To achieve an additional 25% of office floor area a new storey was added on top and the floors at each level were extended around the perimeter. To compensate for the added structure, the existing concrete roof was replaced with a lighter steel framed solution.

One of the key challenges faced during design was the number of cantilevering floor structure to each elevation of the building, which also supported cladding sensitive to deflection.

Deflections were minimised as much as possible using both traditional and bespoke steel framing. However, for the front elevation over the entrance with the largest cantilevered structure, external bracing was used to hang the floor structure from the remote central column. Completely exposed and visible, these bracing members are connected back to the structure with exclusive steel connections with a higher level of architectural finish.

The devil was truly found to be in the detail, the additional primary structure was simple and straight forward in theory, however connecting the new steel structure back to the existing circular reinforced concrete columns posed the greatest challenge.

In addition to supporting sensitive cladding to the building periphery, the steel framing had to fit into a slim spandrel and ceiling zone and be able to fix to the circular face of the concrete columns.

With no room for a typical connection, these had to be concealed within a tight space, with bespoke curved steel connections fitted to the column. Each connection was individually surveyed, all uniquely fabricated to suit. Existing steel reinforcement within the columns had to remain intact, thus flexibility had to be built into the detail in the event of a clash.

One of the many notable features of the building is the sharp ‘knife edge’ roof structure to the new top floor, which was achieved with cantilevering tapered steelwork specially fabricated for the profile. The 5-storey atrium at the heart of the structure was formed by connecting the two southern wings of the structure together and providing a slim steel fabric to the roof to allow as much natural light into the atrium as possible. A new stair core was also added to the rear of the building, interweaved with the existing structure and connecting to an existing fire escape stair from the basement.

With extensive 3D modelling and the coordinated efforts of the design team it was possible to integrate all this into an existing outdated office building.

The new structure and cladding wrapping this existing building to bring it into the 21st century was a feat of engineering and architectural creativity working together.

The project won the Refurbished/Recycled award at the BCO South of England & South Wales Awards.

Photo credit Hundven-Clements Photography