This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol6", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
Apart from panic, the primary cause of nearly every fatality from fire may be put down to the subject being overcome by the fumes of combustion, and as these fumes may spread with great rapidity to all parts of the house it is particularly important that they, as well as the actual flames, should be confined 13 and led away from those parts of the house which are filled with people.
Fig. 7 shows a possible section of a theatre in which the question of air current has not been considered. The auditorium is ventilated with a central "Sun-burner," and every tier is ventilated by exhaust ventilators at the back, while the opening immediately above the stage is practically nil. The direction that would be taken by fumes and flames in case of fire is indicated by arrows. It is probable that plenty of air can enter at the back and sides of the stage, and the fire thus fanned, in burning the inflammable scenery, will produce dense volumes of smoke, which, if the proscenium opening be not properly protected, will immediately enter the auditorium and pass to the exhaust openings, as indicated by the arrows. The effect of this would be most serious in the gallery, where, as has been known to happen, the people may not even have time to leave their seats before being overcome by the fumes. In respect to the gallery, a great danger may be noticed in the form of section given, in that the ceiling over it forms the highest part of the auditorium, and smoke will consequently collect there at once. Having filled the upper parts of the house with smoke, or before this if the air currents be suitable, the fumes will enter the passages and staircases, suffocating those who are struggling to escape.
Fig. 8 is intended to illustrate the direction in which a remedy from the above dangers may be sought. First and foremost may be placed the fire-resisting screen or "curtain" to divide the dangerous source of fire, the stage, from the auditorium. But the screen at the last moment may possibly become inoperative, or if this should not occur air currents which may have received little attention may go far to overcome the protection afforded by the screen.
Assuming that a fire is started upon the stage, and that the fire screen is satisfactorily lowered. The stage will become loaded with smoke, and many of those employed upon the stage, who often amount to hundreds, will in all probability be overcome by it; while, from the same cause, the fire-brigade will be unable to enter in order to extinguish the fire.
It is necessary, then, to supply an opening in the roof above the stage of ample proportions to allow the smoke to pass away. This is illustrated in Fig. 8. The London County Council's regulations in regard to this specify that the roof over the stage shall be provided with an opening at the back thereof equal at the base to 1/10 the area of the stage, the opening being glazed with thin glass and automatically opened in case of fire, or simultaneously opened on lowering the fire-resisting screen. With the provision of the screen and the large ventilator over the stage the fumes will readily escape, and will have no great tendency to force a passage into the auditorium, while the stage will be sufficiently clear of smoke to allow firemen to enter.
With the question of air currents properly considered, an audience may have time to escape even if the fireresisting screen refuses to fall. In order that the fumes may be prevented from passing into the auditorium, air currents must be arranged to pass if possible from the auditorium towards the stage. For this purpose the large ventilator provided in the roof over the stage must be open, while those ventilators in the auditorium must be closed. These two actions should be controllable simultaneously from the stage and house. The arrows in Fig. 8 indicate the direction of the air currents in this case.
In order that the people in the gallery may not be overcome by the collection of fumes, this part of the house should not be higher than the rest of the auditorium, while no seat should be above the level of the proscenium opening.