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the basis of smoke flow rates (i.e., the volume flow in goal is to make it as safe as possible based on previous
the fire plume) using a design fire and local gas tem- experience, on current knowledge, and on technical
peratures downstream from the design fire because they equipment development. Consequences of tunnel fires
determine the ventilation volume flow rates. The design can be disastrous.
fire scenario takes into account all the important issues · Tunnels are generally safe. Tunnel fires are rare events
such as a time factor, ambient conditions, wall proper- and happen less often than fires on open roads. Fewer
ties, and the efficient operation of detection and venti- than 150 people have been killed anywhere in the world
lation systems, which can have a significant influence in road tunnel incidents involving a fire--and that
on the appropriate design fire characteristics. includes those killed by any preceding accident (44)
2. Design fire scenario for egress analysis--Evacuation (collisions). Fewer than 20 tunnels around the world
measures for tunnel users or emergency rescue services have suffered substantial damage as the result of a fire
need to be within tenable environmental conditions, emergency.
identified as breathable gas temperatures and concentra- · Road tunnel fires cannot be completely eliminated until
tion of toxic gases at head height in the tunnel, as well as vehicle fires are eliminated.
hot gases at higher levels in the tunnel that radiate down
onto evacuees. A tenable environment is well-defined in DESIGN FIRE SIZE
NFPA 502. Time is a very important factor. The times
for hazardous conditions to develop at particular loca- Design fire size is one of the most important parameters for
tions as discussed later in this chapter need to be com- tunnel fire engineering. The materials that burn in a fire mostly
pared with occupant egress times. These in turn need to come from the vehicles involved, and they include elements
take into account the time it takes for occupants to real- of the vehicles, such as the seats, tires, plastic materials in the
ize they are in danger and begin their escape. Evacuation finishing or even in the body work itself; cargo; the fuel from
time from buses also needs to be considered. the vehicle tanks, which amounts to hundreds of gallons for
3. Design fire scenario for thermal action on structures-- trucks; and the loading, especially for goods vehicles. The
See timetemperature curve discussion. goods loadings vary and can lead to many different kinds of
4. Design fire scenario for the safety of tunnel fire fires. Some examples of combustion energy outputs are given
equipment--Usually the critical fire life safety equip- in Table 12.
ment is required to be designed for the expected envi-
ronment during a fire emergency. For design purposes it is necessary to choose fire charac-
5. Design fires for work on tunnel construction, refur- teristics corresponding to the traffic that uses a particular
bishment, repair, and maintenance--Fires related to, tunnel. Conditions, such as the allowance of transporting haz-
for example, tunnel boring machines and the refur- ardous vehicles and materials, have to be taken into account.
bishment of tunnels are considered out of the scope of
this report. Tunnel fires differ from open fires in at least two impor-
tant ways:
Based on the tunnel experience and tunnel fire tests,
several observations can be made: 1. The heat feedback of the burning vehicles in a tunnel
fire tends to be more effective than that in an open fire
· Each tunnel is unique because of the confined enclosure. This effective heat
· A tunnel is a risky environment. No tunnel is absolutely feedback often causes vehicles that do not burn intensely
safe regardless of how it was designed. The designer's in an open fire to burn vigorously in a tunnel fire. For
TABLE 12
EXAMPLES OF COMBUSTION ENERGY OUTPUTS
Approximate Energy Content
Type of Vehicle [MJ (MBtu)] Remarks
Private Cars 3,0003,900 (2.83.7) Used for fire tests in Finland
Private Car 6,000 (5.7) Used for EUREKA fire tests
Plastic Car 7,000 (6.6)
Public Bus 41,000 (39)
Heavy Goods Vehicle (HGV) 88,000 (83)
Loads for HGV 67,000 (63.5) Used in the fire tests in the
129,000 (122) Runehamar Tunnel
152,000 (144)
240,000 (227.5)
Tanker with 50 m³ Gasoline 1,000,000 (948) Medium tanker
1,500,000 (1422) Dutch assumption for a l
ì ar ge"
design fire
Source: PIARC (21).
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example, Beard and Carvel (35) concluded that the The magnitude and development of fire depends on:
HRR of a fire within a tunnel could increase by a factor
of 4 compared with that of the same material burning · Vehicle combustion load (often called the fuel load,
in the open. Furthermore, the oxygen needed for com- which is usually greater than the potential fire size),
bustion is not always as readily available in tunnels as · Source of ignition,
in the open (depending on the tunnel geometry and fire · Intensity of ignition source,
size). The fire conditions may either develop to a: · Distribution of fuel load in the vehicle,
Fuel-controlled fire where unreacted air bypasses · Fire propagation rate,
the burning vehicles (typical tunnel fire controlled · The tunnel and its environment (including available
by limited fuel available), or a oxygen), and
Ventilation-controlled fire, giving rise to large · Other factors that will be discussed in the following
amounts of toxic fumes and products of incomplete chapters.
combustion. Essentially, all the oxygen is consumed
within the combustion zone and fuel-rich gases The fire power is measured in megawatts (MW) or MBtu/hr
leave the exit of the tunnel (e.g., extremely severe (1,000 Btu/hr), although it has become more common for
tunnel fires, such as the Mont Blanc fire where oxy- engineers to combine the peak HRR with the fire growth rate.
gen is limited). For example, full-scale tests of HGV loads in the Runehamar
2. As a fire develops in a tunnel, it interacts with the ven- Tunnel showed that the HRR can exceed more than 100 MW
tilation airflow and generates aerodynamic disturbances (341 MBtu/hr) in less than 10 min. This means that the fire
in the tunnel flow. This interaction and disturbance may growth rate will be crucial in determining whether those caught
lead to drastic changes in the ventilation flow pattern, in the fire can escape. Studies showed that the fire growth rate
such as throttling of airflow (buoyancy effects) and is more important than the peak HRR when investigating the
reverse flow of hot gases and smoke from the fire into safety of people in the tunnel. The peak HRR varies between
the ventilation air stream (backlayering). Such effects 1.5 MW (5 MBtu/hr) and 202 MW (689 MBtu/hr) for road
on the ventilation not only complicate firefighting pro- vehicles. The gas temperatures in the ceiling vary from 110°C
cedures, but also present extreme hazards by propa- (212°F) to 1365°C (2489°F).
gating toxic fumes and gases far from the fire. Impact
of ventilation on fire size is discussed in chapter thirteen. It must be emphasized that most of the test results are
dependent on the test conditions. These include low air
Design fires in tunnels are usually given as the peak fire velocities during most of the tests and a cross section signi-
HRR. There are various methods and techniques to calculate ficantly smaller than usually found in road tunnels. This
and estimate the fire HRR of a given vehicle; some could overestimates the heat radiation coming back from the walls
be provided by manufacturers (for cars and buses), others and may underestimate the amount of oxygen available in
calculated; however, there is no common ground on how the tunnel.
to calculate the HRR. One method is the weighting of the
burning components of a vehicle, another is analytical. Some The design fire size selected for design significantly affects
calculations incorporate burning efficiency, which means the magnitude of the critical velocity needed to prevent back-
that the fire may not consume the entire heat load available. layering. Table 13 provides general fire size data for a selection
The leftover content is typically in the form of either a char of road tunnel vehicles. It presents typical fire size data for
residue or as soot and smoke particles displaced by the com- passenger cars and multiple passenger cars, for buses, HGVs,
bustion gas stream (45). and tankers; however, this does not allow for evaluation of
TABLE 13
TYPICAL FIRE SIZE DATA FOR ROAD VEHICLES
Peak Fire Heat Release Rate,
Cause of Fire 106 Btu/h (MW)
Passenger Car 17 to 34 (5 to 10)
Multiple Passenger Cars (2 to 4 vehicles) 34 to 68 (10 to 20)
Bus 68 to 102 (20 to 30)
Heavy Goods Truck 239 to 682 (70 to 200)
Tanker3 682 to 1,023 (200 to 300)
Source: NFPA Standard for Road Tunnels, Bridges, and Other Limited Access Highways (2008) (19).
Notes:
1. The designer should consider the rate of fire development (peak HRRs may be reached within
10 min), number of vehicles that could be involved in a fire, and the potential for a fire to spread
from one vehicle to another.
2. Temperatures directly above a fire can be expected to be as high as 1800°F to 2550°F (1000°C
to 1400°C).
3. Flammable and combustible liquids for tanker fire design could include adequate drainage to limit
the area of pool fire and its duration (see Table 14).
4. HRR may be greater than listed if more than one vehicle is involved.
