09 January 2018

Danish Bridges: 4. Cirkelbroen, Copenhagen

The fourth bridge in this set of six Copenhagen spans is another moveable bridge, although of a very different type to the three previous examples.

Copenhagen appears to have had something of a bridge-building boom. Two of the bridges I've covered so far are recent: Inderhavnsbroen (2016) and the Butterfly Bridge (2015). The Cirkelbroen ("Circle Bridge") was also completed in 2015. Another bridge is in the process of being built across the inner harbour right now, designed by Buro Happold and Wilkinson Eyre.

The Cirkelbroen spans across the mouth of the Christianshavns canal, where it enters Copenhagen's inner harbour. It therefore eliminates the need for quite a long walking or cycling detour, and it's clearly a useful piece of infrastructure. Its construction was funded by a private donor, Nordea Fonden, who appointed artist Olafur Eliasson to come up with the design, working alongside engineers Rambøll.

The bridge consists of five overlapping circular platforms, each ornamented with a tall mast, stabilised with a series of cables, like five intersecting spectral Christmas trees. The designer's intent was to create a non-linear pathway, forcing bridge users to slow down, stop, and look around. Given the problems encountered by speeding cyclists on the highly linear Inderhavnsbroen, it feels like a smart move.

As with the Inderhavnsbroen, the Cirkelbroen's history has not been without incident. In 2013, the main contractor Pihl collapsed, followed shortly afterwards by the steelwork subcontractor VSB.  In the same year, the bridge was attacked in the courts. It had been granted a special dispensation from the municipal development plan in 2011, presumably to allow Copenhagen to take advantage of Nordea Fonden's generosity, but a lawsuit sought to have this declared illegal. In 2014, the lawsuit was successful, and work on the bridge was halted, with much of the substructure already complete, and the superstructure steelwork largely complete but not yet on site.

The project recovered fairly quickly, with a new development plan put in place, and a new contractor appointed. The shenanigans surrounding artist-led, privately-funded development pushed through a planning consent process for financial and political reasons reminds me a little of London's Garden Bridge, of the dangers in allowing private gifts to distort or bypass the proper democratic process, whatever the ultimate public benefit.

The finished bridge is, I think, mostly a success.

It is easy to criticise the masts as unnecessary adornment, on a bridge with tiny spans which simply doesn't need to express any height. I think the structure would have looked absolutely fine without these elements, and they are emblematic of how difficult it is for artists to understand and address sympathetically the arena of architecture and infrastructure. Dispensing with the tent-poles would have allowed more attention to be devoted to the bridge at floor-level. Alternatively, the tent-poles could have been made genuinely structural, and integrated into a lighter-weight bridge floor. In this respect, I think the design is a failure, as all the best bridge designs are better able to integrate their disparate elements.

The bridge at deck-level is an enjoyable structure. I think perhaps it should have been made even less linear, as cyclists are still tempted to cross too rapidly for comfort of nearby pedestrians. The parapet detailing is attractive, and the bridge platform becomes a series of interlinked belvederes.

When I visited the bridge, I was unaware of how it opened. There are clear joints in the deck which indicated some form of rotation, but I could only make sense of it after returning home and viewing videos.

The best understanding of how it works can be gleaned from a paper presented to the Nordic annual bridge conference in 2014. The two southern discs are fixed, while the three northern discs move when the bridge opens. The central disc forms a pivot, and the three moving discs rotate about it: this is a swing bridge, if a very unusual one. To a structural engineer, it's initially baffling, as you assume that the pillars below each disc provide permanent support.

Instead, the two northern pillars are supported on a giant steel box hidden below water level, shaped like a triangle in plan to support those two pillars while cantilevering from the rotating pivot pillar. At first sight, with the absence of any counterweight, it looks physically impossible, but the triangular support box is hollow and hence buoyant - its tendency to float counteracts its self-weight. I assume it cannot be perfectly balanced, as the buoyancy will vary with water level and fluctuations in salinity, but presumably it's balanced enough that the variation in loads on the pivot pillar are tolerable.

You can see the bridge opening here on YouTube (concept and reality):

It's not hard to see this system as being ripe for problems over the structure's design life: the rotating surface is below water level, vulnerable to corrosion, silting and other degradation. I trust that the city of Copenhagen has a good maintenance regime in place.

I struggle a little with the swing bridge concept for this structure, which feels counter-intuitive and unnatural. At the same time, I admire its audacity, and the ability of the bridge to subvert expectations. There's a little hint of magic to it.

Despite its oddities, I liked this bridge.

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