Stocking thriller

Can you imagine walking through a colossal fishnet stocking? In November, you will be able to live out this kinky fantasy when a unique fishnet footbridge is completed in central Manchester. The £550 000 bridge replaces a conventional one damaged by the 1996 IRA bomb. Its extraordinary design is a competition-winning scheme by local architect Hodder Associates with structural engineer Ove Arup & Partners.

There is method in the apparent madness of the fishnet configuration. The previous footbridge traversed an 18 m gap between the first floors of the Arndale Centre and the Marks & Spencer store. It also had to deal with a height difference of 1.2 m between the two buildings. The floor levels could not be changed, but the competition brief asked for the slope to be corrected optically. Hodder’s transparent fishnet tube spans the two buildings horizontally while masking the gradient deck of the bridge inside.

The fishnet arrangement works structurally, too. The double-curving cylindrical structure corresponds to a hyperbolic paraboloid, which allows it to be made of 18 straight steel rods. The rods look curved because they spiral around the circumference in both directions. Likewise, the apparently curved glass skin is composed of 360 flat triangular panels.

Added to that, the thicker 110 mm diameter rods act in compression, while the thinner 25 mm diameter ones that cross them are in tension. The result is that compression and tension cancel each other out, leaving the bridge sitting as a simply supported beam on its bearings on either side, without exerting any sideways pull on the structures of the store or shopping centre.

“This is a world first in structural terms,” enthuses architect Stephen Hodder. “A hyperbolic paraboloid structure has never been used horizontally before.”

The bridge was lifted into position last month – an operation that took only 40 minutes. The steel rods were threaded through holes in steel ring collars fixed at either end and tightened with bolts.

A particularly tricky design problem was posed by the triangular glass cladding panels, which had to bear the weight of maintenance staff. The panels are made of toughened, laminated glass, supported at their three corners by purpose-designed stainless steel clamps. But it was considered possible that, should the glass shatter, it would pull its corners out of the fixing clamps. Accordingly, a particularly tough interlayer of polyester, as used in the hurricane zone of Florida, was specified and securely fastened to the clamps to support people even if the glass shattered.