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Tower topper

Structures - A new Merseyside landmark is gradually being unveiled. Dave Parker reports from Liverpool.

Those lucky enough to move into the distinctive penthouse 'pod' that tops Liverpool's 27 storey Unity residential tower sometime later this year will find themselves practically eyeball to eyeball with the world famous Liver Birds.

The two 5.5m high copper statues, a German sculptor's rendering of the mythical cross between an eagle and a cormorant that has featured on the seal of the city since medieval times, sit on top of the 90m high Royal Liver Building.

When this opened in 1911 it was dubbed Britain's first skyscraper, one of the earliest multi-storey buildings to use a modern style steel and concrete structural frame.

Steel and concrete are also combined imaginatively in the twin towers of the £60M Unity project.

Structural concrete is the dominant solution on the residential tower and the 16 storey ofce tower alongside, but the two storey pod uses steel frame construction. This was chosen because of its lighter weight and potential for prefabrication, according to structural engineer Faber Maunsell project manager Neil Donaldson.

Constructing the cantilevered 650mm thick insitu concrete floor slab that forms the base of the pod was the biggest challenge, however.

'There were serious buildability and health and safety issues to be addressed, ' says Donaldson.

'The slab cantilevers out up to 4m, so major temporary works were needed. We involved the Health & Safety Executive and subcontractor Efco as early as possible, and came up with a solution that worked very well in practice.' Main contractor Laing O'Rourke took only four weeks to get the oor ready to take its first pour of C40/50 concrete, using a grid of built up plate girders to support the formwork. (see box). By then several other buildability issues had been resolved, not least the lack of space on the congested site.

'The ideal solution would have been to prefabricate as much as possible at ground level and lift it up into place in large sections.

But there was simply no room for this, so we had to assemble it piece by piece insitu.' Offsite prefabrication turned out to be the right solution for the 400m 2 roof, however. A last minute approach to Corus Building Systems led to the swift development of a fully prefabricated version of Corus' Hi-Point panel system.

Below the pod, insitu concrete is the material of choice, with C40/50 used for all vertical pours to make the load bearing elements as slim as possible.

Architect Alllford Hall Monaghan Morris designed the tower as a series of interlocking Lshaped two storey apartments; Laing O'Rourke responded to the challenge by opting for tunnel form construction with a jumpformed service core.

This system facilitated production rates as high as one oor per week, aided by extensive reinforcement prefabrication.

Service space was maximised by the at softs produced, and the structural rigidity of the insitu frame minimised the size of the core needed for lateral stability. And, says Donaldson, the superior acoustic performance of insitu concrete was also very desirable on a residential block.

'We did consider both steel and precast concrete for the 16 storey ofce block, ' he adds. 'But Laing O'Rourke reckoned it could achieve the same overall speed of construction using insitu, so that's what we went with.' A slipformed core and post tensioned at slabs with slim insitu columns created an efcient structure. Transfer structures carry loads from both towers over the two stories of basement car park.

Foundation design was complicated by the close proximity of a branch of the Mersey Tunnel, which at one corner is within 4m of the building's footprint. Faber Maunsell's solution was a deep contiguous piled wall.

Currently the focus is on completing the colourful cladding.

Final dismantling of the pod temporary works is scheduled for next month. The end result should be a distinctive addition to Liverpool's skyline, albeit still not quite as distinctive as the Liver Birds.

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