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CONTRARY TO POPULAR OPINION, POST-TENSIONED SLABS ARE NO MORE DIFFICULT TO ALTER OR DEMOLISH THAN OTHER STRUCTURAL FORMS

CONCRETE QUARTERLY/ AUTUMN 05

Concrete can offer real benefits to the developer of high-rise residential towers and also to the high-rise homeowner. High-rise residential towers are an increasingly common feature of many UK cities. Research undertaken by property consultant Savills shows that there are currently 87 residential towers of more than 20 storeys under construction or being designed.

This new-found popularity is due to factors such as the raised profile of city living at a time of increased emphasis on urban regeneration and high-density residential development. There is also the kudos that towers can give cities. Many of them are striking in design with a symbolic role for urban renewal and for civic pride.

The challenge and costs involved in the construction of residential towers, coupled with the fact that most are of individual design, has resulted in developers and contractors closely examining different methods of construction. Concrete is found to offer many benefits, and typically to be the best option. For example, the inherent robustness, fire resistance and sound insulation of concrete mean there is no need for the extra costs of additional fire proofi ng and sound insulation. Another considerable cost advantage is the fact that concrete construction can provide more floors within a given height restriction, particularly through the use of posttensioned suspended concrete floors. These can be some 300mm thinner than alternative methods of construction. Further advantages are that flat slabs are fully able to accommodate irregular grids, that they allow rapid construction, and that the clear, flat soffits enable complete service layout flexibility.

Residential buildings have no need for deep service zones beneath floors so fl at soffits are ideal.

Post-tensioned flat slab concrete construction has been used for the 40-storey Holloway Circus tower in Birmingham by Ian Simpson Architects. Here, the post-tensioned floor slab is just 225mm thick and spans up to 10m. Post-tensioned slab construction has also been used for the 48-storey Beetham Tower in Manchester (Ian Simpson Architects) and the 30-storey Bridgewater Place in Leeds (Abbey Holford Rowe). Concrete was chosen for its overall cost-effectiveness and construction efficiency.

An important contributor to this was the provision of a safe working environment at minimum cost and time, and formwork systems which incorporated safety features. For example, slab-edge protectors are increasingly becoming the norm.

A further benefit is the increased net-to-gross floorspace ratio. For the Holloway Circus development, the cores are located at the rear of the tower and are coupled together with a shear wall.

This takes the lateral loads while concrete columns around the perimeter help to take the gravity loads. By optimising the thickness of the concrete walls and the number of columns, together with lift services and stairs layout, a net-to-gross ratio of 80 per cent was achieved.

POST-TENSIONED SLABS There are three main forms of post-tensioned construction: flat slab; band beams and slab; and ribbed slab. A flat slab, generally spanning 6-13m, provides the minimum construction depth and fastest construction. The slab's depth is controlled by deflection criteria and punching shear. Band beams and slab construction provides large clear spans with the beams spanning up to 20m and the slabs spanning 12m. Ribbed slab construction can span up to 15m and is more structurally efficient than the other two options, but its complex formwork means that this method can be less cost-effective.

Contrary to popular opinion, post-tensioned slabs are no more difficult to alter or demolish than other structural forms.

As for any construction method, the Health and Safety file must contain all structural design and construction information to enable future alteration and demolition. With post-tensioning, small holes can generally be accommodated between tendons, and larger holes can be accommodated away from the column strips, as with conventionally reinforced slabs.

SOUND AND FIRE The benefits of concrete construction for residential towers are not limited to developers and contractors. For the homeowner it also offers particular advantages.

One of the main problems of high-density living in residential towers can be noise from neighbouring apartments.

Here, the heavyweight concrete provides high levels of sound insulation that fully meet the new standards for reduction of sound transmission as specified by the revised Part E of the Building Regulations. New separating/party wall minimum values for airborne sound insulation are 45 dB for purpose-built dwellings and 40 Rw dB for relevant internal partitions within all house types. The Robust Standard Details developed for concrete blockwork walls are designed to exceed these levels and so ensure compliance with Building Regulations while avoiding pre-completion testing of dwellings. A similar set of Robust Details is available for separating floors for both airborne and impact sound requirements.

A further benefit of concrete for residential towers is its inherent fire resistance. A recent fire in a tower in Buckland, Portsmouth, was contained to just one flat. The residents of adjoining properties were able to return to their homes soon after the fire was extinguished. Confirming the fire-resistance properties of concrete construction, the Incident Commander, Assistant Divisional Officer Mark Wittcomb of the Portsmouth Fire Service, stated: 'the concrete construction of the tower block provided considerable fire proofing which aided us in containing the fire to the one flat'.

With so many high-rise projects currently under development or on the drawing board in the UK, concrete construction has much to offer high-rise building, particularly through employing post-tensioned suspended slabs. For the developer and contractor, concrete offers cost-effective and efficient construction. For the homeowner a quiet and secure living space.

Andrew Minson is head of framed buildings with The Concrete Centre

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