BANG!

"It's possibly the most extraordinary thing I have ever been presented with," says Ron Packman director of structural engineer Packman Lucas. "It's a solidified explosion made in steel. When I first saw the drawings, I thought it couldn't be done."

Packman is describing his initial reaction when presented with 3D designer Thomas Heatherwick's drawings for what will be the UK's tallest sculpture when it is finished. Called B of the Bang, it looks like one of those giant starbursts seen high in the air at firework displays – but frozen in time and space. The 180-tonne work consists of 180 massive spikes up to 35 m long exploding out of a central core. At 56 m high, it is almost three times taller than the record holder, Antony Gormley's Angel of the North – but what makes B of the Bang particularly difficult to realise is that it leans over at an angle of 300. It is Packman's job to make the piece stand up.

The sculpture was commissioned to mark the success of the Commonwealth Games held at Manchester Stadium, now home to Manchester City football club. Heatherwick wanted to create something different. "Commemorative monuments tend to be very passive," says Heatherwick. "They are all about peace between nations and people holding hands around the world. They are not about individuals doing something very extreme, where people have to push themselves to the limit. So we felt we should do something very dynamic." Heatherwick also remembers the 100 m sprinter Linford Christie saying that you need to leave the blocks on the "b of the bang" of the starting pistol to win. "That's where the idea of B of the Bang came from – that moment of impulse." The sculpture expresses the explosive energy of athletes as they leave the blocks.

Because the vast stadium stands on an isolated site away from the city centre, Heatherwick felt the sculpture needed to be the same height as the stadium in order to complement it and to have the greatest impact on occasional passers-by.

Heatherwick did not just hand Packman a brief and tell him to sort out the engineering of the sculpture – it has been a joint effort. The two first met when Packman was teaching Heatherwick furniture design at the Royal College of Art. Since then, they have collaborated on a wide range of projects and now Packman has an engineer working full-time in Heatherwick's studio. Packman and Heatherwick have been working together from the initial brief towards the finished design. "Tom is very good at collaborative work. There's no 'I'm the artist' stuff," Packman says.

Packman first heard of the concept for the sculpture when woken at 2am by a phone call. Engineering work had already started on Heatherwick's initial idea, but neither Packman nor Heatherwick were entirely happy with it. Then, while Heatherwick was on holiday in Mexico, he was suddenly inspired by a new idea – hence the early morning phone call. "Tom said he wanted to change the sculpture as he had a much better idea," Packman recalls. "When he told me what it was, I loved it and started work on it immediately."

But Packman, who had assumed the work would be vertical, had a shock when Heatherwick arrived back with the completed drawings. Heatherwick had decided the piece would be more dynamic if it leaned over. "Ron is someone who has worked with the studio for a long time," says Heatherwick. "He has always encouraged me to be bolder, so it was really funny when I showed him the drawings and his voice went up a few pitches." Packman says why he was prepared to go along with Heatherwick. "Intuitively, Tom is an engineer," he says. "He takes the line that, if he thinks it's okay, he can get Uncle Ron to make it work."

The design was refined by Heatherwick by making and adjusting a wooden model of the sculpture until he was happy with it. This was subsequently used for the competition entry to win the commission and to help the team working on the project fully appreciate what it was getting involved with. Turning the model into drawings proved tricky, but it was resolved by defining the position of the outer end of each spike and the position of the centre where the spikes meet. Once the design had been turned into a CAD model, the engineer could start working on the detail. All the time, Packman was conscious of remaining faithful to the original design: "Our job is to make the engineering invisible, to make it 'no hands'," says Packman. "The trouble is, simple is very difficult."

After the drawings were completed, it was time to analyse how the structure would behave once it was built. The effect of the wind on the sculpture was Packman's main concern, because wind does not produce a steady force, but gusts and eddies that cause structures to vibrate. "It's on a plain with not much around it," says Packman. "The stadium produces these wind vortices that come thundering across the plain and clatter into our structure."

The main problem was the torsion or twisting response of the sculpture to the wind forces. Because 180 tonnes are supported on five steel legs, the wind can cause the sculpture to twist about its vertical axis. Once it gets to the point where the natural spring of the legs is stronger than the force pressing on it, the sculpture swings back again like a bungee jumper on the end of a length of elastic.

The frequency of the sculpture's movement had to be defined to prevent the wind from bringing the sculpture crashing to the ground. This was the problem suffered by Tacoma Narrows Suspension Bridge in Washington state in the USA, which collapsed in 1940 as a result of wind-induced vibration. If the natural frequency of the sculpture matches that of the wind frequency, then the swinging gets worse and worse until the structure suffers catastrophic failure. Packman likens this phenomenon to a child on a swing. The swing moves back and forth with a frequency defined by the length of its supporting chains. When the swing is pushed at the same frequency, it goes higher. If the swing is pushed at a frequency that is counter to the frequency of the swing, then it slows down.

Wind tunnel analysis carried out by Flint & Neill Partnership showed that gust buffeting of the structure presented the biggest headache. Armed with this analysis, the engineer was able to tune the sculpture's reaction to the wind by adjusting its weight and the thickness of the support legs so that its frequency was sufficiently different from the range of likely wind frequencies.

The analysis also showed up an unexpected benefit: although the sculpture weighs 180 tonnes, it is very light relative to the volume of space it occupies – like the head of a dandelion. This means that it behaves like a sail and moves with the initial gust of wind, but then acts as a brake to prevent it oscillating too much.

The next job was working out how to make the sculpture economically. The giant spikes are made in the same way as lamp standards used for lighting roads and car parks. "It's a very basic, cheap way of doing it," says Packman. The spikes start off with a wider diameter at the sculpture's core and taper towards the ends, and are facetted rather than curved. The support legs presented a big design challenge as these had to look like the other spikes while supporting the whole structure. The wall thickness of the spikes varies from 3 to 6 mm, but the tubular support legs have a wall thickness of 40 mm except for a section adjacent to the ground. "We couldn't find a way of getting any more metal in, so they are solid near the ground," says Packman. To make the support legs visually match the spikes, they are clad in thin, facetted metal plate.

"We did a massive amount of design work on the core," says Kevin Cumberland, managing director of steel fabricator Westbury Structures, which will make the sculpture. This was because Heatherwick wanted the effect of the spikes exploding out of each other rather than being attached to an obvious centre. "Thomas was adamant you shouldn't see any bald areas on the central core," says Cumberland. "We got it down in size to 1 m, but you could still see the bald areas." The solution was to join the spikes to each other in clusters then attach the clusters to the core so that the core is not visible. Each spike has to be precisely positioned on the core, as the tip of each has to be at least 3 m above ground to prevent pedestrians wandering into them. If the attachment points on the core are not spot on, the spikes could end up too low down. Also, the stresses on the support legs are so great, it is vital they are in exactly the right position.

The prickly nature and scale of the sculpture means erecting it will also be difficult. The 100-tonne central core will be brought to site by road and will be supported 8 m above ground in a temporary frame. The support legs and about half the spikes will be welded to the core in this position as it is easier and safer to work on it lower down. Then the whole structure will be lifted up and rotated to its final position and attached to a concrete pad resting on piles – this will be below ground so the spikes will appear to be coming straight out of the ground. The remaining spikes will then be attached to the core. Work has begun on the foundations and the sculpture should be completed by next April, when the public can judge whether it is the most extraordinary thing it has ever seen.