Tall & Urban News

1960s-Era London High-Rise Undergoes Retrofit

The ambitious cut-and-carve job involves stripping a 16-story late-1960s office block back to its concrete skeleton before extending it upwards by a further 13 stories
The ambitious cut-and-carve job involves stripping a 16-story late-1960s office block back to its concrete skeleton before extending it upwards by a further 13 stories
08 April 2020 | London, United Kingdom

The Hylo mixed-use development under construction on the edge of the City of London provides an intriguing game of spot the difference. Walking around its concrete frame, you can look at the floors and columns, and try to guess whether they are new or old. On this puzzling 29-story building that straddles the decades, it’s trickier than it seems.

Contractor Mace is overseeing an ambitious cut-and-carve job that will involve stripping Finsbury Tower, a 16-story late-1960s office block, back to its concrete skeleton before extending it upwards by a further 13 stories and replacing its original single core with two new cores.

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Melding the old with the new has presented all manner of engineering and construction challenges. The extra weight requires additional foundations; new drainage must be cut into the 1960s basement; and various methods have been used to strengthen the original concrete columns that were poured in-situ more than 50 years ago.

With so much effort required to tune up the existing structure to enable it to support its 2020 extension, you wonder why they didn’t just tear it down and start from scratch. That would surely have been easier, but also far less environmentally friendly, and with modern surveying and construction methods, neither is it necessary. With advances in surveying and digital-modeling technology combining with greater demand for sustainable ways of building, methods used on Hylo could be part of a big future in construction approaches.

The Hylo building sits close to Old Street Station, also known as “Silicon Roundabout,” in reference to the mass of tech firms that have been established in the area. It is being developed by CIT, which acquired the site in late 2015. What it bought was a 16-story office block with shorter three-story podiums on its north and south sides. The developer’s goal was to build a much taller mixed-use building on the site, offering residential, commercial, and office space.

Mace project director Paul Fairhurst says the option of knocking the whole thing down and starting again was considered, but CIT wanted to retain as much of the existing structure as possible. This is not because the Finsbury Tower was an architectural gem. Major-projects construction director Graham Barter says sustainability was an important factor in the decision to refurbish and extend, as it prevents hundreds of tons of concrete being demolished and then remade. Fairhurst says removing all that demolition and concrete work also reduces the time it takes to deliver the new building and cuts the cost.

Making this work required a “cut-and-carve” approach, Fairhurst explains: “In the simplest terms, it’s [about] retaining that existing structure. You’re cutting and carving into it; you might be strengthening it and altering it so that you can create a new form.”

Hylo is the second major cut-and-carve job Mace has done with CIT, following on from the South Bank Tower, which saw a 31-story building extended to 42-stories in 2015. Fairhurst says the approach is not so unusual now. “It’s happening more,” he says. “We’re seeing it a lot in the market.”

Not everything can be saved, however. Most significantly, in the case of the Finsbury Tower, its core had to be deconstructed. The core connected to the north side of the tower like a spine, rather than going up through the middle of the floor plates, and it failed to meet the needs of the new structure on two points. Firstly, it could not support or service the 13 new stories that would be built on top, and secondly, its narrow form meant the staircases would not meet fire regulations. In other words, the bigger building would have too many people trying to fit down the narrow passageway in the event of an evacuation.

Removing a building’s core naturally creates a challenge of structural mechanics. “If you do it without adding in some temporary stability, that building is not going to like it very much,” Fairhurst says. The solution devised by AKT II was to install numerous temporary large steel cross-braces on every floor. This stabilized the building until the two new cores were erected and tied back to the existing floors. Fairhurst says installing this solution was one of the most technically challenging parts of the job, ensuring the bracing was in the “right place at the right time." AKT II constantly monitored the building to ensure the braces did their job and the building kept its form.

The final major step of getting rid of the redundant elements involved Tower Demolition soft stripping the tower, which involved removing the cladding, fittings, and MEP, taking it right back to its skeleton of concrete slabs and columns.

With this done and the remaining building temporarily strengthened, the 1960s structure was ready for an upgrade to prepare it for its 2020 makeover.

The starting point for getting the old building ready to be combined with the new was understanding what the team was working with, and this meant getting good data. The primary source was the original hand-drafted plans, but they did not always reflect what was built.

“There’s always the unknown in there that you’ve got to deal with, in regards to ‘that pile wasn’t exactly where we thought it was going to be’ or ‘that reinforcement wasn’t exactly where it was going to be.’ So, it’s very much an ongoing monitoring piece,” said Fairhurst.

AKT II worked to verify the exact structure and fill in any gaps. It carried out a 3D point-cloud survey that checked and mapped the exact dimensions of the existing building. “You set up a laser on the floor, it spins around, and creates an image,” Fairhurst explains. “I’ve not used a 3D point-cloud survey to the same level of detail [on other projects],” he adds, revealing that more than 80 gigabytes of data were generated just from the survey of the existing structure.

This map of the old building was then combined with the new structural plans to give a comprehensive digital model of the Hylo building, which Barter says was invaluable. “It’s hugely quicker, and it allows you to get on with the design and have the understanding much earlier [compared with] dealing with surveyed information that has to be transposed into a digital format. You’ve got it there at your fingertips.”

Inevitably, some of what was built in the 1960s were not up to the standard needed in 2020. Dealing with concrete that is about 55 years old requires a careful analysis of what materials are still fit for purpose, including meeting today’s more stringent building regulations. The first step involved taking samples for chemical analysis and making sure the reinforcement was in reasonable shape. Fortunately, the concrete was broadly found to be in good condition, with some poured fresh in-situ to remedy areas that required it.

Mace also found that concrete at the edges of some of the existing precast slabs needed to be toughened up. The 2020 cladding is heavier than its 1960s counterpart, which required structurally stronger concrete to fix it to. This saw J Coffey bring the slabs up to the required standard by cutting back the old concrete and adding new reinforcement before reforming the concrete edges and extending them out to their original point.

Building-regulation changes over the past 50 years meant other improvements were needed as well. Today, more concrete is required to encase reinforcement, to ensure it can withstand heat from fire for longer. To rectify this, Mace sprayed five layers of intumescent paint over the concrete to provide a greater fire-resistant barrier.

A further challenge on the stripped-down tower was its uneven profile. Some floors were stepped back from others so that the exterior presented a kind of zigzag line, instead of each floor ending at the same consistent point. To remedy this, about 520 steel plates, which have a stubby ‘T’ shape when viewed side-on, were installed on the edges of the concrete slabs to extend them by 150 to 300 millimeters. These were bolted through the top into the floor and horizontally through the upright of the ‘T’ into the edges of the floors. The result is that each floor now comes out to the same point and presents a flat steel plate to fix the new façade to.

Overall, the issues that Mace encountered with the old structure that needed remedying were minor, according to Fairhurst, with nothing proving significant enough to change the program of works. Here, flexibility was key. “We’ve been mobile enough as a project to deal with it, move around it if we needed to, and keep going, then come back to it,” Fairhurst said.

Aside from ensuring the fabric of the building was up to scratch, other measures were also needed to make sure its structure could take the weight of the 13 new stories. These included stronger foundations with 430 new piles installed across the site.

Most were CFA piles of about 450-millimeter diameter, going into an average depth of about 15 meters, with the deepest extending to 18 meters. Operatives also installed secant piles on the site of the new six-story residential block to create a basement box. About 60 existing piles were also reused, with new capping beams installed to transfer and equalize the load between the new and existing piles.

Fairhurst says the ground engineering was helped by the water table being about 1 meter deeper than the deepest part of the basement excavation on the residential block. But piling under an existing building presented its own challenges. J Coffey, which carried out groundworks, had to find a way to carry out piling in the basement where the existing ceiling was only a few meters high. This made it impossible to bring the big rigs in. “We used mini-piles, and more of them, to offset the fact that we couldn’t have the larger piles,” explained Fairhurst.

The concrete columns that ran through the existing building also needed to be strengthened in many cases so that they could support the 13 new floors.

This was done in two ways. On the lowest levels of the building, where the loads are largest, inch-thick steel jackets were bolted together around the existing columns. Higher up the building, where loads are reduced, fresh concrete was added to the existing columns to thicken and strengthen them.

The team was also conscious of the risk of punching shear, where heavy loads from above can cause the column to punch through the slab it is supporting. To prevent this, the team drilled diagonally into the column and the slab and installed reinforcement.

While the basement of the old building was retained with no further excavation carried out, it did need extensive modifications to accommodate the drainage required by the new building with its extra floors and the layout of the amenities. Cutting in new drainage channels required resin injections, and the installation of water bars and hydrophilic strips in the concrete, to make the modifications waterproof and prevent water ingress. Barter says: “It’s a significant piece of civil engineering, cutting into an existing basement and making sure the waterproofing is maintained.”

Adding stability to this whole extended structure are two new cores. Rising 29 stories. these have a much larger footprint than the original single-core of Finsbury Tower, allowing for bigger stairwells. It was erected, slipform, by J Coffey in the third and fourth quarters of 2019. The new levels from floor 17 upward are comprised of steel.

When the building is completed towards the end of 2021, its 1960s heart will be invisible, a product of modern engineering and construction, but also more modern attitudes to sustainability.

While recycling materials and reducing the carbon dioxide (CO2) emitted in the fabrication of concrete and steel are vital steps in the fight to make construction greener, reusing and repurposing more of the existing built environment has an important role to play, too.

For more on this story, go to Construction News.