A former City of London government building is being painstakingly reduced to its frame and reclaimed with stone salvaged from its former facade.
Outline: Panorama of Sant Pau
Customer: Orion Capital Managers
Architect: KPF
Main contractor: mace
Contract value: £262 million
Initial date: February 2022
Estimated Completion: March 2025
The removal of an existing building to allow it to be reused or repurposed, known as ‘slash and cut’, is fast becoming the common construction method for developers working in busy urban centers where land availability is low and its price is high. .
As the size and quality of development plots shrink and local authority planning departments look more favorably on schemes involving elements of structural reuse, cut-and-slash construction is one method that may make sense. commercial Time, material and cost savings are available in addition to great environmental and sustainability benefits. These can not only help reduce a project’s carbon footprint, but can also make the completed scheme more attractive to landlords or tenants.
But cutting a building down to its skeletal frame is no simple process. It will throw up many challenges along the way, and the reusable percentage of the original structure can vary greatly.
Deep in London’s financial district, a stone’s throw from St Paul’s Cathedral, main contractor Mace is 18 months into a three-year project which it claims will see the team re-engineer up to 75 per cent of the reinforced original. concrete building to provide a new commercial office space of 53,000 square meters.
“We are playing structural gymnastics with the framework. The amount of steel we have here is equivalent to that of a large steel frame building.”
Chris Harrison, Mace
Before Mace began his work, the site featured a robust 10-storey office building – the former headquarters of British Telecom, originally built by the government’s property arm the Public Services Agency (PSA).
The building’s history has allowed the development team to ensure that as much of the original framework as possible is saved, explains Mace project manager Chris Harrison.
“It’s very well built. As an old PSA building, you’d expect that,” he says. “But this is also a fairly young building. It was only in service for about 40 years.”
Old building, new face
The reinforced concrete frame is going through its own mid-life crisis and is subject to more than a few dings and dents here and there. Instead of a botox injection to enhance its features, a steel frame will be used to expand the building to 14 floors of offices, restaurants, bars and public rooftop gardens. Its two-story basement will feature a gym and changing rooms, including an indoor pool, while the original Portland stone facade has been stripped, cleaned, redesigned and refitted to complete the building’s facelift (see box below).
With a 2-metre-deep pool base spanning the building’s footprint, the team was able to focus on work above ground level, although the basement was extended downwards to accommodate the swimming pool As the central line of the London Underground passes beneath the site, extensive vibration monitoring is being carried out throughout the work, with weight limits.
The project team moved in almost as soon as the last of BT’s workers left their desks early last year. While a team from specialist contractor Grants Precast focused on cataloging and removing the facade, the soft strip revealed the solid construction of the frame. With floor plates designed to open at all levels, initial focus was on removing the building cores and part of the west elevation. This meant that some of the 550mm square columns had to be strengthened by introducing steel bands wrapped around them. Some were fully encased in 40mm thick steel jackets with non-cementitious void filler grout injected to close the voids between the column and the jacket.
“Instead of increasing the size of the footing, the structural load is being transferred through the existing building,” explains Andy Gooding, Mace’s construction manager. “We put steel jackets on selected columns to increase the amount of load they can carry, which allowed us to remove some of the beams and replace them with cell beams.”
The city of London takes shape in Doncaster
It may have been originally cut in the south-west of England and spent 40 years in the capital, but South Yorkshire is where the Portland stone of the building’s facade is being given a new lease of life.
Specialist contractor Grants Precast began the three-and-a-half-month process of removing the original cladding as soon as the scaffolding was erected around the building. The company measured and assessed each panel before sending them to its manufacturing facility near Doncaster.
Here, his team painstakingly worked through 1,500 tonnes of Portland stone and granite, cleaning them of 40 years of London grime before re-measuring and cutting. Once cut, the stone is placed in shapes to be cast into the new facade panels, the largest of which are 12m long.
“At first there was a fear that we might be about 50 square metres, so we ordered new stone to accommodate that. We needn’t have worried,” Mace project manager Chris Harrison says with a smile. “The original stone was in very good condition. Grants has cleaned and scraped the surface and trimmed. Each new panel has about 30-40 pieces of the original Portland stone.”
On levels two through nine of the building, each floor measures more than a hectare. Even the smallest floor plate on the upper level measures 4,300 square meters.
To provide these open floors, the team has been removing columns, a task that requires some engineering. Large 18 tonne transfer beams up to 14 meters long, 1200mm deep and with a 50mm thick flange plate have been installed. In total, about 6,500 tons of structural steel are being stitched into the existing frame. “We are playing structural gymnastics with the framework. The amount of steel we have here is equivalent to a major steel frame building,” says Harrison.
Away from the frame, Mace’s team has been using double-wall precast concrete to replace three existing cores in new locations and adapt a fourth. Using 10mm self-compacting concrete, the 6-ton sections are being filled in two stages to prevent hydrostatic pressure from bursting them.
“Obviously we couldn’t use jump or slide shapes because of the existing building,” explains Gooding. This is because the retained reinforced concrete frame was in the way. The jump/slide formation would have meant more breaking up of the existing frame to make room for the formwork.
“The precast sections were filled halfway, the concrete was let loose, and then a second pour to fill,” adds Gooding.
Environmental objectives
With a 75 percent reuse rate, it’s no surprise that the development scores high in the sustainability stakes. Alongside its BREEAM Outstanding target, client Orion Capital Partners also wants 81 Newgate to achieve a NABERS five-star energy efficiency rating and WELL Platinum certification to ensure the health and wellbeing of the workforce.
Mace claims that 465 kg of CO will be saved2 per square meter maintaining the structural framework of the building.
“The massive carbon savings from reusing much of the building has been beneficial from day one, even in the design stages. The boundaries are being pushed here. It’s a leading project and it will be a place great to work on once completed,” says Harrison.
With 18 months or so to go until the building opens, the team is focused on preparing it for the installation of its new facade, ready for its occupants to enjoy the panoramic views of St Paul’s .
The many benefits of slicing and slicing
Mace project manager Chris Harrison believes cut-and-cut construction will become the norm for commercial office developments, particularly in busy city centres.
However, he has a few tips for planning authorities. “I would suggest that any changes should be notified early in development, at the planning stage,” he says.
From day one, all stakeholders at Panorama St Paul’s, including the client, structural engineer, architect and main contractor, have been focused on delivering a showcase project that will highlight the true potential which offer cut and cut schemes.
But far from carbon and raw material savings, there are perhaps more prosaic benefits as well. Through their judicious approach to treating the existing building as a source of material, the project team is convinced that it has saved a great deal of time and money.
“When all is considered, I believe we will have saved around 15 months on the construction schedule and around £150 million in construction costs compared to a similar new build scheme,” says Harrison.
