Greenwich. Monday, 23 March 1998. The first cladding panel is lifted into position 50m up on the Millennium Dome. As the team from cladding subcontractor Danny Construction Company begins to unfold the 225m2 of PTFE-coated glass fibre fabric, the smiles on the faces of the Dome team have more than a hint of smugness. Despite all the ups and downs of the last 18 months, and despite a late change of fabric type and supplier, this first panel is being erected exactly to the day laid down in the programme drawn up in August 1996.
McAlpine Laing project director Bernard Ainsworth says this is mainly down to the successful erection of the cable net that hangs from the Dome's 12 distinctive yellow towers. Unfavourable weather over the Christmas period caused no serious delays, and stressing of the net actually began four weeks early, in the middle of February. 'During bad weather we concentrated on knitting the three cable rings together at ground level,' Ainsworth explains.
'Then each ring was individually hoisted into place and connected together by the 20-strong abseiling team when the weather was better.'
Nearly 70km of steel cables between 25mm and 90mm diameter make up the net. According to Buro Happold project director Ian Liddell, no other structure in the world is quite like the Dome - and not just in terms of size. 'Nothing else uses a radial cable design suspended from masts in this way. During the cable erection we really learned something about cable handling and clamping.'
In practice the worst problems came from the temporary clamps used during erection. These began to slip unpredictably, and only a timely switch to a tougher rubber conveyor belting material for the clamp inserts prevented serious delays.
'We also had problems with the 90mm perimeter cable, which kept on twisting as it was tensioned,' Liddell reports. 'And it turned out to be difficult to pre-position clamps on the cables accurately enough during manufacture.' This was largely due to the sheer scale and complexity of the cable web and the near-impossibility of calculating the net effect of all the individual cable extensions as loads were applied.
Problems were eased, however, by the success of the barrel-pin connector design used for the nodes where most of the cables terminate, which allowed 'lots of rotation'. New Millennium Experience Company site development manager Peter English says that once the temporary clamp problems were identified and solved, the rest of the cable work was relatively plain sailing.
He adds: 'It turned out that getting Watson Steel to supervise the cable erection as well as supplying and erecting the masts was a very good decision, even though Watson had never done anything like the net before.
'The interface between the cable erection and the other structures on the site was so critical, the potential for conflict between two different contractors would have been very high.'
Watson in fact simplified the complex interface by winning the contracts for the six, three storey service cores within the Dome as well. Work on these is well advanced. At the moment, however, the most impressive 'conventional' structure on the site is the 40m by 30m by 50m high central scaffold tower, which consumed 72km of steel tubing and was erected by Kwikform UK in just two and a half weeks.
Radiating out from the top of this tower are the two diametrically- opposed sets of access platforms from which Danny Construction will carry out the majority of the cladding operations (NCE 12 February).
English says there was no real problem with the Health & Safety Executive during the development of the method statement for the cladding phase. 'We have an excellent relationship,' he asserts. 'Although this is a very unusual structure we were received with very open minds.'
Cladding supplier Birdair vice president of operations Stan Kopaskie explains that the wind speed limitations agreed with the HSE are based on a complex formula which includes wind direction in relation to the panel being fixed, height above ground, and frequency of gusting. And he dismisses with good-humoured contempt suggestions that typical winds on the Greenwich peninsula represent any serious threat to the cladding programme. 'We wouldn't like to try it in a hurricane - but yesterday we were working in 25 knots (45km/h) without any problems.'
Kopaskie goes on: 'In terms of area alone, the panels here are no bigger than some we've done before, it's just that they're very long, thin ribbons. And after the success of the ground level trials we foresee no major problems.'
These trials were mainly devoted to checking out the unfolding procedures for the cladding packages and developing the package of electric winches, 'comealongs' and Tirfors used to tension the panels before they are clipped on to the cable net. Panels come in two basic sizes, with the upper of the two courses measuring 75m long and tapering at 5 from a base width of 5m to only 1m at the top edge.
The outer fabric is only 1.2mm thick, but an inner lining of thinner uncoated glass fibre cloth brings the total weight of each upper panel to just under 1t. Panels are delivered from Birdair's Buffalo works in a two-way concertina fold.
Birdair UK general manager Daniel Ptacek explains. 'When the folded panel is lifted on to the web, we position it about a third of the way up from the lowest fixing point. Then we unfold and fix the upper two- thirds first, then the lower third. To minimise the risk of creasing and cracking, it's folded around those inflatable 'noodles' that children use as buoyancy aids.'
In anything but calm conditions some degree of caution is needed during the initial stages of panel fixing. As each fold is straightened and pulled out along temporary radial support ropes, it is fed between temporary transverse restraining ropes at approximately 3m centres.
Unfolding takes about an hour on average. Motive force is provided by electric winches top and bottom. Once these have straightened the panel out completely, 12mm diameter steel cables are fed into pockets running down each side of the panel.
Kopaskie comments: 'We tried pre-fitting these cables at the factory but it didn't work out, so now we include a feeder rope in the pockets to pull the cables through on site.
'Once we've tensioned these cables bar-taut we can pull the panels sideways with Tirfors and comealongs and clip them on to the connectors.'
Simple extruded aluminium sections at 500mm centres act as the edge connectors between panels and net. On the upper section of the net, where slope is minimal, these connectors actually hold the fabric some 210mm above the net proper - part of Buro Happold's elaborate precautions against melt water ponding.
More elaborate connectors are needed at the cable nodes and panel corners, and site splices are needed to accommodate the giant yellow masts. An aluminium 'walkstrip' then goes on top of the edge connectors. The narrow flap of fabric which extends beyond the side cable on one side of each panel is then unfolded over the walkstrip and heat welded to the adjacent panel to form a watertight joint.
With two sets of access platforms, two 10-strong fabric crews and up to 40 abseilers positioning the 26,000 clips ahead of the cladding fixing, Birdair expects to see four panels fixed every day on average. In all, a grand total of 72 upper and 72 lower panels have to be fixed and sealed before 19 June - a date as critical to this time-dominated project as the 23 March panel lift.
As the cladding progresses and the Dome changes from an ethereal spiderweb to something much more solid, public reaction will change again. People will come to realise that the Dome's cost of less than £50M is about the same as a City HQ office building - and it is more than likely to be occupying its present site for many decades to come.