A hybrid dream: the Yavuz Sultan Selim Bridge on the Bosphorus

by Editorial Staff

Compasses Magazine

Something new has appeared in the fascinating landscape of the Bosphorus Strait.

Slender, soaring while at the same time powerful, contemporary, reaching out to the future, this is the Yavuz Sultan Selim Bridge, the new landmark of modernity connecting Europe and Asia.

 Something new has appeared in the fascinating landscape of the Bosphorus Strait. Slender, soaring while at the same time powerful, contemporary, reaching out to the future, this is the Yavuz Sultan Selim Bridge, the new landmark of modernity connecting Europe and Asia. A sophisticated and highly technological structure, a display of sculptural engineering, a combination of a suspension and a cablestayed bridge, spectacular and conceived not only as a structure functional for crossing the water and connecting the districts of Poyrazköy, on the Asian shore, and Garipçe, on the European side, but as an element representing a complex infrastructure system. During the day, its appearance is that of a megastructure worthy of a work of Land Art, while at dusk it resembles a space station that has landed masterfully and with considerable engineering and structural effort between the two shores of the huge estuary, in a rocky point where the hilly terrain advances, creating a narrowing of the stretch of water between the two opposing coastlines.

A bridge, dedicated to the ninth Ottoman emperor, which certainly serves the area and the city, but which aims to reach out well beyond the region, projecting itself toward both continents with a dual motorway and high-speed railway connection. A functional plurality that allows the Yavuz Sultan Selim Bridge, the third Bosphorus bridge, to be considered the only suspension bridge in the world, housing on its deck, all on the same level, an eight-lane motorway and two railway lines, with an incredibly shallow maximum height of 5.50 metres. A speci?city that has also earned it the record as the widest suspension bridge on the international scene, with a width of 58.50 metres. A result achieved partly due to the decision to conceive it as a hybrid cable-stayed, suspension bridge supported by two A-shaped pylons soaring to 322 metres in height, another record that overshadows even the iconic Tour Eiffel in Paris.

Therefore, it is a structure that, in its various constructional elements, has far outstripped recent major projects around the globe. Furthermore, it took an incredibly short time to build: 39 months from the laying of the foundation stone, thanks to the efforts of up to ten thousand people on a site that required organisation akin to that of town planning on the two shores involved as areas of operations, in full compliance with the planned timing and budget. A ?nancial plan backed by the involvement of the major Turkish banking groups that supported this project, in which the Yavuz Sultan Selim Bridge is a substantial part of a broader infrastructure scheme entrusted to the major Turkish group IC Içtas (in a specially set up consortium with the Italian company, Astaldi, holders of a 33.33% share in the formula of the ICA joint venture) but which also includes the building of no less than 150 kilometres of the Northern Ring Motorway (from Odayeri to Pasaköy), with an awe-inspiring system of viaducts measuring 25.40 kilometres and underground tunnels designed to link the European and Asian railway lines. The consortium also has the task of subsequently managing the whole infrastructure project, as a concession holder.

It is therefore an extremely wide-ranging operation of which the impressive Yavuz Sultan Selim Bridge is certainly the iconic element, standing at the far northern end of the Bosphorus, just before it ?ows into the Black Sea. A landmark that takes its place, ideally and functionally, alongside the previous two suspension bridges connecting Europe and Asia, both built in the last century: the 15 Temmuz Sehitler Bridge, formerly known as Bogaziçi Köprüsü, the ?rst bridge over the strait dating from the seventies, and the second, Fatih Sultan Mehmet, built at the end of the eighties. Bridges that, despite their soaring structural lines and functional value, compared to the recently built third bridge, now appear to be from an earlier generation both with regard to size and design.

The first bridge over the Bosphorus, inaugurated on 30 October 1973, stretches between the Ortaköy and Beylerbeyi districts. Designed to connect the two halves of the city, it was the ?rst permanent bridge uniting the two continents; it was made by a British company, which enlisted the services of the engineers Gilbert Roberts and William Brown with the collaboration of the Italian Almerico Meomartini. In the period in which this major bridge was constructed there was also a huge increase in the population of Istanbul (now a city with almost 16 million inhabitants) which, due to high immigration, began to change its urban layout in relation to its two different areas of development, with an expansion of the Asian area. In support of the need to build a third bridge, one only has to consider that at present the 15 Temmuz Sehitler Bridge is crossed each day in both directions by 180,000 vehicles, a considerable ?gure comparable only to a few bridges in the United States.

Standing just 5 kilometres north of the previous bridge and connecting the Rumelihisari and Anadoluhisari districts, where there are very ?ne Ottoman fortresses, is the second bridge, the Fatih Sultan Mehmet, dedicated to the 15th-century Ottoman Sultan who conquered Constantinople in 1453. Designed by the London ?rm Freeman Fox & Partners, and completed in 1988 by a Japanese company, this suspension bridge has similar characteristics to the ?rst. Consequently, in order to reorganize the ?ow of car traffic between the metropolis and its territory, also bearing in mind its exponential growth over the past ten years, the decision to expand the railway and motorway systems converging upon a third bridge now appears even more necessary.

In short, the cityscape of Istanbul is dotted with bridges, and the challenge of structural design has become a distinguishing mark of the city, of its sights and of its territory. The Yavuz Sultan Selim Bridge rightfully emerges as the latest evidence of this leitmotif that has continued down through the centuries.

The third bridge over the Bosphorus is in fact the heir to a speci?c structural and design tradition, that of sculptural engineering of which the two designers, Jean-François Klein and Michel Virlogeux, are certainly two of the most important exponents in this era. It is a design approach in which the bold expressiveness of the structure ultimately restores the prestige of infrastructural works, placing them among the elite of architecture. It is a path paved by forerunners like the great 20th-century concrete structural engineers, from the Spanish, Mexican by naturalisation, Félix Candela to the Italian Pier Luigi Nervi, but also by ?gures such as Eugène Freyssinet from France. The result is a series of structures which, like the Yavuz Sultan Selim Bridge, tell a story.

This bridge is in fact the tale of a structural challenge in the relationship between forces, loads and suspensions, but also of research into the potential of materials – from the steel cables to the concrete – as well as of careful and thoughtful attention to the design of the parts, of the elements that characterise it and make it unique. For instance, in the initial designs the very slender deck was made of box girders on several levels with various distribution solutions for the high-speed railway, placed on a lower deck, and the motorway lanes, central or lateral, on one or more levels. It was later developed, with more than a few problems and with great skill, by designing two motorway carriageways, each with four 3.65 metre-wide lanes separated by a central railway corridor composed of two sets of tracks each 5.25-metre-wide for high-speed and goods trains. The result is the widest deck in the world, with a width of 58.50 metres and a maximum height of 5.50 metres which has no equal in the world. It is a system that naturally underwent, and withstood, testing in a wind tunnel with application of a wind at 300 kilometres per hour, followed by the addition of the load determined by the cars and, above all, by the train under the rails of which reinforcing elements were placed. With this geometric dimensioning, the maximum design speed has been set at 120 kilometres per hour for the two motorway carriageways, while for trains in transit it will be 80 kilometres per hour for goods traffic and 160 kilometres per hour for high-speed trains. Even the “A” shape of the pylons is dictated by the load of the train; indeed, they were conceived in this way by the designers so that the main cable could be inserted in line with the tracks, also placing the secondary cables at the centre, and thus stiffening the deck in the railway zone. Additionally, the pro?le of the pylons, which have a triangular section, is the result of an analysis of the incidence of strong winds present in that part of the Bosphorus on their surface, but also an actual design choice since the triangular pro?le integrates better with the inclination. This also applies to the very long stay cables, connected to the external part of the deck in order to make it more rigid, which precisely because of their “abundance”, are ?anked by the catenary suspension system that counters the perpendicular forces. Here, to avoid risks due to high wind rather than seismic activity, dampers have been designed and inserted to eliminate the vibration of the stay cables when trains pass or when there are strong wind currents. The dampers are hydraulic insulators that contain vibration (the longest cable on the bridge measures 597 metres, with a steel strength of 1,960 MPa, and is also the longest cable in the world). With these a series of important stratagems were adopted, starting from the ?rst step: identifying the site for the on-land foundations on two small promontories jutting out into the strait. This made it possible to avoid contact of the pylons with the water in a stretch crossed by numerous opposing currents and by busy maritime traffic. This was a positive decision that moreover involved relatively shallow excavation of the foundations (with a depth of 20 metres and a diameter of 20 metres) in rock without fractures and therefore particularly suited to receiving concrete pylons made with slipform and automatic climbing formwork systems.

A hybrid cable-stayed suspension bridge, as already stated and as its two designers have rightly de?ned it. Klein and Virlogeux decided to combine the cable-stayed bridge system – frequently used for many European bridges including some built very recently, and able to provide considerable stability to the structure even against stresses determined by the high winds that characterise the Bosphorus and, in particular, that area of the estuary – with a structure with a “catenary” suspension cable that takes us back to the futuristic visions of the early cast iron bridges built by Gustav Eiffel (the Garabit Viaduct 1880-84) and of the metal ones in the United States, able to cross very wide distances and cover unimaginable spans. Therefore, the Yavuz Sultan Selim Bridge has a double identity. It is cablestayed, according to the best tradition, which is that of a structural “genre” that began long ago at the end of the 18th century with the bridge at Freyberg (1784), the work of a German carpenter, Immanuel Löscher, and continues up to today passing through the post-war years with the bridges designed by Franz Dischinger – in particular the iconic 183-m-long Strömsund Bridge (Sweden) in 1955 – as well as the futuristic experiments of the Italian Riccardo Morandi on the Maracaibo Lagoon in Venezuela (1957-1962) or over the Polcevera in Genoa (1960–1964). Yet it is also a “catenary”-type suspension bridge, designed according to the load system studied at the end of the 1800s by the great Catalan architect Antoni Gaudí on which the soaring structures of the famous Sagrada Familia cathedral in Barcelona are based.

In any case, the hybrid structure proposed with full awareness of the problems, and audaciously, by the designers of the Yavuz Sultan Selim Bridge, has a “noble” precursor in the 19th-century iron bridges in San Francisco and Brooklyn, but also, for its sophisticated structural concept, in the highly evolved experiments of the masters of reinforced concrete, who certainly include the Italian Sergio Musmeci, with his project dating 1953 for the Messina Bridge, an example that can be assimilated to that of the third Bosphorus bridge in terms of complexity both with regard to stresses due to wind conditions, and for the need to cover a wide span. To solve these problems, Musmeci envisaged triangular “sails”, a highly original spatial system of suspension to stiffen the structure and limit the perpendicular stresses on the bridge itself, an innovative idea revived more recently for Norman Foster’s London Millennium Footbridge. An emblematic example of this latter solution, in line with the research expressed in the design of the Yavuz Sultan Selim Bridge, is also the Millau Viaduct in France, completed in 2004.

Each case, however, remains separate because each design is produced for a speci?c context, time, and environment, and conditions that are always different from one another. The designers of the Yavuz Sultan Selim Bridge also stated that the Kojima-Sakaide route of the Honshu Shikoku project was a source of inspiration in terms of its impact on the landscape. Here we are on the Bosphorus, a special territory, with a history of beauty dating back to before Istanbul itself. Today it is still an important place from a naturalistic point of view due to the reproduction of birds that pass through it and then migrate, to the unspoilt and dense forests, as well as to the strikingly beautiful rocky coasts. It is also a place of high winds and powerful currents, a landscape and territory in which every action cannot but take into account the forces of nature and the unpredictability of events, complying with them, respecting them, monitoring them and, sometimes, challenging them. Consequently, this third bridge is and will be something more than a means for connection and crossing, a link with the future.

Maria Vittoria Capitanucci

Excerpt from the book: L. Castelli, M.V. Capitanucci (eds.), Yavuz Sultan Selim Bridge. The new gateway between East and West, Rizzoli, Milan 2017.




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