Table VIII. One-Way Metal-Tile Floor-Construction. Volumes of Concrete in Cubic Feet, per Square Foot of Floor-Area 1. Stairs. CHAPTER XII STAIRS, WALLS, PARAPETS, AND CORNICES When REINFORCED-CONCRETE STAIRS are used for industrial buildings, they are usually constructed as inclined slabs, monolithic with treads and risers, and spanning from floor to floor, or from floor to intermediate landing. The thickness of the slab varies from 4 to 8 in, according to the spans and loads. The proportion of tread-width to riser-height is very important and rigidly Fig. 2. Recessing and Bonding Concrete Stair-Slab Supported by Beam. No Lower Run Fig. 3. Face of Riser Set Back to Allow for Safety-Tread. Fig. 4 Lower Run at Landing specified by practically all building codes. In general, the sum of the treadwidth and riser-height should approximate 17 in, and the product of the treadwidth and height of riser in inches should lie between 70 and 75. Wherever possible a monolithic finish is the most satisfactory. Safety treads, with or without a projecting nosing, are the almost invariable rule. As it is usually not practicable in multistory building to cast the stairs at the same time as the floor-slabs, BONDS AND RECESSES are provided at the junctions of stair-slabs and floors. Fig. 1 shows a detail for stairs resting on a floor-slab. Figs. 2 and 3 show methods of recessing and bonding when the stair-slabs are supported by beams, and Fig. 4 a detail for a down-run from either a floor or a landing. It will be seen that the detail in Fig. 3, to be used Fig. 6. Recesses and Bonds for Concrete Stairs for Girderless Floor-Construction where there is a down-run from the same supporting beam, differs from Fig. 2 in that the former shows the face of the riser set back 31⁄2 in from the face of the beam to allow sufficient space to set a safety tread on the top riser of the downrun and still permit an alignment of the riser-faces. Fig. 5 shows a recommended detail for tread and riser for general application, and Fig. 6 shows bonds and recesses for stairs designed for girderless floor-construction. In the computations for the thickness of concrete and sectional area of steel, the slab is considered as SIMPLY SUPPORTED, and its length as equal to the horizontal distance between supports. The main longitudinal reinforcement, preferably of small bars, is usually carried across landings, the bars being bent so that they lie near the lower surface of the landing-slab. Round or square 3/8-in distributing bars, also, should be placed transversely to the main reinforcement, about 18 or 24 in on centers. With the inclusion of these details, the design of stair-slabs is identical with that described for floor-slabs in Chapter V. Table I gives the SLAB-THICKNESS AND REINFORCEMENT required for different spans, with a live load of 100 lb per sq ft (New York City requirement), Fig. 7. Typical Design of Reinforced-Concrete Stairs, Based upon New York City Requirements. Plan based on the stresses indicated. Figs. 7 and 8 show a typical design in accordance with the same code. 2. Concrete Curtain Walls. Curtain walls are ordinarily built of reinforced-concrete, brick, or terra-cotta, or a combination of these materials. Reinforced-concrete curtain walls are usually made 6 or 8 in in thickness, the height depending upon the use for which the building is designed. The reinforcements should preferably be small rods placed 2 in inside of the exterior face of the wall. If 3%-in squares are used, the spacing should be about 12 in on centers horizontally, and from 12 to 18 in on centers vertically, depending upon the height of the wall. Some designers use as much as 0.3% of steel, based on the concrete-section in each direction, but this would seem to be a little more than required under ordinary conditions. Figs. 9 and 10 show two general methods of curtain wall design. If the curtain wall forms the spandrel beam, as in Fig. 10, the framing at the corners may be arranged as shown in |