The west of Scotland is one of the windiest places in Europe. Gales blow off the Irish Sea year round. When a competition was held in 1992 to design a new landmark tower for Glasgow, therefore, most entrants realised that windloads would be important.
But architect Richard Holden and engineer Buro Happold went a step further, designing a tower that actually responded to the wind. The structure now being built on the banks of the Clyde turns with the wind like a giant windvane.
The Glasgow Tower is to be the focal point of the new £75M Glasgow Science centre. Visitors will ride 100m by lift to a cabin at the tower top, taking in a spectacular panorama of Glasgow and 30km of rugged countryside beyond. It is Glasgow's answer to London's Millennium Wheel.
With a width to height ratio of 1:13, the tower is 60% more slender than conventional structures. To prevent wind-induced oscillation, the £8.5M structure has a teardrop section making it, in effect, a giant vertical wing that slips through the air stream.
A tailplane and outrigger wings form a tripod with the main tower.
Developed through extensive windtunnel testing, outrigger wings are necessary to combat turbulence at the tower's trailing edge. 'Without the outriggers, turbulence would produce a buffeting effect on the tower, which would be uncomfortable for the passengers, ' says Buro Happold design team leader Hash Mistry.
Turbulence is generated when low pressure forms on the leeward side of a tower. The aerofoil shaped outriggers serve to accelerate air into this area of low pressure, pushing the turbulence away from the main tower.
A tailplane, which 'steers the ship', will have a string of LEDs which change colour and move in patterns, to create a Las Vegasstyle lightshow at night. The tower will be topped by a 25m pylon complete with flashing aircraft beacon to warn off choppers visiting the nearby heliport.
Despite its aeronautic qualities, the tower will be turned to face into the wind with four 6kW electric motors, at a speed of 18degrees a minute. Once facing into the wind, passengers will experience wind-generated movement of up to 0.05g, a comfort factor derived from London Underground's ride quality guidelines.
If the motors fail leaving the tower side-on to the wind, it will withstand 1 in 50 year, three second gusts of 160km/h, though all the passengers will be evacuated.
'We looked at the possibility of allowing the wind to blow the tower round, ' says Mistry. 'But we found that, without power assistance, the tower would feather at an angle of 40degrees to the wind and buffeting would be a problem, like a sail flapping in the wind.'
Although it soars over some of the biggest steel fabrication workshops in Europe, the Clydebank shipyards, the tower's core structure - a prefabricated steel spaceframe - has been made up in Poland by contractor Rawent, using Ukrainian steel. Erection and much of the detailed design is being carried out by German spaceframe specialist Mero, which is also putting up the Science Centre's exhibition hall and IMAX theatres next door.
Each 12m, 27t section of the tower is bolted together on the ground and then lifted into place.
Buro Happold opted for bolted connections to increase flexibility and fatigue life in the structure. 'Like an aircraft, it has been built for serviceability rather than strength. Fatigue is the issue, and there is a huge redundancy in the strength design, ' says structures director Steve Brown.
Lifting the sections, Mero is using one of the largest mobile cranes in Europe - a behemoth with 1,000t capacity and a 46m reach. At a rental cost of £5,000/day, the contractor has been at pains not to lose time.
Lifting elements into place is a tricky operation in the wind, and Mero has found that conditions are best before 6am or after 6pm.
Meanwhile, ensuring the right tolerances on the steelwork is crucial to avoid time-consuming insitu fine-tuning. The entire tower has already been assembled once on its side to ensure that it fits together.
The tower rotates on a turntable of 24 rubber sprung roller bearings, and is anchored down through an inverted 'root cone' housed in a 15m deep circular chamber within a diaphragm wall. Loads are directed down into a thrust bearing on the chamber floor via the point of the cone, formed as a single 30t steel casting. The whole 450t tower can be jacked up on the chamber floor to change the bearings if required.
Sunk through river sediments, the foundations are tied down into underlying granite with ground anchors to prevent heave of the chamber floor.
Visitors to the Science Centre will arrive at the tower via exhibitions mapping the contributions of Scottish scientists Kelvin, Watt, Lister and Fleming, and 400 years of Glasgow history. Riding to the tower's GRP cabin in groups of 18, they will then see present day Glasgow laid out below them.
However, prevailing winds are from the south west, which means that in the main the Glasgow Tower will be oriented towards the less than scenic ship yards and estates of Govan.
Science centre staff expect that, weather permitting, there will be demands to rotate the tower.
Both Science Centre exhibition hall and tower are due to open in spring 2001, with the IMAX theatre opening mid-October. This week the tower has reached 75m high - 70% complete and on schedule for handover by the end of the year. Wind permitting, of course.
On the spot
Full name: Hash Mistry
Qualifications: IEng AMICE; AMIStructE
Company: Buro Happold
Current job: Wing Tower, Glasgow
Best thing about the job?
Complex design that addresses aesthetic and structural criteria.
Worst thing about the job? There is no worst - this is only job of its kind and requires a high degree of attention. Highly enjoyable!
What is the most useful lesson you have learned as an engineer? Ability to work equally well with designers, contractors and fabricators.
Advice to any young engineers just starting out? Work hard and take on new challenges. Seek out new experiences.
Anything else? In Scotland what else? I play golf.