This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
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 ceiling. The greatest span of the flat 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 concrete to connect the beams; but, as the Philadelphia 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 necessary for structural purposes was required. The Building Code requires that the floors of apartment houses shall be designed to carry safely 70 pounds per square foot.
Fig. 204. Floor for an Apartment House.
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 concrete. 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-concrete 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 compression. 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 reinforced with a 1-inch round bar. The two-inch slab was reinforced with 1/4-inch bars spaced 24 inches; and over the girders and at right angles to the girders, 3/8-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 between 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 1/2 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
Fig. 205. Section of Plaster Block.
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 8 inches apart, which made a saving in the amount of lumber required for forms. It was found necessary to wet the blocks thoroughly 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.
On the Juniper Street side, balconies were constructed nearly the full length of the house. They are 4 feet wide. Structurally they were constructed as cantilever beams, and consist of slabs of concrete 6 inches thick and reinforced with 1/2-inch round bars spaced 6 inches center to center. These balconies are constructed at each floor level. In Fig. 204 is shown a cantilever beam with a span of 6 feet. It is 12 inches wide and 26 inches in depth, and is reinforced with 4 bars 7/8- inch in diameter. This cantilever supports the exterior wall and one end of a simple beam of a span of about 16 feet.
The concrete consisted of a mixture of 1 part Portland cement, 3 parts sand, and 5 parts stone. The stone was trap rock, broken to pass through a 3/4-inch ring, dust screened out; and the sand is known as Jersey gravel, which is a bank sand. The reinforcing bars were plain round bars of medium steel.
386. The McNulty Building.* The columns used in the construction of the McNulty Building, New York City, are a very interesting feature in this building. The building is 50 feet by 96 feet, and is 10 stories high, and was one of the first small-column reinforced-con-crete buildings erected in New York. The plan of all the floors is the same. A single row of interior columns is placed in the center of the building, about 22 feet center to center.
The columns are of the hooped type, and were designed from the formula approved by the building laws of New York City. The formula used was P = 1,600 r2 + (160,000 Ah ÷ p ) x r + 6,000 As, in which P = the total working load, r = radius of the helix, As = the total area of the vertical steel, Ah = sectional area of the hooping wire, p = the pitch of the helix.
*Eengineering Record, Sept. 14. 1907.
The interior columns are Cylindrical in form, except those supporting the roof, which are 12 by 2-inch and are reinforced with 4 bars 3/4 inch in diameter. In all tie other stories except the ninth, they are 27 inches in diameter. Below the fifth floor the reinforcement in each of these columns consists of 2-inch round vertical bars, ranging in number from seven in the fifth floor to thirty in the basement, and banded by a 24-inch helix of 1/2-inch wire with a pitch of 11/2 inches. The vertical bars were omitted between the sixth and tenth floors; and the diameter of the helix was gradually decreased, while the pitch was increased. In the ninth floor the diameter was reduced to 21 inches.
The wall columns are in general 30 by 26 inches, and support loads from 48,000 pounds in the tenth floor to 719,750 pounds in the basement. In the sixth story, the reinforcement in these columns consists of 3 round vertical bars 2 inches in diameter; and in each of the floors below, the number of bars was increased in these columns there being 24 in the basement columns. These are spirally wound with 5/16-inch steel wire forming a helix 23 inches in diameter, with a pitch of 2 1/2 inches. Above the seventh floor, the columns are reinforced with 4 bars 3/4 inch in dameter, and tied together by 5/16--inch wire spaced 18 inches apart. The columns rest on cast-iron shoes, which are bedded on solid rock about 2 1/2 feet below the basement floor.
The main-floor girders extend transversely across the building, and have a clear span of 21 feet. The floor-beams are spaced about 6 feet apart, and have a span of about 20 feet 6 inches. The sides of the beams slope, the width at the bottom being two inches less than the width at the under surface of the slab. The reinforcement consists of plain round bars. The bars for the girders and beams were bent and made into a truss (the Unit System) at the shops of the contractor, and were shipped to the work ready to be put in place. The stirrups were hot-shrunk on the longitudinal bars. The helices for the columns were wound and attached to some of the vertical rods at the shop, to preserve the pitch. The vertical rods in each column project 6 inches above the floor line, and are connected to the bar placed on it, by a piece of pipe 12 inches long.
The concrete was a 1:2:4 mixture. Giant Portland cement was used, and 3/4-inch trap rock. The placing of concrete was begun about the middle of August, 1906, and the building was completed December 20.