14 January 2018

Danish Bridges: 6. Cykelslangen, Copenhagen

This is the last in this series of posts about the bridges of Copenhagen. There are other interesting structures in the city, but I didn't have time to visit them on this occasion.

Opened in 2014, this 230m long bridge was designed by Dissing + Weitling with Ramboll. It is a bridge for cyclists only - no pedestrians are permitted. It forms a key link in a long cycle path, connecting the Bryggebroen at one end to Dybbølsbro at the other, allowing cyclists to pass across Copenhagen's inner harbour, bypass the Fisketorvet shopping mall, and cross over a major railway corridor. As well as making cyclists' journeys easier, it also helps keep them out of the way of pedestrians.

Nicknamed the "bicycle snake", the bridge curves between buildings and above a harbour inlet. I imagine it gives cyclists some great views, but I decided not to get in their way. I could only admire the bridge from below.

The bridge is minimalist in design, with a steel spine box girder below the deck supported on steel tubular columns. The curved layout of the bridge in plan is sufficient to ensure stability against overturning, with the girder restraining torsional movement. There is nothing extraneous in the design, just what's necessary and no more. The only concession to anything even slightly excessive is the adoption of an orange surface for the floor, although that is less evident under bright lighting at night.

It's an admirable design, and much superior to its neighbour, Bryggebroen. It shows that even a functional and economical design can be greatly improved if treated with care and attention.

Further information:

11 January 2018

Danish Bridges: 5. Bryggebroen, Copenhagen

The previous four bridges in this series of posts have all been moveable bridges, and Bryggebroen ("Quay Bridge") doesn't break that trend.

Built in 2006, it connects two quaysides across the Copenhagen inner harbour, hence the imaginative name. The east side is mostly a residential area, with the Gemini Residence, apartments built around former grain silos, dominating the area immediately adjacent to the bridge. The west bank features offices, residences, and a large shopping mall.

The bridge was designed by Dissing + Weitling with Carl Bro as engineer. The contractor's engineer for the D&B phase was COWI. The 190m long bridge is a steel spine-beam structure, with the spine box dividing cycle and foot traffic. The structural form was a consequence both of the traffic segregation and the desire for a low construction depth from floor to soffit.

The opening element is an asymmetric swing span (44m navigation span and 23m back span). Deep boxes sag below the walkway floor to provide sufficient strength and stiffness, and the pivot is tucked away between two sides of the box. As with the similar arrangement on the Inderhavnbroen, it is visually massive, and out-of-keeping with the rest of the bridge.

The designers acknowledge that this was never meant to be an "iconic" structure, but given its prominent position, I think too little effort was made to render it visually attractive. The piers are thin tubular trestle legs, which feel awkward next to the width and scale of the bridge deck.

The spine beam, particularly seen up close, is lengthy and somewhat morose. I'm puzzled as to why nothing was done to prevent people walking, skating or riding along its top: perhaps these things simply don't happen in sober Copenhagen.

Further information:
  • Google maps
  • Wikipedia
  • Structurae
  • Non-iconic footbridges (Jensen, Footbridge 2008)
  • Rethinking Cities (Trojaborg, Jensen and Henriksen, Footbridge 2017)

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.

Further information:

07 January 2018

Danish Bridges: 3. Inderhavnsbroen, Copenhagen

All the bridges I visited in Copenhagen either span the city's inner harbour, or run close by it. The third in this set, the Inderhavnsbroen, is named simply for what it is: the Inner Harbour Bridge. It's the most recent bridge to span these waters, completed in August 2016.

The bridge was designed by COWI and Studio Bednarski, adopting a rare bridge typology and making it even more unique. It carries pedestrians and cyclists across the waterway, and is a double leaf retractable bridge. Structurae only lists seven retractable bridges worldwide, and I think only 4 of those are still in use, although that's certainly an incomplete list.

The Inderhavnsbroen is distinctive in a number of ways. The general arrangement is unusual, with two approaches each arranged in a tuning-fork layout, allowing the retractable decks to slide back and forth between the prongs of the fork. Plan curvature adds significant complexity, with the bridge overall adopting a gentle S-shaped curve.

The bridge had a troubled gestation. This is an understatement, to say the least! The contractor Pihl and Søn was appointed in 2011, with a price reported to be significantly below that of other bidders. As the project progressed, problems were reported with the steelwork not meeting requirements, and later with cracks and other defects in the concrete supports. Errors were reported in design drawings showing supports 0.6m higher than they should have done.

In August 2013, Pihl and Søn collapsed. I believe all the bridge steelwork already fabricated was scrapped, and during the wait for a new firm to be appointed, storm flooding damaged electric motors which had already been installed. Independent engineers were brought in to review the works, reporting inadequate reinforcement in some supports. The new contractor appointed in 2014, Valmont SM, had to undertake extensive remedial works to parts of the structure already completed.

Progress improved, but new problems were still encountered. The wire rope system which pulls the bridge open and closed was found to be faulty. Close to completion of the construction, it was found that the two arms of the bridge did not meet properly in the middle, with locking bolts misaligned. This was attributed to twisting under differential temperature effects, caused by temperature variations across the width of the bridge deck, not allowed for in design standards.

By the time the bridge was complete, it was claimed to have taken twice as long as planned, and cost 50% more than the original budget. The design team went on record to criticise their own client, the City of Copenhagen, who have also gone on record criticising the design engineer. For the most part, the design team's complaints related to the client's procurement and project management process, which led to a number of "unauthorised" or bodged changes to the original design.

Problems during design and construction are often forgotten and forgiven once a bridge is complete. Unfortunately, the Inderhavnsbroen has experienced considerable public criticism even once in service - more on that in a moment.

It's worth commenting on the way in which the bridge was funded, along with three smaller structures including the nearby Butterfly Bridge. It was not funded by the City of Copenhagen, but by the A.P. Møller Foundation, to the tune (in the end) of 241 million Danish Krone (about £29m, or €32m). These are not in my view excessive sums when compared to other expensive and complex spans such as the millennium bridges in Gateshead or London. However, I do wonder whether the lure of private money can subvert normal sound governance, public accountability, and good decision-making. Would the municipality have approached the project differently if it had been entirely their own money?

The basic concept for the bridge has a lot of merit. The arrangement chosen allows pedestrians to get much closer to "the action" when the bridge opens than is often the case with opening bridge. Bascule spans rise up to block the river from view. Swing bridges leave waiting users stranded. The Inderhavnenbroen concept uses the side spans as balconies so that those waiting can have a closer encounter both with the movement of the main span and with any ships that pass through.

The arrangement has some unfortunate visual side effects, however. Moving bridges often suffer from a loss of visual continuity associated with unavoidable breaks in the structure. Here, however, the offset from the side spans to the main spans creates a significant disjuncture in both form and scale, which I felt was quite jarring from many viewing positions.

This is then accentuated further by the design of the moving spans. The side spans are all girders below deck, while the moving spans have massive edge girders which not only sag below deck at the point of support, like the wings of a robotic manta ray, but also rise up above floor level at the same point. This leads to a very distinctive change in level of the edge of the deck at the position where the opening and fixed spans meet, and it has also introduced a complex twist into the opening span balustrades.

Generally, the moving spans seem far too deep to me, giving them a feel of heaviness rather than the lightness that a moveable bridge normally embodies. The design of the edge girders also determines too many other elements of the design. They are angled outwards, leading to some mildly gruesome complications along the join-line between moving and fixed spans: the bridge balustrades are forced to be tilted outwards on the fixed spans, as are gates which prevent access to the main spans when the bridge opens. However, the balustrades on the moving spans are vertical at this point, tilting inwards above the support girders, and then becoming vertical again at midspan. It's all extremely awkward.

The rising barrier gates reportedly became a problem late in the bridge's development, with short segments of triangular barrier added next to them as an afterthought.

The whole area of the ends of the moving deck is troublesome. A median barrier is used to separate pedestrian and cyclists, although they have no barrier on the rest of the main span. Cyclists are obliged to negotiate rapid chicanes to pass from the internal to the external spans, and this has been the source of much criticism, expressed most pitilessly in an article for ArchDaily, and physically embodied by the addition of red and white striped warning stickers on various parts of the bridge.

The bridge appears quite steep, and the chicanes are clearly too tight to negotiate on a bicycle without slowing down. A series of rubber skid-marks attests to the inevitable outcome: cyclists struggling to brake and avoid headfirst impact into the glazed balustrades which form the ends of the moveable span.

The slope of the deck was probably unavoidable given navigational requirements and the general topography of the harbour area. However, the dead-end facing oncoming cyclists was eminently avoidable: the chicanes could have been made less tight, and the ends of the deck angled so that errant cyclists are at least spared head-on impact.

It's hard not to wonder whether the twin-girder solution is also partly to blame. A spine-beam deck would have kept cyclists and pedestrians separated throughout, avoiding the risk of impacting the end of the median barrier. It may also have allowed the visual depth of the opening span to be better integrated with the side spans.

The separation of cyclists and pedestrians also generates a great deal of dead space on the bridge. The balcony on the pedestrian side of the bridge may get some use, but matching balconies on the cyclist side presumably see very little use most of the time: I certainly didn't want to risk crossing the cycle path to get to it.

Perhaps it's not the done thing in Copenhagen, which is criss-crossed with cycleways, but adopting a shared-space approach would have had the advantage of both slowing down cyclists tremendously as well as giving all bridge users better access to all parts of the bridge. With the current arrangement, pedestrians are essentially barred from the north half of the structure.

On the positive side, the bridge clearly fills an important gap in the city's connectivity; I visited both at night and in daytime, and it was always well used. It also escapes the greyness which is characteristic of too much Scandinavian infrastructure, even though only a little. The blue and yellow glazed panels contrast well with the rest of the bridge, and are a nice touch. One panel is currently cracked, but I don't know whether a cycling accident or vandalism is to blame.

The choice of design was ambitious from the outset. The numerous problems encountered during design development and construction are to some extent the result of misfortune and mismanagement. However, some are inherent in the design concept, and others are the sort of thing which is inevitable with any highly complex, pioneering structure, especially where bespoke mechanical engineering is involved. That's an inevitable consequence of a challenging, uncompromising design.

I was very glad to visit the Inderhavnsbroen. It's a very interesting structure, and everyone involved no doubt feels a great sense of accomplishment for its (eventual) completion. It will be interesting to see whether the Copenhagen municipality can find ways to address the design flaws other than simply sticking on red-and-white warning markers. I would hope that there are more creative solutions better in keeping with the rest of the design intent.

Further information: