179. Concrete containing iron or steel wires, rods, netting, etc. embedded in it, is now extensively used for floor construction, and in many respects is equal to tile-arch floors, and is cheaper. One form consists in the use of wires or bars to suspend the floor from wall to wall, or between girders, taking up the tensile stress which the concrete itself is unable to withstand. Another method is to support the concrete on wire netting, or expanded metal, fastened to the beams. The earliest form of concrete floor was the Hyatt system, in which thin plates of iron, set upright, and bound together at short intervals by wires - thus forming a kind of gridiron - were placed in the concrete near the bottom, and rested on the beams at both ends.

180. The Ransome Floor

The Ransome Floor. From the Hyatt floor, above mentioned, was developed the Ransome floor, the difference being in the use of twisted square bars of iron or steel, instead of flat bars set on edge. Owing to the twisting throughout the length of the bars, they are held firmly in place in the concrete, and are as strong as the plates and wire, besides costing much less. These floors have been used without intermediate supports for spans up to about 35 feet, and are remarkably stiff under heavy loads. A floor 15 ft. X 22 ft. was loaded for a month with a weight of over 400 pounds per square foot, the resulting deflection being only about 1/8 inch.

In Fig. 86 (a) is shown the ordinary Ransome floor, a being the concrete, and b the twisted bars, inserted near the bottom of the slab, to take up the tension.

180 The Ransome Floor 213

Fig. 86.

Fig. 86 (b) shows the same system applied to a panel floor, which is considerably stronger than the flat form.

181. The Lee Floor

The Lee Floor. Another method of floor construction, in which use is made of iron or steel rods, is shown in Fig. 87, and is known as the Lee floor. In this system, hollow tiles a are substituted for concrete, and are supported by round rods b, two or more of which are twisted together, the ends being attached to the walls or girders. Quarter-round grooves are formed in the lower corners of the tiles for the cables, which are spaced from 8 to 12 inches apart, depending on the size of the tile. When in place, the cables are covered with cement concrete c, which, with the plaster, forms an effective protection.

181 The Lee Floor 214

Fig. 87.

182. The Metropolitan Floor

The Metropolitan Floor. This system is shown in Fig. 88. At (a) is shown the first stage of construction. Light cables, spaced from 1 inch to 1 1/2 inches apart, and consisting of two No. 12 gauge galvanized-iron wires twisted together, as shown at a, are attached to the upper flanges b of the beams by means of hooks, shown at c, about 3 inches long, and made of 1/4-inch square iron. In order to stretch the cables equally, a bar or pipe is laid centrally between the I beams, as shown at d. Centers are then suspended, under the cables, and a composition made of 1 part of wood shavings and 5 parts of plaster of Paris, mixed to a thin paste with water, is then applied. This sets very quickly, and the floor is sufficiently strong to be used soon after it is formed. In this system the beams require no tie-rods, as floor loads, being transmitted vertically, cause no side thrust, as do arches, while the tendency to pull over the top of the beams is counteracted by the rods and hooks in the adjacent panels. The ordinary thickness of this floor is 4 inches, the floorbeams being spaced about 5 or 6 feet apart. As the beams are generally more than 4 inches deep, wire netting is wrapped around them, as shown at a in (b), Fig. 88, to hold the plaster of Paris composition, which surrounds and effectually protects the beams.

182 The Metropolitan Floor 215

Fig. 88.

At (b) is shown the completed floor, with a panel ceiling beneath. When a flat ceiling is desired, it is made as represented in (c), Fig. 88; a shows a flat bar, set on the lower flange of the I beams, and carrying the wire netting b, that serves as a support for the ceiling composition, which is laid on forms placed under the beams. The thickness of this coating is about 1 1/2 inches, 1 inch being below the lower flanges of the beams.

183. This floor has a great many points of superiority over other floors, the chief ones being its lightness and strength. The average weight of the plate forming the floor is 18 pounds per square foot, and that of the ceiling plate, exclusive of plastering, 6 pounds. The great saving in weight may be seen by comparing these Figures with those for dense tile arches, given heretofore.

The breaking load on a floor of this kind is about 1,500 pounds per square foot, with plates 4 inches thick and 6 feet span. In some instances as much as 2,000 pounds per square foot have been carried safely. This floor construction is entirely fireproof, and, on the whole, is one of the very best systems yet introduced.

184. The principle of the Metropolitan floor is utilized in other constructions, which, however, make use of cement concrete in place of plaster, and expanded metal or wire netting instead of wire rods, to resist the tensile stress.

In all floors made of concrete, plaster, or tile, with steel or iron tension rods, the two materials should be closely united, so that the bars or wires will not draw through the concrete, and thus destroy the rigidity of the floor. For this reason, the bars in the Ransome floor, and the wires in the Metropolitan floor, are twisted, and wire netting, etc. provided in other forms, all serving to resist the slipping tendency. Furthermore, to obtain complete adhesion of concrete to metal, it is necessary that the materials and workmanship be first class in all respects.