all very large jobs, and may, therefore, constitute a part of the regular equipment, the same as do the tools in the fabricating shop.

The cost of their transportation, installation, and removal is, however, a special feature that must be considered in the determination of the economics of each structure. For very-long-span trusses there are necessary enormous steel travelers that, with their equipment, may sometimes weigh 1,000 tons; and, when the job is finished, they are likely to be unsuitable for future work and of comparatively small value for salvage.

For very heavy work it is also necessary in some cases to provide structural-steel falsework and to employ considerable ingenuity in its construction from portions of the permanent structure afterwards to be erected. The repeated use of special erection-metal and its availability for other purposes after the finishing of the job are important elements in the economics of the problem.

As the principal members of long spans have attained a maximum weight of more than 100 tons each, it has been necessary to provide special methods of handling them and of securing them to the hoisting apparatus; and considerable sums have been spent in the construction of steel yokes, clamps, beams, and other special devices intended solely to provide rapid and effective connections to these pieces and to enable them to be accurately and safely handled. Such appliances greatly reduce the amount of hand labor and justify considerable preliminary expenditure.

Replacing Steel Bridges

The replacement of steel bridges almost always involves the maintenance of traffic on the bridge and often of navigation below the structure during the process of reconstruction. In most cases the new structure is on the same alignment and nearly or quite at the same elevation as the old one; and frequently the old substructure is satisfactory with minor modifications to receive the new superstructure. It is often difficult, and sometimes very expensive, to divert the traffic from the old structure while the new one is being erected; hence the problem of reconstruction, especially of long and high spans, thus becomes one of the most difficult and expensive that are likely to be encountered, and the economics vary so greatly that no general determination can be made, necessitating that they be investigated independently for each structure.

For short spans where the whole span or its single complete girders or trusses can be handled as units by travelers, derrick cars, or other apparatus traveling on the ground alongside, or on the structure itself, and especially where the bridge is a double-track railroad structure, it is usually comparatively easy to divert traffic to one track, and to remove the old structure piecemeal, putting in the new parts as fast as the old ones are taken out and gradually rebuilding the entire superstructure.

For viaducts this may frequently be done with two derrick cars handling