Italian and European guidelines for seismic design of isolated bridges

In the past few years official guidelines have been issued in Europe and in Italy to regulate the design of (passively) isolated bridges. Explicity covered is the use of elastomeric and elato-plastic elements. The design philosophy aims at ensuring adequate protection with respect to two Limit-States or Perfomance Levels: the first one being the limit of fully elastic response, the second one being a state where the structure is still undamaged while the isolating elements are close to their ultimate deformation. Capcity design measures are adopted to ensure the protection of the structural parts even for larger-than-anticipated seismic events. Detailed requirements on the behaviour of the isolating elements so that they can reliably fulfil their protective role are given. Modelling, methods of analysis and verification criteria complete the content of the Guidelines. Aspects requiring improved definition are discusses in the conclusions of the paper.(AU)

Effects of multi-support excitation on isolated bridges

Multi-support seismic excitation can affect the response of isolated bridges, and particularly the perfomance of the isolating devices, which are generally designed for synchronous action. Three phenomena are of major interest for bridge structures, denoted as: a) incoherence, b)wave-passage, c)site-response, of which the former two, grouped under the term 'nonsynchronism', have been object of an extensive parametric study in the past. Here, the third phenomenon is addressed, which is due to spatially varying local soil profiles, and its effects on the bridge response are evaluated. The technique adopted is that of a random vibration analysis of linear systems, equivalent to the examined bridges equipped with hysteretic-type isolating devices, whose response is represented through linear elements of equivalent stiffness and damping.(AU)