Fatigue Response of a Fracture-Critical Bridge at the End of Service Life

Abstract

A fracture-critical bridge along Interstate Highway 35 with a relatively high volume of traffic had exceeded its original design life and was monitored before and after a repair was implemented to strengthen the bridge. Strain gauges and crack propagation gauges were installed at critical locations near existing fatigue cracks identified from annual manual inspections. The initial data were evaluated and indicated that the fatigue life of the bridge had been exceeded, such that the bridge needed to be repaired or replaced. The bridge owner placed the bridge on a fast-track schedule for replacement; however, because it would take 2 to 3 years to complete the bridge, the bridge owner also elected to strengthen the bridge in the interim. The girders were strengthened by installing bolted cover plates between the top flanges of the longitudinal girder and transverse floor beams. The response of the bridge was monitored before, during, and after the bridge was strengthened. In the strengthened condition, the fatigue behavior of the bridge was dramatically improved due to unintentional composite behavior between the bolted cover plates and the concrete deck. Developing composite behavior in noncomposite, aging bridges may be a potential method of mitigating future fatigue damage.