Chaos theory analysis of cable-stayed bridge vibration

Cable-stayed bridges are a recent development in bridge structural design in which the cables meet the bridge deck at an acute angle rather than perpendicularly. Some early cable-stayed bridges have exhibited large amplitude stay cable oscillations. One such bridge, the Fred Hartman Bridge across the Houston Ship Channel in Texas displayed two different modes of vibration: a local mode involving independent motion of individual cables and a global mode, in which the cables vibrated collectively under certain wind and rain conditions. This abrupt shift in mode as a function of a change in environmental parameters suggests chaotic behavior. Analysis of the probability density function of maximum accelerations of the cables typically showed a fractal power law distribution at lower values, but also some sharp changes in the tails. The Lorentz Map plots of the data also indicated two regimes: a dissipative one at lower acceleration values and chaotic behavior beyond a critical acceleration value. The plots also imply that the chaotic system is nearly one-dimensional. The working hypothesis is that steady winds impose additional stresses on the stay cables that push them over the boundary into the chaotic regime where random impulses from falling raindrops become amplified into cable oscillations.