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SSDA Awards

AWARD: 100 Liverpool Street, London

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Two existing City of London steel-framed office buildings have been combined, extended and reconfigured into a modern, flexible, mixed-use development.

FACT FILE
Architect: Hopkins Architects
Structural engineer: AKT II
Steelwork contractor: William Hare Ltd
Main contractor: Sir Robert McAlpine
Client: British Land

Forming an initial phase of the City of London’s wider Broadgate redevelopment, 100 Liverpool Street consists of two existing buildings that been reconfigured into a single mixed-use structure.

Totaling 66,000m², the project achieves a 40% increase in leasable area, while reusing 50% of the existing superstructure and 100% of the foundations: an industry-leading ‘tour de force’ of steel’s unparalleled sustainable qualities, according to the project team.

New steelwork was erected to knit the structures together, replace the demolished areas and add four new floors to the top, creating a new 10-storey landmark building.

Sustainability is at the core of this design, as only steel construction would have allowed the project to reuse the foundations, and then strengthen retained steelwork to allow them to support new lightweight steel floors.

“The project shows the unique circular economy credentials of steel-framed construction. The steel industry’s fantastic quality control processes and provenance have enabled the retention of much of the existing frame,” explains AKT II Technical Director David Watson.

Steelwork’s high strength has allowed novel strengthening details to carry the additional loads with minimal new material and waste. Steel has made the project’s shallow transfer beams and super-slim suspended floor-structures possible; allowing these to fit within the existing buildings’ limited clearance zones.

The existing steel-framed buildings were built in the 1980s by main contractor Bovis, with the steelwork contract being undertaken by Redpath Dorman Long. The records of this construction programme and its structural design were readily available and proved to be invaluable when the design team needed to carefully analyse the structure.

“Analysis of the structure allowed us to identify and utilise redundancies in the original design, and work out which areas of the retained steel frames would need strengthening,” explains Mr Watson.

“The lightweight nature of a new steel composite design, using Fabsec cellular beams, meant we were able to reuse the foundations and only had to strengthen 33% of the existing columns to support the new build elements.”

The new floors utilise a composite design, with cellular beams supporting metal decking and a concrete topping. New steel columns are bolted to the existing steel frame where possible, and the building follows the original structural grid based around a 7.5m × 7.5m column spacing.

The new office spaces open onto new terraces and atria, while the station’s western pedestrian artery has become a dramatic, full-height retail mall. Many of the office floors are extended outwards, while the new upper levels progressively set back to preserve sunlight down to the adjacent Broadgate Circle plaza. The cores have been upgraded, with new express-lifts installed.

The project team also developed the structural design within a propriety software package that allowed structural performance and embodied carbon analysis directly from the shared-model geometry. This included generative parametric modelling, to rapidly iterate and verify design options for balancing the new and existing structure in terms of carbon, logistics, buildability, and flexibility.

It was imperative to retain maximum existing structure, in part to minimise the works and any potential disruption to the surrounding infrastructure.

The team first identified ways to distribute the increased loads with minimal intervention, by assessing the buildings’ designed capacity along with the consequent occupational, cladding and finishing loadings to date, and analysing the overall balancing to identify possible areas of opportunity. Through back-analysis of the existing structure using 3D finite element modelling (FEM), several existing redundancies were affirmed.

Tying in a new steel frame to an existing 1980s frame has been done as seamlessly as possible. Floor slab thicknesses vary throughout the scheme, but in areas where new build meets retained structure, the new slab corresponds to the old.

In the old building, services were accommodated below the steel beams, but as the new areas have cellular beams, allowing the services to be placed within the steelwork’s depth, there are areas where the services transfer from one configuration to another.

The one exception to the standard 7.5m grid pattern is the eastern elevation of the building that spans over the Liverpool Street Bus Station, which was closed temporarily during the construction works.

Here a series of 15m-long × 2.9m deep trusses, positioned at level two, have been retained as part of the new design as they create the column-free space for the buses.

Above the bus station, the original structure has been retained with new steelwork added to the top to form the 10-storey new building. Above the trusses the grid reverts back to the 7.5m × 7.5m pattern, with a series of columns supported on the trusses at mid-span.

In summary, the judges say the re-working of two 1980s office buildings cleverly presents itself as a ‘new’ building. On an extremely constrained site, built over a main access into Liverpool Street Station, the team have added floors and reworked the existing steel structure to create an elegant new City office with high sustainability credentials.

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