墩基础与柱刚度比 [双柱墩地震反应的轴力-刚度耦合作用]
摘 要：地震作用下钢筋混凝土双柱桥墩产生的动轴力不仅会改变桥墩的屈服强度，而且也会相应地改变桥墩的刚度，但目前广泛使用的集中塑性模型一般并未考虑这种轴力刚度的耦合作用。以自动计入轴力强度及轴力刚度耦合作用的纤维模型分析结果为基准，研究了轴力刚度耦合作用对钢筋混凝土双柱桥墩地震反应的影响。结果表明，轴力刚度耦合作用对双柱墩弹性阶段的反应有较大影响，但不改变结构的极限承载力；由于拉压桥墩的刚度互为消长，等高双柱墩的地震位移响应在不计轴力刚度的耦合作用时与实际情况只略有差异，但墩柱弹性内力响应则会被较大地低估；不等高双柱墩的刚度受较矮侧桥墩控制，因此轴力刚度的耦合作用对结构的地震位移响应和弹性内力响应都有明显影响，桥梁不规则性越大影响越显著，在进行结构分析时不能忽视这一因素的影响。
关键词：钢筋混凝土；双柱墩；地震反应；集中塑性模型；纤维模型；轴力刚度耦合作用
中图分类号：TU 375
文献标志码：A
文章编号：1674-4764（2012）04-0066-06
Axial Force-Stiffness Interaction in Seismic Analysis of RC Double-Column Bridges
WEI Bin1,2, LI Jianzhong1
(1.Department of Bridge Engineering, Tongji University, Shanghai 200092, P.R. China;
2.Guangdong Communication Group Testing Center, Guangdong 510550, P.R. China)
Abstract:Earthquake induced dynamic axial force in reinforced concrete (RC) bridge bent columns will not only change the yield strength of the columns but also change their stiffness, which is seldom considered by the common lumped-plasticity line model. Based on the fiber element model results that taking into account the influence of dynamic axial force on strength and stiffness simultaneously, the axial force-stiffness interaction effect on the seismic responses of RC double-column bridges was analyzed. The results show that, axial force-stiffness interaction has a large effect on the seismic responses of the double-column bridge in the elastic range, and it does not alter the ultimate capacity of the columns. Since the stiffness of the columns under compression and tension dynamic axial forces offset each other, the global displacement of bridge bent with equal columns is relatively unaffected by the axial force-stiffness interaction, however, the differences of the column member forces are manifest. For the short column controls the global stiffness, the axial force-stiffness interaction has significant influences on both the global displacement and member force responses. The influences become larger as the irregularity of the bridge bent increases, so the interaction between axial force and member stiffness should be sufficiently considered in seismic analyses.
Key words:reinforced concrete; double-column bridge; seismic response; lumped-plasticity model; fiber element model; axial force-stiffness interaction
双柱桥墩及框架结构立柱地震时会产生动轴力的作用，其中轴力的变化对构件屈服强度的影响已为人所熟知［1］。近年来的研究表明，钢筋混凝土构件的截面屈服曲率对动轴力及配筋率等的变化不敏感，其强度和刚度间大致存在一种正比关系［2］。因此轴力变化时，钢筋混凝土构件的刚度也会发生与轴力强度（P-M）类似的耦合作用，即存在轴力刚度的耦合作用。
在实际的地震反应分析与设计中，考虑到模型建立、计算时间及结果解读等的复杂性，一般尽量采用集中塑性的杆单元进行非线性分析。对集中塑性单元，轴力强度的耦合作用可以通过设置塑性铰的屈服包络球来方便地表示，这在商业有限元程序（如SAP2000［3］）中已有成熟的考虑方法。但对于绝大多数的有限元程序，集中塑性杆单元的刚度独立于所受的外荷载，即不能考虑轴力刚度的耦合作用。更为精细的纤维模型将钢筋混凝土截面离散为大量具有实际非线性应力应变关系的单元，可以自动计入轴力强度及轴力刚度耦合作用［2, 4］，但对计算机时和结果处理的要求也大大增加［5］。