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 ptirpose must therefore avoid the two extremes—on the one kind, of being brittle; and on the other, of being 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 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 a considerably harder steel may be used than is usually considered advisable for steel trusses.
If the structure is to be subject to excessive impact, a somewhat softer steel will be advisable; but even in such a case, it should be remembered that the mere weight of the structure will make the effect of the shock far less than it would be on a'skeleton 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.
2S2. Slabs on I-Beams. There are still many engineers who will not adopt reinforced concrete for the skeleton structure of build ings, but who construct the frames of their buildings of steel, using steel I-beams for floor-girders and beams, and then connect the beams with concrete floor-slabs (Fig. 103). These are usually computed on
the basis of transverse beams which are free at the ends, instead of considering them as continuous 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 slab when passing over the floor-beams, in order to develop the additional strength which is theoretically possible with continuous beams, and since this is not usually done, it is by far the safest practice to consider all floor-slabs as being "free-ended." The additional strength which they un doubtedly have to some extent because they are continuous over the beams, merely adds indefinitely to the factor of safety. Usually the requirement that the I-beams shall be fireproofed by surrounding the beam itself with a laver of concrete such that the outer surface is at least 2 inches from the nearest point of the steel beam, results in having a shoulder of concrete under the end of each slab, which quite materially adds to its structural strength. But usually no allowance is made; nor is there any reduction in the thickness of the slab on account of this added strength. In this case also, the factor of safety is again indefinitely increased. The fireproofing around the beam must usually be kept in place by wrapping a small sheet of expanded metal or wire lath around the lower part of the beam before the con crete is placed.
Slabs Reinforced in Both Directions. When the floor-beams of a floor are spaced nearly equally in both directions, so as to form, between the beams, panels which are nearly square, a material saving can be made in the thickness of the slab by reinforcing it with bars running in both directions. The theoretical computation of the strength of such slabs is exceedingly complicated. It is usually con sidered that such slabs have twice the strength of a slab supported only on two sides and reinforced with bars in but one direction. The usual method of computing such slabs is to compute the slab thick ness, and the spacing and size of the reinforcing steel, for a slab which is to carry one-half of the actual load. Strictly speaking, the slab should be thicker by the thickness of one set of reinforcing bars.