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Hard raft

Two considerations dominated the deliberations of Skidmore Owings & Merrill's design team when it started work in late 1997. The first was to maximise the development potential of the awkward site. 'The original 1980s proposals were effectively limited to three storeys by the very long spans needed to clear the rail lines on the western side of the site,' SOM associate director Graham Wiseman explains.

'However, we were able to agree an extra line of columns with Railtrack, which cut the spans down to more reasonable dimensions.'

All columns close to the lines would also have to be designed with the possible high speed impact of derailed rolling stock in mind. Addition of the extra columns, which made a building of up to 13 storeys theoretically possible, was a big early breakthrough. But with the developer looking for maximum flexibility (see main story) the design of the raft was still far from straightforward, Wiseman reports.

'We looked at a myriad of truss-type options - and a solid, 2.5m deep concrete raft that would have made it possible to put the building's columns anywhere.

'This would have needed really massive foundations, however. The solution adopted was much more economic.'

Based on a 6m north-south grid the 'hollow raft' would allow buildings with either 6m or 12m grids - or a mixture of both - to be built on it without further work. 'And there is enough depth in the interstitial space to allow lift or escalator pits to be situated almost anywhere the client wants them, or for extra structural steel to be stitched in later if necessary,' Wiseman adds.

To minimise impact effects, each column is tied to another, and protected by large plinths at track level. Moment connections are used in both directions. And although train speeds in the vicinity are low, the raft is designed to stand up if a column is completely smashed out.

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