United States Department of Transportation
i
Characterizing Design-Basis Fire Exposure for Highway Bridges
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2024-12-19
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[PDF-5.43 MB]
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Edition:Sept. 2021 – April 2024
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Abstract:This report addresses the development of design-basis fire exposure levels for highway bridges, which can define the thermal load in a performance-based structural-fire engineering framework. Several approaches are presented for characterizing the spatial and temporal thermal impact of open-air fires from both vehicles and stationary fuel sources under or near highway bridges. A vulnerability assessment is performed to establish spatial influence zones for structurally significant fire sizes (i.e., fires that are large enough to cause structural damage) based on their heat release rate (HRR) as well as their proximity and orientation to the bridge’s primary structural elements. The frequency of a structurally significant fire hazard is characterized according to the type of primary structural element, its location relative to other roadways, and the traffic conditions on those roadways. A reliability-based design fire load is then stochastically determined in terms of maximum incident heat flux. The most critical parameters that influence the design fire load are the standoff and clearance from the closest roadways or other fuel locations to a given bridge span. A closed-form equation is developed to quantify the design fire load for a bridge span as a function of these two parameters.
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