Assessment of Bridge-Structures Under Fired Impact: A Case Study Approach

This master thesis deals with the behavior of steel girder highway bridges under small vehicle fires. Fire at and below highway bridges are a common problem for bridge structures. Unlike in tunnels or buildings the consideration of fire hazard is no mandatory aspect of bridge design. However, the high amount of fire incidents on highways and the important function that bridges have in the infrastructure resulted in a few case studies to explain the failure process of a bridge. All of these case studies deal with collapses of the bridge structure caused by gasoline tanker fires. The behavior of steel girder bridges under small vehicle fires without a collapse of a bridge is not included in any case study yet. This thesis shall establish understanding of the post-fire behavior of a steel girder under a vehicle fire with several boundary conditions. A special topic in this thesis is the analysis of a micro-crack included in the steel girder. Micro-cracks can be caused by fatigue or by the fabrication of the girder among other things. This is also a first attempt to consider the condition of a highway bridge in the analysis of a steel girder bridge under a fire impact. The fire impacts used in this thesis contains temperatures less than hydrocarbon fires (maximum temperature of 1100 °C), that are used to represent gasoline tanker fires, but higher temperatures than external fires which are not caused by fuel or gas (maximum temperature of 600 °C - 700 °C). The case study provided in this thesis is carried out using the finite element software Abaqus. The structural model of the chosen hypothetical highway bridge is a two span beam. To simplify the numerical model just one steel girder connected to a concrete deck instead of a complete bridge is used. This offers detailed modeling of the structure and the consideration of many possible mechanisms of the structure. The numerical model considers a variation of the fire impact, there location, horizontal restrains and the number of spans of the bridge. The results can also be transferred to different static models of bridges