Monitoring and Assessment Program for Wabasha County Bridge

A 143-m (470-foot) span steel truss bridge, the Wabasha County Bridge crosses the Mississippi River at Wabasha, Minn. In November 1996, the Minnesota Department of Transportation (MnDOT) implemented a retrofit strategy to mitigate perceptible vibrations in several truss members at moderate and strong wind gusts. In this strategy, MnDOT installed a "central cord" of tubular members, halfway between top and bottom cords, to reduce the effective length of the truss members, thereby increasing the natural frequencies of vibration and reducing the amplitude of vibration and the associated strains. This report documents the monitoring and assessment program used to investigate the dynamic response and efficacy of the retrofit strategy for the Wabasha Country Bridge. Researchers determined amplitudes and frequencies of the vibration for the longest diagonal member. The measured frequencies are larger than those estimated before the retrofit and have resulted in reduced strains and displacements from vibration. Maximum strain levels at the quarter point of the member are estimated to be small after the retrofit, with peak values corresponding to 8.6 MPa (1.2 ksi).