11 August 2010

Scottish Bridges: 13. Gogarburn Bridge

Here's another leftover, a couple of photos I dug out from last year, of Gogarburn Bridge, near Edinburgh.

This highway bridge provides slip road access off the A8 dual carriageway into the offices of the Royal Bank of Scotland (whose logo you can see dangling from the crown of the arch). It opened in March 2006, is 25m high, and spans 60m.

It shares its typology with a small number of other bridges, many of which I've covered here before:

All these bridges have a single steel arch spanning diagonally across the deck, with the cables arranged asymmetrically so that from one angle, they give the appearance of criss-crossing, in the manner of a network arch. At Hulme and Gogarburn, the bridges cross busy main roads, and the network arch appearance is therefore visible to many drivers every day. The same does not apply at Newport Street (above a railway) or Clyde Arc (above a river), and that makes me wonder whether the solution is visually appropriate at those sites.

There are also similarities to the Juscelino Kubitschek bridge in Brasilia, although that has a much more efficient (and less visually interesting) hanger layout.

There are a number of reasons why this type of bridge is structurally inefficient. The use of inclined hangers rather than vertical hangers induces axial forces into the deck, and because of the skew of the arch, this induces twisting of the deck in plan. Additional bearings are required to resist this, and Gogarburn Bridge also requires bracing to the hanger outrigger beams.

An arch subject to vertical load in its own plane is subject to very limited bending effects and hence can be made structurally efficient so long as any tendency to buckle laterally is prevented. At Gogarburn (and its siblings), the cables apply substantial asymmetrical forces horizontally to the arch, establishing large lateral bending moments which may control the design.

These inefficiencies are justified by the striking visual effect of the structure, which consists of its faux-network silhouette; the easy visual legibility of a single rather than double arch; and the visual interest created by a profile which varies constantly as you move around it.

At Gogarburn, I think the effect is justified, particularly by the desire of RBS to create a visual gateway to their headquarters site.

Gogarburn Bridge was designed by SKM Anthony Hunt (who also carried out the independent design check at Newport Street), and built by Watson Steel for Sir Robert McAlpine.

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