may be equally extended for erection. Ordinarily the latter is a very simple process, effected by one or by two derrick cars or wrecking cars that unload, transport, and erect the girders in position in single or successive operations at a direct cost of as little in pre-war times as $1.00 per ton.

Derrick cars or their equivalent are not always available in remote localities, and are not generally possessed, except by railroad companies, bridge companies, and important contractors. In their absence the girders can be handled, usually less advantageously, by derricks, gin-poles, and various combinations of jacking, blocking, rolling, and other operations that may be devised, modified, and combined according to the experience and ingenuity of the erector; and although theoretically they are more expensive and less efficient, they may often be more advantageous than the provision of high-class equipment under disadvantageous conditions.

Viaduct Erection

Viaducts generally consist of two or more lines of plate or lattice girders from 30 to 100 feet long on towers up to 100 feet high, although these dimensions have been considerably exceeded in infrequent cases. The ideal method of erection is by means of a derrick traveler, sometimes called a mule, installed at grade at one end of the viaduct and having a reach long enough to erect one tower and one connecting span in advance; after which it moves forward on the completed portion of the structure, erecting panel after panel as it proceeds. Such travelers have at least two main booms of great length and one or more auxiliary booms for hoisting, swinging, and placing the steel that may be delivered either at grade or on the surface of the ground. With a large number of duplicate heavy spans, such as occur in elevated railroad construction, this class of metalwork can be erected with great rapidity, a record of 1,000 lineal feet of double-track structure per day having been made it Brooklyn with one traveler and crew, independent of the preliminary distribution of steel and the subsequent field riveting.

Where the girders have been too long or too heavy, or the towers too far apart for erection with a boom of practicable length, cantilever travelers have been successfully employed. They have been of different types, usually having an elevated horizontal boom overhanging the wheel base by the length of one tower span and one connecting span, and equipped with trolley hoists for unloading material from cars on the tracks in the rear and transferring it to the required position in advance for assembling in the structure.

In a few cases where there has been a great length of viaduct of substantially uniform height above the surface of the level ground, the structure has been erected by a strident gantry traveler, moving on a surface track and provided with several sets of hoisting tackles to handle from the ground (where they were distributed) the span and tower members.