The breakaway friction coefficient of curved surface sliders (CSSs) governs the transition between the sticking and the sliding behavior of the isolators, and hence affects the response of an isolated building during an earthquake. When the inertia forces induced by low-to-moderate intensity excitations are not able to overcome the breakaway frictional resistance of the CSS isolation system, the structure behaves as a fixed-base building, thus experiencing higher accelerations, interstorey drifts and internal forces than the isolated building. The majority of structural analysis programs disregard the static coefficient of friction, and implement the dynamic friction coefficient only throughout the response history analysis, which may imply an increased prediction of displacement demand for the isolation system but may lead to an unsafe design for the superstructure. In this paper, the frictional resistance to sliding before the breakaway is simulated through a bidirectional plasticity domain, which has been coded in a finite element of the isolator formulated in OpenSees to incorporate the transition between the breakaway friction in the sticking phase and the velocity-dependent friction model in the subsequent sliding phase. Based on this formulation, the influence of the breakaway friction on the response of buildings isolated with CSSs is investigated numerically through an extensive parametric study comprising more than 9000 bidirectional nonlinear time history analyses (NLTHAs). The parameters cover a range of friction coefficients, superstructure properties and a large group of natural spectrum-compatible bidirectional ground motions having different intensity levels and frequency contents. The results from NLTHAs are processed statistically to elaborate regression formulae and design recommendations that can be useful to predict the trigger acceleration at which sliding motion starts, as well as to suggest how to achieve more accurate estimates of the seismic response when the breakaway friction is ignored in the structural analysis model.

曲面滑动器（CSS）的脱离摩擦系数决定着隔离器的黏着和滑动行为之间的过渡，因此影响了地震中隔震建筑的响应。当中低强度激发所引起的惯性力无法克服CSS隔振系统的分离摩擦阻力时，该结构的作用就像固定基础的建筑物，因此，其承受的加速度，层间漂移和内力均大于孤立的建筑。大多数结构分析程序都忽略静摩擦系数，而仅在整个响应历史分析中才实现动摩擦系数，这可能意味着增加了对隔离系统位移需求的预测，但可能导致上部结构的设计不安全。在本文中，通过双向可塑性域模拟了分离前的滑动摩擦阻力，该域已在OpenSees中制定的隔离器的有限元中进行了编码，以纳入粘滞相中分离摩擦与速度-之间的过渡。后续滑动阶段的摩擦模型。基于此公式，通过广泛的参数研究，包括9000多个双向非线性时程分析（NLTHA），以数值方式研究了分离摩擦对用CSS隔离的建筑物的响应的影响。这些参数涵盖了一系列的摩擦系数，的超结构特性和一大批具有不同强度水平和频率含量的自然频谱兼容双向地震动。对NLTHA的结果进行统计处理，以拟定回归公式和设计建议，这些建议可用于预测开始滑动运动的触发加速度，并建议在忽略分离摩擦时如何更准确地估计地震响应在结构分析模型中。