much lighter than that of Class Z of the Compromise Standard System; but the said loads will have to be determined for each individual system of elevated railroad, so as to provide for the greatest train load that can ever come upon the structure, but for no more.

IMPACT ALLOWANCE LOAD.

The impact allowance load is to be a percentage of the equivalent uniform live load, found by the formula:

where P is the percentage and L the length in feet of span or portion of span that is covered by the live load, when the member considered is subjected to its maximum stress.

DEAD LOAD.

The dead load is to include the weight of all the metal and wood in the structure, excepting that of those portions resting directly on the abutments, whose weights do not affect the stresses in the trusses; also any other permanent load that may be carried by the structure.

The following unit weights are to be assumed in estimating the dead load:

Creosoted lumber four and one-half (4 1/2) pounds per foot board measure.

Oak and other hard woods four and a quarter (4 1/4) pounds per foot board measure.

Yellow pine three and three-quarters (3 3/4) pounds per foot board measure.

White pine and other soft woods two and three-quarters (2 3/4) pounds per foot board measure.

Rails and their fastenings, sixty (60) pounds per lineal foot per track.

Two thirds (2/3) of the dead load shall be assumed to be concentrated at the panel points of the lower chords in through-