Protection of Cable-Stay Bridges from Accidental and Man-Made Fire Hazards: A Rational Physics-Based Approach to Analyzing Vulnerabilities and Mitigations

The last two years have witnessed a number of new cable-stay bridge design awards in the US, many of those bridges call explicitly for the development of a Threat, Vulnerability and Risk Assessment (TVRA) and for a design to mitigate the effects of improvised explosive devices and accidental or intentional fire threats, typically tankers. The subject of intentional fire effects on bridge has received little attention compared to blast effects. Prescriptive approaches that rely on a rated duration when exposed to a UL 1709 hydrocarbon fire curve are poorly correlated to specific fire threat scenarios which must be addressed in a TVRA. A simple application of such traditional approaches can yield designs that are potentially uneconomical, but more importantly can steer the engineer away from more thoughtful and resilient designs. A performance-based design offers a more rational risk-based approach that rewards good design practices. However, performance-based design requires a rigorous physics-based approach to analyzing vulnerabilities and assessing mitigations. This paper examines a rational approach as it applies to critical components of a cable-stay bridge. It discusses accidental and intentional fire threats, critical bridge members that could be susceptible to such events, physics-based computational analysis methods to accurately evaluate the consequences of such events, and candidate mitigation measures using performance based criteria. This methodology has been applied to actual cable-stay bridge designs in the US. Examples in this paper however are developed for generic yet representative bridge components.