Blowing in the wind
Glasgow Science Centre tower is the tallest building in Scotland; from the top, 120 vertiginous metres above the River Clyde, you can see as far as the distant mountains. It may be a midget compared with other city towers, (Malaysia's Petronas Towers are 452m, the CN Tower in Toronto is 447m) but what is remarkable about the Glasgow building is the sheer elegance of its bladelike structure, so breathtakingly minimal that to ascend to the top takes an act of faith.
The tower is a unique structure; it turns at its base on a turntable to align itself with the prevailing wind and its components are shaped aerodynamically. This has reduced the quantity of structure by about 30 per cent compared to a fixed-base tower, as the turning tower only has to resist wind forces in one direction. It has also minimised strong movement at the top, which would be uncomfortable for visitors.
The design of the tower, by architect Richard Horden, was in response to a competition held in 1992 to produce a new landmark for the city. Horden developed the concept, with structural engineer Peter Hepple and subsequently Ian Liddell of Buro Happold, to produce a slender structure with a viewing cabin at the top, reached by a pair of lifts and a circular escape staircase.
The tower is part of a new complex on the south bank of the Clyde, which includes an IMAX cinema in a silver-clad hemisphere and an adjacent exhibition building, now nearing completion. It is reached by means of a tunnel leading from the exhibition building. A basement podium round the tower turntable houses a gallery and departure lounge for visitors waiting to ascend in the lifts.
The structure derives its form from principles of wind flow and resistance used in the design of aircraft wings. The main shaft, a triangulated framework of tubular columns, 'teardrop'-shaped on plan, encloses a steel circular staircase clad with a cylindrical tube of silver anodised aluminium panels. Steel aerofoil-shaped outriggers stand at each side of the shaft, tapering and gently curving as they rise to reflect the line of forces. The shaft and the outriggers are connected every 12m by a series of K-braces and steel ribs which curve to a tail, which is a circular hollow section, also clad with silver anodised aluminium panels to form a tapered aerodynamic shape. The shaft is topped by a viewing cabin of glass-reinforced gypsum and the tail is extended vertically with a 24m-high carbon fibre mast.
The lifts are mounted on each side of an open lattice mast which is fixed to the rear column of the stair tower at every 12m.
The base of the tower comprises a circular horizontal disc which turns, supported by 24 rollers set in a concrete ring beam resting on a series of raking steel columns. Rubber springs in the rollers provide damping.
Below the disc, the structure tapers to a point, a solid steel casting, supported by horizontal and vertical thrust bearings and enclosed in a cylindrical concrete diaphragm wall bedded in the alluvium of the River Clyde. Visitors to the building will be able to view this remarkable mechanism.
The steel tower was fabricated in Poland where it was laid out horizontally to ensure that it would fit when erected on site. It was lifted in 12m sections, with the escape stair and its cladding fixed on site by a 1,000tonne crane.
Aerodynamic design A key part of the design was the control of wind flow to achieve a steady wake when the tower is orientated towards the wind. The significant elements were the size and shape of the stair tower and the profiles of the outriggers, which had to control the airflow around the shaft to prevent uneven flow (vortex shedding). The tail is curved to help balance the tower. In spite of this, the centre of lift is forward of the tower's centre of rotation and to be stable it must be driven to face the wind. The aerodynamic design of these elements was carried out using a computational fluid-dynamics program and wind tunnel test as well as extensive analysis.
Although the tower is safe in high winds, people will start to feel uncomfortable in the cabin if the maximum acceleration value exceeds 50mg (the equivalent of a tube train's movement). Calculations showed that this may happen, causing the tower to close, on about six days in the year.