by Bill Bryson
To build a full-scale model and test it to destruction would have been essentially to build the bridge itself. So, as is typical in the engineering of large structures to this day, there comes a point when judgment dictates that the model testing must end and the real thing begin. In order to keep the navigation channels of the Menai Strait unobstructed, the longest tubes were fabricated along the banks. When completed, the tubular beams were floated into position between the towers and lifted into place by means of hydraulic jacks. This critical stage in the construction sequence was accomplished in a relatively short period, during which ships used the channel on the other side of the rock.
Although there were some anxious moments in the floating and hoisting process, the tubular girders were finally in place by 1850. However, since they had not been tested at full scale, there remained legitimate questions as to how they would perform. Such heavy girders might deflect so much under their own weight that they would be noticeably bowed and so present to a steam locomotive little better a roadway than the flexible deck of the suspension bridge. In anticipation of this possibility, the towers had been deliberately designed to be tall enough to accept iron chains from which the weight of the tubes could be partially supported. If this were necessary, then the bridge would effectively be a suspension bridge with a very heavy roadway. However, the tubes proved to be sufficiently stiff so that no supplementary support was necessary. Thus, the height of the towers in the finished bridge appeared to serve no structural purpose, a condition that some structural critics have seen as a flaw of its form.
The ultimate structural test would, of course, be when the first trains crossed the bridge. In anticipation of that, it was customary to conduct a ‘proof test’. In the case of the Britannia Tubular Bridge, as the structure came to be known, as many heavy steam locomotives as could be assembled were driven end to end upon it. The girders barely moved under the unreasonably heavy load, and so the design was ‘proven’ to be sound. (There was hardly a thought given to the structure’s ability to resist the wind, for it was so heavy that the strongest winds could no more move the tubes from their piers than a breath of air could a brick sitting on a table.) The structural performance of the bridge established it to be everything that Robert Stephenson claimed it would be. He, Fairbairn and Telford were all elected to the Royal Society within a year of the completion of the respective Menai crossing on which they worked.
The period during which the Britannia Bridge was under construction also saw rapid progress in the development of the new technology of photography. An engineering construction project was a perfect subject for the new art because it provided a static scene that was ideal for the long exposure times required. Indeed, photographs of the building of the Britannia Bridge are among the first of the genre. It was not practical to photograph groups of project engineers, however, because it was unlikely that they could all stay still long enough to capture a sharp image. Thus, the traditional art of painting was more likely to be employed to capture an occasional assembly of engineers.
The famous group portrait, Conference of Engineers at Britannia Bridge, was produced by the artist John Lucas shortly after the structure was completed, though it purports to show the partially completed bridge in the background. In any case, it conveys a sense of how such an ambitious engineering project advances in stages and that it takes a team to bring it to fruition. The completed Britannia Bridge may be attributed to its conceptualiser and chief engineer, Robert Stephenson, but like any other great structural achievement it owes its realisation to a host of other engineers advising and working on various details of the design and construction. To the engineers must also be added the often anonymous foremen and workers. These were represented in the painting by the two men kneeling on the floor and leaning against a wall, but clearly paying attention to the goings on.
Some key figures of the Britannia Bridge project are missing from Conference of Engineers. Neither Fairbairn nor Hodgkinson, without whose physical experiments and empirical formulas a successful full-scale tube design might never have been achieved, is depicted. This suggests that Lucas’ intent was not to capture a scene where all of the responsible parties are assembled, but rather to depict an example of what was probably a not infrequent occurrence at the construction site. On the occasion that Lucas visited, the final tube of the bridge was being floated into place to complete the bridge shown in the background of his painting.
The progress of a project like the Britannia Bridge was followed by engineers and contractors around the world. It was not only the design that interested them but also the manner in which it was executed and the erection of the parts accomplished. Anyone with such an interest who would have been travelling in the vicinity of the Menai Strait would likely have wanted to visit the construction site and experience for himself its scale and the energy and atmosphere surrounding it. Conferences of engineers and others, including everyone from members of the railway’s board of directors to foremen responsible for key operations, would take place regularly.
It was evidently at a board meeting that took place near the Britannia Bridge site that Joseph Paxton daydreamed and sketched on a blotter before him the rudiments of his design for a building to house the Great Exhibition of 1851 – which became known as the Crystal Palace. Though not an engineer himself, Paxton had been responsible for the design of the Great Stove and the Lily House at Chatsworth and believed that structural principles embodied in those achievements could be applied to making an iron-and-glass building to accommodate the Great Exhibition. The following week, with the help of the railroad engineer and Royal Society Fellow Peter Barlow, the design was fleshed out. When Paxton shared his scheme with Robert Stephenson, he declared the concept sound and encouraged Paxton to proceed. Such one-on-one conferences between engineers, architects and interested parties occurred too frequently and privately to be captured by Lucas or, apparently, by anyone else.
Engineers with no direct connection to a project would also visit it, much as those on a busman’s holiday do today. In the conference at Menai that Lucas did recreate in oils, the engineer Isambard Kingdom Brunel is depicted sitting to the extreme right. Brunel and Stephenson, among the most prolific of the great Victorian engineers, had had different views on railway gauges, with Brunel favouring the broad and Stephenson what came to be known as the ‘standard’ gauge. After the initial spate of building independent railways throughout the land, the lack of a common gauge among them made interconnecting them problematic. Brunel eventually lost the battle of the gauges, but he was to best Stephenson in designing a bridge to carry railway trains.
As much of a structural success that the Britannia Tubular Bridge was, it was an economic and environmental failure. The enormous amount of material and labour entailed in riveting relatively small sheets of wrought iron together to form massive tubular girders made the bridge very costly. In addition, since the trains ran through rather than atop the tubes, the ride could be a very hot and sooty experience. When Brunel was faced with designing a bridge to carry trains across the River Tamar near Plymouth, he had to achieve structurally essentially what Stephenson did at Menai, while at the same time doing it more economically and in a more environmentally acceptable way. His solution was to exploit in combination both arch and suspension principles to produce a significantly lighter bridge that was also open to the atmosphere and so presented a more pleasant ride. Brunel’s Saltash Bridge – officially known as the Royal Albert Bridge and bearing the inscription ‘I.K. Brunel, Engineer’ above its portals – as well as the wind-resistant suspension bridge of the German-American engineer John Roebling, proved that Stephenson’s solution to carrying trains over great spans was, in his own words, ‘a magnificent blunder’. Only about a half-dozen tubular bridges would be built throughout the world.
However, just as the design and construction of the bridge itself remains significant as a case study of how an overwhelming problem was solved and an epochal building project acc
omplished, the symbolism embodied in Lucas’ group portrait is timeless.
Confluences of engineers and the physical embodiments of the designs from their mind’s eye have been recorded with conventional optical cameras on many occasions, especially when failures occurred. After the high girders of the Tay Bridge collapsed in 1879, a photographer from Dundee was hired by the investigative body to record on film the remains in place. The set of systematic photographs was generally forgotten for over a century, until Peter Lewis of the Open University came across them in the Dundee City Library. Employing high-resolution and digitally enhanced scans of the old photos, he found on the piers distinct signs of brittle fracture of many of the cast-iron lugs. This led him to his revisionist explanation of the cause of the failure: the repeated movement of the bridge under passing trains and wind caused fatigue cracks to grow, which eventually led to the fractures. This made the cross bracing dependent on the lugs ineffective and the bridge consequently became more flexible. On the late December night in 1879, the combination of a fast-moving train, a howling storm, and a weakened structure proved to be fatal.
Bridge failures have been dramatic both structurally and photographically. At the turn of the twentieth century, the Firth of Forth rail bridge, the world’s first significant all-steel bridge, had the longest spans (1,710 feet) of any bridge in the world. In response to the collapse of the girders of the Tay Bridge, the Forth Rail Bridge had been designed as a robust cantilever structure, an old form that had recently been revived and popularised in Britain by engineers William Fowler and Benjamin Baker. The heavy look of the completed Forth Rail Bridge led some engineers to believe that it was grossly over-designed, and they sought to produce cantilevers lighter in form and fact. In 1907, a cantilever bridge under construction over the St Lawrence River near Quebec was on its way to achieving a record 1,800-foot span. Photographs of the incomplete bridge show it to have been of a very much lighter and lacier design than the Forth. Indeed, the Quebec Bridge proved to be overly slender and unable to support even its own weight. The bridge collapsed before it could be completed, claiming the lives of seventy-five construction workers. Photographs show it to have dropped into a tangled pile of steel.
A commission appointed to look into the causes of the collapse found that the weight of the bridge had been underestimated by the design engineer, who also made errors in his calculations of the stresses in the structure. The principal consulting engineer, Theodore Cooper, who was also the de facto chief engineer, had been remiss in not overseeing the work closely enough. After the causes of the failure were understood, the bridge was redesigned as a heavier cantilever structure and one whose geometry was much more amenable to analysis.
The failure of the first bridge brought uncommon attention to the rebuilding project. In one case, the board of engineers charged with redesigning the structure – the American and Canadian team of Ralph Modjeski, C.C. Schneider and chairman C.N. Monsarrat – were caught by the camera standing in the individual chambers of one of the key compression members (a redesign of the inadequate component that had initiated the collapse) awaiting assembly into the new bridge. In another photo, Monsarrat and Modjeski, along with the engineer of construction G.F. Porter and the chief engineer of the bridge company, G.H. Duggan, are sitting in a line on one of the thirty-inch-diameter pins – as if it were a beast of burden – that were awaiting installation.
When the central section of the redesigned bridge was being lifted into place to complete the structure, a fracture in one of the hoisting devices caused the entire section to fall into the river. A photograph of the impact of the steel on the water, complete with the accompanying dramatic splash, provided a rare example at the time of a failure caught on film. In spite of its troubled construction history, the Quebec Bridge was finally finished in 1917 and has stood for almost a century as the longest cantilever span in the world, a testament to the consequences of a failure. For longer spans, engineers looked to suspension bridges, which thanks to John Roebling and his successful approach to designing wind-resistant structures, were no longer looked upon as the frail descendants of the Menai Strait Suspension Bridge. Indeed, it was Roebling’s Niagara Gorge Suspension Bridge, completed in the mid-1850s, that had become the first suspension bridge to carry railway trains. The principles on which it was based – weight, stiffness and stay cables – also guided the design of his masterpiece, the Brooklyn Bridge. Through the last part of the nineteenth and the first couple of decades of the twentieth century, engineers designed suspension bridges with longer and longer spans, almost always stiffened by a truss.
Among the most watched suspension bridge projects of the 1930s was the Golden Gate Bridge across the entrance to San Francisco Bay. The structure’s 4,200-foot-long main span was to remain the longest in the world for over a quarter of a century. San Francisco had long wanted a bridge to connect it with Marin County across the strait – known as the Golden Gate – and thus with other northern California counties, but engineering proposals came with a prohibitive price tag. When Joseph Strauss, whose bridge company had specialised in movable bridges of modest span and appearance, proposed a hybrid cantilever-suspension bridge that he promised to deliver for a very attractive price, local movers and shakers paid attention. Not only did he assure them that he could design the bridge but also that he could help promote the bond issue needed to pay for it. When he was made chief engineer of the project, he invited engineers who did have direct experience with suspension bridge design to serve as consultants. At the first conference of the engineering advisory board, held in Sausalito in August 1929, the participants posed for a photo on the steps of the Alta Mira Hotel.
The President of the Board of the Golden Gate Bridge and Highway District, William P. Filmer, is naturally front and centre. Close to him, on his right, is chief engineer Strauss, hands on hips, elbows out, in a defiant stance that at the same time signals keeping others at bay. Directly behind Filmer is an army officer; as was the case at the Menai Strait, the approval of the military was essential in allowing any bridge to be built across the strategic Golden Gate.
Charles A. Ellis, the designing engineer, is standing on the same step as Strauss and Filmer, but away from them, a placement that may have been directed by the photographer to keep the tall Ellis from appearing to tower over everyone else. Still, his height emphasises Strauss’ small physical stature – something about which he was reportedly sensitive. Though the difference in their heights is ameliorated somewhat by Ellis’ standing almost off by himself, it is very likely that Strauss’ stance was prompted by this placement of Ellis on an equal footing. The tension between Strauss and Ellis suggested in this group portrait presaged that which would grow as the designing of the bridge progressed.
No chief engineer can be as fully informed about design details as those who are working directly on the calculations. Strauss had never carried to completion the design of a suspension bridge, let alone one that would break the world record for span length. The detailed design work fell to Ellis, working under the consultants, and specifically under the supervision of Leon S. Moisseiff. At one public presentation of progress on the project, questions of substance about the design could only be answered by Ellis, making it clear to all who did not already know it that Strauss was uninformed about critical details of his own bridge. Not one to like being found in such a position, Strauss effectively exiled Ellis from San Francisco by sending him back to the Chicago office to continue the design work out of the public eye.
With little staff help, Ellis worked away on the bridge’s design, but did not work fast enough to suit Strauss. After a confrontation over the design of the towers, Strauss ordered Ellis to take a vacation, from which he was never welcomed back. Ellis was replaced by Clifford E. Paine, who was identified as principal assistant engineer when construction on the bridge was completed in 1937. The engineering team listed on the dedicatory plaque located on the bridge tower did not include Ellis. This omission went general
ly unremarked upon for almost five decades, until the story of Ellis’ involvement was told by John van der Zee in his 1986 book, The Gate: The True Story of the Design and Construction of the Golden Gate Bridge.
While the Golden Gate Bridge was under construction, an even larger and arguably more ambitious project was underway to connect San Francisco with its neighbours across the bay to the east. Comprising two suspension bridges in tandem, a large-bore tunnel, a 1,200-foot cantilever span and a long viaduct, the San Francisco–Oakland Bay Bridge was the most expensive publicly funded highway project undertaken to that time. Since no state highway department possessed within its ranks all the expertise needed to undertake such an ambitious project, California enlisted expert consultants to help with the job. At its completion, which occurred about six months prior to the completion of the Golden Gate Bridge, the team of engineers making the final inspection of the Bay Bridge posed for a photo against the backdrop of one of its large suspension cables. Among the engineers were specialists in foundations, superstructure and traffic, emphasising the multiple disciplines needed to carry out a work of such magnitude and complexity.
With the Golden Gate and Bay bridges finished, there were few large metropolitan areas left in America that needed – and could afford – such spectacular bridges. But there remained the need for more modest suspension bridges in special locations for special purposes. New York City was preparing to host the 1939 World’s Fair, and it wished a new highway link in the vicinity. Elsewhere in the US, remote areas with the political clout and will also felt the need for suspension bridges. These were designed according to a new aesthetic, which dictated that a bridge’s deck should be as slender-looking as possible. One way of achieving this look was to eliminate the trusswork that had become a hallmark of American suspension bridge design.