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VIEWERS STRUGGLE TO WORK OUT HOW THE STRUCTURE IS PROPPED ON 12 COLUMNS THAT LOOK AS THOUGH THEY HAVE BEEN KNOCKED ASKEW

METALWORKS

Alsop Design's current contribution to London's Bankside Quarter is Palestra, a £68 million, 12-storey speculative office development in Southwark, already becoming a local landmark. As it rises out of the ground, the building is challenging passing commuters and tourists en route to Tate Modern, who struggle to work out how such a large structure can be propped on 12 columns that look as though they have been knocked askew during construction and then left to stand.

Palestra sits directly opposite Southwark underground station. When fully let it will provide workspace for up to 3,000 people in its offices. The 38,000m 2 speculative development (see www. palestra-london. com) sits over the quirkily angled perimeter columns, which are described by Will Alsop as 'dancing'. The office space appears as a raised volume that has been divided into two stacked boxes.

The lower box rises vertically nine storeys in height.

Although the floors remain horizontal within it, the clad volume is set at an angle of 2.6º to the horizontal. Glass curtain walling almost touches the ground at the east end of the site, slanting upwards to create a two-storey public space to the western end, propped along its length on the skew columns.

To achieve Alsop's ubiquitous raking columns from second-floor level down to the ground, engineer Buro Happold designed a fabricated steel transfer structure at second-floor level, which allowed variations on the maximum possible raking angle for each column.

The mass of the top three stories appears disjointed and separate from the rest of the building. Planning constraints requiring the building not to overshadow residential properties in the neighbourhood led Alsop to shunt the top section, already overhanging the lower box by 1.5m, one grid square along as well. The end result is the huge upper glass box that oversails the building below on two sides by 1.5m, with a massive 9m cantilever at the western end.

The upper box is separated from the lower one by another sequence of dancing columns, which also acts to separate the two entirely different grids of their respective steel frames. The lower, larger box is constructed to a 10 x 7.5m grid; the upper box switches to a more spacious 12 x 7.5m grid. This design results in none of the perimeter columns in the smaller top box structure sharing any position at all with the columns in the lower mass of the building.

The lower box has to be able to absorb the significant overturning force of the cantilevered three-storey box under which it sits. Forces as high as 20 times the average wind load have been designed for, taken through into the stair and lift cores, all constructed using robust but conventional steel K-braced frames for maximum stabilisation. The cantilevering section of floors 10 to 12 is formed from substantial plate girders, all tied back to the main structure and cores.

Work began on site in January 2004, with the majority of the steel frame being completed by March 2005. The steel frame superstructure comprises composite 508mm circular hollow section columns supporting twin primary cellular beams, which in turn support secondary beams carrying permanent metal decking filled with a 140mm thick in-situ concrete slab on metal decking.

The hollow-section columns are filled with reinforced concrete, which, as well as enhancing structural performance, also increases fire resistance by acting as a heat sink that would conduct heat away from the frame during a fire. The result is a column that requires no external fire-protection measures, even up to a rating of two hours, and only needs a decorative paint finish applied on site. The composite steel columns are structurally very efficient, despite being light, and are therefore easy to transport to site and to fit into place, with each column splice needing just four bolts.

Casting the flange of the cellular beams into the slab gave the steel greater load-carrying capacity within the same structural depth, but without having to resort to using shear studs. The composite floor also reduced the ratio of the heated area of the steel-to-beam section area in the event of fire, requiring only a single coat of intumescent paint instead of the two that a conventional floor, with concrete slabs resting on top of beam flanges, would have commanded.

Palestra is due to be completed in June 2006. Alsop's previous iconic building for south London - Peckham Library - is now a named destination on bus routes. Will this new landmark structure achieve similar celebrity?

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