Ballasted decks on reinforced-concrete slabs are much used for half- through, plate-girder spans. The floor-system can be of the usual stringer- and-floor-beam type, generally with four lines of stringers per track, on account of restricted headroom; or the slabs can rest on rather-closely-spaced, transverse beams. With the latter arrangement it is economic of metal to adopt spacings of 5 feet or more; but, on account of restricted head room, it is frequently necessary to space much more closely, in order to permit the use of shallower beams. If the available depth is very small, these cross-beams can be embedded in the solid slab; or reinforced-concrete troughs resting on the bottom. flanges of the beams can be employed, the tops of the beams being encased in concrete. The relative economics of the last three types depend on whether the steel needs to be encased so as to protect it from locomotive blast or other attack from below. If no encasement be required, the first type is the cheapest; but otherwise there is not much difference in the costs of the three. As before mentioned, in all cases where beams are embedded in concrete, the top surface of the slab must be water-proofed.

Rails resting directly on the steel require floor-systems of closely-spaced, transverse beams-an expensive construction and not good in case of derailment.

In respect to encasement, it is generally economic to put it on by cement-gun, because it is cheaper than the poured covering, and being thinner it saves in the dead load to be carried by the steelwork.

For double-track, deck truss-spans it is most economical for the floor-system to space the trusses 20 feet or 21 feet centers, and to use two lines of stringers about 7 feet centers, resting the ties on the stringers and top chords. Such a floor-system weighs but little more than that for a single-track through-bridge. It may take so much extra metal in the strengthened chords as to absorb most of the saving from the omission of the two lines of stringers, but the floor beams are much lighter than those ordinarily required for double-track bridges. There is also considerable economy involved in the lateral system and vertical sway bracing. In this layout of floor-system any of the three ordinary types of deck can be utilized.

In general it may be stated that for any span-length the floor-system of a deck span will be cheaper than that of a through span.

Stringers are usually plate-girders, except for through, plate-girder spans; and in these rolled I-beams are more applicable. Floor-beams are nearly always plate-girders, although occasionally in narrow structures it may pay to use deep rolled I-beams. Cross-beams in through, plate-girder spans having no stringers are nearly always rolled I-beams, unless the span be unusually wide.

In making economic comparisons between plate-girder and rolled I-beam constructions, it is necessary to take into account the difference in