Winds from Storms Eunice and Franklin have taken their toll on infrastructure around the UK in the last week, but modern construction projects showed the value of clever engineering – with Tottenham Hotspur’s relatively-new £1bn stadium proving why it's widely regarded as the cream of the crop.

Videos emerged of the stadium’s roof billowing up and down in the gale force winds, which is to be expected on a stadium that stands 48m high. In the past this might have been a cause for concern, but would have given the designers and engineers behind it great satisfaction.

The roof of the @SpursOfficial football ground is currently pulsating up and down in the wind!

(Turn your phone landscape and look at the suspended lighting rigs)#StormEunice pic.twitter.com/wzXTpzH4K3

The Tottenham Hotspur Stadium was designed by Populous Architects, had structural engineering from Buro Happold, and was constructed by Mace. But it was German specialists Schlaich Bergermann Partner who can claim most responsibility for the roof. The firm has also lent its expertise to other stadia including the Super Bowl-hosting SoFi in Los Angeles and the curve-roofed aquatics arena in Paris for next year’s Olympics.

Populous was behind the London Olympic Stadium (now the London Stadium), which was the first cable net roof in the UK, and Tottenham Hotspur’s became the second. Initially it was intended to be a steel truss roof, but plans were switched to the modern style to save on weight (about 600t) and for more elegance.

The roof of the Tottenham Hotspur Stadium is based on a spoke and wheel system, designed with form-finder software to fit the irregular geometry. It is supported by a 700m compression ring that sits at the back of the stand connected to two inner compression rings – one higher and one lower. The outer ring is connected to the inner by upper and lower radial cables, supporting them. The outer compression ring is connected to flying columns by tension cables that vary between 12m and 15m.

The bespoke movement joints between segments allow for up to 50mm of movement between them. Concertina type gaskets are between the segments and are spaced to align with the radial roof ties.

The flexible cable roof is connected to the stands by a girder grid system with more than 600 bespoke sliding bearings. That is to say, it more or less floats up and down on this complex cable structure.

It is designed this way to take the force of thousands of football fans simultaneously jumping in the air when the likes of Harry Kane find the back of the net.

Schlaich Bergermann Partner partner Knut Göppert explained: “The roof is based on the spoked wheel principle. Basically, at the outer edge of the roof a compression member is located. This is similar to the rim of a bicycle wheel. The cables of the roof are prestressed as the spokes of the bicycle wheel and spread apart at the inner edge of the roof. The inner roof edge flying masts, spreading the pairs of cables up and down, function as the central hub does in a bicycle wheel arrangement. This structure is the most light weight structure possible. It not only makes use of this superb principle, but also of the high material strength of the cables (three times stronger than structural steel).

“Structures with optimised weight have a low carbon footprint and use an energy efficient erection method. Most structural elements act in tension and are not subject to buckling. One of the specifics of this arrangement is that is shows higher deflections compared to totally rigid structures. The structural flexibility is an advantage since it helps carrying the loads with minimal internal stresses. Therefore, movements of prestressed cable structures are system immanent. To allow these movements, it is essential to design all details for the expected movements without adding any restrain forces to the structural members.

“With this in mind and immediate inspections carried out on Saturday, the behaviour of the roof was as expected and all the detailing worked as planned.

“It should be noted that on such structures and shapes wind is the dominating load. The wind load to be considered for the design was derived by wind tunnel testing. In a storm, wind suction is the dominating load direction, such as it act on an aeroplane wing. Nevertheless, as we all experienced in the last storm, wind is never constant, but dynamic and fluctuating. As a result, the roof can be moved up by wind suction, but it moves down when the speed decreases or when turbulences result in wind load acting downward. This phenomenon was observed during a period of very high wind forces last Friday.”

It took two weeks to lift the structure into place after extensive preparation and laying out of the galvanized steel cabling. Six cables supported the lower ring and four on the upper as it was moved into place.

Populous senior principal Tom Jones said: “When designing large-span roof and façade structures for major buildings like stadiums, it is essential that the Architect designs in appropriate tolerances and movement joints for the predicted movements that the building might experience during its lifetime.

“In the case of the Tottenham Hotspur Stadium, the use of a cassette cladding system in the roof, sitting between each of the primary roof cables, is one design solution that allows the roof to move under wind load, without damaging the overall enclosure to the stadium. Incorporating flexibility to deal with extreme weather conditions, for example strong wind, or heavy snow and rainfall, is a vital part of the design process.”

WSP director and head of stadia Peter Chipchase told NCE: “Long span roof structures like this are naturally flexible and as such will move more than a ‘conventional’ structure both under quasi-static loads, and in response to the wind. This is predicted and quantified as the structure is designed, and accommodated by the structure itself, and those elements fixed to it such as the cladding.”

Commenting on the videos, Aecom buildings and places director Jon Leach told NCE: “This type of roof structure is inherently quite flexible, and under the extreme wind loads experienced during the recent storms this magnitude of movement might not be unexpected.

“If so, it shows the importance of allowing for such movements in the envelope and interfaces of such large structures.”

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Tagged with: Buro Happold Mace Populous Schlaich Bergermann Partner

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