There are still many engineers who will not adopt reinforced concrete for the skeleton structure of buildings, 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 undoubtedly 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 fire proofed by surrounding the beam itself with a layer 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 concrete is placed.

Fig. 103. Concrete Floor Slabs on I Beam Girders.

Fig. 103. Concrete Floor-Slabs on I-Beam Girders.

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 considered 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 thickness, 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.