Engineering Wonders of the World
Volume I

170 ENGINEERING WONDERS OF THE WORLD. therefore uses the arch, which, by dispensing with the tension member altogether, reduces the weight to a minimum. The increased stress thrown upon the supports requires that they shall be of exceptional strength. But that quality is already required in them by the habits of the river. Therefore his design is the most economical for the situation; and his argument has enjoyed general endorse- ment. In the St. Louis Bridge each span consists of four arches, springing from massive steel plates, called skewbacks, built into the face of and anchored to the ma- The Arches sonry of the supports, 7 feet above high-water level. Each arch, or rib, is composed of two parallel tubular members, 9 inches in diameter and 12 feet apart, inter- braced at intervals of about 12 feet through- out their length. The member is made up of a succession of steel tubes, about 12 feet long, secured to one another by composite joints, into each of which are bolted the ends of rigid braces from the two nearest joints of the parallel member. Adjoining ribs are 16| feet apart at the skewback, and converge to a minimum distance of 12 feet at the crown of the arch. Horizontal braces secure the four upper members, and similarly the four lower members, in their relative positions ; while a further system of diagonal bracing between each, rib and its neighbours completes the scheme of resistance against lateral pressure. The construction of the tubes, of which 1,036 were built into the bridge, is worthy of explanation. A steel plate was rolled up into a straight cylinder 12 feet long, and riveted. The internal surface of this envelope- tube was then lined with six close-fitting steel staves of the same length. The adjustment of these straight tubes to the curves of the arch was, of course, effected at the joints. The roadway, which, with the sidewalks for pedestrians, has a total width, between para- pets of 50 feet, passes over the summits of the piers, rising gradually some 13 feet between the abutments and the crown of the central arch. It is carried upon a timber platform resting on iron perpendiculars based upon the socket joints of the upper members of the ribs. The level of the two railway tracks which run between the outer pairs of arches on either side of the bridge, about 13 feet below the carriageway, is approximately that of the lower members at the crown of each arch. On the Missouri side, the road, joined by the railway emerging from a tunnel under the streets of the city, gains the bridge over an approach carried upon handsome stone arches. The Illinois bank is much lower ground, little above ordinary water-level, so a long viaduct of iron trestle-work leads up to stone arches matching those at the other end of the bridge. The whole erection, with its long, sweeping span, its tapering piers, and its graceful lattice-work, conveys to the beholder a pleasing sense of invincible strength com- bined with artistic proportions and out- lines. In the process of construction each arch was built up tube by tube from both ends simultaneously; the weight of the growing section being supported first by a cable carried back over UP a tower built on the summit of the pier or abutment, and, as its length in- creased, by a second cable carried over a minor tower erected above the point of attachment of the first cable. The crucial difficulty was experienced when the halves of the arch met at the crown and the corresponding tubes had to be “ entered.” With the changes of tem- perature throughout the day and night the metal of the ribs and cables was always either contracting or expanding, rendering it almost impossible to get all the tubes in the right position at the same time. The solution was found in making the central tubes adjustable in length by a screw joint. The work was