The highly inflammable nature of wood as a material used in the construction of floors, and the risk to life which it entails, has of late years received particular attention from the architectural and building world; the object of course being to design a floor which can be said to be entirely free from inflammable material, and proof against the attacks of fire in all its varied forms.
Wood has naturally been shunned - except, of course, as a covering, for the sake of comfort - while stone is known to be a most treacherous material under the influence of heat, as it will crack and give way without warning when least expected.
Cast-iron, too, is most unreliable, in fact, dangerous in the extreme; inasmuch as it cannot resist sudden shocks of any kind, which are especially present in cases of fire, when the iron, after having been heated by the flames, is suddenly cooled with cold water from the hose, which makes it crack immediately, entailing the total collapse of the whole structure generally.
Wrought-iron is certainly not so treacherous; but still it is dangerous, on account of its liability to twist and expand under intense heat/thrusting the walls out.
Hence it is always advisable to encase iron parts of structures with some material which is itself really fireproof, before the work can be said to be "proof against the effects of fire." The only materials which can be said to possess these requisite properties are - fire-clay in any burnt form, concrete of cement, and breeze, or other similar agglomerates, as well as plaster of various kinds.
Brick Arches are the simplest and most effective fire proof material, though only suitable for small spans, except where the height of the space which they and their rise occupy is of no consequence. In large spans, and where this is a consideration, their success and usefulness are diminished by the insertion of encased structural parts of iron, whether cast, for columns, or wrought, for girders, whereby the floor-space is cut up into sections, and filled in by arches springing from the encased wrought-iron girders. The spandrils of these arches are levelled up to the crown with cement-concrete, whereon the ordinary floors are constructed The only objections which can be raised against brick arches, apart from the supplementary ironwork, are that they are heavy and complicated, and take up too much room, besides being expensive; but these are held to be serious if not fatal objections; and they are considerably intensified by the insertion of iron girders, which, whether cased or uncased, render the floor very complicated in construction, and dangerous, unless the girders are tied together by rods, etc., to withstand the thrusts caused by the arches themselves.
Consequently, recourse has been necessary to other, lighter, and more simple methods, which will give as much security as possible from the ravages of fire. Thus, the mode of construction that has been settled upon as being most free from complications, and the cheapest, is that of concrete areas, supported by iron joists embedded within the thickness of the concrete itself, which need not be very deep in floors of ordinary spans; and this is an important consideration, as the thicker the floor the higher the building must be to give the same amount of space.
Where the span necessitates the employment of binders, or, girder, to support the iron joists which uphold the concrete areas, care should be taken that these additional members are enclosed or surrounded by a casing of fire-clay, concrete, or other non-inflammable material, to protect the iron.
A great advantage of these and other kinds of concrete or fireproof floors is that a new building, so far as internal arrangements are concerned, can be constructed from floor to floor without so much trouble to secure the placing of wall over wall, and in similar constructive details; though of course the floor must be made strong enough in itself to allow of such liberties.
As before explained, a concrete floor composed of breeze, slag, brick-dust, or other non-inflammable materials, mixed with cement in such proportions as the situation and character of the work call for (usually in the proportion of 4 or 5 to 1 of cement), is the best form of fireproof floor. It is illustrated by Fig.. 522, the iron joists being fixed about 2 feet apart; after which a temporary wood platform is built underneath it, to support the concrete until it has set hard; and this is fixed below the bottom of the joists, so as to allow of the concrete encircling the iron, as shown.
Dennetts system Fig. 523.