encased bottom chords of eye-bars at the elevation of the lower deck, so as to take up the arch thrusts. The available clearance was very small; so that the floor-beams of the lower deck had to be made of steel I-beams encased in concrete and riveted to steel hangers, also encased.

Reinforced-Concrete Trestles for Steam Railways

A number of reinforced-concrete trestles have been built of late years for steam-railways. The usual type for river crossings has been solid-concrete slabs on concrete piles. The economic span-lengths for such trestles are very short, usually from ten (10) to fifteen (15) feet; but most of the structures have had spans of fifteen (15) or twenty (20) feet on account of waterway requirements. Solid slabs have also been used extensively for grade-crossing-elimination work in cities, the substructure generally consisting of either solid cross-walls or cross-girders resting on a row of small columns carried on a continuous footing. The economic span-lengths for such structures are a little greater than those for the slab-pile-trestles; but this economic question is of very small importance, since the span-lengths are generally fixed definitely by other considerations.

The slabs for most of these trestles have been separately moulded and afterwards set into place by derrick cars or locomotive cranes. In some cases this was the cheapest possible method of construction; but in others the need for maintenance of traffic demanded it. In many cases also the headroom was limited, and falsework beneath was not permissible.

For the purpose of determining the economics involved, the author has had made in his offices during the last three or four years a large number of estimates for steam-railway, reinforced-concrete trestles of the slab-girder type, for span-lengths varying from twenty (20) to fifty (50) feet, and for heights varying from twenty (20) to sixty (60) feet, measuring from base of rail to bottoms of footings. Both simple and continuous girders have been computed, the former proving somewhat the cheaper. The substructure considered consists of bents composed of two battered columns with a cross-girder at the top, the columns being supported by either one continuous footing or two separate footings. For the higher trestles it proved to be economical to use longitudinal struts between columns in alternate spans, thus making a succession of towers with a single span between each of them. The economic span-length for such trestles varies from one-half to six-tenths of the height.

In the second edition of "Bridge Engineering" (fourth thousand), which will probably be issued in 1922, there will be a lengthy Appendix for the purpose of recording the results of all of the author's studies (excepting the economic ones reproduced in this treatise) on the subject of bridges made since July, 1916, when the first edition of that work appeared. The said Appendix will contain an extensive series of diagrams giving curves of quantities for different types of steam-railway, reinforced-concrete trestles.