Seismic reliability of isolated bridges

The study evaluates the seismic reliability of isolated multi-span continuous deck bridges considering the influence of the friction pendulum isolators with the aim of providing also seismic reliability-based design abacuses as a function of both the structural properties and reliability level expected. The behavior of these systems is analyzed by employing five degrees of freedom accounting for five vibrational modes of the elastic reinforced concrete pier and a single degree of freedom in order to take into account the response of the infinitely rigid composite deck, isolated by the seismic devices. Finally, the reinforced concrete abutment is modelled as a fixed support above which the FPS device is located. The non-linear FPS response is described also considering the velocity-dependent behavior. The uncertainty in the seismic input is taken into account by means of a set of natural records with different characteristics scaled to increasing intensity levels. The uncertainty on the friction coefficient is also taken into account through an appropriate probability density function. Firstly, fragility curves of both the reinforced concrete pier and isolation system supporting the deck are evaluated and subsequently, in line with the hazard curve of the reference site, the corresponding seismic reliability curves are computed by means of a probabilistic convolution integral. Finally, seismic reliability-based design abacuses for different structural properties are proposed.