Strata, London

At 43 storeys and £147.9 million, Strata SE1 was the first significant development of the £1.5 billion Elephant and Castle regeneration. The structural design of the building was fast, efficient and economic to construct and features 200mm post-tensioned concrete slabs with a compact rectangular core. High-strength external blade columns align with the partitions and there are only two internal columns. At the top and bottom of the tower the columns step in. This was achieved by ‘walking columns’ - stepping the columns gradually to remove the need for deep transfer structures. The slabs used C32/40 concrete whilst the columns used C50/60 concrete, so care was taken in the design of the slab/column junction. The foundations included 37 large-diameter bored piles with a maximum length of 44 metres founded in the Thanet sands.

An important feature of the tower is its sustainability and the building is targeting an EcoHomes assessment rating of “Excellent”. Air permeability is less than half that required in current Building Regulations, a vertical district heating system negates the need for boilers to each apartment and the ventilation strategy includes heat recovery to every one of the 430 apartments and with openable panels hidden in the facade for natural ventilation. However, it is the three wind turbines at the very top of the building that grab the headlines.

Three five-bladed 9m diameter 19-kW wind turbines provide a visible commitment to sustainable design. Located to take advantage of the area’s 35mph wind speed it is anticipated that the turbines will generate 50MWh of electricity per year equivalent to eight per cent of Strata’s estimated total annual electricity consumption. It would need a very large array of photovoltaics in order to generate the same amount of energy. Each turbine is sited on a five tonne inertia base made from concrete and steel and supported on anti-vibration mounts to prevent vibration being transferred to the building’s structure. The use of five blades rather than the usual three reduces operational noise.

Strata is not the only building that uses wind power to generate its own electricity. The Bahrain World Trade Centre has wind turbines placed between its two towers and the Lighthouse Tower in Paris’ La Defense district has wind turbines placed at the top. Meanwhile, in Miami, the Cor building features wind towers as an integral part of the exoskeleton concrete shell.

Further information: Project Review by Mark Walton, WSP Group, Issue 88, The Structural Engineer, July 2010