for the general and comparative economics, so that, by intelligent development and modification, they will cover the field within the limits of justifiable construction.

The subjects considered are divided on broad lines into steel and concrete; long, medium, and short spans; and high and low structures.

Steel Bridges

As far as possible all items of fabrication should be completed at the bridge shop, so as to reduce the amount of assembling, fitting, and riveting in the field to a minimum. No work should be done at the site which can be performed at the shop; and no work should be done on the structure itself that can be performed ashore upon the separate or combined pieces before erection. Standard plant and equipment should be used; the largest possible proportion of work should be done by machinery and power; and the most skilful and experienced labor available should be employed in conformity with the equation of the different costs to a minimum for the completed work, including salvage, rental of plant, cost of transportation, installation and removal of plant, and the greater or less importance of extra speed (as in case of danger from floods), always considering the fundamental requirements for absolute safety and the excellence of the finished work.

It is assumed that the methods, plant, equipment, and service best adapted to the type of structure and the given conditions are available, and erection with them will be considered the economic method; but this decision, of course, is subject to modification when the problem is complicated by artificial conditions or by sudden emergencies that make changes of details, methods, or equipment safer or more practicable, as, for instance, when labor troubles, difficult transportation, scarcity of materials, or accidental physical developments make the original preparation susceptible to delays and to important changes of conditions that may have a vital influence on erection operations; and radical changes in the original programme are sometimes necessary, in order to prevent large increases over the proper estimated cost.

Girder Spans

These are plate girders or riveted trusses of such dimensions that they can be erected complete as units, usually not exceeding 50 or 60 tons in weight, 100 feet in length, and 15 feet in depth. Even these limits are likely to be excessive for transportation by rail from the fabricating shop, the size generally being limited by bridge and tunnel clearances and track curvature, and the weight by twice the capacity of each of the available cars.

If the girders can be shipped entirely by water, the limits for transportation are  very  greatly  increased;  and  under  special  circumstances  they