Floor arches are often tested by applying a heavy roller to the arches after the wood centers have been removed, and by dropping a heavy block of timber upon the arch, a two-inch bed of sand being previously spread over the tiles to prevent mechanical damage.
Concrete Floors. The use of concrete for floors has become an important factor in the construction of fireproof buildings. Con crete floors combine the qualities of strength and ease of manufacture with lightness and less expense than most of the hollow tile construe tions. Concrete so used is generally found in combination with steel or iron in some form or other, to which it owes its tensile strength.
Concrete floors may be considered under two classes, one in which the concrete in combination with steel members forms the whole of the floor, and the other in which steel I-beams are set as for hollow tiles with the spaces filled with concrete upon wire cloth or expanded metal, or with these or strands of wire embedded, as tension members, in a plate or arch of concrete.
Of the first-named class of concrete the Ransome patent is the best known. (Fig. 21S.) 'l'liis consists of the use of twisted square bars of steel running through the lower portion of the plate or beam to give the necessary tensile strength to the concrete; it has been used for spans up to thirty or forty feet without steel beams or girders. Another form of this floor consists of a lighter construction of concrete beams each with its twisted steel member, with a thin plate of concrete between. (Fig. 219.) The other system of concrete floor construction differs from the hollow tile floor construction in the use of concrete for a filling between the beams instead of tiles; steel columns, girders and beams remaining the same as for the tile floors.
These floors are usually to be found in one of two forms—the flat plate of concrete lying between the beams or the segmental arch of concrete formed upon a center of metal lathing. Of the former system, the plate floor of the Expanded Metal Co. is a good example. (Fig. 220.) This floor consists of a plate of cinder concrete, from three to seven inches thick, in which is embedded a continuous sheet of expanded metal which is laid over the tops of the beams and is allowed to sag clown between them a few inches. A flat centering of planks
is with its surface a little below the sag of the metal, and a con crete of cinders and Portland cement is poured over and through the meshes of the expanded metal, and leveled off at an established height above the tops of the steel floor beams. Troughs are formed in the centering at each side of the floor beans which allows the concrete to completely enclose the beam.
On this plate of concrete, a wooden or cement floor may be laid in the usual manner, and a flat ceiling formed below by hanging stiffened metal to the beams; or the underside of the concrete plate may be plastered upon, forming a series of panels. These plates, of seven inches thickness have been used up to seventeen feet of span between the beams; and the average floor will weigh about thirty-, five pounds to the square foot, and is about four inches thick.
Of the types of arched concrete floors, the Roebling patent is a fair example. (Fig. 221.) This construction consists of an arch of wire cloth stiffened with rods, which is sprung between the floor beams with more or less of a rise according to the spacing of the beams. Upon this arch, a concrete of Portland cement and sand is deposited, making, when set, a solid slab of concrete three inches or more in thickness at the top, with the haunches leveled up to the top of the arch. The most economical proportions for this floor have been found to be a basis of ten-inch beams, spaced according to the span and load, with an arch of three or four inches at the crown. This makes a very strong floor, and one not easily damaged, as it may be punctured by holes of any size without destroying the concrete arch, which is kept from shattering by the wire cloth. If a flat ceiling is required, it may be hung to the bottom of the beams of stiffened wire cloth and may be flat or panelled as desired. No centering is required with this floor, as the stiffened arches are bent to the required curvature, and after being set in place, they possess enough strength to receive the loud of concrete or the shock of any falling body. Being open to the air both above and below, the concrete sets quickly, two days being generally enough for safe use.