The economic panel length for this type of floor-system, as far as that floor-system alone is concerned, is always smaller than can be employed for single-track truss-bridges, whether through or deck, as can be seen by referring to the curves on pages 1224, 1229, and 1233 of "Bridge Engineering." This is also true for double-track, riveted-truss bridges, as shown by the curves on page 1239 of that treatise, although there is no great reduction below 25 feet. For double-track, pin-connected bridges, 25 feet is the economic length (see "Bridge Engineering," page 1244). This is due to the extra metal required for the cut-away ends of the floor-beams, making pin-span cross-girders much heavier than riveted-span ones. Where jack-stringers or four lines of carrying-stringers per track are employed, the economic panel lengths would be still shorter. It will be found, however, that variation in panel length will not affect greatly the total weight of metal in span, since lengthening the panels generally reduces the truss weight. For long, deep trusses, longer panels than usual give smaller total weights of truss metal.

In respect to the economic depths of stringers for two lines per track, they vary from 4 feet for 20-foot panels to 5 feet for 35-foot panels with light live loads, and from 5 feet for 20-foot panels to 6 feet for 35-foot panels for heavy live loads. The economic depths for four lines of stringers per track are about one foot less than the preceding.

The economic depths for floor-beams vary from 5 feet to 6 feet, according to panel length, in single-track bridges with light live loads, and from 6 feet to 7 feet with heavy live loads. The corresponding figures for double-track bridges are respectively, from 6 feet to 8 feet, and from 8 feet to 10 feet.

Under-clearance requirements often call for shallower floor-beams than those of economic depth, especially for double-track bridges. Very shallow, double-track, railway-bridge floor-beams are quite expensive. Any decrease in depth of floor-system below the economic one will increase the cost of main structure; but this is partially offset by the slightly- reduced lengths of approaches; consequently, when comparing the costs of floor-systems of different depths, this fact must not be forgotten.

Half-through, plate-girder spans are used only where the headroom is limited; hence the floor-systems for these structures are almost always shallow and uneconomic of metal. With open-timber decks, four lines of carrying-stringers per track are employed in order to provide a shallow floor; and the panels can be short and of economic length. Solid floors are quite common with through-plate-girder construction. The trough floor-system is very thin, and hence is applicable thereto; but it is expensive and otherwise objectionable, as hereinbefore explained. When a ballasted deck on creosoted timber is adopted, four stringers per track can be used, but it is more common to employ closely-spaced, transverse rolled I-beams, especially when the headroom beneath is small. The latter construction is also usually employed with a ballasted deck carried on a steel plate.