16 April 2018

Yorkshire Bridges: 19. Lofthouse Interchange

From Stanley Ferry, I was heading north towards Leeds. My next stop was the Lofthouse Interchange, the magnificent junction between the M1 and M62 motorways, which can be viewed from Long Thorpe Lane on its northern side, for anyone so inclined.

The Lofthouse Interchange may be magnificent, but it's also notorious. Built in 1967, it is a three-level interchange, with the M62 crossing above the M1, and an 800-foot diameter roundabout built above both motorways to accommodate all interchanging traffic. It looks fantastic from above, but it creates a terrible bottleneck, due to the conflict between traffic streams entering the roundabout.

In 1999, major improvements were made by the introduction of new link roads connecting the western arm of the M62 to the northern arm of the M1. However, many conflicting movements remain on the roundabout, and it is known both for congestion and for collisions.

Highways England are consulting on a scheme to improve the junction, although there's no information yet on what this may actually involve. The best long-term solution would be to completely separate all slip road movements, but that is likely to be both hugely expensive and hugely disruptive during construction.

One casualty of any radical change could be the junction's distinctive "banana piers". Judging from the degraded state of the concrete on these, that may not be such a bad thing.

The designers of the Lofthouse Interchange were looking to solve two structural engineering problems, at a time when computer structural analysis was not as ubiquitous as it is today. The first was the concern that mineworkings in the area could lead to settlement of the bridges.

The common solution to this at the time was to introduce as much articulation as possible, so, for example, using a series of simply supported spans rather than continuous beams. The second concern for the engineers was thermal expansion and contraction of the curved bridge decks on the tall support piers.

The issue is not entirely clear: for short simply-supported spans, sliding could be accommodated by bearings on the top of each pier, and the piers designed accordingly. I think the issue here is that the designers wanted the decks to be connected together so that there was only one expansion joint at the end of each bridge: the decks are therefore connected to the pier via fixed bearings (permitting rotation under settlement), all expanding from one abutment.

In any event, the "banana piers" were the solution. The "banana" element is hinged at its base, and supported from a cruciform-section concrete strut hinged top and bottom. The effect of this is that the bridge deck can expand along the direction of its curvature while both deck and pier remain stable. It is a simplistic, statically determinate solution, which today would probably be dealt with by designing the piers to be flexible enough to accommodate thermal movement.

The outcome is one of those highly distinctive structural solutions that the Yorkshire motorways are filled with (see also past posts on Droppingwell Footbridge, Smithy Wood Footbridge, Needle Eye Bridge etc).

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