14 May 2013

Katehaki Bridge, Athens

Late in 2012, I visited Athens, not a city generally noted for its bridges. However, I did get time to visit perhaps its most striking contemporary design, Santiago Calatrava's pedestrian bridge at Katehaki. This 94m long, 50m tall bridge was built in 2004 as an offshoot of Calatrava's other activities in Greece, designing the Athens Olympic stadium.

I had seen photos of the bridge beforehand, and like many of Calatrava's recent bridge designs, felt it was an example of a tendency to take things to extremes, unnecessarily so.

What makes this bridge seem so preposterous is the treatment of the cable-stay back span. In an asymmetric cable-stay bridge, where the main span is longer than the back-span, the back-span cables are generally anchored to the ground to provide the necessary stability. In most such bridges, the back-span cables are angled so that they provide a horizontal force, helping the bridge's mast to resist the horizontal pull from the main-span cables.

On the Katehaki Bridge, the back-span cables are instead vertical, resisting none of the sideways pull from the main span. Instead, that pull is resisted through the curvature of the pylon, as a compressive thrust. The curvature is a rational choice rather than simply a sculptural affectation - the pylon can be thought of as a uniformly loaded arch turned on its side, with the thrust at either end of the arch restrained either by the ground support or by the vertical back-span ties. The pylon is therefore essentially funicular in its form - its geometry responds directly to the forces applied to it.

As with many of Calatrava's bridges, the pylon form is well-sculpted, and the recessed cable anchorages are, in my view, visually a very successful detail. At its base, the pylon comes to a perfect point, with the weight of the entire bridge carried onto a support pedestal through a simple steel plate pivot, which cannot be more than an inch or two thick. The photo (right) shows this detail, and also shows the challenge to the bridge's fabricators of achieving Calatrava's continuous, curved forms - it's essentially impossible without fabrication tolerances making the welded plate joints more visible than would be desirable. The bridge was built by the Greek contractor Metka.

Katehaki Bridge was to be something of a prototype for Calatrava's much larger Puente de l'Assut de l'Or, in Valencia, a 180m long, 125m tall cable-stayed bridge of the same overall form. In both bridges, the pylon is offset to one side of the deck, an arrangement that Calatrava is inordinately fond of and which allows the sculptural nature of his structures to be expressed with great visual clarity. The penalty is that both pylon and deck are subject to significant lateral and torsional forces, adding considerably to the overall cost.

At Katehaki, the tendency of the deck to twist is addressed by building a steel box girder along one edge, incorporated into the parapet height so as to reduce its overall visual impact. The cable stay anchors are on the inside face of this box, which helps to counteract the tendency to twist.

The opposite edge of the deck supports an open post-and-rail parapet, an arrangement which can be used to strong visual effect where there is a desire to orient bridge users towards certain viewpoints. That doesn't seem to be the case here - it's simply an artefact of the prior design decisions.

The decking is formed in short timber planks, all lined up neatly rather than staggered. These are supported on steel ribs, and the whole deck is cross-braced to provide in-plane stiffness. The underside of the deck is given a somewhat scabby appearance by the deteriorating finish of the wood planks, although the top-side still looks very good.

There is some nice detailing at the end of the bridge away from the mast, where a triangular frame supports the bridge deck (I confess, I didn't take any note as to how the bridge may cope with thermal movement). The adjacent lift shaft and staircase structure has an attractive white terrazzo finish.

On the whole, the bridge does not feel out-of-scale to the task which it accomplishes, spanning a highway tunnel and two adjacent two-lane carriageways. It certainly makes crossing a busy traffic junction easier, as I found when having to cross other parts of the junction to take some of these photos.

Although the shape and height of the mast announces clearly the very conscious creation of a landmark (the bridge is unmissable on a car journey into the city centre from Athens airport), the more intimate features of the bridge have generally been detailed with care, and it's pleasant to walk across.

Perhaps the only thing that I find odd, and this is as a northern European visiting a southern European country, are the open-air escalators which provide access at both ends of the bridge. It's impossible to imagine such exposed machinery lasting long in a northern climate, although here they are simply an extension of the adjacent subway station, which is also entered via exposed escalators.

Overall, I very much enjoyed visiting this bridge, it strikes me as a very successful design (at least visually - it cannot have been particularly cost-effective).

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