With his explanatory bridge constructed on paper, Eads led his readers to observe that the pair of canted levers could be replaced by slanted straight members connecting one end with the other, thus saving material and money. At this point, the bridge looked like the roof structure of a house, which every reader of Eads’s report must have recognized as a kind of bridge in which they had long had confidence even if they lacked a complete understanding of its principles. As with larger roofs, the simple triangular arrangement has to be supplemented with cross bracing in order to keep the lines of the timbers from bending and breaking under their burden, and hence the familiar roof truss. In the 1860s, however, when railroad bridges had to span much greater distances than roofs, and without the help of excessively high and costly peaks, the flatter bridge truss made of iron evolved. The forces that had to be resisted by the various members of the truss depended upon its proportions, and these in turn affected the cost of the bridge. The proportions of height to length of bridge that would “insure the greatest economy” depended upon the type of truss, and they had been found to vary from about one-to-eight to one-to-twelve, thus making all sorts of bridges share a certain sameness of outline. Indeed, to Eads at least, the term “truss” included “every known method of bridging except the arch,” which provided the ultimate economy.
To use less material than a truss of triangles, the top of the bridge could be curved, thus leading to the “bow-string girder,” and Eads led his readers to observe that its bottom chord became unnecessary, thus reducing the cost, if the bridge piers or abutments could provide the forces to keep the bow from flattening out. However, for such a long bridge as Eads proposed at St. Louis, such an unbraced arch would be too flexible, especially under the action of heavy railroad traffic, and so he argued that it could be stiffened by means of a form of trussing between it and the roadway. Finally, in the ultimate refinement of his argument, Eads replaced the single heavy arch with a pair of lighter arches, themselves trussed together to provide enough stiffness so that the roadway could be supported on lighter members, thus reaching a further economy of materials.
According to Eads, there were two general kinds of arch bridge—the “upright” one, which he reasoned to from the principle of the lever, and the “catenary or suspended arch,” which was otherwise known as a suspension bridge. Even though Roebling’s Niagara Gorge Bridge had carried railroad traffic for almost fifteen years, some disagreement still remained among engineers about the safety of suspended spans. However, there was little doubt that the suspension bridge, or suspended-arch principle, was the most economical means of spanning long distances, such as that over the Mississippi between Illinois and Missouri. Eads addressed the question of upright versus suspended arch later in his report, and he weighed the pros and cons of using iron in tension and compression.
Though the repeated loading of iron in tension, such as occurs in a suspension bridge, was then known to be capable of leading to failure by fatigue, Eads and other engineers understood that they could avoid this by keeping the loading levels sufficiently low; but this meant using more material, and hence resulted in a greater cost. Since the ultimate strength of conventional iron was greater in tension than in compression, in the final analysis the suspension bridge, which relied on the former, was the most economical type for long spans. However, the upright arch, which Eads clearly favored on philosophical if not aesthetic grounds, would become economically competitive with the suspended arch if it could be made of a material whose capacity to carry a load without yielding in compression was not so inferior as was that of iron to its capacity to carry a load in tension. There was in fact a new material that fell into this category; Eads specified that his upright arch be made of cast steel.
As everyone knew, temperatures in St. Louis ranged from well below zero in the winter to well over a hundred degrees Fahrenheit in the summer sun, and this also had to be considered in bridge building. Since iron and steel expand when heated and contract when cooled, a five-hundred-foot arch would rise and fall eight inches with the passing seasons. Similar movements could result from heavy loads crossing the bridge. Uneven heating of the bridge by the sun, or asymmetrical loading of the bridge, could also push and pull it every which way over time. If this kind of potentially destructive movement could not be accommodated, it could destroy abutments, buckle rails and roadways, and eventually tear the structure apart. Eads was clearly concerned about this aspect of his bridge, as indicated in several patents for improvements in bridges that he took out while the bridge was under construction, in the late 1860s and early 1870s. The drawings for the earlier patents, though clearly showing an archlike bridge, are titled “truss bridge,” which further emphasizes Eads’s view of the arch, especially when stiffened, as a supremely efficient limiting case of a truss.
No engineer, in Eads’s day or now, can conceive, promote, design, defend, and build a major bridge without help. A good deal of the attention of an engineer-in-chief like Eads is necessarily taken up in matters of a political, financial, and public-relations nature, and there is not enough time, let alone experience or talent, in one person’s lifetime of days to carry out all the minute calculations and prepare all the detailed drawings that enable the proper parts to be designed, ordered, and assembled into a finished bridge, whether mathematical or not. Early in his report, Eads acknowledged his debt to some of the most important contributors to the engineering endeavor.
Henry Flad was born in Rennhoff, Germany, in 1824. After graduating from the University of Munich in 1846, he worked for the Bavarian government. He served as captain of engineers in the Parliamentary Army during the 1848 revolution, then fled to America. Flad landed in New York City in 1849 and worked there as a draftsman until getting involved with railroad engineering and moving west with the railroads. He enlisted in the Union Army and served in an engineering regiment, rising from private to colonel. After the war, he worked on plans to improve the water supply of St. Louis, and then was engaged as chief assistant to Eads for carrying out “the mathematical investigations and calculations for the Bridge.” Flad was in turn aided in the design by Charles Pfeiffer, who had recently emigrated from Stuttgart, where his thesis on the theory of arch-bridge design had won a prize. Indeed, in addition to his German degree of “civil engineer,” Pfeiffer may also have brought with him the experience of the Koblenz Bridge, completed in 1864, for Pfeiffer “based his first series of calculations on the equations developed for Koblenz and modeled his first sketches directly on the German bridge.” In 1869, Eads and Flad jointly were issued a patent for an “improvement in arch bridges” that relieved the thrust of the arch on its piers, “thus permitting a light construction of such piers.” Like Eads’s earlier patents, the drawing was headed “truss bridge.” A few years later, Flad was issued a patent in his own name for an invention that enabled the stay cables essential to the construction process to be maintained at a uniform tension even as temperature changes affected a bridge structure under construction.
Drawings from one of Eads’s several patents, this one employing the canted-lever principle and showing how broadly he applied the term “truss bridge” (photo credit 2.7)
As Eads, Flad, and Pfeiffer knew, the essence of sound engineering lay in clearly stating the assumptions upon which calculations are based so that they may be checked at all times for lapses in logic and other errors. It is thus imperative that engineering premises be set down clearly, and that the calculations that follow be systematically and unambiguously presented, so that they may be checked by another engineer with perhaps a different perspective on the problem. Eads explained the procedure that was used in his office:
After careful revisions by Col. Flad, the results obtained from time to time were submitted to me; and, finally, to guard against any possible error in the application of the principles upon which the investigations were made, or in the results arrived at, they were referred by me to the patient analysis and careful examination of Chancellor W
. Chauvenet, LL.D., of the Washington University, formerly Professor of Mathematics in the U.S. Naval Academy at Annapolis. His certificate, affirming their correctness in every particular, will be found appended to this report. For the interest this gentleman has taken in the enterprise, for the care bestowed in examining and verifying the scientific data required for the work, and for many valuable suggestions and simplifications in the investigation, I feel under many obligations.
The title, degree, and affiliations of William Chauvenet, who was serving as consulting engineer to the project, were given, of course, to establish his authority and integrity, just as his certificate was to attest further to the correctness of Eads’s report. In virtually all large engineering projects there is one or more consulting engineers of vast experience and irreproachable authority, who provide either the basis for carrying out the details of the design or the imprimatur that some less prestigious engineer’s design is sound. The profession of engineering, like all professions, rests upon the exercise of sound judgment, and the interaction of engineers of varying degrees of experience must involve a delicate balance between recognizing and accepting something as correct, on the one hand, and recognizing that it might not be done in exactly the same way by another engineer—including the one doing the checking.
Engineers are also human, of course; sometimes they cannot entertain the idea that a competing design may be an equally correct, if different, solution to a problem. An earlier consultant to Eads, Jacob H. Linville, was an undisputed authority on railroad bridges. Having become the engineer for bridges and buildings for the Pennsylvannia Railroad in 1863, he had by 1864 completed the first long-span (320-foot) truss across the Ohio River, at Steubenville, on the West Virginia border. When Eads sent him the preliminary drawings for his own long-span arch bridge, Linville wrote back disapprovingly of the design: “I cannot consent to imperil my reputation by appearing to encourage or approve of its adoption.” He further wrote, “I deem it entirely unsafe and impracticable, as well as in fault in the qualities of durability.” Linville was perhaps more concerned with his own reputation as a proponent of more conventional trusses than with the safety of Eads’s design, and he was perhaps a bit disingenuous in condemning what he may not have fully understood. Not surprisingly, Linville suggested for St. Louis a truss bridge, one that could be assembled on pontoons, floated into place, and then raised into position by hydraulic jacks, the way Stephenson’s Britannia tubes had been, thereby obviating the objection to scaffolding obstructing the river during construction.
Eventually, however, with the opposition of Linville, Boomer, and the Convention of Engineers assembled to discredit Eads a thing of the past, he was able to proceed with the plans endorsed by Chauvenet and the board of directors. First the bridge’s piers had to be built up from the rock below the river bottom, for Eads believed that no other foundation would survive the scour of the Mississippi. Indeed, the need to sink the piers so deep gave Eads additional incentive to construct the smallest possible number of them in the river, and thus he had had to specify the longest practicable spans. This kind of trade-off is common in large bridge design and often has an enormous effect on the total cost of the bridge. However, later in 1868, before any final decisions had been made on how to sink the piers, Eads became ill with a terrible cough and, under a doctor’s orders to seek complete rest, tendered his resignation to the bridge company. But it was not accepted, and so work on the bridge, which had actually begun as early as August 1867 with the construction of the western abutment, was halted when Eads left for a restful trip to Europe.
Eads’s proposal for a bridge across the Mississippi River between St. Louis, on the left, and Illinois Town (photo credit 2.8)
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After six months, Eads came back briefly to New York in order to conduct some financial negotiations, then returned to Europe, where he discussed plans for his bridge with British and French engineers and visited construction sites. It was on this trip that Eads became acquainted with the relatively new technique of using a plenum pneumatic, or pneumatic caisson, for sinking foundations underwater, and he inspected at Vichy a bridge-construction site employing the procedure. The customary method, which Eads had had in mind initially, was to erect a cofferdam enclosing an area from which the water could be pumped out and the riverbed excavated to bedrock. Among the drawbacks to the cofferdam, however, was that it was open to the air, and thus exposed to the elements and subject to flooding. The new caisson method employed what was essentially a gigantic inverted box into which compressed air was pumped to keep the water out while the work went on inside. As the river bottom was excavated, Eads planned to pile heavy stones atop the caisson, thus erecting the pier at the same time that its weight pushed the caisson into the riverbed. The scheme appealed to Eads’s experience with submarine salvaging, being not unlike a gigantic diving bell or inverted and weighted barrel, and the sand pump he had devised for salvage work would also provide an efficient means of getting the excavated debris out of the caisson.
Thus, though the pneumatic-caisson method had been used for almost fifteen years to sink over forty piers in Europe, in America Eads was going to improve upon the concept and extend it to greater depths than ever before. The first caisson was launched from its own construction site, floated into position, and sunk in October 1869. Work inside the chamber was to continue smoothly day and night for five months, through the winter. As the exotic construction progressed, “a visit to one of the air-chambers under the piers was one of the principal attractions that St. Louis had to show to visitors.” One retrospective description captured the experience of descending into the caisson:
For a while one felt perfectly comfortable in this underworld—a world such as no mythology and no superstition ever dreamed of. The transition, indeed, became apparent by pain in the ears, bleeding at the nose, or a feeling of suffocation; but these inconveniences and seeming dangers, inevitable upon such a visit to hell, were insignificant in comparison with the interest which it offered. It was undertaken by hundreds and hundreds of visitors, including many ladies, and none returned from that depth without carrying along with them one of the most remarkable reminiscences of their whole life. Shrouded in a mantle of vapor, labored the workmen there loosening the sand; dim flickered the flames of the lamps, and the air had such a strange density and moisture that one wandered about almost as if he were in a dream. For a short time all this was extremely interesting and delightful, but it was not long before the wish to escape again from this strange situation gained the upper hand over the charm which it exercised. Gladly did the visitor, after a quarter of an hour, re-enter the air-lock, with an unfeigned feeling of relief, to watch the air beginning to escape from this chamber. At once the door behind him leading from the caisson closed by the denser air, and fastened as firmly as if there was a mountain behind it. The compressed element escaped whistling from the air-lock; the air within was more and more equalized with the air without; a few minutes, and they were of equal density; then the door, no longer pressed against its frame by the dense atmosphere, opened to the winding stairs, and the visitor came forth taking a long breath, and, to use one of Schiller’s words, once more “greets the heavenly light,” which shone from far above down the shaft.
A caisson being sunk for the St. Louis Bridge (photo credit 2.9)
When the caisson had penetrated sixty-six feet, a telegraph terminal was installed inside it, “where all things hideous are,” enabling workmen there to communicate at all times with “those that breathe in the rosy light.” Eads used this telegraph as a promotional device to send greetings to bridge directors in New York, and visitors in the caisson sent messages to friends in the “outer air.” Such antics and poetic rantings demonstrate how casually the caisson was treated, and no doubt many of the visitors had a good laugh when a flask of brandy one of them had taken down into the depths, and there drunk from, exploded violently when they returned to the outer air. This, however, was but an amusing
precursor of more serious events to come.
When the caisson reached a depth of seventy feet, the workmen began to experience some difficulty climbing the stairs to the surface. As the caisson was sunk deeper, men suffered increasing attacks of cramps and paralysis, which were thought to be due to insufficient clothing or poor nutrition. In March 1870, when the caisson had reached ninety-three feet, the air pressure inside it was about four times what it was in the open air, and workmen began dying upon emerging from the caisson, or after being hospitalized for an ailment that came then to be called “caisson disease” but today is known as “the bends.” Eads asked his family physician, Dr. Alphonse Jaminet, to look after the workmen, but Jaminet himself became paralyzed one day, having spent time down in the caisson and come up after only a few minutes in the air lock.
Perhaps somewhat to his own surprise, Jaminet recovered, and began to conduct research into these mysterious attacks. He shortly concluded that the major cause was too-rapid decompression in the face of a drastic difference in air pressure between the submerged caisson and the outer air above. Thereupon he placed restrictions on the amount of time the men could work inside the caisson, and on the speed with which the pressure in the air lock could be reduced. The west pier of the bridge, having to go to a depth of only eighty-six feet, was sunk with fewer incidents. Fatalities would also occur in sinking the caissons used in the piers for the Brooklyn Bridge, however, and Andrew H. Smith, a former army doctor and specialist at the Manhattan Eye and Ear Hospital, would be engaged as surgeon to the New York Bridge Company. Smith, like Jaminet, would eventually recognize too-rapid decompression as a prime contributor to the problems being experienced in the deep caissons.
Engineers of Dreams: Great Bridge Builders and the Spanning of America Page 6