the axles of a motor car produces smaller bending moments than that on the axles of a locomotive.

After the distribution of the live load on the various axles of the entire train has been determined, it is well to prepare a diagram of equivalent uniform loads and one of total end shears similar to those for the Compromise Standard System of Live Loads for Railway Bridges given in Chapter XIX, in order to facilitate the computing of stresses and bending moments.

FLOOR.

The style of floor in general use on elevated railroads consists of timber ties with four lines of timber guard-rails, closed floors of buckled plate carrying timber ties in ballast being employed at crossings of important streets and boulevards, so as to prevent dirt and moisture from falling upon people passing beneath. Such a closed floor has been advocated for the entire line, and certainly it would be an improvement upon the open floor; but the increased expense involved is likely to interfere seriously with its adoption for future elevated railroads. The ballast over the buckled plate in tight floors is necessary to prevent noise from passing trains, which, unless some effective sound-deadener be adopted, would be simply deafening. There is one important incidental advantage in employing a closed floor, viz., that the elevation of the grade is thereby reduced about three feet. Of course nearly as great a reduction of elevation can be obtained with the open floor by resting the timber ties on the inner bottom flanges of the longitudinal girders; but this style of structure is objectionable for several important reasons, prominent among which are the necessarily large sections of the ties and the difficulty in replacing them.

ECONOMIC SPAN LENGTHS.

With the ordinary live loads, for structures located on private property the economic span length is about forty feet,