23 Dec Fire Safety for Bridges
Currently, there are no specific provisions in standards for the fire design of bridges, neither in terms of protection, nor from developing fire mitigation strategies in bridges. Peter Stephenson, Business Development Manager, and Doctor Mostafa Jafarian PhD MSc, Technical Manager – Structures & Façades, Warringtonfire examine the current status and consider potential solutions.
Various research conducted around fire safety for bridges [1-7], carried out following real events, demonstrates that fire on bridges can result in major issues for the transportation infrastructure.
Fire safety within the built environment, particularly for building design, is recognised and covered by regulations, codes and standards. However, for bridge design, fire safety is not recognised nor as developed, even though a significant incident involving a bridge failure can cause significant economic damage impacting on a region. In fact, while current codes and standards do not specify any provisions for fire resistance of load bearing structural members in bridges, bridge fires can often lead to substantial losses.
From a fire safety perspective, the capability to identify the primary failure modes of a bridge, subjected to a credible fire scenario based on assumed fire loading and fire exposure risk, requires specialist input from competent fire engineers. Whether a fire occurs on or below a bridge structure, a risk-based approach to cost effective fire protection measures should be documented. This should occur during the design process or as part of the on-going risk management of the bridge lifecycle and infrastructure management [1,5].
Bridges are designed and constructed using various materials and many bridges have been in situ for several years.
There is a common misconception that bridges made of wood or steel are the only types that can be destroyed or damaged by fire. This is based on the fact that steel bridges have a relatively low resistance to high temperatures and wooden ones are built of combustible materials. However, recent incidents on concrete bridges prove these fire-resistant structures can be impacted by extreme heat.
A recent example is when firefighters in Rome, Italy attended an incident in the early hours of 3 October 2021 involving the Ponte Romano dell’Industria, known as Ponte di Ferro – a 19th-century iron bridge. The area under the bridge was reported to have been a “squatter settlement” where a fire was reported, which impacted on the bridge and it was subsequently declared unusable after the fire.
One other well-documented incident happened on 30 March 2017, when a concrete bridge, the 1-85 North Bridge in the Buckland area of Atlanta, Georgia collapsed during a fire incident. Expert analysis determined it took less than 30 minutes from the fire starting for the structure to fail. The impact of such bridge fires and their significant economic impact is typified by the following:
- Complete structures or elements have to be rebuilt or repaired.
- Businesses lose revenue because they’re not accessible to customers.
- Traffic has to be diverted, making travel inconvenient for local residents, travellers, and delivery services.
On the other hand, due to the fact that the nature of fire on a bridge is different than what is traditionally used to test fire protection products, the traditional methods of protection may not be promptly available. Thus, for those cases further structural analysis, in addition to thermal data gathering from a relevant fire test regime, would be required.
The recommendations for fire protection of buildings, which are contained in current codes, cannot be readily used for bridges due to different fire loads and configurations etc. So why has fire safety of bridges lagged behind when considering the economic impact of an unusable bridge? It is often assumed that the likelihood of fire breakout in, on or under bridges is very small and hence it is not justifiable to fire protect all bridges. And a small percentage of bridge fires damage bridge structural members only, thus life safety of commuters is not usually at risk.
While those assumptions would be sufficient for a building, they ignore the primary facts around fire safety of bridges as part of the main infrastructure :
- If a bridge gets damaged, maintenance/reconstruction costs of damaged bridge components would be considerably high, and;
- Interruption in traffic will cause delays and creates a requirement for traffic redirection, which would lead to indirect costs as well as other financial difficulties.
What can be done
The main issue with bridge fire safety can be summarised as the interruption of daily activities, but the full impact is dependent on the bridge type and location, and the renovation cost of the damaged construction.
To address this point, it has been suggested by different researchers [1,7] that it is important to:
- Identify the critical bridges based on risk-based evaluation;
- Provide sufficient level of protection for them based on a structural fire engineering analysis.
Guidance can be found within NFPA 502, Standard for Road Tunnels, Bridges and Other Limited Access Highways, published by the National Fire Protection Association, which provides general recommendations for bridges that are approximately 1,000 feet (30.5m) long or more. However, the code doesn’t address many important issues, including:
Loads that bridges should be able to handle when exposed to fire.
Recommendations for protecting bridges from damage caused by credible fire scenarios and extreme heat.
Inspection and evaluation recommendations after bridges are damaged by fires.
As discussed above, current regulations simply provide too little guidance for engineers, designers, and inspectors. There are four main areas of engineering that are addressing these issues:
1. Risk and safety engineering studies investigating structural risks when bridges are exposed to credible fire scenarios, along with the consequences of bridge collapses. The aim is to determine acceptable levels of risk and what cost-effective risk mitigation measures can be employed to minimise the bridge risk profile.
2. Structural fire engineering analysis to estimate fire scenarios/loads and associated studies to determine the fire resistance of bridge structures to minimise fire-related damage.
3. Fire protection engineering assessments and recommendations to determine cost effective fire protection measures.
4. Forensic engineering analysis to improve the ability to assess bridges damaged by fires and improve decision making about how to make repairs to minimise direct and indirect costs.
These four engineering areas currently work independently and to optimise future bridge fire safety, a competent fire engineer should coordinate these activities to protect the public and minimise the economic impact of bridge fires. The engineer should also consider how risks should be evaluated and which preventative measures are applied to different types of bridges. A good model to follow is how fire risks are evaluated, addressed, and prevented in tunnels. By following this example, bridge fire risk can be mitigated, saving time, costs and inconvenience.