Natural Frequencies and Modes of Suspension Bridges

The natural frequencies of vibration and the corresponding mode shapes of a stiffened suspension bridge are determined, and the changes that occur in these characteristics with respect to parametric changes in the bridge are examined. A single‐span, planer structure with hangers and flexible cable supports is used to model a suspension bridge. The derivation of the governing equation of motion for the model is sufficiently general to be applied to a multispan system; however, only a single‐span case is studied. The use of flexible cable supports enables the single‐span model to represent a typical interior span of a multi‐span structure. The method of analysis is based upon a generalized Holzer method coupled with a linearized form of a discretized initial‐value formulation. Parameter studies are presented in which the vibration characteristics are studied as a function of cable sag, deck moment of inertia, and cable support stiffness. Of particular interest is the phenomenon in which there is a reordering of modal configurations that accompany the sequential array of frequencies in response to changes in certain parameters.