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Structures According to plan

New Concrete Engineering - Innovative design requires meticulous planning and innovative engineering solutions, as Christina Taylor finds out in Spain.

The picturesque Riojan village of Elciego - or Blindman - in northern Spain may live up to its name once the dazzling titanium waves of Frank Gehry's latest creation take their place on top of the £9M flagship hotel for the Marques de Riscal winery.

The four floor substructure covers a built area of around 2,000m 2and sits over a working wine cellar next to the winery itself.

But this is no ordinary hotel.

Gehry's now trademark flamboyant titanium curves cascade over one of the most geometrically complex structures an engineer is likely to come across.

Indeed, it is quicker to ask IDOM structural design engineer Miles Shephard which aspects of the building had not posed technical difficulties, rather than those that did.

The building comprises the basement wine cellar and four hotel floors plus mezzanine.

Three huge 'super columns' pass through the structurally crucial third floor slab up to the fourth floor. Four inclined prisms support the third floor longitudinal cantilevers. From here columns are irregularly distributed - some inclined and discontinuous - forming curved irregular internal walls. The three irregular shaped floor slabs decrease in size towards the top producing an asymmetrical pyramidal shape.

To complicate the load paths a further, 21 self supporting layered titanium covered canopies are attached to the edges of each slab. Each is unique in size, shape and position.

There are myriad engineering challenges. 'There isn't one thing that stands out as the worst - we have had to consider the building as a whole, ' Shephard says.

'The key to success was the extensive and rigorous planning and testing phase, ' adds IDOM structural engineer Miguel Angel Frias.

Testing was carried out on numerous scale models which were then digitised.

The irregularity of the canopies meant that models had to be tested in wind tunnels as wind codes could not be applied.

Structural modelling was carried out using wire frame models which were translated into the program RISA for the steel canopies and ANSYS for the concrete slabs to test different load cases.

'During the development phase, detailed construction drawings were taken from a CATIA master plan where all the outlines of the different layers - concrete, steel sections, cladding etc. - were plotted in 3D, ' explains Shephard. 'The contractor has the full CATIA file loaded onto a PC on site'.

The detailed preparation looks to have paid off, as Shephard remarks: 'The fact that the contractor didn't want to change any details during the construction process shows we got it right.'

Planning on the project began in 1999 but the first phase of works kicked off in May 2001 with construction of the basement wine store. Vineyards planted around the base of the building serve as camouflage.

A 1m deep layer of soil provides heat insulation to allow the cellar to function both during and after the works.

The main building completion date is next February with the extension due to be completed the in July.

The building's three 'super columns' are founded on large pad footings on rock 8m below the reception area. They then pass through the bottle store and second floor reception area up to the third floor slab at a height of 16.5m. From there, three concrete lift cores and a forest of small steel section columns (some inclined) support the fourth, mezzanine and fifth floors plus the roof.

Formed of 300mm thick walls, the 'super columns' are highly irregular, their positioning governed by the building's internal spaces. They are aligned along the central axis of the slab and form the only supports for the substantial third floor slab which has cantilevers of up to 10m in length.

Constructed of 800m 3of concrete and with a steel content of 3.5% by volume, the third floor slab supports the rest of the building above it.

The slab is of variable depth with a 1,300mm deep section running longitudinally. Transverse cantilevers thin to 450mm at the edges while longitudinal cantilevers have a depth of 750mm and are supported by four inclined columns (see box).

'As there are no movement joints in the slab, the main part was poured in three phases to reduce the shrinkage stresses to a minimum, ' explains Frias.

A week separated the first two pours of 290m 2and 260m 2respectively. About three weeks later a third 240m 2middle section was poured. Each pour was then covered in a 30mm layer of water for seven days during curing.

The concrete itself is a Grade HP35 or C40 concrete with maximum aggregate size of 20mm, plus anti-shrinkage agents and plasticisers.

To counteract high loading on the transverse cantilevers the slab was transversely post-tensioned using a grouted duct Tecpresa system with 12mm loose strand cables. Ducts generally followed the profile of the soffit, the changes in slab thickness providing the necessary eccentricity to the cables.

Tensioning was complicated by the zig-zag edges of the slab which have to support canopies with varying loads. To allow access to the cables, the irregular slab edge could not be cast until tensioning was completed.

After the third pour, 55% tensioning occurred - once the concrete had gained a strength of 30N/mm 2(standard cylindrical test - or about 35N/mm 2for a standard cube test). Once the fourth floor had been cast and formwork removed, the remaining 50% were tensioned.

I-section reinforcements were embedded into the slab edges to form connections for the canopy attachments. On the north side, where loads were lighter, the edging was poured on at the fourth pour stage with I sections attached after tensioning (see diagram).

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