The first buildings of reinforced concrete were designed of slabs, beams and girders, similar to wooden construction. A modern type, the so-called flat-slab floor, dispenses with all beams and girders, and uses a continuous flat slab of equal thickness throughout, sup ported on columns with flared heads. At times the slab thickness is increased around the column head, a so-called adropped panel.° The reinforcing consists of bands of small rods g-inch to %-inch diameter; in the 4-way system running diagonally between columns and also directly; in the 2-way system the rods nin rectangularly at right angles to the sides of the panel and are spaced more closely near the column. The rods are bent up from the bot tom of the slab to the top about the Y4 distance of the span to pass over the column in the 4-way, or over the column and the edge of the panel in the 2-way. Rods that are spliced should pass over to the third point of the adjoining panel. The flared head of the column should be 0.225 of the span between column centre, and the flare should be not more than 45 degrees with the vertical. The slab thickness should be not less than 1/32 the span length. The greatest bending moment exists around the column heads. These flat slab buildings offer more favorable installation of sprinkling systems, spread light farther in the interior, provide a given head room with less height of building and offer economy in forms and in construc tion. The General Electric Company's factory building at Fort Wayne, Ind., has a °Corr plateo flat slab floor with panels 25 feet by 25 feet. The slab is 13 inches thick at the centre and 23 inches thick at the columns. The ex panded head of the column is 60 inches in diam eter. For a live load of 400 pounds per square foot, the two-way reinforcement of the panel consists of corrugated bars spaced 10% inches in the centre of the panel, then g-inch bars spaced 9% inches, decreasing to 6g inches spacing near the columns. The Soo Ter minal at Chicago is an elevated railroad yard over 2,000 feet long and 345 feet wide carrying the heaviest engines. Columns 12 feet 10 inches high, spaced 24 feet square, are 32 inches in diameter, and flared out to a 5-foot 6-inch capital. The slab is 18 inches thick at centre and 30 inches thick for an area of 9 feet square, surrounding the columns. The reinforcement is 4-way, consisting of g-inch rods spaced 5% inches. The structure is with out expansion joints.
An excellent type of floor slab is composed of hollow tile spacers between reinforced con crete joists, giving a stiff, light flat ceiling that holds plaster well. Some building-s are con structed by erecting previously and separately molded elements such as columns, beams and slabs. Occasionally a light structural steel framework is erected and supports the forms. After the concrete is poured, the framework becomes the reinforcement.
Finish of concrete floors is similar to wear ing surface in sidewalks. If a wood floor is desired, nailing strips of wood about 2 inches by 4 inches spaced 16 inches are laid on the rough slab and embedded in a lean cinder con crete. Linoleum may also be laid directly on a smooth base. Various special floor finishes have been devised.
Footings in uniform soils should be designed for equal soil pressure so that the settlement of the entire building shall be uniform to avoid cracks. The dead load is chiefly considered.
In firm soils such as gravel, hard clay, footings may be plain, without reinforcement, sloping from the column base at an angle of 60 degrees with the horizontal. Column footings on yield ing soil are spread, and designed as cantilevers to resist flexure. The load on the column plus the dead weight of the footings divided by the bearing capacity of the soil equals the required area of the footing. The rods may run in either 2-way at right angles, or 4-way, i.e. rectangular and diagonal. Often the footing. f or a wall col unr.on a property line cannot project into the adjoining property; then a combined footing for the wall column and the adjoining interior column is constructed of trapezoidal shape to carry both columns. Raft foundations, consist ing of a simple slab under the entire building, float the structure in yielding soil, or support it on piles.
Reinforced or plain concrete piles are either formed in place, as the Simplex and Raymond, or separately molded and driven, as the Gil breth and Chenoweth. Unlike wooden piles, they may extend above the level of permanent ground water. In the Simplex pile, a hollow cylindrical steel tube or form 16 inches in diameter and g-inch thick is driven to a suit able bearing. Then the pile is filled with con crete and the tube withdrawn. The driving point may be either a conical cast-iron point,that is left in place, or more rarely, a hinged cutting edge, an °alligator point," which opens as the tube is withdrawn. For the Raymond pile, a conical shell, made of wire encased in a thin sheet steel shell, is driven along. with a mandrel, to re fusal. The mandrel is withdrawn, and the shell filled with concrete. In these two types it should be certain that the soil does not fall back as the tube is withdrawn, or else deform the green concrete; and that the back pressure does not deform the shell. Apart from simnle reinforced piles, patented forms of separately molded piles are the Gilbreth pile, a tapered reinforced pile with longitudinal corrugations, and a central hole for a water jet ; and the Chenoweth pile, which is rolled. A layer of concrete, two inches or more in thickness, is laid on wire cloth, attached to a mandrel; after rolling the wire cloth becomes a spiral in the pile, which is also wrapped around with wire. Separately molded piles are made under favor able conditions as to concrete. (See Pazs). In the McArthur construction a hole is driven to a firm stratum, and enlarged at the bottom by tamping concrete.
Walls of concrete buildings are preferably of skeleton construction, with wall girders and columns, and curtain walls of concrete 6 inches thick, or of brick or tile. The surface is often veneered with brick, tile, marble, etc., which is tied on with metal ties, previously fixed in the concrete.
Stairs of concrete are reinforced as beams between floor and landings. For common spacings of about 6 feet from floor to landing, the steps are 5 inches thick from the re-entrant angle of tread to bottom, with V2-inch round rods 6-inch centres; at each floor is a girder about 10 inches by 15 inches deep and the landing is a simple reinforced slab 5 inches thick, when the stairs are parallel; if at right angles, the landing is hung by rods to a girder above the stairway.