cantilever should show even a small economy in the comparison, it would be well to adopt it.

The question of what is the economic limit of length of simple-truss spans as compared with cantilevers is still a mooted one. Professors Merriman and Jacoby, on page 119 of Part IV of their excellent treatise on "Roofs and Bridges," state that the economic limit for simple spans was probably nearly reached in the building of the five hundred and eighty-six (586) foot span over the Great Miami River at Elizabethtown near Cincinnati; but the author has had occasion to compare simple-truss spans of somewhat greater length than that with the corresponding cantilever structures and has found them more economic. The continuity of cantilever spans in resisting wind loads lowers the requirement for minimum width from one- twentieth (1/20) to one-twenty-fifth (1/25) of the greatest span-length, and hence, because of substructure considerations, gives an advantage to the cantilever type that in certain extreme cases will more than offset its disadvantage of greater weight of truss metal.

This question of when to pass from simple-truss spans to cantilevers is not affected very much today by the last consideration, because bridges with spans long enough to necessitate the comparison are often so wide as to cause it to be ignored. For instance, one seldom hears any more of a single-track railway bridge having a span longer than four hundred (400) feet; and first-class, double-track, steam-railway bridges have a clearance of twenty-eight (28) or, preferably, thirty (30) feet, thus making the distance between central planes of trusses from thirty-two (32) to thirty-four (34) feet. The limiting simple-truss span-length established by good American practice for the latter dimension is six hundred and eighty (680) feet, and for cantilevers it is eight hundred and fifty (850) feet.

Of still greater importance are the special requirements that govern the layout at each site. Fig. 12a (which is a reproduction of Fig. 55aaa on page 1271 of "Bridge Engineering"), shows typical layouts for cantilever bridges. There is still another type, consisting of equal (or nearly equal) spans with short cantilever arms, that is discussed later in this chapter.

The Type-C-cantilever bridge, which has three spans of practically equal lengths, will first be considered. It will be compared with a corresponding structure having three simple-truss spans. These layouts apply where the distance between end piers is fixed, while the intermediate piers can be placed where desired. Fig. 12b gives the comparing weights for pin-connected, double-track-railway bridges. From its curves one can see that the span of equal cost is about six hundred and thirty (630) feet. It may be possible to reduce the cantilever weights by varying the sizes of the openings and the relative length of suspended span to cantilever arm; but, even with such changes, it is not likely that the span-length for equal weights of metal would be as low as six hundred (600) feet.

In the case of highway bridges, the weights of metal per lineal foot in