Engineering Wonders of the World
Volume I

THE ST. LOUIS BRIDGE. 169 the elevations of the bridge are reckoned, and at low river the water ran 20 feet deep. The use of floating coffer-dams was at that time, in Eads’s words, “ not unknown,” but the present attack upon so dangerous a stream as the Mississippi marked a very bold ad- vance. A visit to Europe in search of health incidentally gave Eads an insight into new methods of working under air pressure in sub- aqueous caissons, and he did not hesitate to discard his carefully prepared designs and work out fresh ones. All material was as- sembled upon the banks in advance, and on October 17, 1869, the caisson was towed to its place and the corner-stone of the great pier laid within it. For five months the work was pressed forward day and night, though storms and ice-packs constantly threatened interruption, and the caisson with its vast load sank steadily between its guide-piles, until, on February 28, 1870, it reached its final bed upon solid rock. The west pier reached rock two months later. The un- qualified success of these two operations decided Eads to settle once for all the ques- tion of the stability of the east abutment, by carrying it also down to bed-rock. The depth to be sunk in this case was greater by 8 feet than for the east pier, but acquired experi- ence simplified the task. When only the last ten feet or so remained un- A. Terrific „ . done, the works were struck by a terrific tornado, said to have exceeded in wind pressure all existing records. Derricks, machinery, timbers—every- thing was swept off the abutment into the river ; the scaffolding of the east pier and part of its stonework came down, and all was chaos. Yet, though the works were swarming with men at the time, the casualties only amounted to one killed and eight injured. The extreme violence of the hurricane may be estimated from the havoc it wrought upon the adjoining railway. A whole station build- ing was torn from its site and demolished. A heavy sleeping-car was carried bodily for seventy yards through the air, and empty freight cars for hundreds of yards. Most extraordinary of all, a 25-ton locomotive was lifted clear off the rails and deposited upside down at the foot of the embankment, without touching ground on the way. This terrible visitation cost the Company $50,000, but it had its value as an object lesson. The design of the superstructure was amplified with a strong wrought-iron wind-truss, reaching from pier to pier, calculated to resist lateral pressure upon the bridge of a storm of equal severity. The piers when finished rose 80 feet above the directrix, the abutments 68 feet. The tallest column of masonry is the east pier, which measures 200 feet from bed-rock to parapet, standing, in the normal condition of the river, with just half its height out of water. The material used is mostly lime- stone, faced from below water-line upwards with granite or sandstone. At the basis of the arches there is a solid course of granite eight feet thick. The parts of the steel superstructure were being prepared meanwhile at the Pittsburg works of the Keystone Bridge Company, one of the principals of which was the well-known Mr. A. Car- negie. The absolute accur- acy insisted upon by Eads, as regarded even the smallest details of material or fitting, exercised the patience of the con- tractors almost to breaking point ; but in every dispute the engineer had his way, and his “ ridiculous ” stipulations in favour of security were carried out. The dominating feature in the design of the superstructure is, as has been said, the ribbed arch. For a better understanding of the mechanics of this contrivance the reader is referred to an article on “ The Development of the Bridge,” in vol. i., pp. 102-107, of this work. The cost of the material necessary in a truss of 500 feet span, argues Eads in his Report, is prohibitive. He The Steel Superstruc- ture.