How Mace built Landsec’s low-carbon hybrid

Targeting the highest sustainability ratings, a Landsec commercial scheme in Southwark, south London, provides an exemplary model of design, setting an industry-wide precedent for the reuse of existing structures and exceeding recognised carbon targets.

Timber Square consists of two buildings that are situated a short distance from the capital’s South Bank and the Tate Modern. One building is a transformed 1950s-built printworks (now simply known as Print), where 80% of the existing structure is being reused and five new floors added to the top, forming a nine-storey office block. Next door, the other structure (known as Ink) is a new-build 14-storey office building.

This is Landsec’s latest steel-framed project in Southwark, following the delivery of the nearby Forge scheme (see below), which is said to be the UK’s first net zero commercial building designed in line with the UKGBC’s framework. It was completed by Mace and its joint venture partner, Sir Robert McAlpine. 

Mace Construct is main contractor for Timber Square and project director Rob Dudley says: “Building on our experience delivering The Forge, we are working closely with the Landsec team to maximise the retention and use of recycled materials.

“Combined with the integration of a complex steel hybrid structure and the application of advanced offsite methods, with Timber Square we are pioneering the next generation of low-carbon buildings.”

To this end, both buildings are steel-framed structures, a solution that has allowed the design to benefit from a lighter and quicker framing solution as well as one that can easily create the long clear internal spans wanted by office tenants.

The retained structure in Print is a steel frame supporting in-situ concrete floors. However, above this for the new floors, a hybrid solution of steelwork supporting cross-laminated timber (CLT) flooring has been used. This solution has also been utilised for the entire superstructure for Ink, making this structure the largest in the UK to be designed with a steel/CLT frame.

Further enhancing the project’s sustainability credentials, approximately 70% of the steelwork fabricated, supplied and erected by William Hare was sourced from electric arc furnace (EAF) production facilities.

EAF steelwork is considered to be much greener and more efficient in terms of energy consumption for the production process, as it can utilise renewable energy from wind farms instead of carbon fuels such as oil and gas.

Another carbon saving has been generated by using 120 tonnes of reused steelwork sourced from Cleveland Steel & Tubes, which has a stock of reclaimed material. This has been fabricated into around 500 beams, used primarily in and around the cores in both buildings.

As Print was originally built for an industrial use, a cut-and-carve construction programme has been undertaken, whereby the retained structure has been extensively remodelled to create a modern office environment.

This work required a significant amount of new strengthening steelwork to be erected within the retained structure, providing support and allowing voids to be cut through the concrete floors. Four new steel-framed cores, for lifts and stairs, have been erected through these voids. Extending upwards into the new uppermost floors, the cores are braced and provide stability to the building.

According to Landsec, the existing commercial building has been sensitively redesigned to cater for a mixed tenure alongside additional retail units at ground floor level. Representing both environmental and social sustainability, the building places the health and wellbeing of its occupants at the centre of the development with increased light levels, natural material palettes and access to outdoor terraces.

On the front elevation, facing Lavington Street, a double-height ground floor reception area has been created from what was originally a large loading bay. Above this, a series of seven large steel transfer beams have been installed to support and create a new set-back and terrace at third-floor level.

The majority of the steelwork erection for both buildings has been carried out using the project’s three tower cranes. However, the transfer beams each weigh 8 tonnes and required a mobile crane to be brought to site for their installation.

Creating the new upper floors, steelwork follows the retained structure’s original column grid, while also wrapping around a new full-height central atrium. As the existing structure is encased in concrete, the old steelwork is in good condition, and once areas have been broken out, new steel-to-steel connections have been made.

Forming the new office spaces are a series of 850mm-deep Warren trusses, up to 14m-long. The trusses each weigh up to 3 tonnes, and were brought to site as fully assembled sections and erected straight from the delivery truck.

In the Print building, finishes are kept to a minimum and steelwork, flooring, structural joints and services are visible, creating a modern industrial aesthetic. This design includes the retained structure, where the predominantly concrete encased columns and floors will be on view, while the upper floors, trusses and the underside of the CLT flooring will also be left exposed.

This exposed design aesthetic is also present in Ink, as this building’s entire frame, services and underside of the flooring will also be left on view.

Based around two centrally positioned stability-giving concrete cores, the steelwork in the 14-storey Ink building is based around a 6m x 10.5m column grid. Sitting above a two-level basement, the steel-framed structure will form 17,781 sq m of new flexible workspace, with floor plates ranging from 733 sq m up to 1,514 sq m. Ground floor retail units can be accessed from a pedestrianised square between the two buildings.

Throughout the Ink building, cellular beams have been used to support the CLT flooring and accommodate the services within their depth. Fabricated from EAF-sourced UB sections, the beams had bespoke service holes cut into them at William Hare’s factory.

As well as accommodating modern office workspaces, Ink will feature a number of balconies and terraces, providing outdoor breakout areas for the tenants.

The largest terrace is on the fifth floor, along the Lavington Street elevation, where the building steps back to accommodate the outdoor space. Two floors above, the seventh floor cantilevers out over the terrace.

This feature is formed by a large single-storey-high (3.8m) truss, measuring 24m long. Positioned along the perimeter of the building, the truss also supports the column lines that extend upwards from the eighth floor to the roof.

James Rowbotham, head of workplace development at Landsec, says: “We’re delivering two highly sustainable buildings centred around a new public square which combine a breadth of amenities with highly efficient space.

“Timber Square, along with our wider Southwark pipeline will deliver much-needed capacity to a supply-constrained market and allow us to respond to the growing demand for space in the borough.”

Said to be the UK’s first net zero carbon commercial development, the Forge comprises two nine-storey office blocks situated behind the Tate Modern on London’s South Bank.

To help achieve its sustainability targets, the scheme used the platform approach to design, manufacture and assembly (P-DfMA).

P-DfMA uses standardised components assembled using a ‘kit of parts’ methodology, whereby most elements are fabricated off-site in repeatable sections.

By utilising this ground-breaking construction method, the job was completed 40% faster than a similar-sized commercial scheme using more traditional construction techniques.

The Forge’s P-DfMA method used a hybrid steel and concrete framing solution to construct the buildings. Steelwork was chosen for the primary structure due to its speed of delivery and because the beams and columns can be fabricated off-site, in line with the project’s industrialised construction-based philosophy.

Severfield fabricated, supplied and erected 1,000 tonnes of steelwork for the scheme.