touched, because the dead load for the latter will be affected by the weight of the former.

Next it is necessary to assume the weight of metal per lineal foot for the trusses, using, if necessary, the formulae given previously in this chapter. This completes the data for the preliminary dead load, which will consist of the following items:

First. Flooring (timber, track, pavement, etc.).

Second. Floor system (stringers, stringer-bracing, and floorbeams).

Third. Lateral system (upper and lower lateral systems, vertical sway-bracing, and portal-bracing).

Fourth. Trusses.

In making up the dead load, the end floor-beams and pedestals must not be included, as their weight produces no bending moment on the span.

The dead-load stresses in trusses are always found analytically for spans with parallel chords and equal panel lengths; but for all other cases they are determined graphically, and are checked by a single numerical calculation at the member where the graphics stop.

Whenever it is practicable, in making arithmetical computations, the slide-rule is employed. For ordinary work, in which the total stresses can be written with six figures, a twelve-inch slide-rule will give the stresses accurately in thousands of pounds; but where the stresses are greater, Thacher's cylindrical slide-rule is employed.

The live-load stresses are found by the method explained in Chapter XIX.

The computation of all stresses found analytically is facilitated by determining the trigonometrical functions involved in the calculations, and multiplying the panel loads by them. By setting these products on the slide-rule and using the proper tabulated coefficients, it is often practicable to read off a large series of stresses without resetting the slide.

The dead-load stresses and the live-load stresses are written on separate diagrams on the calculation-sheets.

The impact stresses are found from the live-load stresses by