Richard Thompson talks to David Satchell, the engineer charged with the job of making Wembley's huge arch and retractable roof work 'WHERE DO you go from here?' asks David Satchell, ruefully. It is a fair question. If you are the engineer responsible for designing one of the most spectacular, iconic and complex structures in history, where do you go next?
How do you follow that?
'I might as well go back home and open a grocer's shop, ' he says.
But before he can contemplate hanging up his hard hat, the Australian structural engineer has the small matter of delivering England's new national stadium. No pressure there, then.
Mr Satchell is roof and arch package manager for Wembley structural engineer the Mott Stadium Consortium (Mott MacDonald, Connell Wagner and Sinclair Knight Merz). He oversees the team of engineers responsible for engineering Norman Foster's 133 m-high, 330 m-wide arch - the longest single-span roof structure in the world - a seven-part sliding roof and some of the longest, most slender roof trusses ever built.
At once, he has the most exciting and the most scary job in structural engineering.
The roof and arch package is roughly 25 per cent of Connell Mott MacDonald's overall contract to design the entire stadium. The remaining three quarters of the work is split between a substructure package being designed by Mott MacDonald and a bowl structure package designed by Sinclair Knight Merz and Mott MacDonald.
While each package contains major engineering challenges, some of the most difficult engineering is actually in the interface between them - where the arch meets the foundations and where the roof connects with the bowl. Mr Satchell is also responsible for managing those interfaces.
Of course, the most striking feature of the new stadium is the giant arch. Many people believe this is just a gimmick created by the architect to provide something iconic enough to replace Wembley's beloved twin towers.
Mr Satchell is quick to refute this.
'There is a common misconception that the arch is not fundamental to the design. It is, ' he says.
Another common misconception about the arch is that it only supports the north roof. 'Actually, it supports a large percentage of the southern roof as well, ' he adds. (See box) The arch was not the only design considered. In Foster's original plan, four huge cable stayed masts, similar to those at the Millennium Dome, supported the roof.
That design was rejected, mainly because the architect felt the look was not unique enough for the national stadium, but also because the cable stays would have to have been anchored outside the footprint of the stadium.
Another alternative was what Mr Satchell calls a 'dumb truss' structure - an enormous steel goalpost spanning the length of the stadium.
This was rejected because it quickly became clear that a 250-300 m span truss would have to be enormous.
'It would have had to have been about 25 m deep, ' says Mr Satchell - making it not only heavy and structurally inefficient, but also ugly.
'The key is to support the front edge of the roof, ' explains the engineer. 'At more than 80 m, it is far too long to cantilever from the edge of the bowl. With the masts and dumb truss rejected, we looked at the arch.
'But, ' he adds, 'what is important is how we got to this roof and arch design.'
It is a challenging thought. If you had to design the new Wembley, where would you start? The answer is surprisingly obvious. . . the pitch.
'It is very much about ensuring good quality of pitch, ' says Mr Satchell. 'Wembley is famous for its turf and there have been lots of problems with the state of the pitches at so many other new grounds. These are the result of the stands being too close to the pitch, so the grass is starved of sunlight and air. The quality of the pitch was fundamental to the design, for cost and public relations reasons.'
It is not until you get into the detail of the roof design that you can appreciate just how central the pitch is. For a start, in order to maximise the amount of sunlight on the pitch, and because the sun is always in the south, the stadium's moving roof only retracts on the southern side (see concept sketch, above).
To talk of a single retractable roof is misleading. It is actually composed of seven independently propelled roof panels - a large rectangular central panel and three smaller triangular panels on either side. This allows the retracted sections to be stacked so they do not protrude outside the perimeter of the stadium bowl.
As a national icon, the new Wembley could be a magnet for terrorists and the threat of a 'disproportionate' collapse was high on the project financiers' agenda.
Mr Satchell's team carried out a rigorous examination of how vulnerable the stadium would be to an attack.
'One obvious terrorist target is the arch base, ' he says.
'So we had to design the arch base for a blast load.
'The conventional regulations are written for conventional buildings and don't deal with this sort of structure.
It took a good deal of interpretation. We had to look at what would happen if elements such as a cable or a section of the perimeter truss were removed, ' he explains.
'Basically, there is a lot of extra capacity built in. We had to design for a live load equivalent to 35,000 people on the roof. For example, the catenary cable (see roof box) is actually two cables.'
One thing is certain. Whatever job Mr Satchell goes to next is bound to be an anti-climax after this one.
The 145 mm-diameter, high-tension cables that link the main roof beams to the arch exert around 1,700 tonnes of pull at the connection points on the arch. Not surprisingly, the connections between the stays and the arch are equally massive (right). 'What is not immediately apparent is that these connections are almost as big as a car, ' says David Satchell.
'They have150 mm-thick connection plates.'