FIREPROOF CONSTRUCTION 251 tinctly favorable, while others, from the stand point of load-bearing ability, are less enthusi astic. To a considerable degree results de pend upon the type of construction. There should also be mentioned the combination terra cotta and concrete floor, on which little data under actual conditions is as yet available.
For two reasons interior columns are ex tremely important factors in fire-resisting con struction; in the first place they support MI N. ft ntled itoullug Trip f_k?halt fell, 0 La) el Concrete Roof Construction. U. S. Public Building, Los Angeles. Cal.
mense loads increasing with the area and height of the building, and secondly, they stand ex posed to fire on all sides. Concrete, among other materials, has been used as a protection. The San Francisco fire furnishes the only im portant test df efficiency under actual conditions and the general conclusion that concrete was only excelled in value by bricks laid in cement mortar. The latter protection, however, is both heavy and expensive.
The use of concrete for building construction has been steadily increasing, and due to the high cost and scarcity of other materials this increase was accelerated in 1916 and 1917. Of the three ingredients of concrete, cement has been most perfected. Stone is also fairly well standardized, and the third factor, water, is now assuming importance. The fire-resisting qualities of the material depend upon its com position and handling. Its principal use at the present time is for the construction of factories and industrial buildings. The destruction in Canada in 1917 of the large Quaker Oats fac tory, composed almost entirely of concrete, is another indication of the fact that the term ((fire-proof° is as yet a purely relative one.
6. Mortar and Plaster.—Many grades of this material are used for fire-resisting purposes, and as the effect of fire upon the material it self is only a question of the duration and in tensity of heat it must be employed with care. Suspended ceilings of metal lath and plaster, however, have demonstrated usefulness purely Concrete Roof Construction. U. S. Public Building, San
Francisco.
as a protection to floors and floor-beams; their destruction is inevitable but the effect of fire upon essential structural parts is delayed and subdued. As a column protection plaster is far inferior to concrete and brick.
7. Sorel Stone.—This artifical stone is chiefly employed in the production of monolithic floors. Those made with sand as a base or the granular variety are often dusty but a satisfactory floor has excellent fire-resisting properties.
8 Fire-retardant Wood.—The process of treating the wood is to withdraw the sap, resin and gum by a vacuum, introduce an ammonia solution under pressure and kiln-dry the wood. Crystals of the absorbed solution remain in the wood and these, fusing at high temperatures, retard combustion. Ignition of hard woods, which would otherwise ensue in a few seconds, has been shown by tests to be delayed for five minutes by this treatment 9. Asbestos.—Asbestos is non-combustible and non-conductive and may be used to delay the progress of fire but loses its life and powders under intense heat. A well-known use is in theatre curtains, where the object is to confine the fire to the stage a sufficient time to permit the evacuation of the building. It is also used in composition with hydraulic cement for the manufacture of so-called asbestos lumber, sheathing and shingles and in such combinations is a valuable fire-resistant.
10. Glass.—Ordinary glass cracks under heat and affords no protection whatever from fire. Wireglass, known since 1855 but only re cently employed for its fireproof qualities, is ordinary glass with a wire mesh imbedded, made with one pouring and rolling. The wire mesh prevents the heat-cracked glass from falling away and renders glass available• for use in windows, doors, elevator shafts and skylights, where other material is unsuitable by reason of light requirements.
It should be noted that in the use of all materials cost, durability and weight are im portant factors from an engineering stand point, to which factors no consideration has been given here.