The data for this chapter were furnished by the author's old friend, Frank W. Skinner, Mem. Am. Soc. C.E., who, for a number of years, made a special study of bridge erection in all its details, and gave the results of his findings in numerous lectures to engineering students of the leading universities and technical schools of this country. Much of the valuable material that he collected was published in Engineering Record, of which paper for a long time he was the chief editor and leading spirit. The author feels that he is exceedingly fortunate in obtaining for the writing of this chapter the assistance of an eminent engineer who has made such a thorough study of bridge erection in all its ramifications. There are very few engineers who are fitted by both experience and temperament to discourse scientifically and practically concerning the essentially specialized subject of the economics of erection; and of these there is probably not one who possesses such a grasp thereof as does Mr. Skinner. What follows is given as closely as practicable in his own diction.

The erection of an important bridge is a function first of the design, second of the location, and third of the available equipment; and its economics are directly related to these fundamentals, variations in which materially influence the total cost of erection of spans of the same length, the same carrying capacity, and the same type. The recognition of these facts has been such that, in this country, the art of bridge erection, particularly of steel spans, has been highly specialized—to an amount comparable with that of the fabrication of bridge superstructures in great shops fitted with costly tools and used almost exclusively for this single purpose. It has resulted in the development of a clearly-defined, standard practice and the perfection of a number of methods for securing essentially the same effects under different conditions and with different appliances arranged to secure the maximum safety and rapidity with the minimum total cost.

These methods, and the special equipments that have been devised for them, make it possible for a given structure to be handled in several radically-different ways, which may either show in some cases comparatively-uniform results, or, generally, will indicate a decided advantage for one method over all the rest.

The physical and mechanical combinations can be readily analyzed so as to give a limited number of principal cases that will here be classified

396