The Unit System.-In the Unit System, which is controlled by the Unit Concrete Steel Frame Company of Philadelphia, all of the metallic reinforcement for each beam or girder is made into a single unit and placed as a unit in the form. This is accomplished by having both the straight and camber bars fastened together by stirrups and clamps, so that each tension and shear member is rigidly held in its proper position. This precludes the possibility of one or more members being omitted or incorrectly placed by workmen at the building, and affords opportunity for inspection prior to use.

The advantages claimed for this system are absolute accuracy in the placing of the reinforcing material; the ease with which it can be inspected and errors, if any, detected and corrected before concreting; the impossibility of omitting any tension or shear member; the additional strength secured by binding the slab concrete to the beam concrete by means of lacing of the slab reinforcement through the stirrups. The girder frames may thus be set in advance of the concrete work, and provision made for shafting or other overhead fixtures.

CHAPTER XX

REINFORCED CONCRETE IN FACTORY AND

GENERAL

BUILDING CONSTRUCTION

Advantages of Reinforced Concrete in Building Construction.-Practical Details of Construction. Slabs, Columns, Floors, Loads, Walls.-Roofs.—Attaching Machinery.

IN the factory, where the primal considerations are serviceability, fireproofness, and cost, concrete has found one of its leading applications. When reinforced with steel, a structure is obtained which is lower in first cost than an all-steel building, which can be more quickly erected, and which is freer from vibration and more fireproof.

As compared with what is known as the "slow-burning," or "mill" type of construction, reinforced concrete is more fireproof, durable, and carries a lower rate of insurance as the mill type is a combination of brick, stone, or concrete walls with timber floors and columns.

Cost. While all statements as to cost of reinforced concrete buildings may be somewhat unreliable, it is safe to figure that in the simple factory building where elaborate forms are not required and building material prices not excessive, the cost will be about 8 cents per cu. foot. This price will increase with elaboration of surface finish and ornamentation and other unfavorable conditions to 12 cents per cubic foot.

The volume includes the building from footing to roof and the price does not include interior work such as lighting or heating plants, machinery, plastering, plumbing, or elevators.

Fire resistance is one of the chief inducements that has led to the extensive use of concrete in factories. The materials to be employed are first-class Portland cement, quartz sand, and broken trap rock. Limestone aggregates are more easily injured by extreme heat and gravel is more readily dislodged. Cinders make a

good aggregate for fire resistance, but the concrete made therefrom is not sufficiently strong for reinforced concrete work excepting for partition walls and short spans.

A reinforced concrete factory is necessarily a very stiff structure, every part being inseparably connected with every other part by continuous beams, girders, and slabs. This permits the operation of the heaviest machinery with much less vibration than equivalent steel structures.

In taking up the question of a concrete factory, the layout and arrangement of machinery should first be made and the building designed to accommodate the resulting loads.

One of the distinct advantages of a concrete factory is the large amount of window space and light thus made available which is due to the inherent strength of concrete and the thin members required to support the windows, etc. In addition to these advantages the floors of concrete may be made absolutely watertight, can readily be flushed with a hose, and are fire- and vermin-proof.

Practical Construction Details.-The essential principles governing the design of girders, columns, and slabs, have already been given in Chapters XVII and XVIII. The following data is of importance in connection with building and factory construction, and is taken from "Reinforced Concrete in Factory Construction," by Sanford E. Thompson.*

Floor Slabs.-The thickness and reinforcement of the floor slabs are determined by the distance between the beams, and by the loading which will come upon them. The most usual thicknesses are 3 1/2 inches to 5 inches, with reinforcement calculated from the bending moment produced by the loads. An economical quantity of steel is apt to be from 0.8 per cent to 1 per cent of the sectional area of the slab above the steel.

A few rods are usually placed at right angles to the main bearing rods of the slab to assist in preventing contraction cracks, and these also add to the strength of the slab.

In a factory or warehouse the most economical floor surface is generally a granolithic finish, consisting of a layer of 1:2 mortar about three-quarter inch thick, spread upon the surface of the con

* Published by the Atlas Portland Cement Co.

crete slab before it has begun to set, and trowelled to a hard finish just like a concrete sidewalk.

Machines are readily bolted to the concrete by drilling small holes in the concrete at the proper points for the standards and grouting the lag screws in place, or else bolting them through the slab.

If for any reason a wood floor is required, stringers may be laid upon the top of the concrete and spaces left between them or filled. with cinders or with cinder concrete.

Stirrups. Besides the ordinary compression and pull in a beam, there are secondary stresses of shear or diagonal tension, which, if

Lil'

FIG. 63.-Ordinary Type of Ribbed Slab.

not provided for, will produce diagonal cracks. These will run in a general direction from the bottom of the beam near the supports on an incline toward the top of the beam, and may cause the beam to fail. To prevent this cracking, unless the beam is so wide that the concrete can take the whole of the stress without exceeding 60 pounds per square inch in shear, vertical or inclined steel bars, of sizes accurately computed, must be placed. The bent-up tension rods take care of a part of this shear, or diagonal tension, but if these are not sufficient, stirrups, which are usually made in the form of a U, must be inserted at the proper locations to take the remainder.

Columns. The most important of all the members of the building are the columns, for if a column fails, the entire building is liable to go down.

If columns, as ordinarily built in building construction, are made of 1: 2: 4 proportions, it is safe in an ordinary building to allow a direct compressive strength of 450 pounds per square inch, provided the columns are at least 12 inches square. A customary manner of designing is to figure the entire compression upon the concrete to the full size of the column, but to place four or possibly six rods of 5/8 inch or 3/4 inch diameter near the corners or sides of

FIG. 64.-Column Reinforcement. FIG. 65.-Reinforced Concrete

Column Footing.

the column, with 1/4-inch wire loops around these rods at occasional intervals in the height, say, from 8 to 12 inches apart.

Vertical steel rods of larger size may be introduced when it is necessary to decrease the size of the columns. These may be computed to bear a portion of the compressive load, but they cannot be figured at their full safe value of 16,000 pounds per square inch because they have a different modulus of elasticity and compressive strength from concrete and can only shorten the same amount as the concrete. Under ordinary circumstances, therefore, they cannot be assumed to bear more than the safe compressive stress in the concrete times the ratio of elasticity of steel to concrete, or about 7,000 pounds per square inch. Because of this small amount of compres