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The client wanted a signature building for a speculativeoffice price. To achieve this, a conventional steel frame was used, with concrete slabs on metal decking to meet the unconventional demands of the architecture. Buro Happold introduced a number of key differences to the structure, allowing it to be built using normal methods but creating a technically different fi nished product.

Examples are the raking columns, column construction, the large cantilever and twin-beam floor construction.

This was accomplished while building directly above the Jubilee Line extension and accommodating existing piled foundations from the previous building. Palestra has raking columns at two levels which exert large horizontal forces on the building. A column set at an angle will exert a percentage of the vertical force it carries as a horizontal force, in direct relation to the column angle.

The large cantilever also exerts a large horizontal force.

We wrote a computer program that allowed Alsop to play with the column angles in all locations, yet know that the building still worked in terms of foundations and column sizes. The design team could adjust the complex column layout until late in the construction programme with confidence that the structure would work. The column construction is very high strength and requires no secondary fire protection. This was achieved using composite columns designed to European Codes. A circular hollow section (CHS) outer tube was reinforced by an internal CHS and both tubes filled with concrete to form a solid section. Under normal use the column works as a steel tube with reinforced concrete internally. Under fi re conditions the outer tube is sacrificed and the column works as a reinforced-concrete column. At the west end of the building, the upper box is offset by one grid width from the lower, creating a three-storey-deep 7.5m cantilever over Blackfriars Road. This impressive feature was achieved without the introduction of any visible diagonal elements in the facade, nor any disturbance to the internal floor space. Twin cellular beams span the standard grid of 12 x 7.5m. The beams pass in pairs either side of internal columns - more efficient than conventional steel beams which stop and start at columns. Palestra takes advantage of continuity, yielding beams up to 35 per cent lighter. Despite various cantilevers, transfer structures and raking columns, the structure was built in a 600mm zone within a 900mm overall structure, services and finishes zone. This is impressive given the geometry and the 12m clear spans.

It is feasible to expect the steel frame would be heavier than a conventional frame, but by utilising continuity and the strength of composite construction, the frame weight per square metre is the same as for a normal frame.

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