The world's largest steel arch bridge, Hellgate Bridge, built for a connecting railway for uniting the systems of the Pennsylvania Railroad and New York is of about 1000 feet span. The great steel arch forms part of a steel viaduct, itself the largest of its type in the world. The whole length of the structure, from abutment to abutment, being 17,000 feet, considerably over three miles long. It will be not only the longest, but considerably the heaviest steel bridge in existence, with over 80,000 tons of steel used for its construction.

The depth of the truss at the ends is 140 feet; at the center, 40 feet; and at the quarters 66 feet. The reverse curve of the upper member of the arch at either end is explained by the necessity of raising the top member of the portal to a sufficient height above the tracks to allow head room for the trains. The lower arch member has a section of 9 feet by 6 feet at the bearing and 5 feet by 5 feet at the center, the width decreasing evenly from the bearing to the crown. The struts are all riveted box sections, and the suspenders consist of eight heavy angles laced together.

The floor system is built on the customary method of heavy cross girders and longitudinal stringers. The floor beams are 8 feet in depth by 80 feet in length. The main arches are placed in vertical planes 60 feet apart, and the longitudinal portion of the floor system consists of eight lines of stringers, or two beneath each of the four lines of railroad track. Above the stringers is laid a solid wood floor of creosoted 8 x 8-inch timbers, packed tightly together, and chalked. Upon this is 14 inches of stone ballast, in which are imbedded the crossties of the regular Pennsylvania Railroad standard track system. The floor beams are the heaviest ever built for a bridge, the section of the bottom flange of the girders at the center being 6? by 24 inches, or 150 square inches.

Wind bracing is carried in the plane of the upper and lower arches from bearing to crown; but the main wind bracing is carried in the plane of the roadway, and it is arranged as a cantilever truss system. The contra-point and expansion joint of the cantilever are located six panels from each end of the arch; so that the temperature strains of the suspended floor system will in no way affect the arches—in other words, the temperature stresses of the arch and the floor have been made independent of each other. The maximum compression in the arch is found at the bearing, where it amounts to 16,800 tons, decreasing from that to 13,600 tons at the crown. This compression is that due to the combined dead and live load, wind pressure, and temperature stresses.

The abutments of the arch is monumental stone and concrete towers, which serve to divide the arch bridge properly from the steel viaduct which forms the approaches to it. The base of the tower is built of granite; and it rests on foundations of a very hard gravel at a depth of 20 feet below the surface. The upper portion of the towers are built of molded concrete, the design of these towers is simple, massive, and dignified, and altogether harmonious with the design of the great arch itself.