In timber decks the economic stringer spacing is about the same as it is in plate-girder spans; and in concrete decks it is five or six feet for spans of 200 feet or under, and less for longer spans. For instance, in spans of 400 feet with 25-foot panels it is four feet, and with 35-foot panels it is four and a half feet.

The stringerless type of floor system with concrete deck for very short panels-say 12 feet-shows the same economy in truss spans as it does in half-through, plate-girder spans. With truss-spans of moderate length, the panels are so long that the chords would have to be built as girders in order to carry cross-girders between panel points. The floor-systern itself, as compared with that of the stringer type, when the panels are 25 or 30 feet long may show for standard deck-widths a saving of from 250 lbs. to 350 lbs., per lineal foot, while the extra metal in the chords required to make them serve as girders would amount to 250 or 300 lbs.; besides which there must be taken into consideration the increase in weight of truss-metal due to the augmented dead load by reason of the thicker slab. On this account the stringer type will nearly always be found cheaper, especially when electric railways are carried. With encasement there may be an economic advantage in the stringerless type, especially if a very shallow floor be called for. With extremely long panels it will be found cheaper to use stringers close to the trusses rather than to stiffen the chords of the latter enough to make them able properly to carry the transverse loading.

For very long spans with concrete slabs it is desirable to reduce the weight of deck to a minimum. In such cases it will be most economic to support the slab on closely-spaced cross-beams carried on stringers rather widely spaced-say 10 or 12 feet centers. Considering both the weight of metal in the floor-system and that in the trusses, it is found that the economic spacing of the small beams is about three feet. Comparing this with the ordinary type having stringers spaced four and a half or five feet, it will be found that the metal in the cross-beam type weighs about three pounds more per square foot for 25-foot panels and about one pound more per square foot for 35-foot panels, while the slab weighs eight pounds per square foot less. The total deck, therefore, weighs some five pounds per square foot less for 25-foot panels and seven pounds per square foot less for 35-foot panels. Considering the extra weight of truss-metal needed to support this excess of dead load, and remembering that the pound cost for the cross beams is a little less than that for the remainder of the floor-system, it has been figured that the cross-beam type is cheaper with 35-foot panels for spans in excess of 200 feet, cheaper with 30-foot panels for spans of 350 feet, and cheaper with 25-foot panels for spans of 600 feet. Evidently, therefore, there is no economy in adopting this type for spans shorter than 250 or 300 feet; but for all long spans it is quite economical.

This type of floor affords a good support for the rails, which must,