But Smith isn't complaining. He's happy that even before the cladding contractor has finished wrapping the buildings in their foil-like overcoats, the design is creating such a stir. But then, two of the buildings will be the UK's first to be clad with titanium, and the site is also home to the first tower in the world that can rotate on its axis through 360º.
From across the River Clyde, looking past Fosters and Partners' "armadillo" conference centre, the area looks more like a sculpture park than a science park. The site is big, almost 5 ha, and surrounded on three sides by water, with the Clyde to its north and the docks of Pacific Quay to the south and west.
Dominating the Clyde waterfront like a giant glass and metal grapefruit segment is the almost complete raking glass facade of the main exhibition centre. Tucked just behind is an egg-shaped building that has been dubbed "Kenny from South Park" by the contractors; this will house an Imax cinema. Nestling between them is a more modest fabric-roofed affair where visitors will be able to buy tickets to visit the centre. To the east, on a promontory jutting out into the dock, and already dominating the site, is the half-completed Glasgow Tower. When the tower opens in spring 2001, it will be the tallest free-standing structure in Scotland. Perched at the top, 104 m in the air, will be a viewing capsule offering panoramic views of Glasgow and the surrounding hills.
On site, the tower now stands at 64 m – well over half its final height. It is being built from 12 m long, pre-assembled sections lifted into position by a huge mobile crane that currently stands idle waiting for the next section. At the foot of the tower, two teams from steelwork contractor Mero can be seen hurrying to complete it. Behind a forest of scaffolding, one team is busy bolting together a Meccano-style kit of steel trusses that will form the skeleton for the section. Close by, a second team is clothing a completed but naked section in curved sections of aluminium cladding. Once the task is finished, this section will be lifted skywards, taking the tower one step closer to completion.
Mero won the steelwork contract for the tower in open tender, having already won the contract for the exhibition building and the Imax. "It was too prestigious a project for us to miss," says Kevin Burke, Mero's project manager. "Besides, I wouldn't have been happy watching another steelwork contractor working next door on the centre's landmark building," he adds.
Constructing a 104 m high tower that rotates to face the wind was always going to be difficult. "Effectively, we were asking construction contractors to build a big machine," says Steve Brown, a partner in structural engineer Buro Happold. To ensure the tower's erection went smoothly, Mero assembled the steelwork horizontally on the floor of the steel fabricator's factory in Poland. This allowed the contractor to make sure that each section of steelwork fitted perfectly into place and to make any adjustments before the steel reached the site. The tower was then dismantled and shipped to Glasgow ready for the riggers to re-assemble it vertically.
On site, the steel sections were bolted together. "We couldn't have had a welder balancing 80 m up in the air," explains Burke. Using bolted steelwork on the tower will also improve the comfort of visitors to the viewing capsule: "It should give the structure some flexibility to help dampen the wind loads," says Brown.
Keeping such a thin structure upright in strong winds needed some pretty spectacular foundation work. The tower stands in a 10 m diameter concrete pit on the end of the promontory. This had to be in position before the tower could be assembled. "The first thing we had to construct was an 800 mm thick, 26 m deep diaphragm wall," says Muir Smith, Carillion's project manager for the tower. The wall was built in a trench filled with bentonite – a runny clay – to stop it collapsing. "Then, once the wall was in place, we had to remove the ground water," explains Smith. Twelve de-watering pumps were installed around the diaphragm wall. "We were pumping out about 120 litres of water a second to get the water level down to 15 m below ground level," says Smith. When the water level had stabilised, concrete was pumped into the wall, displacing the bentonite. Once set, this formed a cylindrical concrete tube 26 m deep.
Having constructed the concrete wall, the soil inside the cylinder was excavated. Ground anchors were installed in the bedrock below and a 3 m thick concrete plug was cast at the foot of the tube and attached to the ground anchors.
"If we hadn't attached it, it would have started to float when the tide came in," says Brown. Finally, a watertight concrete lining was added to the walls of the cylinder to stop the River Clyde re-entering. Only after the concrete pit was complete, and the tower's 95 tonne steel base-cone was in position, could work begin on assembling the tower.
While the tower will have the honour of being the only 360º rotating structure in the world, the exhibition and the cinema buildings will be the first in the UK to be clad in a shiny titanium skin. Once again, the responsibility for the installation fell to steelwork subcontractor Mero. Mero had tendered for the project as steelwork supplier and erector and envelope supplier and installer; the titanium cladding subcontractor was already on board as part of its team.
For the science centre, the finish on the titanium is diamond-rolled. This is intended as an improvement on the blotchy appearance of the titanium on Frank Gehry's spectacular Guggenheim Museum in Bilbao.
Although both the Imax and the exhibition buildings are being clad with titanium, their different shapes meant the cladding had to be installed differently. For the cinema's highly curved 3000 m² surface, Mero's Burke had the cladding installer fit the titanium shingles on both sides simultaneously. The first panels were installed at the base of the facade, with the installation teams working up to meet at the top of the egg-shaped building. To make access easier, Burke made the unusual investment of buying special rubber ladders that would bend around the building's surface. Mero set up a titanium fabrication shop on site to ensure the titanium fitted the surface. "Every cladding panel was different, so we had to measure and fabricate each tile on site," says Burke.
Behind the titanium is an elaborate cladding system. Steel purlins attached to the cinema's space-frame structure carry steel liner-trays. These are stuffed with insulation before being capped with a steel sheet to hold the insulation in place. "The capping layer gives the titanium installer something solid to fix the shingles to," says Burke. Next, a bitumen waterproof membrane is applied before stainless steel clips are riveted onto the capping panel and the titanium shingles are slotted in place.
To protect the titanium, the manufacturer supplied it covered in a white plastic film. This gives the Imax the appearance of an enormous white egg. The unveiling ceremony will take place a week before the cinema opens and the egg will be transformed into a shiny silver ball.
No such protection has been used on the cladding on the exhibition building. Its gently curving facade made the task of fitting the cladding much simpler. The titanium shingles arrive on site already attached to large, diamond-shaped cladding panels, each measuring roughly 4.5 × 4.5 m. The panels have been put together just 500 m from the site, across the road in Govan, by a local company. A pneumatic lifting rig then slots the panels into position.
The Imax will be the first of the centre's buildings to open in October, along with the fabric-roofed ticket office. These will be followed in spring 2001 by the exhibition building and the tower. Some 600 000 people a year are predicted to visit the centre. If the number of visitors eager to view the uncompleted buildings is anything to go by, the centre will have little difficulty meeting this target.
How the tower's structure was designed
The 104 m high tower with its viewing capsule balanced on top will be the focal point of the Glasgow Science Centre. It will also be Scotland's tallest building. But what will guarantee the tower's status as a landmark is that it is so slender.
"Towers are more usually built with an aspect ratio of 6:1," says Steve Brown, a partner in Buro Happold, the engineer responsible for the tower's structural design. "This means that for every six units in height, you have one unit in width to stabilise the tower in the wind." The tower for the Science Centre has an aspect ratio of 10:1, so it is much more likely to sway dangerously in the wind. For the project to go ahead, the engineer had to find a way to minimise this.
"We based the design on a yacht mast that could be rotated to look into the wind," Brown says. "By rotating the entire 104 m high structure, we could always present its minimum profile to face the wind."
Brown explains that the engineers had to contend with three wind issues. "The first is the static wind load – the constant push of the wind on the structure," he says. "Then there is vortex shedding." Vortices are mini-whirlwinds that occur as the wind drives the air around the tower. Under the force of the wind, these spiral away from the tower, causing it to vibrate. "And finally there is buffeting caused by gusts of wind, which cause the tower to flick backwards and forward."
To ensure a smooth airflow around the tower, the engineers used extensive wind tunnel testing and computer modelling before deciding on a tear-shaped section for the tower. The tear is formed by a section of aluminium cladding at the front of the tower that curves around the viewing capsule's spiral escape stair. This will form the leading edge of the tower as it rotates to face the wind. Behind the stair, the structure tapers to a narrow point. A complex series of trusses concealed behind the cladding and a series of K-braces give the tower rigidity and help resist side thrusts caused by buffeting and sudden wind shifts. "We did a lot of studies to see how much movement the capsule would experience at the top," says Ian Liddell, the Buro Happold senior partner responsible for the tower's aerodynamic design.
The tower's tear-shaped core is flanked on either side by two huge aerofoil wings. Each aerofoil is shaped like an aircraft wing in section, so that the wing's upper, curved side faces towards the core of the tower. As the air passes around the tower's leading edge, the aerofoil will help channel the air smoothly to the rear of the tower, increasing its speed and throwing the zone where vortices occur behind the tower's main core to keep vibration on the structure to a minimum.
Set behind the tower's main core is the thin tail section. This helps to stabilise the structure, "like the tail on a kite", explains Brown. At the top of the tower, the tail is transformed into a 25 m high mast. "The problem with the mast," says Liddell, "is that the heavy tower vibrating in the wind would make it flick from side to side." The engineer found the perfect solution in a carbon-fibre yacht mast that was both stiff and light.
Perched on top of the tower is the glass reinforced plastic viewing capsule where visitors will be able to take in panoramic views over Glasgow. "In strong winds the tower will oscillate," says Liddell. The engineer seems at pains to point this out, after Ove Arup & Partners' experience with the wobbly Millennium Bridge. "There's only so much you can do unless you make the tower wider," he explains. "Think of it more as a theme park ride," Brown adds. "Visitors will zoom out of the ground in one of the glass lifts straight up to the viewing pod. Here they'll admire the view for five minutes before stepping back into the lift."
The lifts that will ferry passengers up and down the tower cannot operate at wind speeds above 50 mph, "so we knew the maximum wind speed we had to design to was 50 mph", says Brown. However, the structure of the tower itself has been designed to more than withstand storm winds that occur once every 50 years.
The designers then had to find a way to hold the tower upright while allowing it to rotate. The tower is sited on a promontory of ancient back-filled land between two dock walls, and stands on a cone of plate steel, or, as Brown calls it, "an ice-cream cone". The cone sits in a 10 m diameter, 16 m deep, concrete-lined pit. Around the rim of the pit at ground level, a series of raking columns supports a concrete-encased steel ring-beam 4 m above the ground, lined with 24 roller bearings mounted on rubber springs. The springs will allow the rollers to move slightly as the wind pushes the tower against them, spreading the load over several bearings.
At its base, the weight of the tower is supported on this single casting which, in turn, is positioned on a bearing to take the downward load and allow the tower to turn. Thrust bearings stop any sideways movement at the base.
Four electric motors will turn the tower. The engineer anticipates that the tower will move once every two minutes, although in light winds the operators may give the passengers in the viewing capsule a 360º panorama by rotating the tower about its axis.
- Other, Size 0 kb