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First past the post

Aintree - New grandstands will give race goers stunning views of the Grand National, Jackie Whitelaw discovers.

If you are thinking of going to the Grand National next April, book yourself a place in one of the twin grandstands being built at Aintree by the design and build team of BDP and Laing O'Rourke.

Not only will you get fantastic views over the racecourse at the front but exclusive views over the parade ring at the back.

And you can watch the horses sashay under the Saddle Bar connecting the stands out on to the course for the start of the world's greatest steeplechase.

The stands are being built as Phase 2 of a £27M project to restructure the racecourse.

Client Aintree Racecourse didn't want to take the Ascot option and shift its star race to another venue, so the work has been split over two seasons. Phase 1 included the new parade ring, jockey facilities, stables, media area, infrastructure and piling.

Work stopped for the big event this April but the team was back on site immediately afterwards to get the steel and precast concrete grandstands, ready for next year's race.

The stands have six oors on the parade ring side and three cantilevered levels of raking seating at the front. 'It was a bit of a complex 3D problem to sort out, ' says BDP Engineering structural and civil engineering director Steve Gregson.

'The important thing is that the grandstands don't have vomitories to get people in and out. Instead access is from the inside at the back and via external staircases. It makes circulation easier, the structure simpler and allowed us to keep the size of the grandstands down.'

BDP project structural engineer Phil Simcock:

'The twin grandstands are steel frames with metal deck/concrete oors, measuring 37m wide, 49m long and 25m high. Limiting the horizontal natural frequencies of the stands was achieved using internal steel bracing, an internal lift shaft and a connection to the reinforced concrete lift/toilet cores on either side of the saddlebar.

'The cantilever roof and rear raking columns generate large horizontal forces on the frame that require carefully positioned bracing. The grandstands' curved roofs were modelled on a jockey's cap. However, to cut costs, this has been simplied. Even so, the nal curved roof and curved transition to the rear elevation generates convex curvature in two directions.

'The perimeter frames of the stands are also splayed, which generates complex geometry only resolved using a 3D model to position key points on the frames. The bottom chords of the cantilever plate girders are restrained by the cruciform stiffeners acting in bending, working with the RHS (rectangular hollow sections) purlins above. Steel fabricator Watson was heavily involved in the development of the design.'

Laing O'Rourke project manager Craig Abson:

'The site is restricted at both front and rear. Faced with this dilemma, and the need to manoeuvre 38t sections of precast concrete into a building with an overall height of 25m, it was decided to erect the superstructure by building from back to front and from top down, from within the footprint of the building.

'Overall stability of the steel frame relies on the lower terrace raking beams to be xed and grouted because the incline of the rear elevation and precast units impose a horizontal thrust that has to be catered for.

'The sequence adopted was steelwork to level six, precast concrete to level ve, steelwork to level four, precast concrete to level three, full roof erection, steelwork to ground level, precast concrete to ground level, release of temporary props.'

BDP structural and civil engineering director Steve Gregson:

'At the end of the two upper terraces, the oor of each grandstand cantilevers 3.2m from the steel frame.

Tight headroom beneath and restrictions on the oor levels above means that only a 480mm thick structural zone is available to achieve the cantilever. Using steel cantilever beams with precast units above was not possible within the available depth.

'The cantilever also had to be achieved using precast units rather than insitu concrete because of the short construction timescale. BDP worked closely with Malling PreCast to design precast concrete units that provide both the cantilever and span laterally the 11m to the main frame columns.

'The z-shaped units have torsion, bending and shear forces, but their shape suits these forces. Polystyrene formers were used in the cantilever elements to reduce weight, yet Malling believes that at 38t they are still the largest precast units it has produced.

'The connection of these specialist precast elements to the steel frame is clearly crucial. Very small rotations of the supports would generate large deections at the end of the cantilever.

'Each support is tied back horizontally with two 40mm diameter post tensioned Macalloy bars, while vertical support is on a steel corbel that cantilevers from the main column, within the depth of the precast unit.'

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