NFPA Journal®,  July/August 2008

By Jason R. Gamache

The latest information learned from recent road tunnel fires and research projects reveals that vehicle fires in tunnels are likely to develop more rapidly than expected, degrade the tenability of an environment more quickly than originally calculated, burn for longer periods and at higher temperatures than expected, and resist firefighting operations.

The 2008 edition of NFPA 502, Road Tunnels, Bridges and Other Limited Access Highways, includes revisions that further clarify the categorization of road tunnels and revisions to ventilation and tenable environments, protection of structural elements, hazardous goods transport, and design fire size. The discussion topics on fixed fire suppression systems in the Annex have also been revised.

Fire suppression systems
Although the use and effectiveness of fixed firefighting systems in road tunnels were not universally accepted in the past, they are now seen as effective in controlling a fuel road tunnel fire by limiting fire spread. The 2008 edition of NFPA 502 permits but does not require the use of fixed firefighting systems in road tunnels. Annex E contains responses to the major concerns expressed by tunnel designers, engineers, and authorities worldwide about such systems’ use and effectiveness in road tunnels.

Protection of structural elements
In the 2004 edition, NFPA 502 introduced new requirements for protecting structural elements. In the 2008 edition, requirements were added to further support this function of the tunnel as it pertains to user safety. For example, structures must now be capable of withstanding the Rijkswaterstaat (RWS) time/temperature curve or any other curve acceptable to the authority having jurisdiction.

Tenable environment
The Annex material relating to tenable environment has been completely revised in the 2008 edition to correlate with the material in NFPA 130, Fixed Guideway Transit and Passenger Rail Systems. The purpose of the Annex is to provide guidelines for evaluating tenability in tunnel evacuation paths. Environmental conditions, geometric considerations, and time considerations should be taken into account.

Regulated and unregulated cargoes
Recent road tunnel fires suggest that goods traditionally not characterized as hazardous, such as flour, margarine, paint, and tires, may constitute a greater risk to tunnel users and structures than expected. As a result, Chapter 13, "Control of Hazardous Materials," has been retitled "Regulated and Unregulated Cargoes" to provide guidance on developing rules for all cargoes traveling through a tunnel.

Design fire size
Large-scale fire tests, such as the Runehamar tunnel tests conducted in 2003, have shown that vehicle fires in tunnels are likely to develop more rapidly and have higher peak heat-release rates than expected. The results of these tests have led to the revision of Table A.10.5.1, "Fire Data for Typical Vehicles," which now reads:

Car......................................................................................5 to 10 MW
Multiple-Passenger Cars (2 to 4 vehicles)..........................10 to 20 MW
Bus...................................................................................20 to 30 MW
Heavy Goods Truck.......................................................70 to 200 MW
Tanker...........................................................................200 to 300 MW

There are substantial increases in the values of 20 MW for buses, 20 to 30 MW for heavy goods trucks, and 100 MW for tankers. In addition, a value has been added for multiple-passenger car fires.