Targeting a BREEAM ‘Outstanding’ rating, sustainability is at the forefront of the design for a nine-storey office development in London’s West End.

FACT FILE
Network W1, London
Main client: Derwent London
Architect: Piercy & Company
Main contractor: Kier Construction
Structural engineer: Elliott Wood
Steelwork contractor: Bourne Steel
Steel tonnage: 1,000t

Derwent London’s Network W1 project, on the corner of Tottenham Court Road and Howland Street, is said to have sustainability at its heart. With carbon reduction as a key focus of the build, the scheme is aiming to achieve a better embodied carbon intensity than the Greater London Authority target of 600 kgCO₂e/m².

Once complete, the nine-storey building will provide 12,449m² of workspace and 464m² of retail space in the heart of London’s prime W1 postcode in Fitzrovia.

According to project architect Piercy & Company, the building comprises three volumes: a base with active frontages for new retail and a street-facing work lounge; a middle volume of workspace; and an upper-level pavilion with terraces and green-roof areas around the perimeter of the building.

Designed with a highly flexible floorplate system to accommodate different types of future use, the middle volume of Network will be used as Category A offices.

A number of framing options were considered during the initial design process and steelwork was the preferred choice as it is less carbon intensive, when high levels of recycled steel (see Sustainability Credentials box below) is incorporated. The material also provides a lighter frame, which ultimately has cost savings as there are less foundations works needed.

Starting at ground floor level, the steelwork is founded on a concrete basement substructure with four piled secant walls. Below this, the foundations for the building consist of a 1.2m-thick concrete raft. Stability for the steel frame is derived from a centrally-positioned concrete core, which connects to steel beams via cast-in plates.

Some extra stability has been required during the erection sequence and this has been supplied by temporary cross bracings, which are removed once the floors have been completed and recycled.

As with most steel framed city centre developments, open-plan floorplates are an important part of the design. To create the required spaces, the perimeter columns are spaced at 6.5m centres, while internally there are spans of up to 10m.

The design includes precast flooring planks supported by steel beams and throughout the building there are more than 2,000 of these units, which were all installed and grouted as part of Bourne Steel’s package. The company’s works also include the installation of safety barriers as well as on and offsite painting of the steelwork. The latter is very important as a decorative finish is being applied to the columns and beams as the steel frame will be exposed within the completed scheme, creating modern industrial-looking floorplates.

“The concrete core and the underside of the precast planks will also be exposed,” says Elliott Wood Associate James Hinks. “We wanted a clean and smooth soffit which was best achieved with the precast option.”

As well as long spans and the exposed nature of the interior, the building’s design prioritises wellbeing in the workplace, with 2.95m floor-to-ceiling heights that allow natural light to flood in, openable windows on every floor plus 259 cycle spaces, 27 showers, 180 lockers and a dedicated drying room.

Although much of the steel frame is based around a regular column grid pattern, there are a couple of significant exceptions. One is the double-height reception area, that faces south onto Howland Street.

This elevation is slightly indented to signpost the entrance, with a 2m-wide cantilever overhang starting at the underside of the level two.

Meanwhile, helping to create the open-plan reception, some columns have been omitted from the two lowest levels. A series of five plate girders positioned at the underside of the second floor support the columns’ lines above and allow them to be discontinued in the reception. Measuring approximately 9m-long, the girders weigh up to 6.5t each and represent the longest and heaviest steel elements in the overall package.

Adding some aesthetics to the reception’s internal appearance, column lines that do exist in this area have had their UC sections replaced with CHS members, which have a smooth appearance with no sharp edges.

The CHS columns each weigh 1.2t and were installed using the site’s two tower cranes and MEWPs positioned on the ground floor slab. The MEWPs have helped install all of the steel frame and the precast flooring up to level four, after which smaller scissor lifts, working in conjunction with the tower cranes, have been used.

“The ground floor slab has the capacity to support MEWPs, but they could only reach up to level four,” says Bourne Steel Senior Project Manager Duncan Wyatt.

“For the upper levels of steelwork, where there is a smaller loading capacity, we had to use smaller and lighter machines, positioned on the installed and grouted precast floors.”

The majority of the upper levels of the steel frame is fairly regimented except the two uppermost floors. Here, the building includes set-backs, where the outer bay of steelwork terminates to create terraces that wrap-around the structure.

Summing up, David Rowsell, Managing Director, Kier Construction London, says: “We are proud to be working with Derwent London to deliver this prestigious and sustainability focused commercial development on London’s iconic Tottenham Court Road. This high-profile project forms part of our strategic goal for continued market growth in London’s private sector.

“We are implementing our integrated technical excellence, working with our in-house carbon team to reduce embodied carbon through our delivery and also utilising modern methods of construction and offsite production to deliver this showcase development for our client.”

Network is due to complete in the second half of 2025.

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Sustainability credentials

With high sustainability aspirations, the Network building is a net zero carbon development that will utilise an all-electric strategy with electricity from renewable sources, low energy cooling and renewables. The façade features a sculptural repeating module that will be fabricated offsite to ensure minimal material wastage. Blue roofs and rain gardens contribute to rainwater attenuation and flood management.

Helping to achieve the sustainability aims of the project, a steel framed solution was chosen, which includes recycled content. This is said to be saving approximately 1,280 tonnes of embodied carbon emissions compared to using non-recycled steelwork.

Furthermore, Bourne Steel says more than 50% of the 1,000t of steelwork used on the project has been sourced from Electric Arc Furnace (EAF) production facilities.

This 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.

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Flooring options

The Network W1 project serves as a reminder that precast concrete floor planks should be one of the options considered at the concept stage of a development, writes David Brown of the SCI.

Precast concrete planks are a common floor solution, especially for lower rise buildings. Due to the long-span capabilities of precast planks, fewer steel beams may be required compared to an in-situ composite floor on profiled steel sheet. The look of an exposed precast plank soffit may also be preferred.

There is a wealth of design guidance, including advice on designing the steelwork compositely. Practical issues often have a significant influence on the sizing of members. If the steelwork is to be designed compositely, the resistance of the welded shear studs depends on the gap between planks and the amount of transverse reinforcement. With a given gap, the necessary bearing on the steelwork leads to a minimum flange width.

Precast concrete floor planks can be simply placed on the top flange of a beam, or to minimise the construction depth may be supported on ‘shelf angles’ or (as was the case at the Network W1 project) on stiffened plates welded to the beam web. If planks are supported on shelf angles or plate, the designer must consider how the planks are installed, if they are to be located in the space under the top flange. A longer outstand may make the installation easier, but the increased eccentricity will introduce more bending into the support and potentially more torsion into the beam in the temporary condition.

Installing transverse reinforcement is straightforward if the planks are positioned on the top flange, as the ends of the plank are usually opened up. If planks are positioned within the beam depth, any transverse ties between planks must be cranked over the steel beam or installed through holes in the beam web.

The designer should consider the erection sequence, since loading from one side only – especially if the planks are supported from shelf angles or similar – will introduce serious torsions and twist the beam. Although erecting planks progressively on each side is recommended, the construction sequence may mean that one sided loading in the temporary condition is a critical load case.

SCI Publication P401 is the latest design guidance on composite beams and precast planks. P351 discusses the use of precast concrete planks in non-composite applications.