29 April 2014

Tyneside Bridges: 5. Tyne Bridge

We followed the River Tyne westwards, visiting each of its bridges in sequence.

After the Gateshead Millennium Bridge, we travelled back in time over seven decades, to the Tyne Bridge, completed in 1928. This was only the latest in a series of bridges, dating back nearly two millennia to when the Romans first built a bridge here.

Designed by Mott, Hay and Anderson, the Tyne Bridge (1925-1928) was widely compared to the nearly contemporaneous but much larger Sydney Harbour Bridge (1923-1932). Both bridges were built by Dorman Long and Co., and the French engineer Georges Imbault was involved in both cases (Imbault also designed the Middlesbrough Transporter Bridge, which we had visited a day before). A further common factor was the presence of Ralph Freeman, consultant to Dorman Long, who took the lead design role on the Sydney bridge.

The Tyne Bridge appears structurally more rational than its Sydney contemporary. Both bridges are trussed steel arches, from which a road deck is suspended. Both bridges feature prominent but structurally redundant towers above their arch springings. However, the Tyne Bridge is in the form of a proper two-pinned arch, with its steelwork converging on massive hinges at either end. The Sydney bridge is essentially a copy of Gustav Lindenthal's Hell Gate Bridge (1916), with tall trusses at each end to give the impression that the towers are more than architectural book-ends.

The towers for the Tyne Bridge were originally intended to serve both as warehouse space, and also to house lifts providing access from the riverside to deck level. However, the warehouse floors were never installed, and the lifts have long since fallen out of use. It would be interesting to see whether they could be brought back into use to serve a fresh purpose today.

The arch was built by cantilevering from each side, with the steelwork restrained by an array of temporary cables. McFetrich reports that some of the cables were later re-used for the same purpose on the Sydney Harbour Bridge. The expense of temporary works involved in this form of construction is why such bridges are only rarely built today: it is far cheaper to build a cable-stayed bridge and to incorporate the stay cables directly into the permanent works. It's therefore interesting to imagine how different Newcastle and Gateshead would look if the Tyne Bridge were to be proposed afresh today.

For me, the most interesting element of the Tyne Bridge is not its massive main span, but the approach viaducts, particularly on the Newcastle side of the river. These thread their way across historic city streets and between buildings. On large city-centre bridges like this, the approaches are generally the cause of the greatest disruption and destruction when the bridge is built, and they can come to dominate a cityscape in quite an unpleasant way. The approaches to the Tyne Bridge seem to me to fit very well with the scale of the surrounding buildings, and to fit well in that context.

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20 April 2014

Tyneside Bridges: 4. Gateshead Millennium Bridge

I'm still (very slowly) writing up bridges from the second day of last year's IABSE Study Tour of north east England. At last, we came to the mightiest bridge of the tour, the Gateshead Millennium Bridge.

Because I'm struggling to find time to write for this blog, I'm mainly just going to offer you a series of photos with brief comments. But really, this bridge can speak for itself.

This is the classic view of the bridge, taken from the north bank of the Tyne. The weather was fairly miserable when I visited, but that doesn't detract much from this remarkable view.

From a distance, it's not just the arch which is striking, it's also the slenderness of the deck. There's a lot of metal in that deck, but it doesn't look like it from here.

Viewed from the west, the deck appears less slender, as from this side you're looking at the main deck edge girder, not the sharp edge of a walkway cantilever.

The arch is kite-shaped in cross-section, with a curved front face and a sharp rear edge. This simple feature provides the vast majority of the bridge's visual interest, offering a fresh and interesting geometry from almost every perspective.

The deck offers a game of two halves, separating foot from cycle traffic and employing two different surfaces. On the right hand side of the second image, the deck hides a stiff steel box girder. This curves in plan to make it long enough to gain the sufficient navigational height over the river without making pedestrian gradients too steep. On the left hand side, lighter weight aluminium deck panels are supported on transverse cantilever arms.

The two halves are separated by a step and a perforated metal "hedge", which acts as a windbreak and also incorporates space for seating.

A glass "shed" at the south end of the bridge sits above the machine control room, and provides space for the bridge operators to receive guests. A matching glasshouse at the north end is available for other use, e.g. exhibitions, but was unused when I visited.

Each end of the bridge is supported on a giant steel hinge. Below this, a steel fin protrudes downwards. Hydraulic rams act against this fin to raise (or lower) the bridge, and their action has to be carefully coordinated to prevent twist occurring.

The form of bridge is fundamentally inefficient when considered as a moveable structure: it is unbalanced in almost every position, and therefore the loads on the rams are considerably greater than the loads borne by most moveable bridge machinery. The foundations must resist commensurate forces.

On our visit, we were lucky enough to get access both to the hydraulic ram pit and also to the bridge control room. This is the main control panel.

It's a hugely impressive piece of both engineering and architecture, quite deserving of its many accolades and awards. It has been suggested that this is one of the most expensive footbridges ever built, and that's probably true, especially for the width of obstacle crossed. But it's a hugely iconic structure, a tribute to the ingenuity and perseverance of its designers, and certainly one of the most lasting monuments to have emerged from the turn of the millennium.

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