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.

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