Analysis of a bridge failure due to fire using computational fluid dynamics and finite element models

Highlights

• A steel girder bridge failure caused by a real fire is analyzed using CFD and FE models.

• Numerical models are able to simulate the observed response of the bridge.

• A parametric study is conducted and modeling guidelines are provided.

• Research is valuable for a performance-based approach for bridge fire design.

Abstract

Bridge fires are a major concern because of the consequences that these kind of events have and because they are a real threat. However, bridge fire response is under researched and not covered in the codes. This paper studies the capabilities of numerical models to predict the fire response of a bridge and provides modeling guidelines useful for improving bridge design. To reach this goal, a numerical analysis of the fire of the I-65 overpass in Birmingham, Alabama, USA in 2002 is carried out. The analyses are based on computational fluid dynamics (CFD) for creating the fire model, and finite element (FE) software for obtaining the thermo-mechanical response of the bridge. The models are validated with parametric studies that consider heat release rate of the spilled fuel, discretization of the fire temperature in the transition from CFD to FE modeling, and boundary conditions. The validated model is used in a study to evaluate the influence of fire scenario (CFD versus standard fires), and live load. Results show that numerical models are able to simulate the response of the bridge and can be used as a basis for a performance-based approach for the design of bridges under fire. Additionally, it is found that applying the Eurocode standard and hydrocarbon fires along the full length of the bridge does not adequately represent a real bridge fire response for medium-long span bridges such as this case study. The study also shows that live loads essentially do not influence the response of the bridge.

Introduction

Bridges are a critical component of the transportation system whose loss can result in important social and economical consequences (e.g. Chang and Nojima [1], Zhu et al. [2]). Therefore, a lot of effort has been paid to understand and predict the effects on bridges of accidental extreme load events such as earthquakes, winds, scour, and ship collisions (e.g. Ghosn et al. [3], Cheng [4]). Fire is an additional major hazard in bridges for two reasons. First, traffic on bridges damaged by fire is usually hard to detour and affects the traffic quality in the region. For example, the collapse of two spans of the MacArthur Maze in Oakland, USA on April 29th 2007 due to a fire resulted in repairs and rebuilding operations costing more than US $9 million [5], [6]. Another example is provided by a bridge fire caused by a dump truck in Robbinsville (NJ, USA) on October 3rd 2012. This fire forced to close the Interstate 95 Highway as well as 79 km of the New Jersey Turnpike, one of the major highways in the US East Coast, and affected the traffic in areas located hundreds of kilometers away of the accident in the states of Delaware and Connecticut. The accident also caused serious traffic disruptions for 6 weeks following the event [7]. Secondly, bridge fires are a real threat as shown by data of a voluntary bridge failure survey, which was responded by the departments of transportation of 18 US states [8]. This survey was conducted in 2011 and collected data related to 1746 bridge failures and showed that fire had caused more bridge collapses than earthquakes (seismic states like California participated in the survey).

Despite its importance, bridge fires have got very little attention in the past as proved by Garlock et al. [9]. In fact, fire safety engineering and structural fire engineering have mainly been concerned with building and tunnel fires (e.g. Buchanan [10], Couto et al. [11], Quiel et al. [12], Gunalan and Mahendran [13], López-Colina et al. [14], Moliner et al. [15] and Seif and McAllister [16]), but bridge fires are different to those and deserve a particular approach. This is due to several reasons such as the cause of fire, the fire loads, the fire ventilation conditions, the use of fire protection, and the type of connections among structural members used (see Payá-Zaforteza and Garlock [17] for more details).

Within this general context, and using a case study, this paper (a) delves into the fire response of steel girder composite bridges as this type of bridge is widely used [18] and is especially vulnerable to fire events [9], and (b) illustrates modeling techniques that can be used to predict the fire response of steel bridges. To reach this goal, the authors have performed a numerical investigation of the behavior of the I-65 overpass in Birmingham (AL, USA) during the fire event on January 5th 2002. The event resulted in the demolition of the overpass and the rebuilding of a new structure and affected highways carrying 240,000 vehicles per day. The numerical investigation is based on data provided by the Alabama Department of Transportation (ALDOT) and comprises a fire model of the event using computational fluid dynamics (CFD) techniques with the software FDS [19], and a thermo-mechanical model of the response of the bridge using Abaqus [20]. Numerical results were validated by comparison with the information provided by ALDOT which (a) enables a better understanding of the advantages and the limitations of numerical models to explain the fire response of bridges and (b) paves the way for the use of these models to study the improvement of the fire response of bridges in high fire risk situation. This kind of knowledge is of major importance for two reasons. First, previous research (see e.g. Payá-Zaforteza and Garlock [17], Aziz and Kodur [21]) is scarce and based more on standard fires or predefined fire events, than on the analysis of real cases and therefore has limitations. And second, it is difficult to conduct full scale experimental studies on bridges because of the dimensions of their structural members and the fire loads required.