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In this paper, a comprehensive probabilistic framework is proposed and adopted to perform seismic reliability and risk analysis of existing link slab (LS) bridges, representing a widely diffused structural typology within the infrastructural networks of many countries worldwide. Unlike classic risk analysis methods, innovative fragility functions are used in this work to retrieve more specific and detailed information on the possible failure modes, without limiting the analysis to the global failure conditions but also considering several intermediate damage scenarios (including one or more damage mechanisms), and providing insights on the numerosity of elements involved within a given damage scenario. Reliability analyses are performed on a set of LS bridges with different geometries (total lengths and pier heights) designed according to the Italian codes enforced in the 1970s. Accurate numerical models are developed in OpenSees and Multiple-Stripe nonlinear time–history analyses are carried out to build proper demand models, from which fragility functions are determined according to two limit states: damage onset and near-collapse. Mean annual rates of exceeding are thus estimated through the convolution between the hazard and the fragility. The results shed light on the main failure mechanisms characterizing this bridge typology, highlighting how different levels of risk (hence safety margins) can be associated with failure scenarios that differ in terms of elements/mechanisms involved and damage extension. Such a higher level of detail in the risk analysis may be useful to better quantify post-earthquake consequences (e.g., costs and losses) and define more tailored retrofit interventions. A comparison of the reliability levels associated with bridges of the same class with different geometries is finally presented. [ABSTRACT FROM AUTHOR]

The seismic assessment of existing bridges is of the utmost importance to characterise the main structural deficiencies, estimate the risk, prioritise retrofit interventions, or estimate losses and repair costs in case of earthquakes. The above tasks require information on the damage mechanisms likely to occur as well as on the damage extent over the structure. Such types of information are generally not provided by classical fragility analysis, which is mainly focused on the evaluation of the global performance of the bridge. In this paper, a systematic probabilistic methodology for the evaluation of bridge fragility is proposed. The methodology aims at offering insight into the failure mechanisms most likely to occur and the evolution and extent of damage within the bridge structure. First, a mathematical description of the proposed analysis methods is given, then an application to a realistic case study—a reinforced concrete multi-span simply supported deck link-slab bridge—is provided to illustrate the applicability of the tool. A nonlinear 3D finite element model is developed, and a multiple-stripe (nonlinear dynamic) analysis is performed by using a stochastic bidirectional seismic input. The results highlight the suitability of the proposed methodology to reveal the main structural deficiencies, the relations among different failure mechanisms (involving piers, bearings, abutments, etc.), and the expected damage extent. [ABSTRACT FROM AUTHOR]

Current practical approaches for probabilistic seismic performance assessment of structures rely on the concept of intensity measure (IM), which is used to decompose the problem into hazard analysis and conditional seismic demand analysis. These approaches are potentially more efficient than traditional Monte-Carlo based ones, but the performance estimates can be negatively influenced by inadequate setup choices. These include, among the others, the number of seismic intensity levels to consider, the number of structural analyses to be performed at each intensity level, and the lognormality assumption for the conditional demand. This paper investigates the accuracy and effectiveness of a widespread IM-based method for seismic performance assessment, multi-stripe analysis (MSA), through an extensive parametric study carried out on a three-story steel moment-resisting frame, by considering different setup choices and various engineering demand parameters. A stochastic ground motion model is employed to describe the seismic hazard and the spectral acceleration is used as intensity measure. The results of the convolution between the seismic hazard and the conditional probability of exceedance obtained via MSA are compared with the estimates obtained via Subset Simulation, providing a reference solution. The comparison gives useful insights on the influence of the main parameters controlling the accuracy and precision of the IM-based method. It is shown that, with the proper settings, MSA can provide risk estimates as accurate as those obtained via Subset Simulation, at a fraction of the computational cost. [ABSTRACT FROM AUTHOR]

Highlights • Effect of uncertainty in dampers properties stemming from manufacturing tolerances. • Reliability-based optimization method used as tool for sensitivity analysis. • Hybrid probabilistic approach employed to speed up the reliability analyses. • Seismic risk of a steel building with viscous dampers with variable properties. Abstract Damping and isolation devices are often employed to control and enhance the seismic performance of structural systems. However, the effectiveness of these devices in mitigating the seismic risk may be significantly affected by manufacturing tolerances, and systems equipped with devices whose properties deviate from the nominal ones may exhibit a performance very different than expected. The paper analyzes this problem by proposing a general framework for investigating the sensitivity of the seismic risk of structural systems with respect to system properties varying in a prescribed range. The proposed framework is based on the solution of a reliability-based optimization (RBO) problem, aimed to search for the worst combination of the uncertain anti-seismic device parameters, within the allowed range of variation, that maximizes the seismic demand hazard. A hybrid probabilistic approach is employed to speed up the reliability analyses required for evaluating the objective function at each iteration of the RBO process. This approach combines a conditional method for estimating the seismic demand at a given intensity level, with a simulation approach for representing the seismic hazard. The proposed method is applied to evaluate the influence of the variability of the properties of linear and nonlinear fluid viscous dampers on the seismic risk of a low-rise steel building. The study results show that the various response parameters considered are differently affected by the damper properties and unveil the capability of the proposed approach to evaluate the potentially worst conditions that jeopardize the system reliability. [ABSTRACT FROM AUTHOR]

This article describes the structural design, nonlinear modeling, and seismic analysis of prototype single-storey non-residential steel buildings made of moment-resisting portal frames in the transverse direction and concentric braces in the longitudinal direction. Various design parameters (building geometry, seismic hazard, foundation soil category) and different modeling assumptions (bare frame model, model including cladding elements, ground motions including vertical accelerations, and modeling uncertainties) were considered to investigate their effects on the simulated seismic performance. [ABSTRACT FROM AUTHOR]

• Investigation of the fluid viscous dampers (FVDs) ultimate capacity on the seismic reliability of building structures. • Advanced FVD model adopted to account explicitly for both the end-stroke attainment and the brittle failure of devices. • Parametric study considering low-rise and medium-rise steel buildings equipped with linear and nonlinear dampers. • Different values of safety factors for stroke and forces are analysed. • Probabilistic study performed via multiple stripe analysis and demand hazard curves provided for several demand parameters. Anti-seismic devices should be designed with proper safety margins against their failure, because the reliability of the structural system where they are installed is strongly influenced by their reliability. Seismic Standards generally prescribe safety factors (reliability factors) amplifying the device responses at the design condition, in order to reach a target safety level. In the case of Fluid Viscous Dampers (FVDs), these factors are applied to the stroke and velocity, and their values are not homogeneous among seismic codes. This paper investigates the influence of the values of the safety factors for FVDs on the reliability of the devices and of the structural systems equipped with them. An advanced FVD model is employed to account for the impact forces arising when the dampers reach the end-stroke and the brittle failure due to the attainment of the maximum force capacity. The effect of damper failure on both the fragility and the seismic risk of the structural system is investigated by performing multiple-stripe analysis and monitoring different global and local demand parameters. In particular, a parametric study has been carried out, considering two case studies consisting of a low-rise and a medium-rise steel building, coupled with a dissipative system with linear and nonlinear properties and studying the consequences of different values of safety factors for stroke and forces. The study results give evidence to the potential brittle behaviour of the coupled system and provide information about the relationships between damper safety factors and effective structural reliability. Some preliminary suggestions are given on possible improvements of current design approaches and on the values of the reliability factors to be considered for future code revision. [ABSTRACT FROM AUTHOR]