Engineers of Dreams: Great Bridge Builders and the Spanning of America

by Henry Petroski

  The Britannia Bridge, in northwestern Wales, with the Menai Strait Suspension Bridge visible about a mile to the north (photo credit 2.1)

  John Roebling was educated as an engineer in Germany, having received a degree in civil engineering from the Royal Polytechnic School in Berlin in 1826, but for philosophical reasons he emigrated to America in 1831 with the intention of starting an agrarian community. He settled first in western Pennsylvania; when the utopian experiment did not work out, he turned to manufacturing wire rope for towing barges on canals and some small suspension bridges that carried the canals over rivers. In 1841, he published a paper discussing the “comparative merits of cable and chain bridges,” in which he described many of the then widely known failures of suspension bridges, arguing that the incidents showed an engineer what he had to design against. By 1854, Roebling had completed a bridge with an 810-foot span over the Niagara Gorge, which demonstrated incontrovertibly that an efficient and economical cable suspension bridge could indeed be built to carry heavy railroad trains. Shortly after the Niagara Gorge Suspension Bridge was completed, Roebling proposed a very long-span suspension bridge over the Mississippi at St. Louis, but there was not sufficient financial support for any kind of bridge at that time. More than a decade later, Roebling would propose several other designs, including combination suspension-and-arch types, but there was little support for these proposals either.

  After the Civil War, however, the City Council declared that it had “become indispensably necessary to erect a bridge across the Mississippi River at St. Louis, for the accommodation of the citizens of Illinois and Missouri, and the great railroad traffic now centering there,” and the city engineer, Truman J. Homer, was instructed to draw up possible plans and estimate costs. He did report, only four days later, and was able to make a recommendation based on an idea he had actually conceived several years earlier and had since spent “much thought and extended inquiry upon.” Homer condemned suspension bridges and proposed the building of a tubular bridge, slightly larger than Stephenson’s Britannia, with carriageways on either side of the main tube and footpaths above it. For forty-nine weeks of the year, the clearance above high water would have been at least forty-four feet; Homer argued that steamboats could pass under such an obstruction, and that, in any case, the chimneys of steamboats could be made so they could be raised and lowered at will. The cost of Homer’s tubular bridge was given at over $3 million. An earlier scheme, proposed by the Mississippi Submerged Tubular Bridge Company, for a tunnel under the river, might have cost even more. Neither was built.

  The urgency for a bridge at St. Louis had been driven by earlier developments elsewhere. In 1856, the first rail bridge across the Mississippi was completed at Rock Island, Illinois, which was just about due west of Chicago, on the Chicago & Rock Island Railroad, thus promising an uninterrupted route westward. St. Louis boatmen reacted by filing lawsuits, “charging that bridges across navigable waterways were public nuisances, navigation hazards, and unconstitutional restraints on interstate commerce.” In the meantime, the river had also been bridged by railroads at Dubuque and Burlington, Iowa, and at Quincy, Illinois, only a hundred or so miles upriver from St. Louis. The Missouri River was also bridged, at Kansas City, thus allowing St. Louis to be bypassed entirely by railroads on the way to its historic trade territories. Although complaints were rising that “it cost nearly half as much to ship a barrel of flour fifteen hundred feet across the river as it did to ship it upstream twelve hundred miles from New Orleans,” ferryboat interests at the city without a bridge insisted they could continue to serve St. Louis commerce by floating entire railroad cars across the Mississippi on barges. However, the expenses and interruption of continuous rail service created bottlenecks in Illinois Town, and business was lost to the northern routes. One newspaper editor is reported to have said that geography had been undone by technology.

  Even though the population of St. Louis was on a par with Chicago’s two hundred thousand in the mid-1860s, in the commercial race the Missouri city was trailing and falling further and further behind. Illinois ranked second in railroad-track mileage in the early 1860s; Missouri was fifteenth among the thirty-seven states, with only 983 miles laid by 1867. Still, there were five railroads from the east and three from the west converging on St. Louis, and no continuous river crossing to serve them. Local newspapers and civic leaders began frantically to call for a bridge, which they argued not only would help St. Louis replace Washington, D.C., as the nation’s capital but also would enable it to become “the future Great City of the World.”

  Neither in the heat of community boosterism nor in calmer times can bridges be erected wherever one pleases. In order to throw a bridge over a navigable waterway between two states, one has first to secure the appropriate enabling legislation. Thus, as an initial step, bridge promoters had obtained a charter for the St. Louis and Illinois Bridge Company, having secured the authorization of the two states in 1865 and that of the federal government in 1866. Like many a bridge charter, this one made certain specifications about the structure, which “might be a pivot or other form of drawbridge or else one of continuous spans.” If the bridge did not pivot or open, it had to have spans of no less than 250 feet and it had to provide no less than forty feet of headway above the city directrix, which was a curbstone at the foot of Market Street indicating the level that record flood waters had reached in 1828, and which defined “the datum plane for all city engineering in St. Louis.”

  James Buchanan Eads, as he was pictured in A History of the St. Louis Bridge (photo credit 2.2)

  It was not uncommon, once one private company was formed to build a bridge and obtained a charter, that a rival firm soon also was established and sought a charter of its own. In the case of St. Louis, the competition was embodied in a Chicagoan, Lucius Boomer, and his Windy City backers. They exerted pressure on the Illinois Legislature to rescind the charter of the St. Louis and Illinois Bridge Company and give Boomer’s deliberately named Illinois and St. Louis Bridge Company the exclusive right for twenty-five years to build a bridge from the Illinois shore. If such a bridge were actually built, toll revenues would effectively flow from St. Louis business interests to Chicago investors. Even if Boomer’s group did not complete a bridge, or if it sold its charter to St. Louis steam- or ferryboat operators, the effect would be to cause St. Louis to fall further behind Chicago in mercantile activity.

  Among the contemporary movers and shakers in St. Louis was James Buchanan Eads, whose interest in bridges to this time was mainly in how they might obstruct the waterway. However, he took the potential commercial threat from Chicago as a call to action, and since a bridge was believed to be inevitable, he encouraged support of the original, and local, bridge company. A committee went to the Illinois state capitol in Springfield to lobby against what came to be known as the Boomer bridge bill. Southern Illinois legislators, who understood the importance of St. Louis to their own economic future, helped to get compromise legislation passed specifying that the exclusive building rights of Boomer’s firm would lapse if a bridge was not begun in two years or finished in five.

  Early in 1868, Boomer began to make noises about what kind of bridge his company would build. An earlier bridge of his had collapsed in 1855, killing Calvin Chase, one of Eads’s original salvage partners, along with many other prominent St. Louis businessmen and politicians en route to a convention in Jefferson City. This time Boomer involved a consulting engineer, Simeon S. Post, of Jersey City, New Jersey, whose reputation was sound; his proposed bridge was to consist of six spans of an iron-truss design he had patented in 1863. The term “truss” designates any arrangement of beams, rods, cables, or struts that are connected together to form a rigid framework, thus enabling relatively long and stiff bridges to be built with a minimum of material. Wooden-roof trusses are of such construction, but, perhaps because they are concealed, they were an often overlooked kind of bridge. The idea of a truss as a bridge in its own right had been mad
e explicit in the Renaissance.

  In his sixteenth-century book on architecture, the Italian architect Andrea Palladio illustrated the wooden truss as a “most beautiful contrivance.” In eighteenth-century England, wooden bridges resembling Palladian designs came to be called “mathematical bridges,” presumably because of the forethought and calculation that had to precede the cutting, assembling, and bolting together into an effective structure of the many different wooden pieces. Today, the Mathematical Bridge that allows the residents of Queens’ College a very convenient route across the River Cam is one of Cambridge’s tourist sites and one of the most photographed, sketched, and painted of its structures.

  A truss bridge and some terminology used to describe its various parts (photo credit 2.3)

  A variety of truss types employed in bridges (photo credit 2.4)

  With the increasing production and application of iron in the nineteenth century, trusses naturally evolved into a plethora of types and styles employing the new material. Iron-truss bridges, unlike the Britannia tubular bridge, were relatively light and open structures, and yet, if properly designed, were just as well suited to carrying heavy railroad trains. How to arrange the various parts of a truss was the subject of many patents dating from the 1840s on, and Simeon Post’s patented design incorporated modular arrangements of iron rods and struts. Like most patents, this was an improvement on the prior art. Post’s arrangement of the components allowed for the expansion and contraction of the iron so that traffic and temperature changes would “not produce injurious effects upon the structure, and in this manner obviating one of the most serious objections to the universal use of such bridges.”

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  James Eads had never built a wooden bridge, let alone one of iron, and perhaps he had never even dreamed of doing so. His interests were more in and on the water than over it. Indeed, to Eads, putting the piers of a bridge in the water meant providing obstacles to river traffic, as did the superstructure they supported, but his concern for the commercial future of St. Louis overcame his preference for an unobstructed river. He never did lose sight of the importance of river traffic to the city, however, and so he could not imagine or endorse any bridge design that would have obstructed the waterway more than it had to, even temporarily during construction. Thus Eads would certainly have balked at the traditional means of building stone-arch bridges and even some of the newer iron-truss bridges, whereby a timber scaffolding known as “centering” or “falsework” needed to be erected first, and might have to remain in the main waterway for an unconscionable length of time until the great bridge being assembled atop it reached the point where the stone or ironwork was self-supporting and the centering could be struck or the falsework disassembled.

  As early as 1866, Eads’s recommendations for bridge legislation included a minimum width of six hundred feet between piers and a minimum headroom of fifty feet above high water, “measured in the center of the span,” to minimize interference with river traffic. Such a specification, which effectively ruled out a truss bridge of any kind, may well have reflected Eads’s reading of the unrealized proposal made early in the century by the British engineer Thomas Telford for an iron arch to provide a clearance of sixty-five feet above the water at the crown while spanning six hundred feet over the Thames River at London. Legislation as strict as Eads’s suggestion did not pass, but a minimum span length for a crossing of the Mississippi below its confluence with the Missouri, just above St. Louis, was fixed at five hundred feet. This was the size arch Eads himself eventually proposed to build.

  Even though no arch greater than about four hundred feet had ever actually been constructed, the reputation of Telford, who was the first president of the Institution of Civil Engineers and who is buried in Westminster Abbey, made his dream almost as good as reality, at least to Eads; besides, the iron bridges of modest span that Telford did complete in Wales and Scotland were masterpieces. However, aesthetic models were ideals to be challenged by competition and economics. A convention of civil engineers, including Post, which was lavishly hosted by Boomer in St. Louis in August 1867, praised his bridge proposal while cautioning investors that Eads’s had “no engineering precedent.” Eads countered with an appeal to Telford’s generally acknowledged sound judgment and authority, which he felt did indeed provide “some ‘engineering precedent’ to justify a span of 100 feet less,” six decades later. He went so far as to state that it was “safe to assert that the project of throwing a single arch of cast steel, two thousand feet in length, over the Mississippi, is less bold in design, and fully as practicable, as his cast iron arch of 600 feet span.” Eads’s own reputation for sound engineering judgment, albeit with projects other than bridges, and his confidence, coupled with Boomer’s increasingly transparent attempts to manipulate public opinion, led to negotiations between the two rival bridge companies, which consolidated in 1868. Their stock was combined, a new board of directors was formed, with equal representation from each side of the river, and the name Illinois and St. Louis Bridge Company was adopted, with Eads as engineer-in-chief.

  Thomas Telford’s 1800 proposal for an arch bridge across the Thames (photo credit 2.5)

  Some insight into what must have helped sway support to Eads can be gained by reading his report of May 1868 to the president and directors of the company, in which he consistently writes of “your Company” and “your Bridge.” As are virtually all reports by successful entrepreneurial engineers like Eads, the twenty-five-thousand-word document is technically concise and sound, a model of clarity and persuasion, and totally accessible to the general reader. Eads said as much in his opening statement to the directors:

  In view of the great importance of your enterprise, the deep interest manifested in it by our citizens and the public generally, and because the plans adopted by you have been frequently misrepresented and unfairly criticised, I have deemed it proper that everything of interest connected with my department should be placed in such form as to be clearly understood, not alone by your stockholders, but also by every person of ordinary intelligence in the community. I have, therefore, endeavored to explain the plan of the structure, the principles involved in its construction, and the reasons for its preference, in the simplest language I can command, and with an avoidance, as far as possible, of the use of all technicalities not understood by every one.

  Under the topic of location, Eads explained why his bridge was to be sited at Washington Avenue, rather than a few blocks north, where “Mr. Boomer’s bridge” was to be located. The convincing arguments had to do with access to the center of population in St. Louis, the pre-existence of streets that were able to absorb all the traffic that would concentrate at the bridge approach, the cost of the connecting tunnel needed to carry the railroad trains through the center of the city without interfering with carriage and foot traffic, and the location along the wharf that would minimize interference with riverboats. Eads also led the reader through an elementary discussion of the principle of the lever in order to demonstrate “the economy of the arch, over the truss, for long span bridges.” In fact, Eads saw the arch as a kind of limiting case of the truss, with stone abutments serving to take the thrust that in a truss would otherwise have to be resisted by increasing amounts of iron, which naturally added weight and thereby cost to the structure.

  To show that his bridge design was not “needlessly extravagant,” Eads proceeded to illustrate “enough of the general principles involved” in bridge building to allow “anyone with ordinary intelligence” to judge for himself the value of the arch over the truss for the St. Louis crossing. He began his remarkably concise and well-illustrated exposition with the “simplest of all the mechanical powers, the lever,” which is familiar to everyone in the form of a simple balance scale. The action of a large weight on a short arm is balanced by a small weight on a longer arm, with the ratios of weights and arm lengths simply related to each other. If the short arm of the lever was bent, or canted, relative to the long arm,
Eads argued that the same ratio of pulls or pushes was necessary to maintain equilibrium, and the supporting wall or abutment into which the canted lever was anchored could supply whatever force was needed. This canted-lever principle was the same as that articulated by Galileo in his seventeenth-century investigations into the strength of materials, and it would play a central role in bridge building later in the nineteenth century. But for Eads in his 1868 report to the president and directors of the Illinois and St. Louis Bridge Company, it was only a means to an end.

  Three figures from the report of James B. Eads, showing the principle of the lever, the “canted lever,” and hack to hack canted levers tied together by a lower chord to make a truss bridge (photo credit 2.6)

  Eads next argued that, if two canted levers were placed tip to tip, with their short arms resting on piers and tied together with a structural element known as a chord, there would result an elementary truss bridge, in which the pull in the chord would be opposed by a compression of the longer lever arms. Like all successful engineers, Eads understood the important and nontrivial practical implications of the truism that the structure worked if it did not fail: “If the upper member fails to resist the crushing force, or the lower one is rent asunder, the truss must fail.” Such simple reasoning provided the basis for calculation of how much material, and hence how much money, was required to build a bridge.