DEMOLITION AND MATERIAL PRODUCTION WASTE
3. fIre load
It is necessary to define the character of the fire and the heat loading to the material when investigating the structural materials. Furthermore the distribution of the heat versus time and space should also be known.
The heat mass dissipated from a single vehicle can be determined with a reasonable degree of certainty. Fig. 6 shows the equivalent heat load (Putz, 2005) and average heat mass evolved due to the total burn out of different kinds of vehicles.
The exact mass of the evolved heat can be calculated for each kind of vehicle if the individual components are known.
However, this calculation is only applicable for underground networks where the same kind of vehicle has been tested. For example: by a total burn out of a type NF 10 underground railway wagon (as applied in Dusseldorf, length 40 m, width 2.4 m) the evolved heat mass generates 91 592 854 kJ energy
Fig. 2: Damage in the Highway Tunnel, Hamburg (1968) (Haack, 2002)
Fig. 3: Damage in the Channel Tunnel (18. 11. 1996) (Haack, 2002)
Fig. 4: Damage in the Mont Blanc Tunnel (24. 03. 1999) (Haack, 2002)
Fig. 5: Fire in the St. Gotthard Tunnel (24. 10. 2001) (Schlüter, 2004)
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(Blennemann and Girnau, 2005). However, this evolving energy is distributed in time and space. Thereby the total energy mass is definable for every kind of vehicle if the components can be summated.
It is important to note that real fire protection begins with the applied materials of the vehicles. The total mass of the flammable materials in a vehicle design conforming to modern standards and considerations is smaller, so the evolving of heat and (toxic) smoke puts less strain on the human organism and
the structure. Fig. 7 shows that the heat-time distribution is also more advantageous.
When designing tunnels, there is no opportunity (with some rare exceptions) to calculate and analyse the individual heat evolving. Instead of describing of fires in terms of a “correct”
standard fire, characteristic curves were established to describe the characteristic of an average tunnel fire. The common used fire-characteristic curves are shown on the Fig. 8.
Each curve is based on one or more of the following:
various research, suppositions, national standards, flammable material compositions and the results of small or large scale tests (Promat, 2006).
ISO Standard: The characteristic curve used mostly in the design of buildings, which is defined by many European National Standards (ISO 834, BS 476:part 20, DIN 4102, AS 1530). It is based on the continuous ignition and natural burning of flammable materials surrounding the fire. The equation of the curve is shown in (1).
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(1) The maximum temperature can be calculated on the basis of the fire load time defined by the National Standards.
The maximum evolving heat mass is around 30 MW. It is not proposed that the ISO standard “ISO” curve be used in the design of tunnels. Fig. 8 shows that the standard “ISO”
curve has a relatively long heat accumulation time which is not acceptable to describe the tunnel fire theory of fast heat accumulation.
HydroCarbon: Hydrocarbon fires demonstrate an alternate characteristic behaviour. Both the characteristic behaviour and the extreme of the curve differ from the commonly used standard “ISO” curve. The faster ignition of the hydrocarbons causes a faster increase and larger combustion fires produce higher maximum temperatures. The equation of the curve is shown in (2). T C
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(2) HydroCarbon Modified: Analogue to the “Hydrocarbon” fire,
the French Standard defines a fire characteristic curve with a maximum temperature of 1300 °C as opposed to the 1100 °C shown above. The equation of the curve is shown in (3).
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fire characteristic curves came from the petrol chemical industry into civil engineering. Their use could be demonstrated because of the huge amounts of benzene and gasoline around the fire, especially in road tunnels.
RABT-ZTV: The “RABT-ZTV” fire characteristic curves have been determined and standardized in Germany as a result of large scale tests. In contrast with the foregoing theories, the curves are described with break points instead of equations.
Different characteristic curves were determined for both railway and road tunnels. Both curves rapidly reach the maximum temperature (1200 °C) within 5 minutes. The maximum temperature is then held for 25 to 55 minutes depending on the type of curve. The cooling phase is 110 minutes for both curves (Table 1).
The German fire characteristic curves are used for design as well as for research because of the linear drive. Curves well define the rapid increase of the temperature in the first
Fig. 6: Equivalent heat load for different kinds of vehicles (Putz, 2005)
Fig. 7: Vehicles of small and large combustion heat (Blennemann and Girnau, 2005)
Fig. 8: Standardized fire-characteristic curves; air/gas temperatures around
the fire (after Blennemann and Girnau, 2005)
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few minutes (Fig. 9). However, this is the only characteristic which determines the parameters of the cooling phases.
The speed of the cooling is also important, not only for the model, but also for the residual properties of the material.
RWS, RijksWaterStaat: Characteristic curves from the Netherlands determined at 1979. This curve shows the highest maximum temperature of all the curves above. It describes the worst case scenario on an accident in a road tunnel when a tanker with 50 m3 gasoline explodes. As a consequence of the accident, 300 MJ of energy evolves in 180 minutes. The behaviour of the curve shows rapid increasing during the ignition period, deceleration in rate of temperature increase in the second period, a significant maximum point, slight recession and stagnation (Table 2) Apart from the lack of the cooling phase (the flammable material is burn out after 180 minutes) the RWS fire characteristic curve has the greatest upper bound of almost 100% of the tunnel fires.
The curves above represent the distribution the maximum design value of air or gas temperature in a tunnel versus time.
The spatial changes of temperature were also examined. It was verified that the maximum temperature develops at the roof of the tunnel; lower temperatures are noticeable at the walls. Figs. 10-12 shows the distribution of developed air temperatures during the fire flaming out from different kind of vehicles.
The longitudinal distribution also shows the spatial changes extending away from the fire. However by the time of ignition, the secondary ignition of flammable materials around the fire is also possible. Thereafter the maximum points of the temperature curve are extended over the tunnel’s longitudinal
Table 1: Significant break points of the “RABT-ZTV” curves (Promat, 2006)
Table 2: Significant point of the RWS, RijksWaterStaat fire characteristic curve (Promat, 2006)
Fig. 9: Large scale test in Germany (1998) (Haack, 2002)
Fig. 10: Maximum air temperatures in the walls during a railway tunnel fire (Richter, 1993)
Fig. 11: Maximum air temperatures in the walls during an underground railway tunnel fire (Richter, 1993)
Fig. 12: Maximum air temperatures in the walls during a road tunnel fire (Richter, 1993)
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Fig. 14: Distribution of air temperature parallel to the longitudinal axis, away from the fire (Blennemann és Girnau, 2005)
section, and increasingly more tunnel linings will be subjected to the maximum thermal load as shown on Figs. 13-14.