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An exact science

Ultra-precise timber construction has been needed to build a bridge like no other, discovers Andrew Mylius.

Scaling up the beautiful but bizarre 9.6m long, 1:10 scale model of Taiwan's Alishan Mountain Bridge for the real thing will be 'one hell of a challenge', says Gordon Cowley, founder of Lincoln based Timber Engineering Connections (TEC).

But if the full scale version gets the go-ahead, Cowley is more than keen to have a go.

TEC constructed the geometrically intricate model from 3mm plywood in a frenziedly short period this summer after being contacted, through consultant Arup, by the bridge's architect, New York firm Reiser & Umemoto. The designer had been searching for a firm capable of fabricating its brainchild in time to be exhibited at the international showcase of avant-garde art and architecture, the Venice Biennale.

Its aim - to show that the crazily undulating timber form could be turned from CAD image to physical reality. The two firms started talking in June, with construction only set in motion in July.

Alishan Mountain Bridge is described by Cowley as 'somewhere between a preindustrial bowstring arch and a gridshell'. It obeys no conventional bridge building rules, with bending forces in the central span counteracted by hefty side span cantilevers.

Its edge rails dip to form the bridge's legs and swoop up at centre span creating the main arch. They bend in three dimensions and as they curve towards the deck's centre line at mid-span, bending forces attempt to close the gap between them:

'The arch suffers from a fair bit of clamshell action, ' Cowley notes.

And its legs are unbraced.

'When we picked it up from its cradle [the formwork over which it was constructed] and placed it on the floor it behaved like a newborn calf - unsteady on its pins.' Hardly surprising, perhaps, given that the entire structure weighs only 30kg.

Considerable structural refinement and strengthening will be needed to make a full scale version of the bridge viable, Cowley ventures. This is largely because, to date, the engineering focus has been on making the bridge work geometrically.

For construction, the 3-D CAD model was 'unwrapped' and laid out flat by Reiser & Umemoto.

Next individual laths were separated out from the structure, enabling TEC to cut them from plywood sheets on its CNC machine.

In all there are 188 diagonal ribs and 16 longitudinal strings, which, laid out in two dimensions, snake from side to side. In order to assign each its correct place in space, TEC pre-drilled all of the bridge's bolted connections - Cowley could not leave any node connections to chance for fear that errors in alignment when the laths were overlaid would throw out the pinpoint precision required to achieve the form.

'We normally work to +/-2mm tolerances but with this we had to achieve unmeasurable degrees of accuracy, ' Cowley notes.

Edge geometry was defined by laminating together two lengths of 4mm ply, cut to shape in plan, to achieve the correct elevational alignment. TEC introduced a 30mm deep, 25mm wide 'keel' element to give the bridge a solid backbone.

And five templates were cut to define the bridge's section at end, quarter and mid-points.

The amazing lattice was overlaid by a web-like deck unit machined from sheets of plywood so that it perfectly mirrored the underlying grillage.

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