Comparing such structures with those composed of steel girders carrying ballasted decks on concrete slabs, there is very little difference in cost, the reinforced-concrete trestles, pure-and-simple, being a trifle cheaper for twenty-foot spans and a little more expensive for fifty-foot spans. But if the steel girders are encased in concrete, the reinforced-concrete trestle will always be found the cheaper.

Retaining Walls

Reinforced-concrete retaining walls show a small saving over plain-concrete ones for even small heights. However, it is better to use plain concrete up to heights of ten (10) feet, measuring from bottom of footing to grade, as the section of a low reinforced-concrete retaining wall is too small, for the reason that a certain amount of massiveness is necessary in such constructions.

In respect to the economics of cantilevered and counterforted walls, the former type is the cheaper up to about twenty or twenty-five-foot heights, above which limit the latter type is the more economic. The discussion of "Footings" previously given in this chapter will apply to the footings of retaining walls.