Buro Happold Ltd
William Hare Ltd
Blackfriars Investment and Royal London Asset Management
Palestra is a state-of-the-art, 370,000 sq ft speculative office building comprising 12 floors and two basesments, situated on the corner of Blackfriars Road and Union Street, Southwark.
Ground level and above are formed with an innovative steel frame on a 12m x 87.5m grid, comprising of double beams and concrete filled columns, that offered all up steel weight for the typical levels – including columns of 58kg/sqm. However, what is of more architectural interest is the way the building is supported by “dancing columns” (inclined columns) at both the ground and 9th floor levels. Furthermore, on the Blackfriars Road frontage, the top three floors of the building cantilever out some 9m over the street pavement.
To accommodate the changing geometry imposed by the dancing columns and the various steps in the width of the building, whilst maintaining a total floor depth – including raised floor and lighting zone – of 900mm, an innovative solution combining double beams with composite columns was developed. The solution adopted not only simplified the installation of the building services by maximising the cells through the beams, but also maximised the efficiency of the beams by using them in double bending.
Twin cellular beams span 12m and are arranged in pairs that pass either side of internal columns and as such the beams’ design takes advantage of continuity. By utilising the sagging moment capacity of the beams past the columns, this yielded beams up to 35% lighter than otherwise would have been the case. In effect each beam is acting as a single section across the width of the building with simple web plate shear, or pin splices located at the natural points of beam contraflexure. Similarly, the beams are simply connected to the columns via web plates that pass through the columns, thus ensuring that no moments are transferred into the columns.
The column construction consists of an external steel CHS filled with concrete grout with a further smaller diameter CHS placed at the centre. The design produces very high strength columns, enabling the structural engineer to provide a slim column carrying the high axial loads and an architecturally pleasing solution. The columns require no secondary fire protection even for the 120 minute fire period required.
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 façade or any disturbance to the internal floor space.
This headquarters building, on a strategic site south of the Thames, exemplifies modern intelligent office space of today.
Within a challenging architectural concept, the engineers have rationalised the floor structures to minimise the depths by using twin floor beams, spanning continuously over two bays, with external cantilevers. Large floor plates have been achieved on a restricted site.
This innovative solution has successfully met the unusual demands of the building form, in a fine example of steelwork for commercial offices.