[Working detail 25.11.10] Perimeter bracing to provide lateral stability
Although the primary structure of the buildings is a concrete frame, the perimeter bracing serves three key purposes: to provide lateral stability under wind load contributing up to 75 per cent of the overall stability; to reduce the requirement for sheer walls allowing greater flexibility of internal planning and servicing arrangements; and to support the winter garden elements at the prows of the building.
The grey bracing elements are formed from 400mm x 200mm oval hollow sections (OHS), to maximise their structural capacity in the working plane. They are joined at every third floor by pinned connections or ‘nodes’ which were developed with Waterman Structures as a single and universal design which would work in all the different nodal conditions.
These include connections in plane but also across the folded facade, at the winter garden prows, and to the ground-level tie-downs. The nodes transfer the lateral forces applied to the structural frame by wind loads on the cladding into the bracing system, and allow for the transfer of bracing forces between OHS members in the plane of the framework.
Lateral loads from the intermediate floors are transferred to the nodal floors by a pair of reinforced concrete walls arranged symmetrically around the core, which act as vertical beams. In the prows the nodes also transfer the north/south winter garden hanger support loads into the bracing system.
The connections are all pinned which not only creates a visual articulation of the parts, but is a requirement of the buildings’ differential movement and the need for adjustability during construction. The offset of the bracing from the facade means that in the event of a fire inside an apartment, the fire would never reach sufficient temperatures at the bracing members to require intumescent treatment. In addition the system has been designed to remain stable even if up to three nodes failed due to a fire. At ground level the OHS elements are concrete-filled to protect against impact.
Tracy Meller and Simon Davis, project architects, Rogers Stirk Harbour + Partners.
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