With planks resting on ties, a spacing of five feet, or even more, can be used, depending, of course, on the size and strength of tie and the assumed concentrated live load. This wide spacing reduces materially the weight of metal in the stringers, thus saving more than enough to compensate for the cost of the ties.

With a concrete deck it is economical to space the stringers from six to ten feet, there being very little difference in total cost for variations between this range. It is better, as a rule, to adopt the smaller limit so as to reduce the cost of the slab-forms. The economic panel-length is small, running from ten or twelve feet for a twenty-foot spacing of main girders to fifteen feet for a thirty-foot spacing, and to eighteen feet for a fifty-foot spacing. It is specially short for closely-spaced stringers with plank floor. In general it may be said that the greatest economy obtains when one of the standard beams just barely figures as a stringer. For instance, if in choosing between a six-foot and a seven-foot stringer-spacing, the 20" 65 lb. I just figures for the latter and must also be used for the former because the 18" 60 lb. I is not strong enough, it will be found economical to adopt the greater spacing. On the other hand, if the 18" 55 lb. I just figures for the six-foot spacing, and the 20" 65 lb. I is required for the seven-foot spacing, the former will prove the more economic.

The question of extra metal in girders or trusses to carry heavier slabs is not important with short spans, but is very much so with long spans.

With concrete deck on plate-girder spans it will sometimes be found cheaper to use no stringers, but to employ rather-closely-spaced cross-girders on which the slab rests directly. The economic spacing of these girders figures out twelve feet or more, so far as quantities of materials are concerned; but it is better to use ten feet or even less, because the supporting of the slab-forms for the long panels is expensive. This type of floor is specially adapted to through, plate-girder spans and will be discussed further in connection therewith. It does not save quite so much with deck plate-girders, as the girders themselves form two or more lines of stringers; hence the, total weight of stringer metal which can be saved is less with deck plate-girders than with through plate-girders. Again, an outside stringer may, in any case, be required to support the hand-railing.

When the structure carries electric railway tracks, a stringer must be placed under each rail. The addition of such stringers would practically eliminate the saving above discussed; and, therefore, the standard type of floor-system with stringers and floor-beams should be employed.

Sidewalk slabs should be carried on longitudinal stringers, even if the cross-girders are spaced closely. The outside stringer should generally be a channel so as to provide a flush surface for the attachment of the hand- railing; and with short panels the inner stringers may be of the same section. For five-foot walks two stringers will give the cheapest construction, the curb  stringer  of  the  roadway  sometimes  being  utilized  as  one  of