becomes too great for heavy loadings. It is true that this limit is sometimes made as high as thirteen feet; but then the ties are difficult to secure, and the maintenance cost thereof is large.

Resting ties on the bottom flanges or on special shelf-angles in half- through, plate-girder spans, although economic in first cost, is not good construction; because the removal and replacement of the said ties is abnormally troublesome and expensive. This practice, which was quite common two or three decades ago, is pseudo-economical, especially in view of the rapidly-augmenting prices of timber. Moreover, this detail has a tendency to distort the webs of the girders; and the depth available for cross-struts is small. These are needed for a proper staying of the top flanges of the main girders, and they have to be stiff in order to be effective. Open timber decks can be used on I-beam spans, on deck, plate-girder spans, and on deck truss-spans without floor-systems, as can also ballasted decks supported by treated timber or reinforced-concrete slabs. In short I-beam spans the beams can be spaced closely and a thin slab can be employed, or the beams can be embedded in the slab, or there can be used longitudinal, reinforced-concrete troughs resting on the bottom flanges of the I-beams with the latter encased in concrete. For I-beam spans with either timber or ballasted deck, it is cheapest to adopt the minimum number of beams that will carry the load; but with limited headroom it will be found necessary to employ shallow beams and space them closely. If it be desirable for the sake of protection to encase the said beams in concrete, it is usually most economic to rest the slab on the beams and encase them separately; but for very thin floors it is cheapest either to embed the beams in the solid slab, or to use reinforced-concrete troughs.

Floor-Systems

The standard floor-system consists of two stringers per track riveted to the cross-girders, with the latter riveted in turn to the trusses or main girders. In general, this construction is by far the most economic type of floor-system; and it provides substantial floor-beams at panel points to serve as lateral struts. It can be used with any of the ordinary types of deck previously described. Auxiliary stringers, often termed "jack-stringers," are sometimes added so as to take care of derailed trains. Four carrying-stringers per track are occasionally employed, either because it is the policy of the road to do so or in order to permit the adoption of a shallow floor, especially in half-through, plate-girder spans. It is more economic to adopt four lines of carrying-stringers per track than to have only two of them and two lines of jack-stringers, for the reason that, except in the case of derailment, the latter are idlers. Four carrying-stringers, of course, require more metal than do two, especially with long panels; but with ballasted decks on concrete slabs they permit a reduction in the slab-thickness; and in spans of considerable length the reduction in dead load involved will nearly compensate for the extra stringer metal.