The retention of London’s listed façades has become a widespread feature of construction projects in the capital during the last decade.
Whether projects involve partial or complete façade retentions, the importance of keeping these historic, and quite often listed street frontages is vital in maintaining London’s traditional streetscape.
Developers and tenants alike want modern spacious offices and the best way of attaining this on a plot, which has a listed façade, is to demolish the building’s innards while propping and retaining the perimeter walls. A new modern structure can then be built behind the walls and incorporate the historic or listed façade.
One of Europe’s largest retained façades is currently being incorporated into the redevelopment of LSQ London in central London.
Formerly known as Communications House, this 1920s building, which overlooks one of London’s most famous squares, was said to have many attractive features. However, over the past 90 or so years it has been enlarged several times and had become inefficient in terms of maximum utilisation of space.
“Our brief was to create a high-spec office building, and we felt that the goal was not just to reimagine but reinvent, while also retaining the look and feel of the original building, which is liked and admired by so many people,” says Make lead architect Frank Filskow.
“Our design makes the best of the existing building by retaining the historic façades, and sensitively restoring them to maintain the integrity of the original architectural features and details.
“The design of the building naturally leant itself to using steel for the primary structural elements. The design of the new steel structure introduced a new central core and enabled clear, open-plan floorplates, improving the office spaces within the building.”
Waterman Structures director Jody Pearce agrees: “One of the key aspects of a façade retention scheme is the alignment of new floors with existing window openings. We promoted the use of a steel frame as it offered the flexibility to suit the various interfaces that occur with the existing façade.”
Depth of floor zone
“By integrating the suspended services within the structural downstand beam zone, the depth of the floor zone against the façade was minimised, thus assisting the alignment of new floors with existing windows further.”
The project’s main contractor Brookfield Multiplex started on site during November 2014, by which time the demolition work had been completed, leaving four propped façades surrounding a cleared site.
“Our first task was to construct a secant piled wall and the main central bearing piles. Then we excavated and enlarged the existing basement into a two-level deep facility, removing 13,000m3 of overburden in the process,” explains Brookfield Multiplex project director Asif Hashmi.
Once this preliminary work and construction of the two main raft slabs were completed, steelwork contractor Bourne Steel was able to begin erecting the new steel frame that begins at lower basement level.
Getting the steel frame erected and subsequently tied into the existing façade is vital, and it is one of the main drivers of the scheme
Asif Hashmi, Brookfield Multiplex
“Getting the steel frame erected and subsequently tied into the existing façade is vital, and it is one of the main drivers of the scheme,” adds Hashmi.
“Once the frame connecting into the retained façade was erected and the concrete floors cast, we were able to begin removing the extensive façade retention steelwork that surrounds the site and start work on renovating the original stonework and installing new windows.”
The new steel frame is structurally independent and gains its stability from two steel braced cores. Once the temporary propping was removed the new steel frame supports and restrains the four retained façades, and so before the propping could be removed, a large number of connecting brackets had to be installed.
Steel framed basement
The project’s new steel frame forms two basement levels and a ground floor, which will accommodate high-end retail outlets and a main entrance lobby.
Above the ground floor there are seven floors of office accommodation, five of which are incorporated into the retained façade.
An elegant new curved roof will enclose the two uppermost floors, offering a unique 21st century interpretation of the traditional London mansard style.
The steelwork has been erected around a regular grid pattern with internal spans of up to 15m.
Cellular beams have been utilised throughout for service integration and to minimise the structural void between floors.
The new fifth floor will be clad with Portland stone to integrate with the retained façade below. This floor level’s steelwork is topped with a ring beam that goes around the entire perimeter of the building.
The ring beam is formed from jumbo box sections measuring 650mm by 450mm with a 25mm thickness.
The sections were brought to site in 3.5m-long sections each weighing 3t.
“The box section ring beam performs two functions,” says Bourne Steel divisional manager Kevin Springett. “The columns for the feature roof are supported by the beam, as these are not aligned with the main columns for the rest of the building, and the stone cladding for the sixth floor is also hung from the beam.”
The steel feature roof slopes outwards from the two centrally positioned cores and is formed with a cranked steel frame, which in turn supports a lightweight aluminium frame and glazing.
This new and elegant curved mansard roof encloses the building and offers a modern interpretation of the traditional mansard style where arch geometry sits atop a classical base.
“This respectful, contemporary addition to the building composition reduces the existing top-heavy visual mass. The curve also seeks to ensure the building blends in seamlessly with the surrounding buildings of Leicester Square,” sums up Filskow.
LSQ London is scheduled for completion in the third quarter of this year.
Project: LSQ London
Main client: Linseed Asset Management
Client Development Manager: Core
Main contractor: Brookfield Multiplex Construction Europe
Structural engineer: Waterman Structures
Steelwork contractor: Bourne Steel
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