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SSDA Awards

Commendation: The Post Building, London

Reusing much of the original steel frame of a former 1960s Royal Mail sorting office, a modern mixed-use development has been created

Architect: Allford Hall Monaghan Morris
Structural engineer: Arup
Steelwork contractor: BHC Ltd
Main contractor: Laing O’Rourke
Client: Brockton Capital LLP and Oxford Properties Group
Located on London’s Oxford Street, a former Royal Mail sorting office has been redeveloped into a new mixed-use scheme by incorporating large retained steel elements within a new steel frame.

Unlike many city centre schemes, this project’s demolition programme included retaining a large portion of the original 1960s-built steel-framed building.

A horseshoe-shaped zone in the middle of the site containing ground, first and second floor levels was left in place.

These floors were originally used for mail sorting duties, while the building’s upper four floors, now demolished, accommodated administrative offices and a plant level.

Keeping some of the original steel frame also fitted into the overall design aesthetic, which features exposed steel beams and columns creating a modern ‘white collar factory’ office building.

Retaining a large steel frame required steelwork contractor BHC to use more than 200t of temporary steel propping and bracing, as the frame’s original stability system had been demolished. The stability system was completely remodelled to remove the existing cores from the key corner floor areas and create a new one in the central part of the site.

Once the steel core was erected, the retained steelwork was connected to this new stability-giving element and this then allowed the temporary props and bracing to be dismantled.

“A number of factors came into play when we chose a steel core instead of a concrete one. The site’s basement and raft foundations have both been reused and this lighter steel option helped avoid the need for new piles,” says Arup Project Engineer Tim Bennett.

“The former post office underground railway runs directly beneath the site and so it was also important not to add unnecessary loads.”

Having stabilised the retained steelwork BHC then set about reconfiguring the large steel beams in readiness for the insertion of new steel mezzanine levels.

The original grid pattern for the Post Building’s ground floors was 12m × 20m to suit post office vehicle movements. Consequently, a series of deep transfer beams was originally installed to support these spans. These transfer beams had the effect of concentrating the original building loads into heavily-loaded, widely-spaced points on the raft foundation.

As these long spans were no longer necessary, new columns were added to reduce the spans and spread the increased overall building mass more evenly on the existing foundations.

The now redundant transfer beams have been slimmed down from 2.0m-deep to 600mm-deep members to allow mezzanine floors to be inserted and maximize the available headroom within the existing floor-to-floor heights.

An entirely new steel frame was then erected around the retained portion completing the lower three floors and filling up the entire site’s footprint.

In summary, the judges say this is a great example of a steel-framed building being adapted to give a new life for a different use. The existing steel frame was retained wherever possible to produce impressive and unusually generous commercial spaces. Maximising the reuse of the existing structure resulted in a build with a much smaller carbon footprint.

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