The North Shore footbridge is starting to emerge cautiously from the River Tees. The four steel legs for the central pier are in place, sitting on four 3m diameter concrete columns, barely visible above the waterline. The columns sit on a 2.5m thick concrete slab which in turn is supported by hollow steel piles founded on rock.
More prominent than these is the pale grey steel frame which stands proud of the water by some 18m. The frame is part of the temporary works to support over 150t of bridge steelwork which will arrive over the next few months. So prominent is this temporary structure, locals could be forgiven for mistaking it for part of the new bridge.
The finished structure will be much more elegant though. Starting on the north shore, the first arch rises to a height of 32m, before falling back down to a pier, which sits 120m across from the first support. A smaller arch then takes the footway the remaining 60m to the south shore.
The intermediate pier for the bridge is not in the centre of the river and has been carefully positioned to accommodate leisure craft and watersports on one side. This has dictated the twin arch form of the bridge with one arch about twice the size of the other.
It is truly an engineer’s bridge, with materials being used to maximum efficiency. The arches soar across the water from each bank, overlapping slightly as they plunge into the water taking on the role of the bridge piers.
The two arches are immediately recognisable as a sort of hurried letter "M", but the swooping line of the M acts as a link between the two arches that can transfer bending moment. This means that under its own weight, the structure acts as two arches, but as soon as people walk on it, it can work as a two span beam.
The arches start at between 200mm to 300mm wide at each bank but increase in width as they leave shore with the thickest part of the bridge around the pier.
"We put the meat of the structure where the stresses are highest," says consultant Expedition Engineering director, Chris Wise.
"It’s unbelievably slender. We’ve spent the last few years proving it will stand up, checking footfall vibrations."
Wise is famous for having worked on the fateful "wobbly" Millennium Bridge in London, so understands the phenomenon only too well.
Expedition has designed every element of the bridge to respond and act efficiently. As the arch tries to spread, the loads can be resisted by the foundations, or by tying the arch internally, explains Wise.
"By tying the arch, we saved on the foundation [costs]. We also used it to carry the deck. We stressed the cables so that the deck was in compression and increased its stiffness."
Stockton Borough Council
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