Ambient Vibration of Two Suspension Bridges

The dynamic characteristics (i.e., normal mode natural frequencies, damping, and shapes) of two suspension bridges are estimated from experimentally measured ambient motion time histories. These characteristics were estimated using random vibration theory and the power spectral densities of ambient time histories. Twenty normal modes of vibration were observed in the 0 cycle per min-60 cycle per min frequency range for the Newport Bridge in Rhode Island. These modes include nine vertical, six lateral, and two torsional deck modes, plus three longitudinal tower modes. For each mode, estimates are obtained of natural frequency of vibration, damping, and shape. For the other suspension bridge, the William Preston Lane Memorial Bridge, 13 normal modes of vibration were observed. These modes are six vertical, six lateral, and one torsional deck mode. No tower modes were observed in the 0 cycle per min-60 cycle per min frequency range.