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Fabric roofing solution helps a grand race get grander

Exteriors, Features | September 1, 2007 | By:

BDP’s Aintree Racecourse redevelopment in Liverpool is a winner.

Aintree, Liverpool, is the spiritual home of the UK’s steeplechase community. The track is hallowed ground for many, not least because of its hosting every year of the Grand National, a challenging event known in racing circles and beyond as the most famous race in the world.

This April, the drama of the Grand National was played out to packed crowds lining the famous Aintree racecourse—and populating, for the first time since the completion of a major £34m redevelopment program, the iconic grandstand that sits at the heart of the trackside complex.

UK architect BDP designed two new grandstands for the world-famous racecourse as part of the overall redevelopment, which also included a parade ring, weighing room, equine areas, and a media suite. The new master plan for the site now places the stables, parade rings, and entrance to the course in a logical sequence, providing safety for horses, jockeys, and the public. Standing at the heart of the site, the two new grandstands provide cracking views over the Grand National start area and finishing straight for the crowds, as well as state-of-the-art hospitality accommodation. Each of the two main buildings is clad in a metal finish, an eye-catching architectural expression that stamps Aintree Racecourse as a world-class center for steeple chase events.

The two new stands are joined by a six-story link building. Originally, BDP thought to cap the roof in metal, to provide a continuing link between the two main structures on either side. As thinking evolved, the expression of the flat roof took on a more three dimensional form, eventually arriving at a three cornered roof expressed as a continuous wave form with a continuous edge beam. It soon became apparent that supporting this form elegantly was not realistic, and a rapid rethink was required.

By this time, the substructure was on site and coming out of the ground rapidly. UK specialist design firm Fabric Architecture was brought on board to work alongside the BDP team to arrive at a more realistic, elegant fabric roofing solution to the area that was named, appropriately enough, the Saddle Bar.

Fabric Architecture (not connected with this publication) specializes in bespoke tensile fabric structures, and with 25 years of experience behind it, was able to immediately bring new light to bear on the project, suggesting a move from a parabolic to a conical form.

The new solution saw a roof created where a bi-pod perimeter mast system works in tension to pull the fourth-floor pre-tension slab of the link building into compression. “Add three supporting masts, pulling the second floor slab into tension, and the math started to work,” designer Nigel Browne says. “Fabric can achieve far greater spans than conventional roof materials, with minimal supporting structure. For the roof to the Saddle Bar on the link building, a fabric structure was the obvious solution.

“The remaining engineering problem was how to get the edge gutters linking both grandstands to work,” Browne says. “Our calculations showed that the tensile membrane roof was exerting massive asymmetric loads that neither building had been designed to absorb.”

The solution lay with the introduction of an innovative double hinge to each end of the 1.5 tonne gutters, with the other end of each gutter anchored onto a floating bi-pod. This solution effectively dampened the loads transferring from the tensile structure to the grandstand buildings, allowing up to 120mm of movement from the tensile fabric roof anticipated for extreme weather conditions.

One final area that needed to be engineered into the equation was water management. With its location on the northwest coast adjacent to the Irish Sea, Aintree (and Liverpool) receive a generous quota of rain in any one year, in amounts significant enough to become a serious issue when designing a fabric roof to such a scale. Gutter dams were incorporated into the design to direct the rainwater into two gutters, taking it off the canopy at agreed points that would avoid drenching the horses and the public below.

Selecting Type 3 (1,500g/m2) architectural PVC-coated polyester with a flurotop laquer as the most appropriate architectural fabric for the roof (as opposed to ETFE or even a silicone/fiberglass solution), Fabric Architecture checked all batches of the material extensively to ensure a perfect fit when the pieces were cut to pattern and welded together. The lifting time—the middle of a gloomy UK winter—was not exactly brilliant, with potential delays from inclement weather presenting a problem.

To ensure that the massive 600m2 doubly curved fabric structure could be lifted safely under potentially poor conditions, a bespoke lifting system was designed that earned the nickname “the Christmas trees.” The assembly bolted onto the top of the three pin-mounted masts, which had previously been linked together and stabilized through a cobweb of cables and winches.

Before the masts were erected, the fabric for the canopy had been laid out on the fourth floor. This meant that once the masts had been installed above, the “Christmas tree” could be used to winch the material up into an intermediate position. As the masts had been designed as a two-part telescopic system, they could then be deployed, jacking up the fabric under tension into its final graceful doubly curved form.

Helen Elias is Fabric Architecture’s contributing editor based in the UK. Her piece on an outdoor classroom appeared in the July/August issue.

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