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Can we create a best in class, retrofit first headquarters building, suitable for the future of workspace?

Location:        City of London
Status:            Planning

Client:             CastleforgeCa
Architect:    Orms

Structural Design Brief

Add an additional 170,000 sq ft NIA to the structural frame with no new foundations or foundation enhancement.

Time history sequential loading analysis of foundations from 1960’s and 1990’s to assess the capacity for additional loading.

Lightweight, long span structures to match the structural grid of retained frame.

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The proposal consists of the redevelopment and repositioning of the dated 1990s concrete framed building. The design includes terraces and biodiverse green roofs every floor from 2nd floor, with a mix of trees, planters and vertical greening utilised providing amenity spaces for the office users.  The large terrace at 13th floor provides views over The City and beyond. The development includes new public realm space with the Prior’s Garden offering landscaped cultural forum space adjacent to the new auditorium.  Additional commercial units and secure short stay bicycle parking offer extra amenity to the building users at ground floor, whilst the B1 level basement boasts 879 bicycle parking spaces for the office tenants with showers, lockers and gym, converting the historic carpark space.


The existing building has nine storeys of superstructure with two full basement level and a third part level, centrally in the building.  The structure was formed with reinforced concrete vertical elements and a combination of reinforced and unbonded, post tensioned slabs and beams. The existing post tensioned slabs are to be retained up to 6th floor, where the building currently steps in and new steel framing extends the building to 14 storeys.  The steel frame will consist of cellular beams to allow for the services to be within the depth of the structure and maximise floor to ceiling heights.  Steel will be formed in a mix of standard steel sections, westok beams and fabricated plate girders to optimise the frame while minimising the embodied carbon usage.

The basement was formed in the footprint of an office block and carpark dating from the 1960s and the existing piled foundations were retained and reused.  Utilising the redundant capacity of these existing piles, the additional five floors can be added without requiring strengthening works to the foundations.


A bespoke script has been developed to enable rapid optimisation of structural solutions, working within the foundation constraints, to define the potential massing of the new building. This same script can be utilised to produce the Revit model and perform the connection design, enabling a smooth digital journey through the design development, with a consistent source of data for the structural solution.


The existing stone façade is retained up to 5th floor, with the existing stone repurposed into new façade panels.  As the new design steps back at upper floor levels the façade changes to a precast system and then unitised curtain walling, which follow the rhythm of the existing façade but providing a complimentary modern design.


A section of the south west corner of the building will be demolished to allow for a new extension.  The extension sits over the public realm space, so only one column was allowed to sit within the public area.  Various options were developed and studied for this space but the proposed solution has an asymmetric steel “T” running ground floor to second floor, that supports the columns on the façade line of the building over. 

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