26 January 2018

"History of the Modern Suspension Bridge" by Tadaki Kawada

It feels like only yesterday, but apparently it was actually 2012 when I reviewed Richard Scott's book "In the Wake of Tacoma", a wide-ranging, very thorough history of 20th century suspension bridges. I praised Scott's wealth of detail, while noting the book's hesitance on key aerodynamic concepts, obsession with factual minutiae, and lack of diagrams and photographs.

Scott's book was published by ASCE Press in 2001. The following year, Tadaka Kawada published a similar book in Japan, tackling essentially the same history from a slightly different angle. In 2010, ASCE Press published this English translation of Kawada's book, edited by Richard Scott. The two books make for a very interesting comparison.

Kawada's book's full title is "History of the Modern Suspension Bridge: Solving the Dilemma between Economy and Stiffness" (ASCE Press, 2001, 246pp) [amazon.co.uk]. Despite the title, it also has very good coverage of earlier suspension bridges, with the first three chapters covering early suspended bridges (up to and including James Finley's structures), 19th century spans in Britain and France, and 19th century American spans. These, and later chapters, are very well illustrated, with hardly a two-page spread going by without some kind of image, whether a diagram, photograph, or historic paintings and engravings.

Much of the early history of suspension bridges is a history of failure. Finley invented the modern form of suspension bridge, with towers and a level deck, and around 40 bridges were built using his patent. However, his first bridge, at Jacob's Creek, Pennsylvania, survived only from 1801 until 1825, when it failed under load, and by that time many of his other bridges had also already collapsed for a variety of reasons.

British engineers lagged a few years behind, with various early cable supported bridges built from 1816 onwards. Captain Samuel Brown pioneered modern suspension bridges in Britain, with the Union Bridge completed in 1820. Thomas Telford's Conwy and Menai Bridges followed not long after, completed in 1826. As with Finley, understanding of the structural behaviour of these bridges was extremely limited, based on simple theory supplemented by experimental trials. Brown and Telford's bridges set new span records but were plagued by failures: several of Brown's structures collapsed or were damaged, caused variously by high winds, dynamic crowd loads, and an over-ambitious attempt to carry rail traffic on a structure initially designed only for highway loads. Telford's Menai Bridge oscillated severely and was seriously damaged by winds, and the bridge as seen today has been radically altered from the original design.

French engineer Louis Henri Navier studied the British designs and published an extensive report on the subject, but his single suspension bridge, the Pont des Invalides was never completed, after the anchorages were found to have moved during construction. A replacement design was built but lasted only 21 years. Other French engineers, such as the Seguin brothers and Louis-Joseph Vicat, were more successful, pioneering the use of wire instead of chains, and inventing aerial cable spinning. The French were better theorists than the British, and built far more suspension bridges in this period, but their understanding of how the bridges behaved was still extremely limited. Several French bridges suffered problems with vibrations, with the most notorious instance leading to the collapse of the Basse-Chaîne Bridge under marching troops, killing 226 people.

Kawada's writing is very clear and to-the-point, and accompanied by useful direct extracts from original literature and extensive referencing. Reading these early chapters, a trend of ever-more ambitious bridges being built under conditions of significant ignorance emerges. As the Americans regained the lead in suspension bridge construction, the same theme continued, with a notable disaster befalling Charles Ellet Jr's Wheeling Bridge in 1854. This 308m span, the longest ever built, collapsed under wind loading, with torsional undulations reported as rising nearly to the height of the support towers.

John Roebling's hybrid stayed suspension bridges, most famously including the Brooklyn Bridge, were significantly more successful. Kawada states that Roebling "understood the meaning of 'stiffness' in modern suspension bridges". Several earlier structures had incorporated stiffening trusses, largely as a pragmatic measure, but Roebling's adoption of measures to ensure significant stiffness resulted in bridges far less prone to problems under either live or wind loads.

As the 19th century came to an end, suspension bridge theory began to mature significantly. Joseph Melan's Theory of Steel Arches and Suspension Bridges, published in 1888, became for some time a definitive text, setting out the so-called Elastic Theory. Kawada is conscientious in explaining both the Elastic Theory, and its later successor, the Deflection Theory, with diagrams and equations. The Elastic Theory ignores deflection of the suspension cable under live load, treating it simply as a means of support which relieves load in the bridge deck's stiffening girder or truss. The Deflection Theory, popularised by Leon Moisseiff, also takes account of the deflection of the main cable under live load. This increases the overall calculated stiffness of the system (by adding the cable stiffness to the deck stiffness), giving both a more accurate result and also a more economic design.

Moisseiff used the Deflection Theory to design the Manhattan Bridge, completed in 1909. Kawada compares it to Leffert Buck's Williamsburg Bridge, complete in 1903 using the older theory. The Manhattan Bridge has a much shallower truss.

The rapidly improving understanding of bridge behaviour opened the way to steadily larger and less expensive structures. In 1931, the George Washington Bridge nearly doubled the world record span, at 1067m. Initially, this was built without any stiffening truss at all, reliant on its massive weight for stiffness (the bridge was only stiffened in 1962 to add a second deck and accommodate more traffic). Other large spans were also under construction, the largest being the 1280m Golden Gate Bridge in 1937.

Any student of bridge design will know what happened next. In 1939, work was completed on Othmar Ammann's Bronx-Whitestone Bridge, again with no stiffening truss, but relying largely on weight for stiffness. Leon Moisseiff took the same approach for the Tacoma Narrows Bridge, completed the following year: as with its immediate predecessors, the road deck was carried by two simple edge girders. The decision was disastrous, with the bridge collapsing when subject to moderate winds only four months later.

The bluff profile of the edge girders led to the creation of wind vortices, which induced oscillation of the bridge deck. Wind tunnel tests were rapidly undertaken on the bridge's cross-section in the months between completion and collapse, confirming the section to be highly unstable under wind effects, and plans were made to install fairings on the girders to reduce the vortex shedding. The bridge failed before the fairings could be installed.

Kawada explains the aerodynamic issues with clear diagrams, including charts and graphs taken from the contemporaneous studies. These are particularly helpful in seeing how an understanding of the critical wind phenomena emerged and then developed further. A major report into the bridge failure was completed in 1941 by Ammann and others, largely exonerating Moisseiff on the grounds that he had simply followed the general trends in suspension bridge design.

However, the trend towards narrower bridges with less stiffness had brought designers back to the types of structure which had repeatedly failed in the 19th century, any lessons from the past having been forgotten or ignored. It is perhaps no surprise that Ammann's investigation report held Moisseiff largely blameless, when it is noted that Ammann's own Bronx-Whitestone Bridge had suffered from wind oscillation problems of its own, although less dramatic in magnitude.

Indeed, there were further lessons to be found in other contemporary bridges: the Thousand Islands and Deer Isle suspension bridges had been completed in 1937 and 1939 respectively, and both shallow-girder designs had encountered serious wind-induced vibration soon after completion. Both these bridges were stiffened by the addition of cable-stays, sufficiently to resolve the problems. Designer David Steinman had reported the problems to other engineers, but it seems that Moisseiff and Ammann had paid little attention.

The Tacoma Narrows disaster led to a huge retrenchment in American suspension bridge design, with deep trusses rapidly returned to favour. Some of these adopted new approaches to providing aerodynamic stability, introducing grids and gaps in the bridge deck, which greatly reduced instability. This trend also continued in the majority of Japanese suspension bridges built in the later parts of the 20th century.

Back in Europe, designers retained a degree of boldness. The Forth Road Bridge (1964) and Tagus River Bridge (1966) largely followed the safe truss-stiffened philosophy, although the latter was designed by Americans, including Steinman. However, Fritz Leonhardt had proposed a radical innovation for the Tagus design competition, an aerofoil box girder design, and this idea was taken up by the British for the Severn Bridge (1966).

The idea of eliminating aerodynamic disturbance, rather than resisting it, was not entirely new, as was clear from the proposal to add fairings to the Tacoma Narrows bridge. However, the Severn Bridge was revolutionary in the completeness of its design conception, using its aerodynamically sleek profile to achieve substantial economies in the amount of material required. Compare, for example, the American Verrazano-Narrows Bridge, completed in 1964. At a span of 1298m, it was significantly longer than the Forth Road Bridge (1006m) or Severn Bridge (988m). However, the weight of the bridge deck is many times higher: 45200 tonnes for Verrazano-Narrows, as against 16300 tonnes on the Forth, and 11400 tonnes on the Severn.

The Severn Bridge was bold, but problems with the structure were rapidly discovered, including issues with hanger vibration, the strength of the towers, and fatigue in the deck box girder. Kawada analyses these bridges in detail, concluding that the Severn Bridge's problems can be attributed directly to its economy, specifically its lightness of weight. He argues that engineers had forgotten that the stiffening effects of mass could be a virtue. In this sense, the pursuit of slenderness had again led to failure. His basic point is well made, but I think it is not entirely fair in the case of the Severn Bridge, with many of the problems resulting mainly from rapidly growing traffic volumes, well in excess of the original design specification.

Kawada's book comes up-to-date with examinations of the world record holding Akashi Kaikyō Bridge (designed on the American heavy-truss principle), the Storebaelt Bridge (designed on the European aerofoil principle), and the London Millennium Bridge (designed on the "we-know-nothing-about-suspension-bridges principle"). He ends by looking at possible future bridges, such as the Messina Strait Crossing.

In concluding, Kawada quotes with approval the American professor David Billington:
"History, for structural engineers, is of an importance equal to science".
This is undeniably the value of this excellent book. I don't think you have to be a designer of enormous suspension bridges to grasp the significance of the history which is recounted here: it is a story of ignorance and complacency, and of the unavoidable surprises which await pioneers of any stripe. These issues appear in many guises in other areas of structural engineering, but are seldom recounted with such thoroughness and clarity.

"History of the Modern Suspension Bridge" is clearly worth reading for any bridge engineer. If you haven't already read Richard Scott's "In the Wake of Tacoma", I would recommend it just as much, although for different reasons - the two books are complementary. Kawada is good on the engineering, the diagrams, and has commendable brevity. Scott is better on the personalities, and has a level of detail that Kawada doesn't match. I enjoyed both books, very much.

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