Seeing Further: The Story of Science, Discovery, and the Genius of the Royal Society - Bill Bryson (2010)

10. HENRY PETROSKI

IMAGES OF PROGRESS:
CONFERENCES OF ENGINEERS

Henry Petroski, the Aleksandar S. Vesic Professor of Civil Engineering and a Professor of History at Duke University, is the author of more than a dozen books on engineering and design, including To Engineer Is Human: The Role of Failure in Successful Design and Engineers of Dreams: Great Bridge Builders and the Spanning of America. His newest book is The Essential Engineer: Why Science Alone Will Not Solve Our Global Problems. He is a Distinguished Member of the American Society of Civil Engineers; a Fellow of the American Society of Mechanical Engineers, the Institution of Engineers of Ireland, and the American Academy of Arts and Sciences; and is a member of the American Philosophical Society and the US National Academy of Engineering.

AS CELEBRATED IN THEIR DAY AS THE STATESMEN OF SCIENCE WERE THE GREAT ENGINEERS OF THE NINETEENTH CENTURY. HENRY PETROSKI EXPLAINS HOW THEY BUILT THESE AWESOME STRUCTURES AND WHY THEY ATTRACTED SUCH ACCLAIM.

One of the great engineering achievements of the nineteenth century was the expansion of the railways into an ever-widening network. Extending the right of way across major bodies of water naturally presented especially difficult problems for engineers, and so early railways often relied upon ferries at these locations. But this solution was not in keeping with the developing image of a fast and uninterrupted journey in a string of carriages pulled by a steam locomotive, and so bridges were built whenever possible. The most daring of these bridges, symbolic of the creativity, resolve, and integrity of the engineers that designed and built them, proved to be great engineering achievements in their own right, especially when the body of water to be crossed presented unique challenges, as it did at the Menai Strait.

This strategic strait, which separates the isle of Anglesey from the mainland of north-west Wales, was controlled by the Royal Navy, and so the Admiralty required that any bridge that was to cross it had to provide a clearance of at least 400 feet horizontally and 100 feet vertically so that tall-masted sailing ships of the day could pass between its piers and beneath its roadway without hindrance. Furthermore, because of the importance of the strait, temporary supports were not allowed in the water during construction. This virtually ruled out the choice of an arch bridge, which traditionally required the use of an elaborate system of falsework upon which the arch was assembled until it was self supporting.

Thomas Telford had already been presented with this problem when he was charged with completing the highway that connected London and Dublin and thereby providing a reliable route for the delivery of, among other things, the royal mail. The Irish Sea could only be crossed by ferry. The ideal location for a terminal was at Holyhead, which is on the west side of the island of Anglesey.

To carry the road from London to Holyhead meant bridging the Menai Strait. Telford initially wanted a cast-iron arch, which in 1811 he proposed to support by cables from above and thereby not obstruct ship traffic during construction. This untried method would have worked, as would be proven a half-century later, but it was not to be tried first at Menai. Instead, Telford designed the only other then-known bridge type that could span the distance and provide enough headroom: a suspension bridge.

The Menai Strait Suspension Bridge was completed in 1826 and remains an aesthetic paragon of what can be achieved with the form. Telford’s early experience as a mason enabled him to design graceful viaducts and towers bracketing the main span, which was a record-shattering 580 feet. He employed wrought-iron chains that were tested before installation, and the completed bridge was a structural marvel of its time. Unfortunately, the wooden roadway of the bridge proved not to be as substantial as its stone towers and viaducts and iron chains. When the wind was especially unfavourable, the roadway was susceptible to being tossed about, and on occasion it was destroyed.

When the Chester & Holyhead Railway was being laid out, routing its tracks across the Menai Bridge seemed the natural thing to do. However, as the wind had demonstrated, the structure’s roadway was light and flexible, and this would not serve the purpose of the contemporary railway. As well as the possibility of the road being destroyed in a storm, there was also the problem of a heavy steam locomotive causing the roadway of the bridge’s main span to deflect so much that the engine would have had to climb out of a valley of its own creation. The engineer George Stephenson suggested decoupling the train of carriages from its locomotive and using horses to pull the carriages to the other side of the bridge, where they could be coupled to another locomotive for the continuation of the journey. This was not what engineers would call an elegant solution.

Stephenson’s son, Robert, had a different idea. It involved designing a bridge that relied on neither the arch nor the suspension principle. Stephenson identified a site about a mile south of Telford’s Menai Suspension Bridge, where a large rock formation divided the strait into two wide navigation channels. Since this natural formation, known as Britannia Rock, was a recognised and accepted obstacle to shipping, there could be no reasonable objection to constructing a tall stone tower upon it. Similarly tall towers could also be erected outside the navigation channels on either side of the rock. Massive wrought-iron girders could then be installed at a sufficient height between these towers so that the vertical clearance was equal to that beneath the suspension bridge.

Robert Stephenson’s scheme was acceptable to both the railway company and the government, and so the detailed design and construction of the bridge was begun in the mid-1840s. Since no such structure had ever been designed, let alone built, it fell to Stephenson to organise what would today be called a research-and-development project. In order to keep the weight of deep girders exceeding 450 feet in length within acceptable bounds, it was decided early on that they should be hollow. At the time there existed no structural theory sufficiently advanced whereby the design of such girders could proceed by calculation alone. An experimental programme was thus embarked upon.

The experimentalist-engineer William Fairbairn, who had established a shipyard and had tested cast-iron beams years earlier, was responsible for conducting scale-model strength tests to establish the preferred shape and detailed design of the wrought-iron tubes. He began with small-scale models to compare the relative strengths of different shapes and arrived at the conclusion that a rectangular cross-section was the best. The model tubes were tested by hanging from their centre weights that represented the load of a heavy locomotive. Weights were added until the tube failed, which revealed the weakness of the structure and thereby provided guidance for how to modify it in the next model. By progressively increasing the scale of his models, Fairbairn was able to establish trends of behaviour, and from the experimental data the theorist Eaton Hodgkinson established an empirical formula by means of which he could extrapolate to the requirements for the full-size tube.

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 accomplished, 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 generally 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.

The first significant departure from the use of a stiffening truss had occurred in the design and construction of the George Washington Bridge, which opened in 1931 and crosses the Hudson River between New York and New Jersey. The exceptional width of this bridge’s roadway and the consequent weight did make a truss unnecessary in this case, but in regions where light traffic meant that only two lanes were required, a narrow and shallow bridge deck meant also a much lighter and more flexible structure. Suspension bridge designers sought to fit their structures with ever more slender decks. By the end of the 1930s, this trend produced bridges whose roadways moved a suprising amount in the wind. There was no satisfactory theoretical explanation for this behaviour, but engineers felt confident that their bridges were in no danger of collapse.

They were disabused of that in 1940, when the Tacoma Narrows Bridge, whose deck had been undulating in the wind for months, began to twist and soon collapsed. Since the undulations had been occurring for some time, the bridge was the object of an ongoing study. Its misbehaviour was being investigated experimentally through a scale model, and the real bridge was being filmed. On 7 November, when the vertical undulations changed over to torsional oscillations, a film crew was despatched to capture the new behaviour. The twisting lasted for hours, and the final writhing of the steel structure caught on film made the bridge infamous. Indeed, before the collapse of the New York World Trade Center twin towers, the failure of the Tacoma Narrows Bridge was the most widely viewed structural collapse in engineering history.

Today, bridges of unprecedented scale and unchallenged beauty continue to be designed and built worldwide, and they require no less of a team than did their predecessors. The seemingly unrelated aims of functional strength and aesthetic appeal had been not only successfully integrated in many of the classic suspension bridges of the past two centuries but also commonly achieved by engineers alone or leading teams. Thomas Telford was in fact both engineer and architect of his Menai Suspension Bridge, and John Roebling was both engineer and architect of his Brooklyn Bridge. That these engineering structures especially have come to be regarded as architectural icons demonstrates the aesthetic heights that an engineer can achieve.

Engineers less artistically confident than Telford and Roebling have engaged consulting architects to advise them on the design of everything from the façades placed on massive anchorages and skyscraper-high towers to the finishing details like deck railings and lampposts. Othmar Ammann, the chief engineer of the George Washington and many other New York City bridges, often sought the help of famous architects. When the George Washington was but an idea on paper, Ammann engaged Cass Gilbert, the architect of the Woolworth Building and other landmarks, to depict how the towers might be finished in stone. Since money was tight when the bridge was being completed, however, the steel-framed towers were left bare - a look that the Swiss architect Le Corbusier found extremely appealing - and bare steel became the new aesthetic standard for monumental bridge towers. For his Bronx-Whitestone Bridge, Ammann engaged the ‘architect to the elite’ Aymar Embury II in designing the structure’s anchorages. It was Embury’s suggestion that they express the force that they exert against the pull of the suspension cables and show its trajectory into the monolithic bookends of the bridge proper.

But relationships between architects and engineers were generally strained in America in the 1920s and 1930s. There had been continuing tensions over which of these professionals should control bridge projects. The architects argued that they were better prepared to choose the form and site for a bridge, leaving it to engineers working under them to figure out how to build the structure. But, unlike large buildings, long-span bridges had traditionally been sited, designed and constructed under the direction of a chief engineer. The increasing structural challenges presented by long-span bridges kept the engineers in control.

In a series of articles in Civil Engineering magazine, the architect Embury described his working relationship with the engineering team for the Bronx-Whitestone and made it clear that the chief engineer always had the final decision. According to Embury, in a bridge project engineers and architects alike were ‘instruments’ of the one chief engineer and ‘were guided by his desires as to the lines along which we should proceed’. But neither was Embury uncritical of his colleagues in either camp. He did not approve of engineers pursuing ‘design by drawing instruments’, by which he meant that they tended to use certain angles in their structures because they were the ones of the drafting instruments close at hand. He was also critical of his fellow architects, who he felt too often followed ‘the easiest way’. In an attempt to promote a meeting of the minds, Embury believed that ‘engineers should be good architects, and architects good engineers’. Who could argue with that?

In more recent years commissioning organisations have tried to force engineers and architects to be equal partners in bridge design. The design competition guidelines for the Gateshead Millennium Bridge, the strikingly original arch-and-cable ‘blinking-eye’ movable structure that carries pedestrians over the River Tyne between Gateshead and Newcastle, made it clear that multidisciplinary teams were expected to produce entries ‘of sufficiently high technical and aesthetic merit’.

The design competition for the London Millennium Bridge, the low-slung suspension bridge for pedestrians that spans the Thames to tie together St Paul’s Cathedral and the Tate Modern museum, went further than the Gateshead one. For the London crossing, it was required that design teams comprise not only engineers and architects but also artists. The winning entry was a collaboration among the engineering firm of Ove Arup, the architectural firm of Norman Foster and the sculptor Anthony Caro. The resulting long, slender-decked bridge has been described as a ‘blade of light’, which it resembles when viewed from a distance up or down the river. As was the case with the Tacoma Narrows Bridge four decades earlier, aesthetics dominated structure, and the unconventional design of the Thames crossing allowed its deck to move sideways excessively under the crowds of pedestrians that flocked to its opening in June 2000. After three days of movement that was deemed potentially dangerous for people, if not the bridge itself, the structure was closed. Much of the public blame for the fiasco fell on the engineers, who were sent back to the drawing board. After being retrofitted with dampening devices, some of which may be said to compromise its aesthetics, the bridge was reopened and has become a popular tourist attraction.

Artistic designs like the Gateshead and London Millennium bridges may not be suitable for large-span bridges that carry vehicles as well as pedestrians. But that is not to say that such large-scale bridges cannot also have a strikingly innovative aesthetic component. The relatively new bridge form that has become a favourite for achieving striking profiles and dramatic effects is the cable-stayed bridge. In contrast to the suspension bridge, from whose two or four main cables a roadway is hung, the cable-stayed bridge employs multitudes of cables that stretch directly from towers to deck. The great number of cables allows for a wide variety of arrangements, and so each cable-stayed bridge can have a distinctive look. This characteristic has led to the design of unique bridges known as ‘signature bridges’.

Among the most widely admired new bridges of this type is the Millau Viaduct, which carries a very high roadway across the Tarn Valley, formerly a traffic bottleneck on the road between Paris and Barcelona. The Millau is a breathtakingly striking design that is commonly attributed to the architect Norman Foster, and it certainly is an architectural achievement in its sculptural form and the way it harmonises with its dramatic natural setting. However, the structural design and construction of such a towering bridge are not architectural but engineering achievements. Unfortunately, the French bridge engineer Michel Virlogeux, who was responsible for the structural design, is largely forgotten when the bridge is marvelled at.

Architects may be more extroverted and therefore the more visible members of a bridge design team today, but they are not always the most essential. Perhaps we ought to revive the grand tradition embodied in John Lucas’ Conference of Engineers to remind us of what was obvious in the nineteenth century, but may now be forgotten.