Leonardo da Vinci Bridge – a Piece of Applied Art
The following article was written by Tormod Dyken, Norwegian Public Roads
Administration, Bridge Department
In the Municipality of Ås, about 20 kilometres outside Oslo, a rather special structure is situated, being both a footbridge and a sculpture. It makes it possible for pedestrians and bikers to cross the main road E18 in a safe way, as well as it offers the car drivers a pleasant view of beautifully shaped arches. The bridge appears more as the product of an artist than that of an engineer, although much engineering effort and creativity were required in order to materialize the concept.
During an exhibition of the works of Leonardo da Vinci the Norwegian artist, Vebjørn Sand, got the idea of translating the original bridge concept of Leonardo into a smaller, modern pedestrian bridge. Leonardo’s original concept was to build an enormous stone bridge across the Golden Horn, a horn-shaped estuary dividing the European Istanbul. With its free span of about 234 metres and a vertical clearance of about 40 metres, the structure would have been one of the great wonders of the world if it had been built.
Leonardo’s original bridge concept
The concept dates back to around 1502 as sultan Bajazet II considered replacing a floating timber bridge across the Golden Horn with a more permanent one. In a letter, Leonardo made a proposal for a stone bridge and description of how to construct the bridge. In combination with a small sketch showing plan and elevation, it is possible to get an idea of the construction principles. Investigations made in our days conclude that the construction of the bridge would have been technically feasible. However, the sultan probably did not dare to carry out the project or he considered it too expensive – nevertheless, the bridge was never built.
Even though it was never built, the concept of the bridge in it self is fascinating. The bridge span would have exceeded anything which had been built up to then. In order to comprehend the magnitude of the project one must consider that at that time, the largest stone bridge span was 37.5 metres, and even at the end of the stone bridge era, at the beginning of the 20th century, the largest span was only 90 metres – Friedensbrücke (1905), Plauen, Germany. Today the Wuchao River Bridge (1990) in Hunan Province of China, with a span of 120 metres, has the longest stone arch bridge span in the world.
Leonardo’s bridge is not only a vast structure, but a beautifully shaped structure as well – and the shape serves a purpose. The basic load carrying system consists of three arches; one vertical arch carrying the vertical loads and two inclined arches – one at each side. The vertical arch, with its height to span ratio of about 1:6, seems to follow the thrust line. However, the required width of the bridge way would have given a very slender bridge with regard to lateral loading. In order to achieve lateral stability, Leonardo provided the bridge with the inclined arches, giving the bridge its characteristic appearance.
The artist’s interpretation
The timber bridge at Ås in Norway should not be regarded as a large-scale model of Leonardo’s bridge concept, but as an interpretation expressed in wood. The artist Vebjørn Sand was fascinated by the renaissance and in particular by the boldness and beauty of Leonardo’s bridge concept. He studied it thoroughly and was the driving force to materialize the idea into a real timber bridge. In cooperation between the artist and the Public Roads Administration, the glulam manufacturer, the architects and the consulting engineer, the basic principles of the concept were translated into a beautifully shaped wooden model.
The shape of the footbridge follows loyally the course of the lines of Leonardo’s little sketch, but in a more pronounced way. The use of glued laminated timber requires a lighter and more minimalist structure – in reality a new structure. Still the new and modern glulam timber structure possesses all the main structural elements of the old stone bridge; the bridge way, with its sag at both sides, vaults over the arches in an elegant way. The main thrust arch carries the vertical loading and the inclined arches provide lateral stability – just like the stone bridge concept of Leonardo.
The timber bridge
The static system of the bridge consists of three non-hinged glulam timber arches with a rounded, triangular cross section decreasing in size from the abutments to the apex. At the abutments there are fixed ends provided by bonded-in steel rods. The centre arch is made up of four segments, the other two of tree segments, which are rigidly connected by slotted-in steel plates and dowels. The arches re-present a great example of the possibilities in shaping glulam timber. The complex surface of the arches was specified by the co-ordinates of a 150x1150 mm surface mesh, and from these co-ordinates, the wood was shaped by a computer aided grinding machine.
The bridge deck is supported by the main arch in the middle, following the convex shape of the arches. At both sides, the deck has sag as indicated by Leonardo’s sketch and the necessary support is provided by slender steel pillars. The use of stress laminated glulam beams for the bridge deck makes it possible to follow the smooth lines of the concept. The beams are pre-curved and clamped together by pre-stressed, high strength steel bars forming a continuous ribbon like slab. The deck is provided with a water tight membrane and a wearing course.
The bridge is highly exposed to weathering as it is provided with very little protection by structural means. Metal cladding, plastic coating, wooden panelling et cetera, were all turned down for aesthetic reasons, leaving chemical protection as the only option for obtaining a reasonable operating life. Among the chemical alternatives, creosote was ruled out for optical reasons (too dark) and CCA-treatment was ruled out for environmental reasons, ending up with a number of environmentally friendly systems in order to compensate for less effect and lack of long term experience.
The applied systems were:
each single lamella in the arches is pressure treated with Scanimp, which is a heavy metal free agent of class AB, i.e., not recommended for soil contact
the glued, grinded and finished arch elements are pressure treated with Ultrawood, which is a water based wax emulsion giving the member a water-repellent surface
In order to further prevent ingress of water, the finished structure was provided with two coats of oil stain with some pigmentation. This treatment has to be repeated with an interval of a few years.
In order to provide the areas of the glulam timber arches, which are particularly exposed to moisture, with additional protection, boron bars were inserted into pre-drilled holes near the abutments. These bars will be inactive as long as the timber stays dry, but with ingress of moisture, the boron will gradually be dissolved and locally prevent rot. The boron bars have to be regularly inspected and replaced if necessary.
The Norwegian design rules for bridges require a design service life of 100 years for all bridges – including timber bridges. With the above treatment, this requirement cannot be fulfilled. The service life was estimated to about 40 years without any reconstruction works. An exception from the design rules was given on this point on artistic and aesthetic grounds.
After two years of exposure, the structure still looks well. However, the massive cross-section of the arches and the change in temperature and moisture has caused some cracking. The cracking is still not alarming, but in the long term fissures on the upper surface, collecting water and allowing it to penetrate into the cross section will cause decay. Filling the fissures has been considered, but has been rejected. Usually a new fissure opens up in the interface between the filling material and the wood, and thus makes the ¬situation even worse. However, the cracking is monitored and if the situation becomes worse, measures such as cladding, has to be taken.
Apart from durability concerns, one may conclude that the Leonardo da Vinci Bridge was a successful project. The bridge has attracted considerable attention and many positive comments. It is neither an engineer’s bridge nor the most efficient way of bridging a highway, but it serves its function, provides the road-users with a beautiful sight, the municipality with a sculpture and pedestrians and bikers passing over the bridge with an experience. Last, but not least, the unusual and beautifully shaped bridge may lead to increased public interest for bridges in general.
Facts about Oslo Bridge Project | Article - Norwegian Public Roads | Article - Structure Magazine