costs of excavation. As explained in Chapter XVII of this work, the labor involved in making such computations as the above-mentioned is by no means onerous.

The greater the span-lengths which are necessitated as minimum by the conditions of the crossing, the more favorable is it to the steel structure in the economic comparison; and when a certain span-length has been reached, the reinforced-concrete structure becomes impracticable. What this limiting span-length is, designers have not yet determined with general satisfaction; but the author is of the opinion that it is not very far from seventy (70) feet for girders and three hundred (300) feet for arches. While it is practicable to build reinforced-concrete arch-spans of greater length than the latter figure, much trouble would be involved during their erection by the unequal and abnormally great settlement of the falsework, which settlement tends to distort the arch rings and, in consequence, to give the structure an unsightly appearance. Such settlement can be reduced, if sufficient care be taken; but an excessive amount of the latter adds to the time required for fieldwork and, consequently, to the first cost.

In general, it may be stated that a high-level crossing is usually more favorable to a steel structure than to one of reinforced-concrete, as are likewise deep foundations and perilous erection conditions; also that, other things being equal, a great width of deck generally militates in favor of the reinforced-concrete type of construction; as do, too, the remoteness of the site from the source of the structural-steel supply and the cheapness of common labor available for fieldwork.

It may be that some reader of this chapter will claim that it is rather indefinite in its conclusions and deals mainly with glittering generalities. Possibly there may be some justice in such a claim; but it must not be forgotten that the economics of the two contrasted types of bridges is primarily dependent upon such arbitrary and uncertain conditions as the relative prices of steel and cement, the availability of concrete aggregate, the comparative costs of high-grade and low-grade labor, and the distances of the bridge site from the various sources of supplies. This fact makes it impossible to come to any fixed or reliable conclusion concerning the relative economics of steel and reinforced-concrete bridges.