Several proposals of seismic design methods for base isolated structural systems focus on the development of simplified procedures based on the assumption of a single mode of vibration dominating the response. Numerous analytical studies have shown that such an approximation is acceptable to predict peak displacement demand, but the effects of higher vibration modes on force demand to some structural components may be significant. This paper deals with the seismic response of partially-isolated continuous girder bridges, with abutment restraints and non linear isolation/energy dissipation devices located onto the intermediate piers. A simplified calculation procedure employing an equivalent linearization is firstly illustrated. Non linear dynamic analyses carried out for a number of case studies are utilized to prove that the combination of appropriate displacement shapes and effective damping ratios can lead to sufficiently accurate, yet simple, prediction of peak displacement demand. However, the method may be inaccurate to capture peak values of abutment reactions as well as deck bending moments. It is also found that for a bridge with n intermediate piers, odd vibration modes up to the (n+1)-th should be considered for an acceptable estimation of such peak values
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