20 November 2013

Teesside Bridges: 4. Infinity Bridge

Okay, it’s white elephant time.

A design competition was launched in 2003 for a new footbridge across the River Tees at Stockton. The intention, as in so many instances, was to use a landmark structure to signpost the local authority’s commitment to developing a new area, on the north bank of the Tees, and attract in outside investment. A budget for the bridge of £4.5m was stated.

The competition was won by Stephen Spence Associates with Expedition Engineering, who developed the concept for a twin arch bridge, with one arch twice the span of its neighbour. The asymmetric arrangement kept the central part of the river free from obstacles, this part of the river being used regularly for rowing training. There was to be some controversy over authorship of the design.

The bridge was completed in 2009, at a cost of £15m, an enormous increase over the original budget, and one which presumably involved the promoter having to plunge their hands deep into their pockets. This is above average for a landmark bridge, but not unreasonably so given the spans involved. The original budget was never enough for a truly iconic structure, and the Infinity Bridge is certainly that.

Visiting four years later, all that our tour group could find on the north bank was mud and grass – none of the hoped-for development has materialised, perhaps unsurprisingly given events in the wider economy. It’s in this sense that the bridge is a white elephant, and it remains to be seen whether the money was well spent.

Nonetheless, the bridge is both spectacular and elegant, and its architectural impact is largely a consequence of engineering requirements rather than the driver for the design. The “reflex curve”, or sag arch, which links the two main arches, originates as a gesture on a sketch. Expedition can make the claim that it is a functional element, as it serves to transfer bending between the two arch ribs, making them both much stiffer and improving behaviour overall. This is, however, probably rationalisation after the fact, a happy but not inevitable outcome.

Several things strike me about the structure. The deck is quite exceptionally slender, comprising a series of precast concrete slabs, prestressed together by the action of the arch’s horizontal bowstring cables.

There is very little stiffness in the deck, and although probably heavy enough to be difficult to excite, it still requires the presence of five enormous tuned mass dampers tucked below its soffit to ensure that vibrations are acceptable. The dampers are impressive but slightly odd – one above the south bank is set horizontally at a location where the deck has a longitudinal gradient, making it look as if the damper is peeling off the soffit and ready to fall.

The arch is tall yet narrow. Most designers, I think, would have adopted a wider arch to provide greater transverse stability. This narrowness is enhanced by the tripod form of each arch, where it bifurcates into two ribs at the central support, but comes to a single rib at the end support. The central support is therefore required to provide all the resistance to wind or eccentric loading. The effect is visually striking, but as an engineer I also found it a little disconcerting. It also means that the deck has to incorporate significant transfer elements to brace between the end supports of the arch (with an outward thrust) and the tie cables along the deck edges (with their inward tension).

The arches are formed from painted structural steel box sections, and generally are shaped attractively. In elevation, their shape was determined by form-finding, to minimise bending moments under dead load. However, the bracing between the twin arch ribs near the main support, which include huge circular sections resembling offcuts from a pipe factory, is much less attractive.

There’s also an odd disjuncture between the plain, minimal profile of the arch ribs, and the centre-pier struts which support them. Both above and below deck, these struts have a “fluted” form, which contrasts markedly with the arch ribs. I don’t particularly like the shape of these elements, although I do think that the high contrast was the visually appropriate decision. It is the shape of the arches including the reflex curve which needs to be highlighted and given visual continuity, even if this is structurally dishonest, in the sense that the primary flow of forces is from the arch rib down towards the point of support. I think this is therefore an instance where the popular maxim that form follows function would have it wrong.

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