Few structures anywhere pose more of a challenge to the engineer than the dramatic Mind Zone. A set of statistics says it all - 366t of structural steelwork, 392 joints, 344 separate connection drawings. Nearly 90% of the structure is made up of unique elements, a nightmare for designer and fabricator alike.
'It was impossible to draw up the traditional plan, elevations and sections,' says Ove Arup associate and Mind Zone project manager Colin Jackson. 'We had to work in three dimensions throughout, using both computer and scale models to clarify the structure.'
When Arup received the design brief in January 1998 the 2,000m2 structure was little more than a set of architect's graphics. Then, the immediate problem seemed to be the foundations - close to the line of the Blackwall Tunnel - and the fact that architect Zaha Hadid wanted to wrap one of the Zone's dramatic cantilevers around the nearest Dome mast.
Jackson says Dome structural engineer Buro Happold had no objection to this, provided there was no actual contact. 'But three of the outer roof tie-down cables running from the foot of the mast would pass through the tallest cantilever, which was planned as a giant LED screen at that point. These had to be diverted and attached to the edge of the cantilever.'
A standard grid of piles had already been installed but, with the Mind Zone being one of the largest and heaviest structures within the Dome, Arup hoped to locate extra piles within the exclusion zone alongside the underlying Blackwall Tunnel. But this was not to be. Instead, a complex of cantilevered pilecaps up to 1.5m deep was drawn up to develop maximum capacity of the available piles.
Faced with the mind-blowing complexity of the structure, Arup decided to divide it conceptually into three 'strips', each ending in one of the cantilevers. 'Actually, the 23m Strip A cantilever began its design development with an end prop,' Jackson recalls. 'But we were asked if we could do without it to make room for a disabled access ramp to the central high level walkway.
'This was no problem in terms of strength, but we were worried about vibration, as the cantilever had a low natural frequency. By restricting pedestrian access to 15m and installing three tuned mass dampers in the outer end we managed to keep everything under control.'
Floored in plywood or glass fibre-reinforced plastic/ aluminium honeycomb composite panels with timber or metal web beams spanning up to 6m, and clad in lightweight GRP panels, the complex structure is far from stiff and rigid. Particular attention had to be paid to ensuring the floor beams had a minimum natural frequency to eliminate resonance caused by pedestrians.
To ensure that fabricator Watson Steel was able to detail and manufacture the hundreds of joints and members in the time available, a team from Arup was based at Watson's Bolton headquarters for nearly five months. Designer and fabricator also worked together on the erection sequence and method statement. Erection began early in June 1999, and was completed by mid-August.
Sponsors: British Aerospace, GEC
Architect: Office of Zaha Hadid
Structural and M&E engineer: Ove Arup & Partners
Main contractor: McAlpine Laing