A common way of providing smoke extraction systems in tunnels is the construction of a smoke extraction plenum in the tunnel roof space, the so called transverse ventilation system. In case of an emergency the smoke and hot gasses will be extracted into the plenum through smoke inlets or hatches.
Such a plenum can either be constructed out of concrete or steel. Regardless of the selected construction method, the structural integrity of this plenum during fire is of utmost importance as the ventilation philosophy is depending on it. In case (part of) the plenum would collapse during the fire event, the intended smoke management approach would be lost, with all possible major implications as a result, let alone the hampering effect it would have on emergency response teams.

Such a smoke extraction plenum gets exposed to tunnel fire temperatures from both sides, e.i. from below but also from the top because the hot gasses are pulled into the duct. Especially at the location of the hatches the temperature exposure will be equally high from both sides.
Therefore, regardless of the selected construction method, such a plenum system
requires thermal protection from both sides and not only from below.

In case the plenum is constructed out of concrete we refer to the section on this
website detailing concrete protection.

The other option is to construct the plenum using a steel frame, which would span from wall to wall, with intermediate hanger rods if mechanically required. As described above, such a frame requires thermal protection from below and from the top. Also the hanger rods do require thermal protection in order to prevent that these would elongate due to thermal expansion, potentially causing unwanted deflection and sagging of the plenum.