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Room for a view

A new £9M grandstand helped Newbury racegoers get a better view of last Saturday's Hennessy Gold Cup. NCE focuses on the unusual structural system at its heart.

Most grandstand roofs are basically cantilever structures.

For the new Tattersall stand at Newbury architect Foster & Partners went for something different, a 'clear, bold, expressive' X-frame system with the lightweight steel profile roof spanning 36m between the tips of the X itself.

At an early stage the temptation to turn the roof into a viewing area had to be resisted: however good the view of the racecourse might have been the extra cost of the much heavier roof structure could not be justified.

Below the roof, the composite concrete floors and 'steppings' - or terraces - span an unobstructed 12m between X-frames.

Service cores are attached at the rear. Structural engineer Whitby Bird & Partners associate director Charlie Benson says the Xframe design has many advantages, as well as a few unusual challenges.

'Connecting the horizontal, vertical and diagonal elements of the frame at discrete node points meant that forces were mainly axial and bending stresses were minimised. But the layout created significant out of balance forces that had to be stabilised.'

These out of balance forces come mainly from concentrated loading at the outer edge of the top floor restaurant, with its large balcony. Resistance to the tendency for these forces to cause the upper section of the Xframe to pivot around the central node comes from an exposed vertical tie between the rear upper and lower tips of the Xframe and a stub connection to the adjacent column at the rear of the restaurant.

A special touch of elegance is added by the three stage 'taper' of the main tubular X-frame members, from 660mm diameter through 508mm to only 273mm.

At foundation level the compressive forces in the members resolve into an outward horizontal force of 600t - which would have needed unfeasibly large foundations.

'We therefore decided to use a 356mm by 406mm UC to tie the legs of the X-frame together, ' Benson explains. 'This is cast into the ground floor slab, and transforms the X-frame into a discrete element. Only vertical and lateral wind loads are transmitted to the piled foundations.'

Unlike, say, football fans, racegoers move around a lot between races. This imposes unusual dynamic loads on what is essentially a lightweight structure. The X-frames, says Benson, have interesting dynamic characteristics, in that vertical deformation tends to produce sway.

Luckily, a finite element analysis showed that even with more than 5,000 spectators jumping up and down in unison there was no real risk of excessive movement.

A fast-build 12 month construction management contract was the chosen method of procurement. Heery International took on responsibility for managing the project, with Watson Steel winning the £1.3M contract to fabricate and erect the steelwork. Bachy was the concrete contractor.

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