72 Steel Construction

It cannot be said to be conservative practice in any of these systems, much to exceed eight feet in the span of the arches. The uncertainty of the quality of the concrete when cinders are used, and the uncertainty of set in the deeper slabs, together with numerous other circumstances likely to affect the uniformity of the product, make it important to keep within this limit.

As will be seen from the illustrations, nearly all the concrete systems require furring clown to give level ceiling.

Tests of Floor and Roof Arches. The

most severe test of all forms of floor arch is their exposure to fire and water when under load. As above stated, one of the functions —and a very important one — of all fireproof materials is to protect the steel ; for, if the covering falls off, leaving the steel members exposed to fire, the steel frame will soon fail. None of the materials used— terra cotta or concrete in its various forms—are of themselves combustible. Failure, when it occurs, is generally due to expan sion and contraction caused respectively by the intense heat and by the chilling effect of the stream of water, and to the force of the stream knocking off pieces that become loosened. All of the systems in general use have been subjected to very severe tests of this character without collapse, before being accepted by the differ ent Building Departments; and it is probable that when failure occurs in actual building fires it is due to constructive defects, there having been less careful construction than was used in the tests.

If only a small portion of the covering becomes detached, the whole adjacent construction is seriously endangered. It will be seen from the above that failure is more likely to start from de tachment of the covering of beams, girders and columns, than in the body of the arch, and such covering should be as substantial as possible. For this reason, haunches or a solid filling protecting the beams and girders are preferable to wire lath wrapping the same.

Tests by the New York City Building Department on floors having suspended ceilings of wire lath and plaster, resulted in these ceilings being entirely destroyed. Tests of different floor systems having rolled shapes, such as T bars or special-shaped bars, imbedded in the concrete slabs, showed less deflection under loading than when a mesh of wire rods was used.

The method of testing floor arches is as follows : A brick furnace is built, having a large combustion chamber, the top being of the floor construction to be tested. This arch is loaded with a load generally four times that specified. Measurements of deflec tions due to the stress are taken before and after exposure to the fire. Daring this exposure, which generally lasts several hours, a temperature of from 2,000° to 2,500° is constantly maintained. After some time a stream of water from a fire nozzle is played on the arch, thus reproducing as nearly as practicable actual condi tions.

After the test, the load is removed to see how great the per manent deflection is. It is important in all loading tests to have the load applied over a definite area, so that the exact load per square foot can be determined, and to avoid all possibility of any portion of the load bearing on the beams instead of on the arch.

The results of some tests made under different conditions are here given: A fire and water test on a concrete expanded metal floor composed of 61 inches of concrete mixed in the proportion of 1 part Portland cement, 2 parts sand, and 5 parts cinders, showed the following results: • The slab was of a type similar to that shown in Fig. 55, the beams being 20-inch beams and spaced about 12 feet center to center, with the span of the beams about feet 9 inches.

The slab of concrete was loaded with 400 lbs. per square foot, under which it deflected .30 inch. Under exposure to fire the deflection increased to 2} inches, and when the test was completed remained about inches.

A portion of the under side of the concrete was knocked off by the stream of water.

A test under practically the same conditions as above was made of the Columbian system. The type was of the general form shown by Figs. 65 to 67. The spans were the same as above. The slab was 8 inches in depth, composed of 1 part Portland cement, 24 parts sand, and 5 parts broken stone. The bars were 5-inch bars, spaced 2 feet center to center, and fastened to the beams by angles.