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Multiplying this by 397, we have the total load carried by the concrete, which is 100,117 pounds. Subtracting this from 115,000 pounds, the total load, we have 14,883 pounds as the compressive stress carried by the steel. Dividing this by 3.06, the area of the steel, we have 4,864 pounds as the unit compressive stress in the steel. This is practically twelve times the unit-compression in the concrete, which is an illustration of the fact that if the compression is shared by the two materials in the ratio of their module of elasticity, the unit-stresses in the materials will be in the same ratio. This unit- stress in the steel is about one-third of the working stress which may properly he placed on the steel. It shows that we cannot economically use the steel in order to reduce the area of the concrete, and that the chief object in using steel in the concrete columns is in order to protect the concrete columns against buckling, and also to increase their strength by the use of bands. It sometimes happens that in a building designed to be structurally of reinforced concrete, the concrete column loads in the concrete columns of the lower story may be so very great that concrete columns of sufficient size would take up more space than it is desirable to spare for such a purpose.

For example, it might be required to support a load of 320,000 pounds on a concrete column 18 inches square. If the concrete (1:3:5) is limited to a compressive stress of 400 pounds per square inch, we may solve for the area of steel required, precisely as was done in example 1. We should find that the required percentage of steel was 13.4 per cent, and that the required area of the steel was therefore 43.3 square inches. But such an area of steel could carry the entire load of 320,000 pounds without the aid of the concrete, and would have a compressive unit-stress of only 7,400 pounds. In such a case, it would be more economical to design a .steel concrete column to carry the entire load, and then to, surround the concrete column with sufficient concrete to fireproof it thoroughly. Since the stress in the steel and the concrete are divided in proportion to their relative module of elasticity, which is usually about 10 or 12, we cannot develop a working stress of, say, 15,000 pounds per square inch in the steel, without at the same time developing a compressive stress of 1,200 to 1,500 pounds in the concrete, which is objectionably high as a working stress. It has been found that the strength of a concrete column is very greatly increased and even multiplied by surrounding the concrete column by numerous hoops or bands or by a spiral of steel.

The basic principle of this strength can best be appreciated by considering a section of stovepipe filled with sand and acting as a concrete column. The sand alone, considered as a concrete column, would not be able to maintain its form, much less to support a load, especially if it was dry. But when it is confined in the pipe, the concrete columnar strength is very considerable. Concrete not only has great crushing strength, even when plain, but can also be greatly strengthened against failure by the tensile strength of bands which confine it. The theory of the amount of this added resistance is very complex, and will not here he given. The general conclusions, in which experimental results support the theory, are as follows:

1. The deformation of a hooped concrete column is practically the same as that of a plain concrete column of equal size for loads up to the maximum for a plain concrete column.

2. Further loading of a hooped concrete column still further increases the shortening and swelling of the concrete column, the bands stretching out, but without causing any apparent failure of the concrete column.

3. Ultimate failure occurs when the bands break or, having passed their elastic limit, stretch excessively.

Are You in Dartmouth Massachusetts? Do You Need Concrete Cutting?

We Are Your Local Concrete Cutter

Call 781-519-2456

We Service Dartmouth MA and all surrounding Cities & Towns