84 FIRE-RESISTING BUILDINGS. 



such material loses strength, when heated, more than mild steel, 

 and is, therefore, not favoured for fire-resisting construction. 



Reinforcement. — Various types of bars, expanded metal and 

 wire mesh fabrics are used as reinforcement of concrete, and 

 small members are generally favoured as they give a greater 

 proportionate surface for adhesion, and are more easily manipu- 

 lated. Each has its strong and weak points, according to the 

 work allotted to it, which alone can be judged by the designer. 



Structural Allocation of Materials. — Generally speaking, it 

 is admitted that stone should be avoided where there is any 

 •chance of exposure to excessive temperature or flame. Sand- 

 stones, limestones, and granites are dangerous, as they crumble 

 away or split into pieces if water is suddenly applied when they 

 •are subjected to continuous extreme heat. For this reason 

 they should never be used for staircases or as piers to support 

 superincumbent walls. Cast-iron, wrought iron, or steel, when 

 exposed to heat, appear at first to increase slightly in tensile 

 strength, but as the temperature rises their strength falls rapidly. 



This is especially the case with wrought iron or steel, some 

 descriptions of which may lose strength to the extent of 70 

 to 80 ])er cent, before a distinct red heat is reached. Cold water 

 suddenly applied to cast-iron, when highly heated, may cause it 

 to crack or fly to pieces. Wrought iron or steel, under similar 

 circumstances, would become considerably twisted or distorted. 

 Irregular heating, such as may take place when a stanchion is 

 partly buried in a wall and one or more faces exposed to the 

 heat, creates internal strains that tend to induce distortion or 

 twisting, and the same applies to floor beams that are partly 

 enclosed in concrete, and have their lower flange exposed. 



The efficient j^rotection of metal from excessive or irregular 

 heating is therefore essential. 



For the convenient allocation of fire-resisting or protective 

 materials to the several constituent portions of a building, it 

 will be well to sub-divide the same into sections, separating the 

 horizontal from the vertical units. The former consist of floors, 

 ceilings, roofs and staircases, and the latter of walls, piers, and 

 partitions. 



Floors. — Solid wood baulks, hollow terra-cotta slabs and 

 reinforced concrete cast in situ are generally accei^ted as the 

 most effective materials for the construction of fire-resisting 

 floors, but the latter, on account of its cheapness, is generally 

 adopted. The following remarks upon concrete will, of course, 

 apply generally to piers and other horizontal and vertical units. 



Fire-resisting concrete recjuires an aggregate of material 

 that has already been burnt, such as broken brick, burnt ballast, 

 pumice, furnace slag, clinker of a hard and weight-bearing 

 structure, and metamorphic stone generally. 



Concrete is, compartively speaking, strong in compression 

 and weak in tension, the ratio of compressive to tensile strength 

 being about as from 6 to 10 is to i. Its lack of tensile strength 



