30 October 2016

Swedish Bridges: 6. Västerbron, Stockholm

This massive bridge spans the Riddarfjärden, a major waterway splitting the north of Stockholm from the city's southern part.

Completed in 1935, the main part of the bridge comprises two enormous steel arches spanning respectively 204m and 168m, along with approach structures on each bank. It's reported to be the largest steel arch span in Sweden.

The bridge was the result of an international design competition, with 72 entries, which concluded in 1930. The winning entry was credited to the Berlin architects Otto Rudolf Salvisberg, Wilhelm Büning and Wilhelm Maelzer. However, the actual design contract was awarded to the 3rd-prize winning architect  Paul Hedqvist, who worked with his partner David Dahl and with the structural engineers Ernst Nilsson and Salmon Kasarnowsky, a Swedish team. The contractors were Dortmunder Union and Motala.

The Swedish Wikipedia article has some good photos of the bridge under construction.

The arch comprises riveted steel box girders, connected in plan with K-bracing. Pin-footed tubular columns support the deck grillage. Reinforced concrete skewbacks carry the arch thrust into the foundations.

This is a simple and admirable bridge, the type of structure where it's hard to think what alternative solution would have been appropriate. The span lengths are quite different, but the layout did not look out unbalanced from anywhere that I stood.

The columns are pleasingly slender, ensuring that the deck and arch elements dominate from a distance, and the columns don't present too much of a "forest" appearance when viewed at a closer angle.

It's pleasing to see that the bridge parapet has attracted numerous love locks (see link below), especially given its reputation as a "suicide bridge", at least until anti-suicide fencing was added in 2012.

Further information:

21 October 2016

Swedish Bridges: 5. Stadshusbron, Stockholm

As I reached the end of my walk down Klara Sjö in Stockholm, heading towards the junction between this waterway and the larger Riddarfjärden, this was not the bridge I was expecting.

There has been a bridge here connecting mainland Stockholm to the island of Kungsholmen since 1672. At that time, less of the city had been built out into the water, and the bridge was apparently a 500m long pontoon bridge, called the Nya Kungsholmsbron. As the waterway was narrowed, the bridge was rebuilt on several occasions, including as a steel swing bridge in 1868.

That was replaced around 1919 with the present structure, which consists of a stone-clad concrete causeway punctured by shallow flood relief culverts, with a twin leaf bascule span. The bascules were fixed in place in 1949. In 2012, the bridge deck was completely replaced by a new single span structure fabricated in Poland.

Before reaching the Stadshusbron, I'd passed under the tall viaduct Barnhusbron, plus three other spans, each of which was significantly larger than the Stadshusbron, which resembles more of a barrier than a bridge. However, the adjoining ground levels here are lower than those I had passed, and the bridge is situated immediately adjacent to Stockholm's City Hall, the Stadshuset. Constructing a higher level bridge would have been inappropriate next to such an architecturally prominent building, and there was presumably little reason to provide a larger span opening.

It's interesting how this choice completely changes your perception of the area. It makes Klara Sjö feel like a broad canal ending in a cul de sac. It's a useful reminder that bridges should sometimes be distinguished only by their modesty.

Further information:

17 October 2016

Swedish Bridges: 4. Klarabergsviadukten, Stockholm

I'm presenting bridges heading south-east along Klara Sjö in Stockholm, a water channel which separates the island of Kungsholmen from the Norrmalm district.

Klarabergsviadukten is a lengthy viaduct which carries a highway across another road, Vasagatan; a series of railway tracks; and over the waterway. The span over Klara Sjö is the longest, at 41m. The section above the railway is a steel structure, while the other parts are reinforced concrete.

Built in 1961, it was apparently originally proposed as part of a major highway which was never built, with plans eventually cancelled in 1974. However, it still forms part of a key traffic route today.

I found this to be a very attractive bridge, largely due to its lightly curved portal-frame shaping, which puts it a cut above the cheaper and more straightforward designs that predominate today. The way the pier legs extend into very lightly ribbed elements just projecting below the main structure is well done, and I admire the honest finishing of the concrete.

Further information:

15 October 2016

Swedish Bridges: 3. Blekholmsbron, Stockholm

This is not the most interesting footbridge I've ever visited, but I'm including it here for completeness. It's the next bridge to the east along the Klara Sjö in Stockholm, after the Barnhusbron and Kungsbron.

Depending whether I believe Wikipedia or Structurae, this bridge is either 55m long with a 32m central span, or has a main span of 41m.

Its name translates as "Bridge of Bleaching Islet", supposedly because there was a small island here in the past where fabrics were laid out to bleach.

Anyway, here are some photos, as I have little else to add.

Further information:

11 October 2016

Swedish Bridges: 2. Kungsbron, Stockholm

Heading east from the Barnhusbron, the next bridge in Stockholm is the Kungsbron, which carries the Kungsgatan ("King Street") over the Klara Sjö ("Lake Clara").

This has been the location of many bridges in Stockholm's history. A timber structure originally spanned the Lake (since narrowed to more of a channel), before being replaced by a steel swing bridge in 1881. In 1907, a steel arch bridge was added at a higher level, and in 1944 the concrete arch bridges which remain today were constructed.

The Kungsbron consists of two spans, one carrying each direction of the Kungsgatan highway. The reinforced concrete arches span 68m and support a 14m wide bridge deck.

The structural form is best understood from below. Each bridge comprises two parallel concrete arches, integral with the deck over the crown area. Over the remainder of the arch, the deck is supported from the arch on circular concrete pillars, which are pinned top and bottom. This arrangement allows the deck to expand and contract without inducing high stresses into the columns, however, at this span, I would have thought that very slender crosswalls would have been sufficient.

The overall form of the two bridges is attractive, but they are terribly let down by the details. The circular columns don't look like they belong with the squared-off arch and deck. The length of the combined arch and deck section at midspan appears excessive. Downstand crossbeams above the columns are a distraction (presumably a legacy of the construction method). Probably the most serious flaw is the complete lack of visual continuity of the deck between its central and end sections.

Further information:

08 October 2016

Swedish Bridges: 1. Barnhusbron, Stockholm

I recently visited Stockholm for the 19th Congress of the International Association for Bridge and Structural Engineering. This was a very good conference, but I don't think I'll have time to report it on any detail.

I did get a chance to explore a handful of Stockholm's bridges, and I'll share the photos here over the next few posts.

The first is Barnhusbron (translation: "Orphanage Bridge"), which connects the island of Kungsholmen to the Stockholm mainland. This is a tall highway bridge, some 290m long and 23m wide, constructed in post-tensioned concrete in 1969. The main span is 64.5m.

The bridge is notably taller than the next bridge along the waterway. This is reportedly because of an aborted plan to relocate the riverside Klarastrandslen highway above the numerous railway tracks below the bridge. However, the bridge's height does also allow it to connect directly into the Upper Norrmalm district.

The bridge parapets have been raised in height quite recently, although without taking the opportunity to address corrosion to the original parapet. The alteration was to improve bicycle safety. I have to note that these parapets don't look strong enough to me to survive any significant vehicular impact.

The bridge consists of two prestressed concrete box girders, which from the formwork marks appear to have been cast in-situ rather than precast segmentally. The void between the two girders is used for a number of utility ducts.

The girders are haunched over the main piers, in what I find quite an unattractive manner: the haunches are highly cusped ("pointy"), and the lower edge of the girder overhangs the edges of the pier. This gives the visual impression that forces in the girder are left hanging in mid-air, a defect which would normally be prevented by more careful design of the girder and/or the pier.

Further information:

03 October 2016

Shortlist announced for Upper Orwell Crossing competition

Suffolk County Council have announced shortlisted design teams for this bridge design competition. The shortlist is:
  • Adamson Associates (Toronto) with William Matthews Associates and Ney & Partners
  • Foster + Partners (London)
  • Knight Architects (High Wycombe)
  • Marc Mimram (Paris)
  • Wilkinson Eyre (London) with FHECOR and EADON Consulting
These are some pretty high-powered designers, and it will be very interesting to see what they come up with. Designs are to be submitted by mid-December with a winner announced in January 2017, a slight slip from the original timetable.

I understand that the promoter relaxed their original financial requirements at the prequalification stage, which has allowed some smaller practices to jump the opening hurdle. Sadly however, they did not relax the requirement for only registered architects to participate.

The engineer (WSP|PB) is already appointed, and whichever architect wins the contest will be appointed to work with the engineer to develop designs for two new highway bridges and a swing bridge refurbishment.

Judging of submissions is on the basis of two design concepts for each of the bridges, plus a fixed lump sum fee proposal, so it will be interesting to try and guess whether the winner has succeeded on creative merit or on price, or a mixture of both. The financial element will work against certain entrants, I think, as my experience is that architects' fee levels can vary significantly.