The approach to the structure developed from a desire to read a simple, elegant cross-section highlighting material changes where needed. Given that the low-energy concept relies on providing thermal mass, in situ reinforced concrete was selected as the load-bearing material.
The key was to agree a repetitive module that coordinated with the architectural proposals.
The flexibility of the internal spaces allows for vertical structures at 8m centres longitudinally and 3.5m centres laterally across the building. A framed structure was selected in lieu of load-bearing construction with columns on a structural grid of 8m x 3.5m. The cantilevers in the circulation spaces at first- and second-floor levels' form the main beam support; this helps balance the slab forces and reduces the need for downstand beams. This approach allows the use of thin concrete slabs 175mm thick, spanning between downstand beams, that reduce the load on the foundations but maintain exposed soffits for use as thermal mass. The link space at first-floor level between the buildings is suspended on in situ RC hangers.
This maximises the space at ground-floor level in the main entrance and primary circulation area.
The timber structure that supports the external envelope acts as a three-pinned arch that is propped from the primary RC frame; this ensures that a basic structural concept is retained, with the wind and snow loads transferred simply. Designing the timber structure in this way allowed the use of simple shear plate connectors between the rafter and column, avoiding the need for haunch braces. The detail at the interface between the RC and timber frames facilitated the ease of construction and adjustments for tolerances. The repetitive nature of the timber frames allowed the main contractor to set up a small workshop dedicated to their production.
As the perimeter loads are relatively light, the foundations are thickenings in the groundfl oor slab, with the more heavily loaded internal columns having mass-concrete pad foundations.
John Gerrard, Adams Kara Taylor