取值范围较低，缺乏对该荷载分项系数值在可变荷载效应占高比重下适用性的充分考虑的情形，以钢拱架结构模型和RC框架柱模型为例，分析了可变荷载效应比值的取值范围，揭示了可变荷载效应比值将会较高的现象；基于可靠度理论并结合设计约束条件建立了荷载分项系数的计算方法；通过数值分析，求解得到了荷载分项系数值；最后分析了按多国规范用荷载分项系数进行结构设计的可靠指标随可变荷载效应比值的变化规律。结果表明：可变荷载效应占高比重时，按我国规范用荷载分项系数设计的混凝土和钢结构构件的可靠指标低于目标可靠指标较多；而美英两国规范用荷载分项系数的适用性要好于我国规范，但对应的钢结构构件的设计可靠指标仍偏低。从可靠性的角度为近年风、雪灾害中若干结构容易破坏的现象提供了补充解释，同时也为相关规范修订提供参考。

Research on values of load partial factors with large ratios of variable load effects

The current unified standard for reliability design lacks full considerations of the applicability of load partial factors with large variable load effect ratio, due to the values of the current load partial factors that are obtained with low variable load effect ratio. To address this shortcoming, the range of variable load effect ratio was studied after some analyses of steel arch truss and reinforced concrete (RC) columns. It is found that the variable load effect ratio can adopt large values. Then, a computational method for optimum load partial factors was proposed based on the reliability theory and some design restrictions. Using this method, the optimum load partial factors are obtained through numerical calculation. Finally, the reliability analysis was performed with different variable load effect ratios for the structures designed by multiple groups of load partial factors in several national codes. The results indicate that when the variable load effect ratio is large, the reliability is much lower than the target reliability for the reinforced concrete and steel members which designed with the load partial factors in the current Chinese codes. The results also show that in terms of applicability, the load partial factors in the current American or British codes is better than that in the current Chinese codes, although the corresponding reliability of steel members is still low. This gives some complementary explanations that some structures are more likely to fail under snow or wind disaster, and also provides some useful references for the revisions of corresponding codes.