limited, requiring close spacing, so as to permit the adoption of shallower sections than the economic ones. The beams are frequently encased in concrete in order to protect the metal from the fumes of locomotives passing below. If a timber deck should be adopted with planks resting either directly on the stringers or on nailing-shims bolted thereto, the spacing should be some two and a half feet, more or less, depending upon the size of the concentrated wheel-loading; but, if the planks are carried on wooden ties, the spacing may be made about five feet.

In deck, plate-girder spans also, the floor-system can sometimes be omitted, especially in short spans, with either concrete deck or planks on ties; but in long spans of this type the steel floor-system will generally be found more economical. The limiting span-length for omission of floor-system is sixty or seventy feet, as far as the economics of the superstructure alone are concerned; but such extreme lengths may increase too greatly the cost of the supporting parts. For instance, with only two lines of main girders and a floor-system there will be required only two columns per bent and two pedestals to support them; whereas with several lines of girders there must be several columns and pedestals per bent, or else a heavy cross-girder or a continuous concrete pier, either of which is more expensive than a pair of columns with their pedestals and bracing. If for any reason it be decided to use continuous shafts in the piers, there may be no extra expense due to supporting several lines of girders; in fact, there may be a reduction in cost, because it might be possible to employ narrower piers or seats.

In deck, plate-girder spans with floor-systems it is economic to use only two lines of main girders up to a fifty-foot total width of deck or in some cases even sixty feet. The girders for economy should generally be placed at the quarter widths, so as to obtain, in both substructure and super-structure, the full economic benefit of the cantilevering. In narrow structures the spacing should be proportionately greater for the sake of rigidity and stability. With three lines of main girders, the spacing should be about one-third of the total width of the deck.

The standard floor-system in deck, plate-girder construction has stringers carried on floor-beams. The portions of the latter outside of the outer girders are cantilever beams with a strap-plate over the top of the main girder and some kind of properly designed detail below to carry the compression from the bottom flange of the cantilever to the bottom flange of the floor-beam. The stringers are generally rolled I-beams and the cross-girders built beams, although occasionally it will be found economic to employ a deep rolled-I-beam. With planks resting on stringers, either directly or by interposed nailing strips, the stringer spacing will, of course, be determined by the size of the specified concentrated live loads and the assumed thickness and kind of material of plank. It is generally about two feet for three-inch plank and two and a half feet for four-inch plank. Closer spacings than these involve an extravagant amount of metal for stringers.