摩擦摆隔震层的瞬时偏心与扭转反应分析

Transient Eccentricity and Torsional Responses of Base Isolation Layers of Friction Pendulum Bearings

摩擦摆隔震技术具有隔震周期与上部建筑的自重无关、自带摩擦耗能能力、隔震层没有初始偏心等优点,正逐渐被我国工程界所接受。但是,由于摩擦摆支座的水平刚度与其所受竖向力成比例,上部结构在水平地震作用下产生的倾覆力矩会使摩擦摆支座的竖向力发生变化,从而导致隔震层刚度中心偏移并可能使隔震层发生扭转。以一个摩擦摆隔震原型建筑的地震反应分析为例,说明现行有关规范中采用的偏心率无法正确反映摩擦摆隔震层的扭转反应特性。通过推导上部结构倾覆力矩和摩擦摆隔震层偏心距之间的关系,证明只有当上部结构在两个正交水平方向上受到的地震作用沿高度分布不一致时才会引起隔震层产生扭转反应,并在此基础上定义了"瞬时偏心率"以更好地描述摩擦摆隔震层的扭转反应。具有不同摩擦系数的摩擦摆隔震建筑对不同频谱特性水平地面运动作用的动力反应分析表明,与窄频简谐激励相比,宽频的实际地震动作用更容易引起摩擦隔震层的瞬时偏心和扭转,且随着摩擦系数的增大,摩擦摆隔震层的扭转反应表现出先减小后增大的趋势。尽管如此,在本文算例中,最大瞬时偏心率均小于3%,相应的隔震层扭转角均小于0.005°,其对上部结构地震反应的影响可以忽略不计。

Friction pendulum bearings(FPBs)are having a more application in seismic isolated buildings in China,because they exhibit weight-independent period,designable frictional energy dissipation and make the isolation layer free from accidental eccentricity.However,the overturning moment of the superstructure under horizontal earthquake actions may lead to the shift of the stiffness center of the isolation layer by changing the lateral stiffness of the FPBs,which is proportional to the vertical force,and thus introduce torsion to the isolation layer.Through the seismic response analyses of a prototype building isolated by FPBs,this paper presents that the initial eccentricity ratios in orthogonal directions do not correspond to the torsional responses of FPB isolation layers.It is proved that the torsion of an FPB isolation layer takes place only if the heightwise distributions of the lateral forces in the superstructure differed in the two orthogonal horizontal directions.A transient eccentricity ratio is then defined to describe the transient movement of the stiffness center of the isolation layer.The results of the seismic response analyses show that broadband earthquake excitations are more likely to stimulate the torsional responses of FPB isolated building than narrowband excitations,and the maximum torsional responses exhibit a non-monotonic relationship with the friction coefficient.The maximum transient eccentricity is less than 3% and the maximum rotation of the FPB isolation layer is less than 0.005°.Their impact on the seismic response of an FPB isolated building can be negligible in engineering practice.