Seismic reliability‐based ductility demand evaluation for inelastic base‐isolated structures with friction pendulum devices

Abstract

Summary The aim of this work is to propose seismic reliability‐based relationships between the strength reduction factors and the displacement ductility demand of nonlinear structural systems equipped with friction pendulum isolators (FPS) depending on the structural properties. The isolated structures are described by employing an equivalent 2dof model characterized by a perfectly elastoplastic rule to account for the inelastic response of the superstructure, whereas, the FPS behavior is described by a velocity‐dependent model. An extensive parametric study is carried out encompassing a wide range of elastic and inelastic building properties, different seismic intensity levels and considering the friction coefficient as a random variable. Defined a set of natural seismic records and scaled to the seismic intensity corresponding to life safety limit state for L'Aquila site (Italy) according to NTC08, the inelastic characteristics of the superstructures are designed as the ratio between the average elastic responses and increasing strength reduction factors. Incremental dynamic analyses (IDAs) are developed to evaluate the seismic fragility curves of both the inelastic superstructure and the isolation level assuming different values of the corresponding limit states. Integrating the fragility curves with the seismic hazard curves related to L'Aquila site (Italy), the reliability curves of the equivalent inelastic base‐isolated structural systems, with a design life of 50 years, are derived proposing seismic reliability‐based regression expressions between the displacement ductility demand and the strength reduction factors for the superstructure as well as seismic reliability‐based design (SRBD) abacuses useful to define the FPS properties. Copyright © 2016 John Wiley & Sons, Ltd.

Seismic Reliability-Based Design of Inelastic Structures With Different Values of the Post-Yielding Stiffness Isolated by FPS

“…Note that in the case of softening systems, assumed as the equivalent model representative of multistory frame taking count of the P effect [38], [39] the abovementioned behaviour factor q represents only the ductilitydependent term strength reduction factor because overstrength capacities, assumed with the equivalent model, are completely absent. Therefore, the reduction factor or behavior factor, hereinafter denoted as q, is consistent with the code provisions [22]- [26] in the case of a unitary overstrength factor and with the one discussed by [31] for the case of softening systems.…”

“…Coherently with the PEER-like modular approach [47] and performance-based earthquake engineering (PBEE) approach [48]- [49], the uncertainties related to the seismic input intensity are separated from those related to the characteristics of the record (record-to-record variability) by introducing an intensity measure (IM). The approach, as also discussed in [20], [31], is based on calculating the convolution integral between the fragility curves and a seismic hazard curve, expressed in terms of the same IM, related to a reference site, in order to define the mean annual rates exceeding the limit states and, next, using a Poisson distribution, evaluate the exceeding probabilities in the time frame of interest (e.g., 50 years). Superstructure properties are not included as random variables as discussed in [31], [50].…”

“…Furthermore, [29] and [30] proved that the equal displacement rule and the equal energy rule cannot be used for isolated systems. Reliability-based relationship between the ductility-dependent strength reduction factors and the displacement ductility demand are proposed by [31] for a perfectly elastoplastic structural system equipped with FPS and located in L'Aquila site (Italy) for different structural system properties.…”

“…The aim of this work is to further advance the study of Castaldo et al [31] evaluating the seismic reliability of non linear base-isolated systems with hardening and softening post-yield stiffness and for increasing strength reduction factors.…”

Optimal Friction Coefficients for Isolated Structures Depending on the Soil Condition

“…[18]- [25]) have been more oriented to develop design approaches for the isolators and to identify the optimal isolator properties. In this context, Castaldo et al [18]- [19] proposed a seismic reliability-based design (SRBD) criterion to define the isolator dimensions in plan based on the evaluation of performance curves for the isolator and the superstructure. Bucher [25] proposed a reliability-based design approach for the bearings accounting for the probabilistic performance properties in terms of maximum and residual isolator displacements, and the maximum interstorey structural displacement.…”