RELIABILITY OF PIECEWISE LINEAR SYSTEMS SUBJECT TO STOCHASTIC EXCITATIONS K. Perros & C. Papadimitriou, University of Thessaly, Greece ABSTRACT This work investigates the reliability of nonlinear elastic and inelastic systems arising in mechanical and civil engineering applications due to impacts between structural parts. These systems are modeled by single and multi-degree of freedom models with piecewise linear elastic stiffness elements and often involve strong inelastic behavior in parts of the system. In order to gain useful insight into the behavior of these systems, one degree of freedom piecewise linear elastic and inelastic systems are first analyzed and the behavior to long duration transient stochastic excitations is investigated. Using subset simulation method, probabilistic response spectra characteristics and estimates of the sensitivity of these spectra to uncertainties in system and loading parameters, such as initial modal frequency, stiffness ratios, size of gaps, inelastic parameters, damping values, excitation strength and frequency content, are obtained. It is shown that the performance of such systems to uncertain stochastic excitation (sinusoid pulse or earthquake type) can be enhanced by optimally designing the system parameter values. The methodology is used to investigate the reliability of the four-span Kavala bridge (Greece) under stochastic earthquake excitations. The bridge deck is supported on columns through elastomeric bearings, allowing impacts to occur between the deck structure and the piers. Short duration sinusoid pulse excitations with uncertain characteristics as well as white noise stochastic excitations are used to simulate the short and moderate duration earthquake excitations and the sensitivity of the reliability to the size of gaps affecting the behavior of the bridge is explored. 1. INTRODUCTION Nonlinear elastic and inelastic systems with impacts arise in mechanical and civil engineering applications. In mechanical engineering applications, the behavior of the systems with impacts are often analyzed using single or multi degree of freedom mechanical models with piecewise linear elastic stiffness elements [1,2]. The interest concentrates on the response and stability of piecewise linear elastic systems to periodic excitation and it has been shown that these systems manifest complex nonlinear behavior. In civil engineering applications, such systems arise in the analysis of bridges with seismic stoppers [3-5] or the analysis of pounding of adjacent buildings. These systems are represented by single and multi degree of freedom models with piecewise linear elastic stiffness elements that often involve strong inelastic behavior in parts of the system. The present study focuses on the analysis of bridges that involve impacts due to the seismic stoppers designed to effectively withstand earthquake loads and reduce the size of the piers. A simple bridge with seismic stoppers is shown in Figure 1. The bridge deck is connected to the piers by elastomeric bearings and seismic stoppers are added on the pier caps that have a small gap with the deck structure so that the elastomeric bearings are free to move under ambient or traffic loads, while they impact on the stoppers only under moderate or strong earthquake loads. Activation of the stoppers due to impact results in sudden increase of the stiffness of the structure. The gaps between the stoppers and the bearings are usually selected such that the impact with the stoppers occurs before the pier yielding. Assuming a heavy undeformed deck of mass M and representing the stiffness of the piers and the elastomeric bearing by massless linear or inelastic springs, one can construct a single degree of freedom (SDOF) simplified model of the bridge as shown in Figure 2. For the case of stopper activation but no pier yielding, the springs are linear and the simplified system in Figure 2 behaves as a SDOF piecewise linear elastic system. For the case of elastoplastic spring representing the inelastic behavior of the deck, the system in Figure 2 behaves as a SDOF piecewise linear inelastic system.