185. The Roebling System

The Roebling System. In this floor, the concrete is used as an arch, and not as a beam. The method of construction is shown in Fig. 89. The first step is to spring between the beams an arch of wire cloth, shown at a, stiffened at short intervals by steel rods b, the ends of the arch resting on the lower flanges of the beams. On this base is laid the arched concrete filling shown at c, which, on becoming hard, transmits the loads directly to the beams. The ceiling is held by a similar netting of wire /, fastened to stiffening rods d, which are attached to the lower flanges of the beams by clamps, and also to the arch netting by tie-rods, as at g.

185 The Roebling System 216

Fig. 89.

This floor weighs, exclusive of beams, but not including the ceiling and two thicknesses of wood flooring, from 47 to to 59 pounds per square foot, according to the span. The carrying capacity is from 1,000 to 2,400 pounds per square foot, on spans of from 4 1/2 to 5 feet. 2-18

The Roebling floor possesses the following advantages: first, a flat ceiling; second, a continuous air space between the floor arch and the ceiling; third, its lightness as compared with most tile ceilings; fourth, the ease of adaptation to any building or any load; and, lastly, no wood centerings are necessary - the wire arches forming the support for the concrete during the laying.

186. The Columbian system, shown in Fig. 90, resembles somewhat the Metropolitan floor, but with the difference that the concrete, instead of being stiffened and supported by wires, is carried on steel bars, having the section shown at a, in (a), Fig. 90; these are inserted in similarly shaped openings in the stirrups (a) which set over the I beams. The bars are from 1 1/2 to 2 1/2 inches deep; the 2-inch bar is the usual size for office buildings, the 2 1/2-inch bar being used for warehouses, etc. The stirrups are usually spaced about 20 inches apart, and are made of 2" X 3/16" steel plate. The spacing of the floorbeams is from 5 to 6 feet, and as the loads are transmitted vertically, no tie-rods are required.

185 The Roebling System 217185 The Roebling System 218

Fig. 90.

When the bars are in place, forms are hung beneath them, and a layer of cement concrete is formed, completely embedding the bars. On this the flooring is laid in the usual manner.

187. In (b), Fig. 90, is represented the construction when a flat ceiling is desired. At a is shown the ribbed bar; at b, the steel stirrup; at c, the concrete casing of the floorbeams; at d, the air space; and at e, the 1-inch bar laid on the lower flange of the I beams, and forming the support for the ceiling, which is put on similarly to the floor layer. In (c), Fig. 90, are shown the I beams encased in slabs of concrete a, with air spaces, as at b, on the sides and under the beams. At d, in (d), is shown a beam protection made of hollow tile. Both the concrete and the tile are fastened to the beams by clamps or ties, as at c, in (c), which are well protected by the casing.

188. The safe loads and weights of this floor, per square foot, exclusive of I beams, plastering, and flooring, are as follows:

Depth of Bars.

Span.

Thickness of Concrete.

Weight, Lb. per Sq. Ft.

Safe Load, Lb. per Sq. Ft.

2 1/2

inches.

6

feet.

4

inches.

40

600

2

inches.

6

feet.

3

inches.

30

200

1 1/2

inches.

5

feet.

2 1/2

inches.

24

150

The level ceiling shown in (b), Fig. 90, weighs about 20 pounds per square foot.

As an instance of the strength of this floor, it is said that a mass weighing about 240 pounds was dropped several times from a height of 8 feet, upon the middle of an 8-foot span, without causing any injury to the floor.