General Remarks

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.

Brickwork and hard burnt clay are the best fire-resisting materials.

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.

Wooden floors will resist a considerable action of fire if well imbedded in mortar, which, however, leads to their premature decay.

Wood may be rendered partially fireproof by being coated with cyanite, Asbestos paints, or other substances mentioned in Part III.

Concrete is generally a good fire-resisting substance, but this depends to some extent upon the materials of which it is composed.

Gypsum (sulphate of lime) is weaker than Portland cement, but resists fire better, as it does not lose its cohesive power even when raised to a white heat and then drenched with water.

Broken brick or stone, for the aggregate, stand fire better than breeze, which will burn away under very high temperatures.1

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

Plaster also resists fire well, especially when it is nearly entirely made with gypsum, as in the Hitchins Company's and Robinson's plasters.

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.

In the Chicago fire, sandstone was found to stand better than limestone. Granite was quite disintegrated, or, under less heat, scaled.2

Silicate cotton, slag wool, and similar materials are fireproof, and very useful for pugging floors or partitions.

c, Cost. - Under this head must be considered not only the cost of the floor itself but the expense it leads to.

Thus a deep floor will involve extra height of walling, and an arched floor, having a thrust upon the walls, will necessitate their being of extra thickness.

d, Strength. - The floor must of course be strong enough to bear the weights it may be required to carry.

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.

Different Forms Of Fireproof Flooring

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.