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Downhill all the way

Structures Dubai mall

One of the world's hottest cities will have the coolest attraction. Report and pictures by Adrian Greeman.

In the steaming, 50degreesC August heat of the Arabian Gulf, when humidity is 100%, nothing could be more inviting than to immerse yourself in ice.

That, anyway, is the hope of property client Majid Al Futtaim Investments which is building a giant 'snowdome' as the central feature of its new retail mall complex just outside central Dubai. The artificial ski slope will be the first in the region and should be a big attraction.

'Shopping malls need something extra, ' says Nigel Rutylo from consultant Mott MacDonald, structural engineer for the overall Mall of the Emirates project (see box), 'and these days restaurants are not enough. Malls are becoming like theme parks and clients demand something fresh.'

For this mall freshness is literally the answer. Consultant Hyder Middle East is collaborating on the project. Via its one time sister company Acer Snowmec, Hyder has built up experience in artificial skiing centres in places like Tokyo and Madrid. It is designing and supervising construction of the snow run section of the project.

The Snow Centre will be one of the largest so far, running for 400m, says Lewis O'Brien, Hyder's project manager. With an average slope of 15infinity, it will be 62m high. The overall structure will top 80m.

'That is equivalent to a 20 storey building so it will be a dominating feature on the local skyline, ' he says.

The structure is essentially a giant cold-store, consisting of an insulated box held inside an outer cladding. The metalroofed structure is clad with GRP contoured into a curved wing shape. Services are housed in the void between inner structure and outer skin.

The inner box, which has 125mm thick PIR insulation panels, is kept at around -2infinityC most of the time for skiing, but can be chilled to -6degreesC for night-time snow making.

It uses special air coolers mounted in the ceiling of the ski-slope and freezer guns which spray a fine mist.

'It makes a perfect skiing snow, ' O'Brien claims.

As freezers go the snowdrome is large. 'We have spans of up to 80m for some of the steel trusses across the main box, ' says O'Brien.

The box houses two ski ramp sections articulated like an elbow at mid-point. There is an intermediate station at the elbow where skiers can stop, and where the ski run splits to provide a fast lane for the skilful on the inside and a slower course outside.

'The highest part is essentially a steel bridge about 100m long and 55m wide supported on two large concrete towers, ' says O'Brien. Two huge trusses form the outside edges of the ski slope, with the composite steel and concrete decking slab floor of the slope supported on transverse 3-D tubular steel trusses.

At the mid-point station the ski course widens out to 80m, creating room for nursery and play slopes and snow boarding.

'There will be a 9m high double-glazed wall, giving views in from the mall' says O'Brien. Some of the hotel rooms, which is part of the overall development, will also have views onto the slopes.

At present the project is at foundation stage with main contractor Al-Naboudah/LaingO'Rourke installing a variety of bored piles between 600mm and 1m in diameter to about 18m depth. The largest are below the 7m 2section pylons which support the top part of the ski-run. Each has 32 piles.

Some concrete columns have begun and steel is in fabrication by Eversendai.

'One of the biggest challenges is three-dimensional contouring of the slopes which are deliberately complex to give skiing interest, ' says O'Brien.

Motts innovative mall functions

For the main mall, UK consultant Mott MacDonald has been working in close collaboration with local architect Holford Associates on structural design and implementation of an overall concept design from US architect F+A, and with UK consultant WSP for all mechanical and electrical services. Project manager is Mace.

The $445M project being built by main contractor Khansaheb includes a lot more than the ski slope; there is a 20-floor luxury hotel with 400 rooms, and three levels of shopping arcades with Italianate and Arabic themes. Multi storey car parking for up to 7,000 cars contribute to a gross built up area of over 600,000m 2.There are also outdoor performance areas, water features, a multi-screen cinema, and indoor theatre and entertainment areas for children and teenagers.

On completion it is expected to be the largest mall outside North America.

A number of innovations by Mott MacDonald include a switch to unusual reinforcement-free mass concrete foundations.

'We would have needed several thousand piles for a conventional foundation' explains Rutlyo, 'and that could have meant delays waiting for subcontractors and rigs in this exceptionally busy construction environment.' A few piles have been used at one end of the site where ground conditions were problematic and some reinforced footings remain for shear walls and lift shafts.

Avoiding delay is vital on a job with a construction programme of just 23 months from the start up October last year. The client wants completion before the Ramadan period 2005. 'It is the most important holiday period in the region with a peak in visitor numbers, ' says Rutlyo.

But the system has other advantages. Dubai suffers from very aggressive groundwater with high levels of sulphate and chloride in the calciferous sandstones and compacted sand layers. On most jobs membranes or waterproof dense concrete must be used to prevent attack of reinforcement.

An alternative is simply to take out the steel, says Rutlyo. It means using much more concrete but locally made, quality ready-mix is cheap.

'You save on the complexities of waterproofing, but also on steel fixing and bending and even some complex design.' The soaring price of imported steel, up over 50% in recent months, has been an additional cost bonus.

Footings are sized using a simple 45degrees load spread which produces minimal tension at the base and hence no need for rebar, he says. Some of the larger bases (up to 5.8m square and 2.8m deep) could be considered to be 'mass pours' and might cause heat problems.

So, in addition to the usual cooling measures such as ice in the mix, cement replacement and night pouring, the aggregate size has been increased to 40mm.

Even with such measures, core temperatures in the concrete can reach 65degreesC. As winter night temperatures drop to 13degreesC, insulation has been needed to reduce the temperature gradient and prevent significant cracking.

'Polystyrene mats sometimes got dislodged so we switched to a 300mm sand layer which worked well, ' says Rutlyo.

To judge when to remove it test foundations were made with embedded thermo-couples.

Other complexities for Mott MacDonald have included design of some long span steel bridges inside the three level airconditioned precincts. These have to be exceptionally stiff to avoid unsettling low frequency vibrations - 'people in a high finish interior environment may be more sensitive than usual', says Rutlyo.

The job is not made easier by the fact that escalators from above and to the floor below, have to spring off the 24m span bridges.

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