Fire Effects on Suspension Bridge Main Cables

Fire resistance design and analysis is an under-studied and under-codified area of bridge engineering. With the lessening of conservatism in bridge design, the aging or our bridge infrastructure, and the increase in the ground transport of highly-flammable and -combustible materials, it is essential that the bridge engineering community better understand and incorporate methods for modeling the effects of fire on bridges. Typical fire resistance analysis looks at the response of individual structural components. Analysis for the component of a bridge is nowhere more important than for that of the main cables of suspension bridges. As such, we will survey and introduce the necessary analysis techniques to provide the bridge engineering community with the knowledge and tools to understand fire modeling and both rapidly and accurately assess their effects on suspension bridge main cables. The work of this dissertation is twofold. In the first portion, we address proper fire modeling techniques for bridge conditions and apply them in a sequential thermal-mechanical analysis of a three-dimensional model main cable with thermally-dependent material and mechanical properties. Although fire modeling has been addressed in a variety of scenarios, including extensive studies for building design and analysis as well as tunnel design and analysis, the types of fires, fire geometries, and air conditions associated with bridge fires vary drastically. Our work identifies the time to failure for our particular main cable example and subsequently compares both the temperature and strain distributions for temperature-dependent and temperature-independent models. Although the three-dimensional analysis is complete, we hope to emulate and expand on the work done in the building fire engineering community and bring to the literature methods to produce significant two-dimensional temperature distributions for when a main cable component is either partially or fully-exposed to fire. As such, the main fire modeling analyses mentioned in the previous paragraph lay the groundwork for our pursuit of closed-form analytical solutions necessary to rapidly and accurately assess the time-dependent temperature distribution within a cable cross-section exposed to fire. These solutions are formed with different approaches depending on the fire scenario in question. They include a separation of variables (eigenfunction) approach, sinusoidal transforms, Laplace transforms, Green's function solutions, and a semi-analytical hybrid method. We validate each of the approaches numerically using three different fire models.