Title:

Decreased Load-Carrying Capacity Assessment of a Vertical-Lift Steel Truss Bridge

Over a bridge’s service life, physical damage, such as material deterioration, accidental impact, and overloading, can decrease its load-carrying capacity. To assess the integrity and serviceability of in-service bridges, accurate and efficient structural health monitoring is required for as frequent intervals as feasible. Based on an integrated decision-making protocol, collected dynamic-response data of a bridge can be postprocessed continuously to notify the bridge owners of any noticeable change in the in-service condition of the structure. Therefore, the location and severity of damage can be determined, and the bridge load-carrying capacity based on the given damaged component(s) is calculated. The case study for this protocol is an instrumented vertical-lift truss bridge, the Memorial Bridge in Portsmouth, NH subjected to some simulated damage scenarios. For both the healthy and simulated damage conditions, a verified structural model is used as a baseline to investigate the bridge load-carrying capacity according to current standards for load resistance factor-rating approach. Once the source of the damage is identified, the impact on the bridge performance must be assessed. The accepted means for this assessment is to calculate the load rating according to the AASHTO Manual for Bridge Evaluation. The Load and Resistance Factor Rating (LRFR) method defines three distinct levels of evaluation: design-load rating, legal-load rating, and permit-load rating. The focus of this study is on the design inventory level rating factor using an HL-93 live load and the strength design 1 load combination. To predict whether the damaged members of the bridge satisfy the AASHTO design specifications for the effects of axial forces and bending moments, analytical investigations are performed for different percentages of damage.

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