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Checking in with the checker

London 2012’s Olympic Stadium has made heroes of athletes and engineers alike. But unsung heroes involved in the creation of the 2012 Games’ centrepiece remain.

Mo Farah’s double gold; Super Saturday; Usain Bolt’s double-triple; London 2012’s Olympic stadium certainly was the place where heroes were made and lauded on those heady nights last August. The team that built it have also had a fair smattering of praise; Highly Commended in the 2011 British Construction Industry Awards, the project took top honours one year later in the Structural Steel Design Awards.

And rightly so; the project, led by client the Olympic Delivery Authority and delivered by the Team Stadium consortium of architect Populous, main contractor Sir Robert McAlpine and designer Buro Happold truly excelled.

Olympic Stadium

Special delivery: The Olympic Stadium’s unique roof structure provided the role of the design checker

But there were other, less celebrated, firms also playing key roles; steelwork contractor Watson Steel Structures was recognised at least in the Structural Steel Design Awards - and quite rightly given the complexities of erecting the stadium’s lightweight cable net roof. But equally, given the uniqueness of the structure, the role of checking engineer was crucially important. That role fell to Flint & Neill, a specialist designer with expertise in cable structures though its long history of working on long span structures around the world, and who had previously performed the same role for the same team on the Millennium Dome in 1999.

That independent expertise was invaluable on the Olympic Stadium. Its roof consists of a PVC coated fabric membrane supported on a cable net with an inner tension cable ring and an outer steel compression truss. It is complex: the outer ring truss is approximately 900m long and 12m deep and is supported at 32 positions by inclined raking tubular columns down to ground level.

The inner cable tension ring then consists of 10, 60mm diameter cables connected by steel brackets at 6m centres, which in turn support a continuous walkway. The tension ring is supported by 80mm diameter suspension cables connected to the top boom of the compression truss, and the whole system is tensioned by 70mm tie down cables connected to the bottom boom of the compression truss.

“The structure itself is unique and the key thing was getting the correct geometry”

Paul Sanders, Flint & Neill

Finally, and adding to the challenge, sitting atop the inner cable ring are 14 large pyramidal lighting towers, each 30m high and weighing a massive 34t; these are restrained to each other and back to the compression truss with a secondary cable system.

On structures like this, the codes just do not apply, explains Flint & Neill project manager Paul Sanders. “On special structures you can’t just apply the codes. You have to think outside the box.

“The structure itself is unique and the key thing was getting the correct geometry. The stresses in the structure very much drive the geometry and stiffness of the structure, which are vitally important to understanding its behaviour.” If the geometry is wrong then one of the biggest problems faced is the risk of ponding, where water and snow melt can rapidly accumulate, the consequences of which are obviously potentially severe.

“So there is great value in having someone completely independent doing the same analysis and design checking; someone who can ask the difficult questions,” notes Sanders. “And this is particularly true when you are looking at special structures like this that fall outside of convention - where you are very often applying first principles so that you can understand the behaviour of the structure, and where you need to understand the limitations of the codes and standards.”

“Because codes and standards can’t cover all the scenarios,” he adds, “It’s all about increasing certainty and reducing risk”.

Reaching consensus on the correct wind loading to be applied to such a lightweight and unusually shaped structure absolutely highlights this point, notes Flint & Neill team leader Amar Bahra. “Wind codes are naturally based on idealised structures and do not readily lend themselves to a unique structure such as this. Buro Happold came at it from one angle and we came at it from another and, not surprisingly, there were differences,” he notes. In the end, wind tunnel testing was used to justify the design.

But the wind loading example serves well to highlight Sanders’ final, key point: “The key thing to emphasise is the close collaboration that existed throughout the team”. And it really is crucial, he says. “You wouldn’t expect the designer and checker always to come up with exactly the same answers, and this is where you need a good working relationship. You need to be able to have an open exchange and understand why you have differences and then resolve them. You have to challenge, but in a constructive and justified way.”

Clearly, it worked: the Olympic Stadium was completed within budget and handed over three months early.

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