Effect of Damaged Structural Members on Performance Degradation of a Vertical Lift Truss Bridge

The presence of viable alternative load paths is potentially a vital property of redundant structures such as truss bridges as it permits redistribution of internal forces in case of failure or removal of main structural elements throughout the superstructure. The effects of members’ strength on the load carrying capacity of a gusset-less truss bridge, the Memorial Bridge in Portsmouth, NH, is investigated. A calibrated structural model of the bridge is subjected to variable loads as well as progressive damage levels to quantify the effects of damaged members on the bridge loading capacity. Multiple damage scenarios are simulated incrementally for each truss member by reducing its cross-sectional area. For each damage scenario, the maximum load carrying capacity of the bridge is defined as the maximum applying force for which the first structural member exceeds its compressive or tensile yielding. To assess the safety and serviceability of the bridge, estimations are made by introducing a residual redundancy factor as the load carrying capacity ratio of the damaged and intact states of the bridge. While the redundancy factor of one implies the healthy condition of the bridge, the factor less than one signifies the presence of damage and loss of performance. This paper facilitates detection of the most critical structural components to degradation of the system load carrying capacity. It is shown that the less the redundancy factor is, the more likely the failure of the corresponding damaged truss member affects the bridge load carrying capacity and hastens its potential collapse.

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