Structural instability of nonlinear plates modelling suspension bridges: mathematical answers to some long-standing questions

TL;DR

This paper models suspension bridge decks as nonlinear plates, analyzing their oscillation modes and stability, revealing energy thresholds for instability and addressing questions from the Tacoma collapse.

Contribution

It provides a detailed mathematical analysis of nonlinear plate models for suspension bridges, identifying stability thresholds and explaining structural instability phenomena.

Findings

Stable at low energies

Instability at higher energies

Answers to Tacoma collapse questions

Abstract

We model the roadway of a suspension bridge as a thin rectangular plate and we study in detail its oscillating modes. The plate is assumed to be hinged on its short edges and free on its long edges. Two different kinds of oscillating modes are found: longitudinal modes and torsional modes. Then we analyze a fourth order hyperbolic equation describing the dynamics of the bridge. In order to emphasize the structural behavior we consider an isolated equation with no forcing and damping. Due to the nonlinear behavior of the cables and hangers, a structural instability appears. With a finite dimensional approximation we prove that the system remains stable at low energies while numerical results show that for larger energies the system becomes unstable. We analyze the energy thresholds of instability and we show that the model allows to give answers to several questions left open by the Tacoma collapse in 1940.