Concrete was chosen for this office development of three interlocking blocks of 10, 14 and 18 storeys for its ability to create the wide spans and thin floor plates required by large open-plan office space. A method of post-stressing was evolved to span the 16m gap between columns. The resulting reduction in floor depth was sufficient to allow an additional floor to be incorporated within the structure's height. Structural stability is provided by a sophisticated system of external H bracing in conjunction with the post-tensioned floors. This bracing, a prominent feature of the design, is joined to the concrete frame, and the massive H girders are encased in concrete and tensioned in situ with diagonal Macalloy bars.
A mix design of 60N/mm2 allowed the concrete to be pumped 80m vertically and still achieve the required design strength. Special welded steel column shutters were designed to produce smoothness and consistency of colour of the exposed surfaces. Extensive off-site trials determined the optimum design mix for the concrete. These involved trailing the grading of aggregates, aggregate type and mix proportions to eliminate sand runs.
Long-span tableforms provided by sgb were developed to give a stability of pour and allow the 70m-long tendons to be placed to within 4mm.
The final designs resulted from extensive r&d which included the construction of a £40,000 mock-up before work began. The result is a high-quality landmark office development.