sidewalks. The live loads for the floor system are Class 25 for the electric railway, Class B for the rest of the roadway, and Class C for the side-walks. Class A over the full width of the deck was employed for the trusses.

RÉSUMÉ OF INVESTIGATION

First. For exclusively railroad bridges, the economic limit for the cantilever type of structure, or, in other words, the main span length for a cost equal to that of the corresponding suspension bridge is that length which requires 4 1/2 pounds of metal to carry 1 pound of live load.

Second. For modern highway structures, carrying also incidentally electric railway tracks, this span length for equal cost is 1,000 feet.

Third. For combined-railway-and-highway structures the limit is intermediate between the limit for railway structures and that for highway structures, the interpolation being done in direct proportion to the ratio of railway-truss-live-load to total-truss-live-load.

This may be expressed by formula thus: If G is the span length of equal cost for strictly-railway bridges, and R is the ratio of railway-truss-live-load to total-truss-live-load, then, for combined-railway-and-highway structures the span length for equal cost will be given approximately by the equation:

For instance, if G=2,700 feet for nickel steel railway bridges and R = 2/3,

This checks fairly well with the value shown in Fig. 13i, where

Fig. 13k is a diagram from which can be found at a glance the span length for equal cost for any proportionate combination of railway and highway live loads, under the assumption that nickel steel is employed for the principal portions of the structure. In case, though, that carbon steel alone be used, which is unlikely, the limiting span length for cantilever construction is to be taken at about 2,000 feet.

While it was not intended to do any figuring concerning the comparative economics of cantilever and suspension bridges when alloy steels having higher elastic limits than 60,000 pounds per square inch are employed, it was surmised that the span length for equal cost for strictly-railway bridges will not differ essentially from the limiting lengths for cantilever main spans determined by the author in "The Possibilities in Bridge Construction by the Use of High Alloy Steels," viz.: