A methodology for scaling non-linear force-displacement behavior of elastomeric isolators for design

Abstract

Base isolation systems may perform differently in practice than in theoretical predictions due to factors such as environmental conditions, aging, temperature fluctuations, scragging, and the natural variability of rubber materials and the manufacturing processes. These factors significantly influence the mechanical behavior of base isolators, affecting their characteristic strength, damping, and stiffness, which consequently impacts the performance of both the isolation system and the superstructure. Most standards emphasize that isolator design should not rely solely on nominal values, leading to the introduction of property modification factors, commonly referred to as λ-factors, to account for the variability in the mechanical behavior of base isolators. These factors were originally defined for bilinear or trilinear force-displacement behaviors. Their application to more complex models has been theorized without clear guidance on how they should be implemented. This study presents a method for scaling the effective stiffness, K_eff, and enclosed hysteresis area, W, in the force-displacement behavior of isolators for complex numerical models such as the Bouc-Wen model, modified Bouc-Wen model, and the algebraic model, which are commonly used for modeling elastomeric isolators. Unlike characteristic strength, Q_d, and post-yield stiffness, K_d, which are typically employed in bilinear and trilinear models, the K_eff and W were chosen because they offer a more general and practical representation of highly nonlinear devices, which often lack well-defined Q_d and K_d values. The proposed approach can independently scale the K_eff and W within the ranges specified by AASHTO, ASCE 7–22, and ASCE 7–41. This approach can be extended to other isolator mechanical properties and numerical models.