Transverse versus Longitudinal Eigenperiods of Multispan Seismically Isolated Bridges

This paper is motivated from the wider need in system identification studies to identify and interpret the eigenvalues of seismically isolated bridges from field measurements. The paper examines the transverse eigenvalues of multispan bridges which are isolated in both transverse and longitudinal directions at all supports including all center piers and end abutments. The paper shows that regardless of the value of the longitudinal isolation period of the deck, the length of the bridge, and the number of spans, the first transverse (isolation) period is always longer than the longitudinal isolation period of the deck. This result cannot be captured with the limiting idealization of a beam on continuously distributed springs (beam on a Winkler foundation) which yields the opposite result of the first transverse period always being shorter than the longitudinal isolation period. This fundamental difference between the response of a flexural beam supported on distinct, equally spaced springs and that of a beam supported on continuously distributed springs has not received the attention it deserves in the literature of structural mechanics-dynamics. Finally, the paper shows that the first normalized transverse eigenperiod of any finite-span isolated deck follows a single master curve and the solutions from all configurations are self-similar and are not dependent on the longitudinal isolation period or on whether the deck is isolated on elastomeric or spherical-sliding bearings.