Hylo Tower

Albert Williamson-Taylor explains how AKT II changed the rules on retrofits and added 13 floors to a 15-storey tower

This story really begins about 15 years ago, with the South Bank Tower. That project broke the mould in terms of intense analysis of buildings and how we understand towers. People said that there was no way you could repurpose and add to towers. We showed that you can.

I was on the Southwark Design Review Panel at the time. George Kyriakou from the developer CIT approached me and asked me to look at this brutalist concrete tower [the 30-storey IPC Tower on the South Bank of the Thames, designed by Richard Seifert and completed in 1972]. There was a pre-planning scheme in place that proposed demolishing the building and replacing it with four 24-storey towers. The original engineers had also done due diligence and calculated that no more than four storeys could be added to the top. George asked me what I thought.

I had a quick look and said, “You don’t have to knock it down. I’m sure we could add at least eight storeys. Let me do some numbers.” We spent three months analysing everything from scratch, learning how the building actually worked rather than how it was designed to work. We developed our own bioclimatic CFD [computer fluid dynamics] tool that could take raw wind data and impose it on any kind of structure. We put anemometers on top of the building to see how much it swayed.

In the end, we added 11 floors to the tower and three floors to the six-storey podium – everything above the ninth floor was essentially new space. We strengthened the core by about 7% and redirected the load path of the structure over the podium. But we didn’t need to strengthen the foundations. When I was called in to the planning department to present the scheme, I had to tell them that it wasn’t a joke. Engineering is not just about following the codes: if we want to progress, we need to go back to first principles and challenge the thinking behind them.

A few years later, George phoned me again because CIT was about to buy a 15-storey exchange tower near Old Street – another concrete building designed by Seifert for British Telecom in the 1960s. They thought they could add eight storeys to it. When we looked at the numbers, we worked out that we could actually add 13 – making it 75% taller. We would have to strengthen the columns at the top, as they weren’t designed to support additional load, but our geotechnical and stress analysis indicated that the foundations were fine.

Then the architect, Stephen Cherry at HCL, said that the floorplates wouldn’t work for modern office space, and that the core, which was on the north-east corner of the plan, would have to be removed and replaced. We calculated that we could demolish the core and the existing structure would remain stable without the need for external bracing. The cladding was stripped off and diagonal bracing was placed between the floor slabs, and then the core could be taken out – we were monitoring closely the whole time for any movement.

When we got to the substructure, some additional foundations were needed for the new cores, so we threaded a series of mini-piles between the existing, under-reamed piles. The cores, on the northern corners of the tower, were then slip-formed to the new height. To minimise loads, the new structure on the top is a steel and composite frame. The brick-slip cladding is also lightweight: fixed to an ultra high- strength precast-concrete panel which is just 75mm thick, about half that of a standard panel.

As with the South Bank Tower, we have also expanded the podium. The existing structure has been extended by 50% to six storeys and a new reinforced-concrete residential block added. An underused site on the edge of Silicon Roundabout now contains double the floor space, with state-of-the-art offices, retail and 25 social-rent homes. We’ve done this while using all of the existing foundations and most of the frame of the original – saving 35% of the embodied carbon of an equivalent new-build. HCL has done such a great job that everybody thinks that it’s a brand-new tower! 

Albert Williamson-Taylor is co-founder and design director of AKT II. Interview by Nick Jones

Photos and drawing Jan Friedlein, AKT II