By Ignacio Paya-Zaforteza
ICITECH, Universitat Politècnica de València
TPF Getinsa Euroestudios S.L, Madrid
Fire protection engineering has mainly focused on two areas: buildings and tunnels, which have their own standards and about which a large number of studies have been published. What about other types of infrastructure, such as bridges? Are they outside the scope of fire studies? One could think that the probability of a bridge being affected by a fire does not merit special study, but is this really so?
Can a Bridge Actually Collapse Due to a Fire?
Recent studies have shown that fires are a real threat to bridges. Garlock, et al., and Peris-Sayol, et al., reviewed 11 and 154 fire incidents on bridges, respectively, between 1997 and 2015, in which nine steel bridges and 15 wooden bridges collapsed after fires and 35 others were so badly damaged that they had to be replaced by new structures.
From previous studies, it could be inferred that collapses only occurred in the case of steel bridges, due to their low resistance to high temperatures, or in wooden bridges because they are built of combustible material. However, recent incidents such as the one in Atlanta on March 30, 2017, when a concrete bridge collapsed, shows that this is a general problem on our roads and motorways, which makes it important to analyze this type of incident. It should be understood that bridge fires are mainly an economic problem and a public nuisance when bridges have to be closed to traffic, more than a life safety problem, since the authors have not found any reported fatality during a bridge fire. (Fatalities have happened, but as the result of the traffic accident that caused the fire, not of the bridge fire itself.)
Can the risk of a bridge collapse be assumed?
Bridges are critical elements of transport infrastructures whose disappearance can have a serious economic impact, not only in regard to the direct cost of repairs or replacement of the structure, but also in terms of the indirect costs of closing a bridge, which can substantially exceed the direct cost in bridges built at strategic points.
The most significant example is the collapse of the MacArthur Maze interchange in Oakland, California, in April 2007, when a fuel tanker caught fire below the bridge and caused the collapse of two spans, after which the interchange remained closed to traffic for 26 days. The economic impact of the closure was estimated at $6 million per day, for a total of $156 million, which was 17 times greater than the direct cost of reconstructing the bridge.
A similar case happened in Rouen, France, in 2012, when there was a fire at the Mathilde Bridge. Although it did not collapse, its resistance was seriously compromised and the section affected by the fire had to be rebuilt. This meant closing the bridge for 22 months while the work was carried out and caused the city many problems. In this case, the direct cost of the repairs was €8 million, while the indirect costs estimated from the claims made by the businesses affected were €10 million.
These two cases highlight the problem of bridge fires and how important it is to prevent them.
Figure 1. Real Incidents (left): bridge in Baltimore, Maryland, USA, October 2012, image courtesy of Renewable Fuels Association; (right) MacArthur Maze collapse, Oakland, California USA, April 2007, image courtesy of Robert Campbell.
Are There Any Standards for These Cases?
In spite of the frequency and consequences of bridge fires, they have been the subject of very few studies and are neglected in the different international bridge design standards (e.g., Eurocodes and AASHTO code). One of the few documents to contain information about this aspect of bridge design is NFPA 502, “Standard for Road Tunnels, Bridges and other Limited Access Highways,” which provides recommendations to be applied in bridges more than 300 m long in very general terms. However, it does not indicate the fire loads that should be considered, how the bridge should be analyzed structurally after a fire, nor how the structure should be protected. These recommendations are, thus, not a great deal of help to the engineers charged with preventing the possibility of bridge fires.
What Should Be Done?
Four types of engineering are involved in this topic:
- Risk and Safety Engineering, to analyze structural fire risks and the possible consequences of a bridge collapse to determine whether the risk is acceptable and, if not, apply preventive measures.
- Structural Fire Engineering, to estimate fire loads and study the resistance of the structure to fires.
- Fire Protection Engineering, to apply the necessary fire protection measures.
- Forensic Engineering, to improve the analytical capacity and decision making about the damage to a bridge after a fire and to minimize its direct and indirect costs.
These four engineering areas must be combined to achieve an efficient design. The first step is to decide on the method to be used in analyzing the risks to identify the preventive measures to be used, similarly to the method used to analyze fire risks in tunnels. The types of structure most vulnerable to fire should also be identified, as should the design parameters that affect their resistance. Finally, effective and affordable protection measures should be studied that are appropriate to the most unfavorable scenarios considered and validated by experiments or numerical modeling to ensure the bridge’s fire-resistance capacity.
Diverse studies have recently been published on these lines of action: Kodur, et al., and Kim, et al., focused on risk analysis; Davidson, et al., indicated the passive fire protection measures required in transport infrastructures; and a team led by Professor I. Paya-Zaforteza carried out fire tests at the Universitat Politècnica de València in Spain (Alós-Moya, et al.[8, 9]) on a composite bridge. A large number of studies have specifically analyzed the structural behaviour of bridges in fires either by mathematical models or computer simulations.
Different methods have been developed by the various research groups to evaluate the fire load as a first step in obtaining the structure’s response to a fire. These methods can be classified into three main groups:
In the first group are those that propose the use of standardized curves for the fire load, whether this be the hydrocarbon curve, the standard curve or a specially designed curve (Paya-Zaforteza and Garlock; Liu, et al.; Aziz, et al.; and Whitney, et al.).
The second group includes those that propose the use of analytical methods to obtain temperature distributions in beams from the fire parameters (size, load, material, etc.), such as Bennets and Moinuddin; Quiel, et al.; and Yanagisawa, et al.
In the third group are those that carry out Computational Fluid Dynamics (CFD) studies, including the bridge and its surroundings, to obtain the temperatures in the gas surrounding the structure. This group is reflected in the studies by Wright, et al.; Gong and Agrawal[18, 19]; Alós-Moya, et al.; Wang and Liu; and Peris-Sayol, et al
Whatever method is used to assess the fire load, temperatures in the structure and its times and failure modes in most of the published studies are obtained by means of finite element models (FEM) that analyze the thermo-mechanical behaviour of the bridge considering temperature dependent material properties. Some of these methods have been tried out in real cases, such as the fires on I-65 in Birmingham in 2002[17, 20], the MacArthur Maze in Oakland in 2007[23, 25], the Ed Koch Queensboro Bridge in New York, the Mathilde Bridge in Rouen and the Lazienkowski Bridge in Warsaw. These studies have shown that advanced numerical models are capable of reproducing the failure modes of actual fire incidents and therefore can be used to analyze the possible fire protection measures.
Figure 2. Numerical Simulations (left): CFD Fire Simulation (Fire Dynamics Simulator); (right) structural response of a bridge (Abaqus; Peris-Sayol, et al. ).
Events in Bridges: a Future Guide
These facts show that there is a lack of standards and directives for solving the bridge fire problem and that this type of incident can bring serious economic consequences in its wake. Although several research groups have carried out studies of the subject, these have lacked coordination in the methods used. For this reason, the International Association for Bridge and Structural Engineering (IABSE) recently created Working Group 12 for the Design of Bridges Against Fire Hazards with the aim of drawing up guidelines to minimize the risks associated with fires in bridges, recommend measures to protect bridges against fires and guidelines for engineers who have to assess the structural safety of a bridge after suffering a fire. It is hoped that this guide will be help those involved in such projects and contribute to reducing the damage caused to bridges and prevent collapses such as those of in Atlanta, Hazel Park in Detroit or MacArthur Maze in Oakland.
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