14 Cornhill, London

14 Cornhill

Architect

DLG ARCHITECTS

Structural Engineer

RAMBOLL WHITBYBIRD

Steelwork Contractor

GRAHAM WOOD STRUCTURAL LTD

Main Contractor

ISG INTERIOR EXTERIOR PLC

Client

IVG DEVELOPMENT (UK) LTD

This project is about engineering to save a grade II* 1920s cellar building, and to transform it into a fully open plan and highly serviced building. The listed features include a triple height banking hall and a period fifth floor panelled boardroom. The building had to be ‘re-built’ underneath the boardroom, which involved opening up the honeycomb of small cellar offices and replacing thick columns with thin ones, one at a time. Two additional floors were also hung above the top of the old building. A complex series of transfer beams had to be devised to hang the new floors.

Working within tight floor to floor heights meant a high level of structural and services integration. The existing 100+ riveted plate beams had to be notched – all of which had different cross-sections – to fit the services in.

With the exterior of the building already determined the architectural focus looked inwards, including an iconic staircase in the glazed ‘prow’ of the building, finely detailed and engineered it works as a ‘restrained’ cog.

The foundations have been re-used which meant the weight of two new floors had to equal the weight of the removed thick columns and associated masonry. Steel was the only usable material to bridge the 15m wide lightwell. Tight construction depth within the existing floor to floor height meant only composite steel could be used, which had the added benefit of integrated service provision.

The roof construction sequence was critical as the existing structure could take the weight of the steelwork for the new floors but not the weight of the steel and floor concrete unassisted. A sequence was used whereby the steel for the two floors and the roof was put in and then connected to a number of new mega columns running through the lightwell – it was then possible to disconnect the weight of the new floors from the existing structure. With the mega columns taking the weight of the new floors the floor concrete could be poured.

The big challenge in designing the roof truss was to understand the complex range of movements and consequence deflections involving the floor beams, cantilevered trusses and hangers that could result from different floor loadings. Part of the elegance of the engineering solution was the realisation that to make the glazing work, it was not necessary to control the building’s absolute movements, only the relative movements within each glazed aperture, thus a solution where the glass moves by 60mm in the middle of the elevation with only 10mm relative differential movement across any one bay was found, and which also gave important weight and material savings.

The anti-corrosion work was all done off site as the steel was fabricated. Fire protection was applied on and off site.

It was possible to re-use 75% of the existing building, including the foundations. This has conserved the embodied carbon-based energy in the existing structure and avoided additional energy expenditure in reconstruction. The fact that the building’s new elements are lightweight and steel with a recycled component both reduces further the total energy cost.

Judges’ Comment

s:

This commercial restoration has modern spaces sensitively inserted within and above the historic listed bank headquarters in the heart of the financial district. The original building had new steel members inserted to open up large clear areas in existing floor