The concrete was composed of 1 part Portland cement, 3 parts sand, and 5 parts stone. The stone was graded in size from t inch o 1 inch. "Johnson" corrugated bars were used as the reinforcing steel. A panel, 16 by 20 feet, of one of the floors, was tested by placing a load of 300 pounds per square foot over this area. The deflection was so slight that it could not be conveniently measured. In Fig. 203 is given a view of the under side of a floor, showing the connection of the girder and beams with the column.
There is a criticism that may be made in the details of the girder shown in Fig. 201. The bars, which are turned up at the end, should have been long enough so that the bars could be again bent parallel to the floor line and be ex tended through the column. This would have tied the girders together in a more secure manner; and these bars, being near the top of the slab, would have resisted any negative bending moment.
385. Apartment House. In designing a reinforced-concrete apartment house which was constructed at Juniper and Spruce Streets, Philadelphia, it was desirable to have a floor system that was flat on the under side, except for the beams connecting the columns, so as to avoid the expense of a suspended ceil ing. The greatest span of the fiat construction necessary to avoid having beams in the ceiling of the rooms, was about 18 feet. It was at first intended to use a slab of reinforced con crete to connect the beams; but, as the Phila delphia Building Code requires that the depth of reinforced concrete must be at least three fifths of an inch per foot of span, to fulfil this condition a slab much thicker than neces sary for structural purposes was required. The Building Code requires that the floors of apart ment houses shall be designed to carry safely 70 pounds per square foot.
This apartment house is 40 feet by 127 feet, and eight stories in height. There is also a basement under the entire building. In taking bids on this building, it was found that a steel frame, not including the fireproofing, cost more than a reinforced-concrete structure. It was therefore decided to construct the building of reinforced con crete. The walls were of brick, except the eighth story, which was concrete. The concrete wall is hollow, having a total thickness of 16 inches; and it is composed of two slabs, each six inches in thickness, with an air space of four inches between the slabs. These slabs are
reinforced with steel bars placed longitudinally and vertically.
The- type of floor construction used is shown in Fig. 204. Rein forced-concrete girders were constructed, connecting the columns; and the space between them was filled with small reinforced-con crete beams and plaster blocks. The girders were designed, when possible, as T-beams; and as a certain amount of concrete was required in the slab to take the compression, the hollow block construction was omitted for a sufficient width on each side of the girder to allow for this com pression. This feature is shown in Fig. 204. The beams were 4 inches wide, 6 inches to 8 inches deep, depending on the span, and were connected with a 2-inch slab of concrete. The beams were spaced 16 inches center to center, and each beam was rein forced with a 1-inch round bar. The two-inch slab was rein forced with ±-inch bars spaced 24 inches; and over the girders and at right angles to the girders, -inch bars 6 feet long were spaced 16 inches; that is, one of these bars was placed in the top of each of the beams. The span of these beams varied from 12 feet to 18 feet.
A hollow plaster block, 12 inches wide, was used as a filler be tween the concrete beams. These blocks were made of the required depth, 6 and 8 inches, and were 12 inches wide at the top and 11-1 inches wide at the bottom. The object in sloping the sides of the blocks was to key the blocks between the beams. The block, in section, is shown in Fig. 205, and is known as the Keystone Fireproof Block. The coefficient of expansion of plaster blocks is very small compared with that of the terra-cotta block; and also the plaster block is more efficient as a non-conductor of heat. The blocks were spaced 4 inches apart, and therefore served as the forms for the sides of the beams. The planks on which the blocks were placed were spaced S inches apart, which made a saving in the amount of lumber required for forms. It was found necessary to wet the blocks thor oughly by means of a hose, before the concrete was placed, as the dry blocks quickly absorbed the water from the concrete. About one per cent of the blocks were broken in handling them. The partitions in the building were made with the blocks. When the floor forms were removed, the ceilings and walls were plastered.