Civil Engineering Reference
In-Depth Information
Introduction
All traffic tunnels in the Netherlands must comply with fire safety require-
ments. The Ministry of Transport, Public Works and Water Management
demands that the structures must withstand a (hydrocarbon) fire for two
hours in which the temperature rises up to 1,350 °C, a situation which is
reflected in the temperature-time curve for simulation of fires by the
Directorate-General for Public Works and Water Management (RWS curve).
In order to protect a structure against such a fire, the immersed tunnels in
the Netherlands are provided with a heat-resistant cladding which is approx-
imately 27 mm thick. This cladding prevents too high an increase in the tem-
perature of the concrete and particularly the reinforcement inside it which is
of vital importance to the strength of the structure.
For bored tunnels, thanks to the circular shape and the rather massive normal
forces present, the reinforcement is of lesser importance for the strength of
the structure in the operational phase than for immersed tunnels. Here the
reinforcement is particularly necessary to limit damage during the building-in
of the segments. In case of fire, it is therefore not so important for bored tun-
nels if the reinforcement maintains sufficient strength and stays put in
'healthy' concrete - whereas this would be essential for immersed tunnels.
For bored tunnels, the criterion particularly applies, that in order to guarantee
the structural integrity, the concrete segments may not be too disintegrated
in thickness through the spalling of concrete as a result of high temperatures.
Further, the damage must be reparable in a safe manner.
The spalling factor is a phenomenon which goes hand in hand with the
quality of the concrete: the greater the density of the concrete, the more
sensitive it is to spalling. The concrete of immersed tunnels is not so sen-
sitive to spalling because of the lower concrete quality.
In any case, at the start of the Westerschelde Tunnel project, very little
was known about the spalling factor, and that meant that a great deal of
knowledge had to be gained in a short space of time, in order to protect
the tunnel against massive fires.
The brief: the RWS fire curve
The traffic tunnels in the Netherlands where the transportation of hazardous
substances are permitted, comply with the requirement that the structural
integrity of the tunnel (wall) remains guaranteed during and after the fire of
a (petrol) tanker with a contents of approximately 45,000 litres of petrol.
Generally speaking, this means that the tunnel may not collapse, while the
damage caused can be safely repaired.The curve is related to a fire duration of
2 hours which is a consequence of the rate of heat release which is related
to the 'size of the pool'. For such a fire, the burning load has an average
value of approximately 200 MW (with a top value of approximately 300 MW);
this scenario has led to the definition of the RWS curve.
The RWS curve is the most stringent in comparison to some other curves
such as the ISO-834 standard fire curve, the Eurocode 1 for a hydrocarbon
fire and the German RABT curve. The curve rapidly rises to a temperature
higher than 1,200 °C.
In order to protect concrete structures, insulation materials can be applied.
However, many standard insulation materials in the building industry are not
resistant to temperatures above 1,200 °C and would melt. It does happen
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