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Typically in circular tunnels the tunnel roof space can be utilised to create an escape
route above the tunnel tube by means of constructing a suspended ceiling system.
Because of the lack of space to provide a means of egress alongside the tunnel tube,
this method is commonly used in this type of tunnels.
The escape route can be reached by the stairwell which connects to the tunnel tube
at road deck level. The escape door leading to the stairwell should be fire proofed to
prevent that the fire could spread into the escape route. Also the spread of smoke
and toxic gasses into the escape route should be prevented. To achieve this, the
escape route area is pressurised with fresh air, creating an overpressure to the
surrounding atmosphere. |
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The area above the road deck can be used for escape route purposes only but can
also be combined with a smoke extraction duct, in which case a fire rated wall
separates the escape route area (fresh air) from the smoke extraction duct. This wall
requires fire proofing because it will get exposed to tunnel fire temperatures through
the hatches in the smoke extraction plenum system. |
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Such an escape route ceiling can either be constructed out of concrete or steel.
Regardless of the selected construction method, the structural integrity of this ceiling
during fire is of utmost importance as this provides the most important means of
egress in a fire emergency. |
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In case the escape route is constructed out of concrete we refer to the section on this
website detailing concrete protection. |
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The other option is to construct it using a steel frame, which would span from wall to
wall, with intermediate hanger rods if mechanically required. The separating wall can
also be constructed such that it functions as a hanger rod. For obvious reasons,
hanger rods should be avoided in the escape route area. |
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Apart from its structural integrity in case of fire an escape route ceiling has an
additional thermal criterion being the maximum allowable temperature on the nonexposed
face of the specimen, e.i. the temperature on the floor should not exceed a
certain tenability level.
The French tunnel fire safety standard provides guidance to
address this. The maximum allowable absolute temperature on the floor is set at
60ºC. This is not a temperature rise above ambient but an absolute maximum. |
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The escape route ceiling system is constructed using a trapezoidal steel sheet as the
load bearing layer. From below, PROMATECT®- T boards are screwed to Z-profiles
and are combined with high density mineral wool thus providing for the required
thermal insulation of the system. On top of the trapezoidal steel decking a metal grid
is foreseen to provide for a convenient surface to walk on.
The system as described
above satisfies the thermal requirement of 60ºC on the floor surface as mentioned
above.
An additional PROMATECT®- H board can be applied between the trapezoidal steel
sheet and the metal grid to obtain even lower temperatures on the floor surface.
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