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Manchester's record taker

Structures - Tall buildings

Construction of the UK's tallest residential tower has just begun in Manchester.

Bernadette Redfern reports.

As an address, Deansgate is probably the most desirable in Manchester. Which is why developer The Beetham Organisation was keen to build there - and to maximise return on its investment by building tall.

Beetham has already sold the 22 floors of apartments in its 171m high, £150M sky scraper, and the Hilton hotel group is setting up home in the posh new landmark too.

The building will soar 43m higher than the country's current tallest residential building, the 128m Barbican tower cluster in central London. It will overshadow its nearest rival in Manchester, the CIS office building, by a good 50m. It is not only its stature that makes the new structure remarkable, though, but its slender, efficient design. 'It has an aspect to height ratio of 1:11, which is very thin, ' boasts designer WSP project director Kameron Moazami.

The tower's slenderness is a direct result of its function.

'Offices tend to be inefficient because property agents require so much space on each floor. An office block has to have around 20,000 square feet (7,000m 2) per floor whereas here the floor plates are 8,000 square feet (3,000m 2), ' says architect Ian Simpson of Ian Simpson Architects. Buildings used for living rather than working in need less maximum letable floor area than attractive spaces with views and light.

Starting at foundation level, the entire structure will sit on a 3m deep concrete raft.

Above this, at ground level, the first three of the tower's 48 storeys form a podium accommodating shops and restaurants open to the public. The tower itself is set back on the podium, putting a bit of distance between itself and the main road.

The structure consists of twin concrete shear cores measuring 8m by 9m and standing 9m apart, with shear walls running from front to back. These walls, supplemented by reinforced concrete columns around the perimeter, will support glass curtain walling.

At hotel level, from the fourth to 22nd floors, the rectangular tower floor plate measures approximately 16m by 40m. At the 23rd floor, however, the north facing facade cantilevers out by 4m. 'This is to break up the facade and introduces clarity to mark the transition from hotel accommodation to residential properties, ' says Simpson.

Initially it was thought that the towers' shear walls could support the eccentric load from the cantilever. However, this would have made them unacceptably thick. WSP instead decided to run columns up from the shear wall at floor 22, gradually stepping them outwards so that by the 28th floor there is a true cantilever of only 2m.

To counter the remaining rotational effect of the cantilever the two shear cores are positioned nearer to this side of the building. Meanwhile, as they rise through the structure and loads decrease, the shear walls will reduce in thickness from 500mm at the base to 300mm at the top. Concrete strength will also diminish from C60 at the base, through C50 to C40.

Only one of the cores contains lifts, stairs and services. The other provides a purely structural function and the space within will be used as part of the floor plan of the hotel rooms and apartments. 'A residential building has less people going in and out and is used more sporadically so we can use a smaller core, ' says WSP Fire Engineering technical director Simon Lay.

Even so, use of a single core would not have been possible had the fire engineering team not designed the ventilation system so that each apartment or room will act as a separate 'fire box'.

In a fire smoke would flush out of the area quickly and there is an unusually low risk of fire spreading, claims Lay.

The structure will be crowned with a 22m tall glass blade, bringing the tower's overall height to 171m. To resist lateral wind loading the blade's concrete base will be bolted fast to a reinforced concrete frame anchored into the building at the 47th storey.

In such a slender structure wind-induced oscillation was a concern. To ensure that movement would not upset the tower's residents, the team built a 1:400 scale model and tested it in a wind tunnel. Acceleration of the tower was found to be between 0.010m/s 2and 0.015m/s 2- well within acceptable range.

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