10–4 Pentonville Road.

In the refurbishment of these dark, outdated 1980s office blocks on either side of a public passageway in Angel, Islington, we stripped the original buildings back to their RC concrete frames and then unified them while adding a modern brickwork façade. We added a rear extension and increased the overall internal area considerably, from 4,000 to 4,900 m².

Location

London, UK

Client

Derwent London

Architect

Stiff + Trevillion

Project Value

£8.5 million

Floor Area

4,1900 m²

Status

completed 2012

Expertise

Structures

The existing buildings both comprised six storeys set in regular 6 × 6 m column grids, and were limited in height by planning restrictions. In order to maximise internal lettable area, therefore, we chose to increase the floor plate by bridging the areas above the existing road using columns embedded in the existing cladding line, which bear the weight of beams overhead. By unifying the two structures we were able to increase stability and remove one of the cores, which further increased available space.

Through demolishing an existing two-storey extension at the rear of the building and infilling an existing light well, we added a new section of column-free floor plate at each level, opting for lightweight steel construction. This choice of material allowed the extension to be supported on the existing piled foundations and avoided the need for costly transfer beams.

We carefully assessed the existing structure to minimise the need for additional foundations. Since concrete gains strength incrementally as it ages, and since there was a tendency for overprovision in office structures in the 1980s, the intention was to use this inherent spare capacity to resist the new loads. In a few cases, we introduced concrete ‘jackets’ to strengthen existing columns, but for the most part this was unnecessary. In terms of foundations, we were able to save the client considerable cost by undertaking an investigation which proved to Building Control that the piles and pile caps improved the bearing capacity of the ground.