Characterization of Nonlinear Dynamics for a Highway Bridge in Alaska

This research studies the nonlinear dynamic feature of the Chulitna river bridge in Alaska USA. The acceleration response of the bridge under moving truck loads are evaluated for dynamic invariants. The Fourier spectrum of a free-decay response (after the vehicle passes) exhibits a salient line spectrum, which is correlated to calculated linear modes from a 3-D finite element analysis. The bridge deck is used shell elements and truss members are simulated with beam elements. At some locations on bridge, the Fourier spectrum of the bridge response to vehicle loading exhibits a complicated narrow bandwidth suggesting the possibility of nonlinear characteristics. Chaos dynamics invariants of the response are examined based on the largest Lyapunov exponents, correlation dimension, and K-entropy and used to identify the nonlinear dynamic features. The bridge was found to have following characteristics: (a) modal parameters extracted from free-decay response exhibit linear properties. The free vibrations have relatively small amplitude which is not sensitive to possible structure damages; (b) at some locations, measured responses to moving loads exhibit chaos nonlinear dynamics properties with specific signatures. All of the derived nonlinear dynamics invariants including Lyapunov exponents, correlation dimension, and K-entropy support this observation; and (c) in addition to linear modal parameters, the multiple nonlinear dynamics invariants with damage-sensitive features can be used as additional candidates for the ongoing bridge health monitoring.