- Historic Sites
Mr. Eads Spans The Mississippi
The odds were all against him. His only credential seemed to be self-confidence—and who had ever heard of a steel bridge?
August 1969 | Volume 20, Issue 5
Eads was not the first engineer to encounter caisson disease. Several French and British engineers had observed it and, after various experiments, had discovered how to prevent it. But medical news travelled slowly, and Eads had never heard of the European solution. One day during work on the west pier, Dr. Jaminet himself suffered an excruciating seizure after a visit to the caisson. The doctor was fortunate enough to recover; analyzing his experience, he decided that the trouble might lie in the rapidity of the decompression. Although he did not thoroughly understand the cause, his conclusion was right, and the introduction of gradual decompression greatly relieved the problem. (Later investigation was to show that a sudden reduction of atmospheric pressure releases body nitrogen into the blood in the form of tiny bubbles that can block the supply of oxygen to vital organs and thus cause severe disorders.) On the east and west piers there were ninety-one cases of the bends, including thirteen fatalities and two permanent disablements. On the last and deepest foundation, the east abutment on the Illinois shore, only one fatality occurred, despite pressures that reached forty-nine pounds per square inch. The east abutment struck bedrock at the astonishing depth of 127.5 feet below the directrix.
Just after Christmas, 1870, an ice gorge came piling down the river. Eads had foresightedly provided a powerful pointed breakwater, which he now rapidly fortified with rock rubble. Then as the weather turned warm and the gorge subsided, the flood began. Roundthe-clock work was rushed to build the masonry high enough to keep above the crest. This peril was narrowly escaped but was promptly followed by a new one. A tornado raged in from the southwest, uprooted trees, hurled trains off embankments, levelled buildings, and, in a matter of seconds, crumpled the superstructure of the east abutment. By good luck, there were only eight injuries and one fatality. The damage was repaired, and a few weeks later the giant column—45,000 tons of masonry—was complete. After three and a half years of unremitting struggle, the four foundations stood unshakable on the Mississippi bedrock, ready to receive the three great arches.
These had already caused almost as many problems as the foundations. Ironmasters of Europe and America had avidly sought the contract, but one after another had been discouraged by Eads’s specification of strict uniformity. Linville, the skeptical president of the Keystone Bridge Company, probably would have refused the contract had it not been for his ambitious vice president, thirty-five-year-old Andrew Carnegie, who subcontracted the steel to the Butcher Steel Works of Philadelphia and reserved the wrought-iron skewbacks for his newly formed Carnegie and Kloman Company. The Butcher superintendents were soon vehemently protesting Eads’s rigid specifications, and Carnegie was angrily complaining, “Nothing that would and does please engineers is good enough for this work.” Ultimately Butcher had to give up. The Chrome Steel Works of New York took on the job, but demanded more money, which brought on a minor financial crisis. Eads’s estimate of five million dollars for the bridge was proving too low, a normal occurrence then and now in large engineering projects.
However, an independent engineer appointed to do a cost study found no important changes to recommend, and Eads pushed ahead. Through the spring and summer of 1873 the steel tubes of the arches moved farther and farther out over the river, supported by cantilever cables made of steel bars an inch thick. Now a final problem was presented: How could the arch halves be joined at midpoint? As usual, Eads had already planned the solution. He had ordered each steel tube to be lengthened by a factor of 1.000363, to produce the extra necessary to take the compression after removal of the cantilever cables. The final two steel tubes of each arch rib he would truncate by five inches, and have screw threads cut inside. A short wroughtiron plug, fitted with two sets of threads, could be screwed into one tube, then screwed out to screw into the opposing tube. A steel band would confer superior strength on the joint.
Late that summer Eads again sailed for Europe, this time to float a new loan on the London market. He undertook the negotiations with Junius Morgan, the enterprising Yankee who had founded one of London’s leading financial houses. Before agreeing to the additional financing, Morgan asked for new evidence of progress in the construction—the closing of an arch. Eads told him that the first arch would be closed by September 19.
In St. Louis, Eads’s assistant Colonel Flad had thought up his own solution to the arch-closing problem. Flad’s idea was to hump the arch slightly, by pulling back on the cantilever cables. Once the joining ribs were brought together end to end, the cables could be loosened and the arch would assume its correct shape. Eads had given approval to the idea, holding his own method in reserve. Unfortunately a spell of unseasonably warm weather hit St. Louis that September, causing the steel tubes to expand rather than contract as Flad had calculated they would. He could not hump the arch high enough to achieve the joint. The Colonel did not give up easily. He built a wooden trough under the ribs and filled it with thirty thousand pounds of ice; then he tried again.