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The author has lately written for the American Society of Civil Engineers a lengthy paper on this subject. It has been very thoroughly discussed by the engineering profession, and the discussions have been answered in an exhaustive resume by the author and his assistant engineer, Ira G. Hedrick, Assoc. M. Am. Soc. C. E. The original paper, the discussions, and the resume have been published in the Transactions of the Society for 1897, Vol. XXXVII ; and any one who desires to make a special study of the subject of elevated railroads will do well to read all that has been published thereon in the said Transactions.
There will, however, be given in this chapter a compendium of the contents of the paper for the use of those who have no time or inclination to wade through the two hundred pages that it occupies.
LIVE LOADS.
The proper live load to assume in designing an elevated railroad is the greatest that can ever come upon it, and is determined by ascertaining the weights of engines and empty cars that are adopted at the outset, then computing how many passengers can be crowded into the latter and assuming that the average weight per passenger is one hundred and forty pounds. The live loads for elevated railroads, unlike those for surface railroads, do not increase from time to time, but remain constant. In fact the late tendency to operate the roads by electricity rather decreases them, for the weight on
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