Fireproof floors are of great service in preventing flames from spreading throughout a building.
A great many different systems of fireproof construction have been proposed during the last few years; and before describing the most important of these it will be desirable to state what characteristics should be looked for in a good fireproof floor.
Characteristics of good Fireproof Floors.
In estimating the efficiency of any system of fireproof flooring the undermentioned points should be attended to.
a, Protection of Ironwork.
The structure of a fireproof floor is generally dependent upon the ironwork; if that is destroyed or gives way the floor must follow. Iron girders and columns may be protected by terracotta blocks, see Tigs. 236, 255, etc, or by concrete, see Figs. 251, etc.
b, Resistance to Fire of the Material composing the Floor.
Wrought-iron, if not protected by a non-conductor of heat, will warp and twist under the action of fire and destroy the structure.
Cast-iron cracks and gives way suddenly, especially when it is heated and then drenched, as it is likely to be during a fire. In the Chicago fire the ends of cast-iron columns were actually melted off.
Timber in large scantlings will resist the action of fire for a long time if the flames cannot get round its sides or ends. After it becomes charred to a certain depth the charcoal formed on its surface, being a non-conductor, protects it.1
1 Lawford, Transactions, Society of Engineers, 1889, p. 43.
Concrete is generally a good fire-resisting substance, but this depends to some extent upon the materials of which it is composed.
Slag cement is likely to be largely used for concrete floors in this country. It is cheaper and lighter than Portland cement, while its fire-resisting properties exceed those of Portland cement or gypsum.2
Stone is a very bad material for fireproof structures ; when subjected to great heat it suddenly cracks and gives way without warning. For this reason arches of fireproof construction should never rest upon projecting stone corbels.
Silicate cotton, slag wool, and similar materials are fireproof, and very useful for pugging floors or partitions.
This can easily be arranged for, as it is a mere question of the thickness of arches and dimensions of the girders, and other parts of the floor.
e, Permanency. - Floors of materials subject to decay from dry rot and other causes must of course be carefully avoided.
Recapitulating, we see that a good fireproof floor should be of good fire-resisting material - all ironwork being protected by nonconductors - that it should not lead to expense in other parts of the building, should be strong enough to carry the weights required to be placed upon it, and not liable to decay.
1 Lawford, Transactions, Society of Engineers, 1888. 2 Gass, Transactions, R.I.B.A., 1886, p. 134.
The student should test each floor described by seeing which it possesses of the characteristics described above. In selecting a floor for any particular purpose some of these characteristics will be more important for that purpose than others, and it is impossible to say abstractly that any system is the best under all circumstances.
A great many different forms of fireproof flooring have been proposed and made use of during the last few years. They may be generally classed under four heads.
A. Arches of brick or concrete supported upon walls or girders.
B. Hollow bricks or tubes supported between girders and filled in with concrete.
C. Concrete filled in between and around girders.
D. Solid timber of considerable thickness.
E. Iron plates resting on girders.