Client: Brockton Capital and Oxford Properties
The former Royal Mail sorting office on New Oxford Street, London has been redeveloped into a mixed-use development containing commercial, retail and residential elements.
The main contractor Laing O’Rourke began demolition works in late 2016, initially, undertaking a large-scale demolition programme, however, they retained a large portion of the original 1960’s steel frame. Therefore, a horseshoe-shaped section in the middle of the site containing ground, first and second floor levels were left in place. The original steel frame fitted in with the design aesthetic, which sees the Post Building having exposed steel beams and columns to create a modern ‘white collar factory’ office building.
As a large portion of steelwork was being retained, BHC was required to use more than 200tonnes of temporary steel propping and bracing due to the original stability system being demolished. The remodelled stability system removed the existing cores from the key corner floor area and a stability system was installed in the central area of the site. Careful consideration was required concerning the sequence of work and load transfer. Temporary works were required to allow construction of the new frame over the existing live network substations which could not be moved before erection.
Once the temporary props were in place, BHC began constructing the new central core that sits within the open end of the retained structure’s horseshoe shape. After the steel core was erected, the retained steelwork was connected to the new stability structure allowing the temporary props and bracing to be dismantled. Having stabilised the retained steelwork BHC began reconfiguring the large steel beams allowing them to be inserted into the new steel mezzanine level.
A steel core design proved to be the lightest option avoiding the need for new piles while keeping in with the desired design aesthetic of exposed steelwork throughout the building.
The buildings original ground floor grid pattern was 12m x 20m to suit post office vehicle movements. Due to this, a series of deep transfer beam was originally installed to support the 20m spans. As vehicle movements are no longer required, new columns were erected to cut down the long spans, increasing the overall building mass, spreading it more evenly on the existing foundation.
By removing the transfer beams’ concentrating effects, the widely-spaced high point loads are replaced by more frequent, lower point loads. This helps limit punching shear and bending forces within the raft, therefore the increased building size can be carried, and the now redundant transfer beams are 500mm deep instead of 1.8m deep.
A new steel frame has been erected around the retained section, completing the lower three floors and filling up the entire footprint of the site. Elements of the retained building were integrated at level one, two and three and this dictates the floor-to-floor height of each storey. Above this, new construction is provided following the original grid levels four and five. These floors step-in at levels five and eight, where the transfer has been installed to support the column location changes. Substantial transfer structures were included at level five to permit the upper floors to follow a more attractive spatial planning grid.
Level eight has a slightly higher floor-to-ceiling height than the other floors and includes a mezzanine. The roof profile was stepped back to minimise visual intrusion and trusses spanning along the step line have been used to reduce the number of internal columns, maximising clear internal floors.
At the Post Building, significant site fabrication and welding was undertaken to reduce the depth of the existing transfer beams.
In total, the redevelopment of the Post Building creates 44,000m2 of floor space, with 8 floors of offices and a basement containing a variety of public uses including shops, cafes, galleries and a GP surgery.