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A steel tower with aerofoil outriggers

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The steel structure of the 120mhigh tower derives its form from principles evolved in the design of aircraft wings. The main shaft, a triangulated framework of 168.3mm diameter tubular columns braced with 100 x 100mm square hollow section members, is 'teardrop'-shaped on plan; it encloses a steel circular staircase supported by a central 139.7mm diameter steel column and enclosed with a cylindrical tube of silver anodised aluminium panels.

An aerofoil-shaped steel outrigger on each side of the shaft stands tapers and gently curves as it rises, stabilising the shaft and reflecting the path of forces.

The two outriggers are braced, at 12m intervals, to the main column by a diagonal 168.3mm diameter circular hollow section K-braces and curved 250 x 250mm square hollow section 'ribs' that meet at the 'tail' of the tower. The tail is a 180 x 180mm square hollow section column, clad with silver anodised aluminium panels to form a tapered aerodynamic shape.

The profiles of the outriggers were calculated to create a smooth flow of air around the tower, preventing 'vortex shedding'. They change in profile as they rise, tapering from a 3.43m long and 452mm wide aerofoil-shaped profile at ground level to a 'fatter' 1.34m-long profile at mid-point and a 345mm long and 227mm wide profile at the top. The outriggers were factory-welded in 12m lengths from curved steel plates with 20mm thick internal stiffener webs. The bull-nosed edges are formed of silver anodised aluminium panels bolted to the ends.

The base of the tower comprises a circular horizontal disc which turns, supported by 24 rollers set in a concrete ring beam which in turn rests on a series of raking steel columns.

The rollers are mounted on rubber springs to provide structural damping.

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