11 March 2014

Tyneside Bridges: 3. Byker Metro Bridge

The third and most recent high-level bridge to cross the Ouseburn Valley is the Byker Metro Viaduct, completed in 1982 for the Tyne and Wear Metro.

It is 815m long and curves in between the Ouseburn Viaduct (1839) and Byker Bridge (1878).

I only had time to visit its western end, which spans the valley. Much of the viaduct, at its eastern end, runs at about 6-7m above ground level, and is visually less imposing.

The bridge was the first glued segmental precast prestressed bridge to be built in Britain. The main spans are 69m long, with haunched concrete box girders supported on twin-stem piers. The bridge's appearance was considered significant enough for the design to be presented to the Royal Fine Art Commission in 1975.

The form of construction lends itself to a site where there was highly restricted access for plant. The concrete segments were precast off site, and winched down the valley slopes on temporary tracks. The length of each segment was just narrow enough to allow it to pass through an opening in the bridge piers, allowing the segments to be manoeuvred along a very narrow construction strip. They were then lifted and erected using the balanced cantilever method. Each segment was bonded using epoxy glue, and stressed in place with short length bars. Further prestressing was added in later stages.

The segments were "match-cast" i.e. each one was cast against its preceding segment, to ensure a perfect fit. Sawteeth keys in the webs, still visible in the completed structure, help align the units during erection. In practice, this method requires very careful measurement and control, as errors have the potential to accumulate over multiple segments. This is exactly what happened with the Byker Metro Viaduct, with a significant twist detected as more segments were added. Special segments had to be cast to correct the twist.

The finished bridge is aesthetically striking, and I think very successful. The plain finishes to the concrete deck contrast with a vertically ribbed finish on the piers and precast parapets. The ribs are nicely detailed, splaying out as the piers widen out towards their base. Horizontal feature grooves have been included on the piers to disguise construction joints.

The splayed piers both give an impression of considerable stability (required to resist centrifugal loads from the metro vehicles), but also make what are quite substantial concrete sections look quite elegant.

I think this is easily one of the most impressive and well thought out bridges on this scale to have been built in Britain.

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