essentially from that pertaining to the designing of the other structures.

CALCULATIONS.

After the leading features of any proposed structure have been determined, and after the general layout thereof is completed, the next step to take is the making of the calculations necessary to determine the stresses in all the parts and the proper sizes for same.

For convenience in making to correct scale pen-sketches of the various portions of the design, the author, uses a cross-section paper divided into one-quarter-inch squares, the sheets being ten and a half inches wide by sixteen inches long, which size experience has shown to be the most satisfactory. At the head of each page are written the date, title of structure, and, name of computer.

At the beginning of each set of calculations the following general data for spans are given:

First. Length-of span.

Second. Number of panels.

Third. The various truss depths.

Fourth. Perpendicular distance between central planes of trusses.

Fifth. Live load or loads to be used.

Sixth. Wind loads for both upper and lower lateral systems.

Seventh. Spacing of stringers.

The dead load from the track and ties in railroad bridges or from the timber floor or pavement in highway bridges is first determined, using the unit weights of materials given in Chapter XIV; then the stringers or longitudinal girders are figured and proportioned, after which their weights and that of their bracing are computed.

Next the floor-beams or cross-girders are proportioned, and their weights are figured. From all these weights the weight per lineal foot of the metal in the floor system is next found.

As the lateral system can nearly always be designed before the trusses, it is generally best to compute the weight per lineal foot of the entire lateral system before the trusses are