{"id":738070,"date":"2024-05-13T06:24:10","date_gmt":"2024-05-13T05:24:10","guid":{"rendered":"https:\/\/www.architectsjournal.co.uk\/?p=738070"},"modified":"2024-05-15T15:25:44","modified_gmt":"2024-05-15T14:25:44","slug":"in-practice-urban-quarrying","status":"publish","type":"post","link":"https:\/\/www.architectsjournal.co.uk\/specification\/in-practice-urban-quarrying","title":{"rendered":"In practice: Urban quarrying"},"content":{"rendered":"

\u2018It\u2019s like a jigsaw puzzle,\u2019 says Patrick Lam, associate principal at KPF. \u2018The idea is very simple: you just have to put all the pieces together in the most efficient way.\u2019 We\u2019re discussing the practice\u2019s retrofit of BT\u2019s former headquarters \u2013 now known as \u2018Panorama St Paul\u2019s\u2019 \u2013 which is currently on site and due for completion in Q2 next year. The 1980s office opposite St Paul\u2019s Cathedral is being transformed through what the practice describes as a \u2018reuse and adapt strategy\u2019 into a mixed-use development featuring a public roof terrace and a new \u2018street\u2019 through the site. It will become HSBC\u2019s HQ following its move from Canary Wharf.<\/span><\/p>\n

The building is being updated through a series of major architectural interventions to create workspaces with much larger floor plates and an abundance of outside space. At every level, office floors will have access to external terraces created by a composition of stepped bays.\u00a0<\/span><\/span><\/p>\n

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The retrofitted building is currently on site and due for completion in 2025<\/p>\n\t

Source:Uniform<\/p><\/div>\n

The most interesting aspect is how the project majorly improves upon the existing\u2019s environmental performance and minimises future environmental impact by reusing all its Portland stone, a first for KPF. In addition, the scheme employs modular construction methods, specifies low-carbon materials and adopts a zero-landfill policy. At completion, the building at 81 Newgate Street will be net zero carbon-enabled. It\u2019s a convincing story.\u00a0<\/span><\/span><\/p>\n

The original General Post Office building on the site, dating from 1874, became the Central Telegraph Office. Heavily damaged in the Second World War, it was eventually demolished in 1967. The BT Headquarters that succeeded it was built in 1985 by the Property Services Agency, with Portland stone and granite facings inspired by the previous building. This was occupied by BT until 2021. KPF was commissioned for its transformation by Orion Capital Managers. The strategy from the outset was one of reuse, keeping approximately 70 per cent of the existing concrete structure intact.\u00a0<\/span><\/span><\/p>\n

\n

\u2018It\u2019s like a jigsaw puzzle; you just have to put all the pieces back together in the most efficient way\u2019<\/p>\n<\/blockquote>\n

Indeed, KPF says that from the moment they were approached for the project the assumption was that the building should be substantially retained, as it had been so well-built. \u2018The PSA was known for seriously robust and well-detailed buildings,\u2019 says principal Paul Simovic. \u2018Irrespective of how it ended up looking, it was quality work.\u2019<\/span><\/p>\n

Provided the floor to ceiling heights could be made to work for modern offices, services could be pushed through and the frame could take the weight of additional structure on top. There was no need to demolish. And there were big programme and cost advantages to saving the frame, not just in terms of carbon. Visually, however, a lot has changed. The practice has taken away the south-west portion of the building, tapering the massing back to improve views to St Paul\u2019s, filled in an original full-height glazed atrium at its centre with new cores, extended the basement out to create space for a 25m-long pool, added four floors on top and stepped the block to add 2,500m2<\/sup> of open terraces.\u00a0 <\/span>The existing building had Portland stone to its main fa\u00e7ades and granite facings at ground floor level, providing two sources of stone. \u2018We had such good-quality stone from the get-go, we wanted to reuse as much as possible,\u2019 says Lam.<\/span><\/p>\n

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Left to right: Demounted Portland stone awaiting processing in the yard of subcontractor Grants of Shoreditch; a sample undergoing cleaning; panel mould ready for a concrete pour<\/p>\n\t

<\/p><\/p><\/div>\n

How did the practice go about this? From the start, it just looked at how many pieces of stone they had available on the building to understand the quantities they had to work with. This was done through rigorous calculations and by tallying up the various bays \u2013 understanding how each was made up.<\/span><\/p>\n

\u2018Obviously, the redesigned building was a different shape and massing, so we had to figure out how to use the same amount of stone redistributed across a bigger building,\u2019 says Lam.<\/span><\/p>\n

This was achieved by making larger openings in the fa\u00e7ade while following the same structural rhythm as the original. This also made the building look \u2018less oppressive\u2019, While admitting more daylight and improving its thermal performance. The building\u2019s bays were also panelised, so that they could be constructed quickly.\u00a0<\/span><\/span><\/p>\n

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Bay redesigned to have increased daylighting and natural ventilation<\/p>\n\t

<\/p><\/p><\/div>\n

The original also had no insulation to speak of. \u2018Although it looks like we\u2019ve left the fa\u00e7ade intact on the building, the reality is that it had to come off anyway to insulate the frame,\u2019 says Simovic. \u2018It still follows the same structural rhythm as the original, but it\u2019s a completely new fa\u00e7ade.\u2019<\/span><\/p>\n

The next step was to figure out how to reuse each piece of the fa\u00e7ade\u2019s stone. Working closely with specialist stone subcontractor Grants of Shoreditch, the team conducted a detailed survey to determine where each piece could fit into the new configuration. \u2018As we did this exercise iteratively, eventually we ended up with an optimal arrangement where we could use each of the pieces in the most appropriate way,\u2019 says Lam.\u00a0<\/span><\/span><\/p>\n

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Colour-coding demonstrates where each stone has been relocated<\/p>\n\t

<\/p><\/p><\/div>\n

The stonemasons dismantled the stone over the course of three and a half months in an \u2018all-in-one process\u2019. It was dismantled from the concrete structure, packaged, barcoded, then transported back to the subcontractor\u2019s manufacturing facility in Doncaster, where it was stored and then cleaned of 40 years of London grime. Each piece was individually cleaned, pushed through a sander and then manually buffed. And after that the stones were cut down to the right sizes and laid upside-down in moulds to be cast into the new fa\u00e7ade panels. Pins long enough to extend into concrete were attached and concrete was poured into the back of the mould. Each new panel contained 30 to 40 pieces of reclaimed Portland stone.<\/span><\/p>\n

The addition of insulation meant the radiuses of curved pieces of stone had to increase. The practice had originally been unsure whether this would be possible, but the stonemasons were able to adjust them to correct the radius.\u00a0<\/span><\/span><\/p>\n

In the end, almost all the granite was reused and a very significant portion of the Portland stone. A tiny amount of new stone was required for the new building.\u00a0<\/span><\/span><\/p>\n

KPF says 81 Newgate Street provided an \u2018ideal opportunity\u2019 for the practice to use this \u2018urban quarrying\u2019 technique. \u2018It\u2019s a new kind of reimagining for us,\u2019 says Simovic. \u2018This is the first of our projects where materials have been very visibly reused. It was a very forensic exercise \u2013 not something we had really done before. Certainly not at scale.\u2019<\/span><\/p>\n

\"\"<\/p>\n

So, what were the main challenges? \u2018It was probably working out how to use the curved corner stones,\u2019 says Lam. For Simovic, it was the risk of breakage during the removal of the stone: particularly when opening the structure, as they didn\u2019t know how it was all fixed and whether it was in a good enough condition to be saved. \u2018Actually, the wastage was very small, and they did a good job of removing it,\u2019 he recalls.<\/p>\n

\u2018It was a geometric challenge,\u2019 he adds. \u2018Once you\u2019ve set yourselves the rule that you\u2019re going to minimise processing and resizing, it\u2019s a big job to put that into effect. To work out the most effective configurations, with the least amount of work, took a lot of time.\u2019<\/span><\/p>\n

The scheme comes nearly 10 years after the practice\u2019s retrofit of Southbank Tower. Originally designed in 1972 by Richard Seifert as an office block, the scheme was transformed by KPF into a residential tower with offices below, adding 11 storeys to the existing and replacing and repositioning the original cores.\u00a0<\/span><\/span><\/p>\n

A very different type of scheme, with no stone cladding, it did share certain construction features with the former BT HQ, such as its lack of insulation.\u00a0<\/span><\/span><\/p>\n

\u2018There, the concrete cladding was integrated with the structure and so there wasn\u2019t really a chance of removing it,\u2019 says Simovic. Instead, the practice had to insulate behind, which was far more challenging, particularly with respect to the building\u2019s inconsistent thermal line.\u00a0<\/span><\/span><\/p>\n

\u2018Every building is different,\u2019 says Simovic. He adds that, had there been a smaller amount of stone on the existing building at 81 Newgate Street, there might still have been alternative ways to repurpose it \u2013 as flooring, for example.\u00a0<\/span><\/span><\/p>\n

\u2018Panorama St Paul\u2019s has been a very useful test case for this kind of repurposing on this scale,\u2019 says Simovic when asked what the practice has learnt. \u2018This style of building is inherently reusable. So, in future, when there is a similar opportunity, we will be pushing hard for our clients to be doing it.\u2019<\/span><\/p>\n

What this showcase scheme demonstrates is that using an existing building as an \u2018urban quarry\u2019 is not as simple as it sounds. It can throw up challenges along the way and the reusability percentage can vary, depending on how well the existing was built. A lot is down to luck as much as anything. However, there is hope that this type of \u2018cut and carve\u2019 construction will become more normalised for city-centre offices. Reuse such as this can be fiddly, but it\u2019s worth it: it saves cost in terms of carbon, money, and time.<\/span><\/p>\n","protected":false},"excerpt":{"rendered":"

\u2018It\u2019s like a jigsaw puzzle,\u2019 says Patrick Lam, associate principal at KPF. \u2018The idea is very simple: you just have to put all the pieces together in the most efficient way.\u2019 We\u2019re discussing the practice\u2019s retrofit of BT\u2019s former headquarters \u2013 now known as \u2018Panorama St Paul\u2019s\u2019 \u2013 which is currently on site and due …<\/p>\n","protected":false},"author":59337,"featured_media":738182,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_oasis_is_in_workflow":0,"_oasis_original":0,"ep_exclude_from_search":false},"categories":[721],"tags":[4286,1334,1786,2981],"yoast_head":"\nIn practice: Urban quarrying<\/title>\n<meta name=\"description\" content=\"Nearly four years after the AJ spoke to KPF about its plans to \u2018mine\u2019 the 1980s former BT HQ\u00a0 in the City of London for its Portland stone, Fran Williams looks at\u00a0how its ambitious plans are turning out\" 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