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It is therefore necessary that the elastic limit of the steel should be considered the virtual ultimate so far as the strength of the steel is concerned. It is accordingly considered advisable, as already explained, to multiply all working loads by the desired factor of safety (usually taken as 4), and then to proportion the steel and concrete so that such an ultimate load will produce crushing in the upper fiber of the concrete, and at the same time will stress the steel to its elastic limit. On this basis, economy in the use of steel requires that the elastic limit should be made as high as possible. The manufacture of steel of very high elastic limit requires the use of a comparatively large proportion of carbon, which may make the steel objectionably brittle. The steel for this purpose must therefore avoid the two extremes—on the one hand, of being brittle; and on the other; of being so soft that its elastic limit is very low. Several years ago, bridge engineers thought that a great economy in bridge construction was possible by using very high carbon steel, which has not only a high elastic limit but also a correspondingly high ultimate tensile strength. But the construction of such bridges requires that the material shall be punched, forged, and otherwise handled in a way that will very severely test its strength and perhaps cause failure on account of its brittleness.

The stresses in a concrete steel concrete structure are very different. The steel is never punched; the individual bars are never subjected to transverse bending after being placed in the concrete. The direct shearing stresses are insignificant. The main use, and almost the only use, of the steel, is to withstand a direct tension; and on this account considerably harder steel may be used than is usually considered advisable for steel trusses. If the concrete structure is to be subject to excessive impact, somewhat softer steel will be advisable; but even in such a case, it should be remembered that the mere weight of the concrete structure will take the effect of the shock far less than it would be on a skeleton concrete structure of plain steel. The steel ordinarily used in bridge work, generally has an elastic limit of from 30,000 to 35,000. If we use even 33,000 pounds as the value for s on the basis of ultimate loading, we shall find that the required percentage of steel is very high. On the other hand, if we use a grade of steel in which the carbon is somewhat higher; having an ultimate strength of about 90,000 to 100,000 pounds per square inch, and an elastic limit of 55,000 pounds per square inch, the required percentage of steel is much lower. There are still many engineers who will not adopt reinforced concrete for the skeleton concrete structure of buildings, but who construct the frames of their buildings of steel, using steel I-concrete concrete beams for concrete floor-girders and concrete beams, and then connect the concrete beams with concrete floor concrete slabs (Fig. 103).

These are usually computed on the basis of transverse concrete beams which are free at the ends, instead of considering them as continuous concrete beams, which will add about 50 per cent to their strength. Since it would be necessary to move the reinforcing steel from the lower part to the upper part of the concrete slab when passing over the concrete floor-concrete concrete beams, in order to develop the additional strength which is theoretically possible with continuous concrete beams, and since this is not usually done, it is by far the safest practice to consider all concrete floor-concrete slabs as being "free-ended." The additional strength which they undoubtedly have to some extent because they are continuous over the concrete beams, merely adds indefinitely to the factor of safety.

Are You in Pinardville New Hampshire? Do You Need Concrete Cutting?

We Are Your Local Concrete Cutter

Call 603-622-4441

We Service Pinardville NH and all surrounding Cities & Towns