Existing high rise can go higher as London’s South Bank Tower has proved, following the addition of 11 extra storeys.
With space in urban areas at a premium and the cost of demolishing and building from scratch rising, engineers are increasingly looking to reuse and extend existing structures. Work at the South Bank Tower in London is one of the most ambitious schemes to date .
Consultant AKTII last year added an extra 11 storeys or 155m to the 31 storey 1970s office block at the same time as it was converted into residential apartments.
Structural steel frame
The new storeys comprise a structural steel frame wrapped around an extended concrete core above an original concrete framed structure.
Also part of the circa £200M project was a three storey upward extension to an adjacent six storey structure that has been refurbished for office space and to connect the two buildings.
“From a client’s perspective, the main benefit of doing this is time,” explains AKTII design director Hanif Kara. “At South Bank Tower, one option would have been to knock down the original structure, but it could have taken three years to get the planning permission for a replacement which couldn’t have been guaranteed, never mind the noise and the dust of demolition or the bigger issue of sustainability. And developer CIT would have missed the property cycle. Values are dropping in London now, we had the space ready for the peak.”
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AKTII has worked out the key elements that made the project possible. Extending the existing structure meant it used design and construction solutions it had never tried before, but now the work has been done successfully, the firm anticipates that there will be many more tower extension schemes.
“As well as saving developers time and money, communities suffer less disturbance, architects are able to use historical context to enhance their design and a city gets to keep more of its history. The engineering is also a lot of fun and it is a more sustainable approach to design,” says AKTII design director Ed Moseley.
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“South Bank Tower was designed by architect Richard Seifert and engineered by Arup. It was a high quality building. Opened in 1972 as King’s Reach Tower, the headquarters for publisher IPC, the then 111m high building was a landmark on the South Bank of the Thames from the day it was completed,” says Moseley.
The record drawings were available, bought by the developer from Arup, with the client on record as saying it was the best money he ever spent.
“We evaluated the drawings and the archived articles of the building’s design and used our judgment as to what would be feasible for the concept at the beginning rather than doing site investigations – they came during the work,” says Moseley.
“Original planning permission was for a six storey extension to the tower but we knew there was scope for more.
“As residential values went up in London, so the decision was made during construction to add 11 storeys, and take the building to a height of 155m in total, which created 18 more apartments than the original 173 proposed,” he explains. “In hindsight, we could have got a couple more floors out of the foundation capacity but not out of the structure.
Ground consolidation over the building’s life helped the engineers.
“We were fortunate with our timing,” says Moseley. “The tower was founded on clay and this had had 45 years to consolidate and get stiffer as the pore pressures increased under the weight of the structure.”
This meant the load on the 79 under-reamed piles could be increased by up to 25%, from 5,000kN per pile in the original design to nearly 7,000kN. This coupled with detailed analysis meant that the tower extension could be delivered with no additional piles.
Waterloo & City line
The associated low rise T-shaped in plan podium building sits on a mixture of piled foundations and a raft foundation where it passes over the Waterloo & City line. This means that extending the tower involved a degree of complex load redstribution.
“To enable us to extend on top of this raft foundation we had to take some weight off to balance the loading effects, so a new basement was dug to about 10m above the tunnel and a slab quickly put over that, engaging the weight of the existing structures to help hold the tunnel down,” says Moseley.
The stiffness of the clay meant that this mixed mode of foundations – some existing raft, some new raft, some existing piles and some new piles could be balanced to control relative movements.
‘Designed for its time’
Moseley disputes any suggestion that the original tower and associated podium building were over designed.
“It was designed of its time, using the software available for an expressive, brutalist architectural vision that governed the design of some of the structural elements. The building has exposed precast columns that don’t reduce in size the higher up the building you go for instance,” he says.
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Source: Timothy Soar
“This worked in our favour when it came to adding height, enabling us to evaluate redundancy in the original design and utilise that, avoiding areas that were already highly stressed, he says. “The cores too, were designed for the big, heavy lifts of their time and we were left with residual structural capacity due to this. This helped to drive their location within the extended buildings.”
The podium building had originally been designed for cellular offices, all with window space, arranged around a central core. Part of the project was to convert the office space into an open plan arrangement.
“In modern office design, open plan is the norm, with light and views available from wherever people sit, so we studied how we could move the stairs and lifts outside, create an atrium inside and some extra floor space,” says Moseley.
Positioning was worked out with contractor Mace to make the cores easier to build, with access requirements for piling rigs driving some of the core location considerations.
Some delicate strengthening work was also required at the second storey, where the building envelope stepped out, creating a weak spot by interrupting the vertical load transfer through the building.
“This was the only weak spot in a structural design that otherwise had been sized to look similar to the tower,” he says.
“We strengthened it by installing inverted steel corbels on the inside face – it’s not a detail I have seen anywhere else. AKTII calls these design critical locations ‘fuse points’ – limiting factors in the original structure that are causing a proposed modification to fail. Where they are identified, be they a connection detail on a particular column, the team looks to find ways to make loads bypass structural design work. So we try to find a way to short circuit the fuse and release the potential of the building.
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There were fuse points on the main tower too. These were caused by the original tower’s staggered, stepped elevation generated by steps in many of the columns. These steps affected any proposed increase in load.
However, because of the plan configuration there were four locations where there was no stagger and AKTII could add weight to the columns to create the floors.
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“Fortunately they were on the orthogonal axis of the structure. Our solution was to create four, six storey insitu fin walls branching at right angles from the core to the full height columns of the building with steelwork for the glazed walls connecting the fins and creating the walls,” says Moseley.
This concept enabled the existing and new structures to remain structurally separate with the exception of the fin walls and the core.
Hangers were used to tie the new floors together, limiting inter-storey movement and creating a combined stiffness to hold the extension still before the cladding went on.
Getting the construction sequence right was the next challenge. Because there were few structural connections between the outer edge of the new steel frame and the existing structure, it was expected that the floors would deflect as the cladding was added.
To counter this and to prevent the glazing from cracking, the consultant decided to preload the floors with water filled barrels, which were gradually removed as the cladding was added.
“We had barrels of water filled to represent the proposed loads and once the glass was in on each floor we emptied the water out of the barrels. It allowed building and cladding to go ahead concurrently, freeing up any programme constraints,” says Moseley.
“The tower was a significant proportion of the floor plate. As work progressed, and we got used to the building, we realised that we could modify it more than we had originally thought possible and put bathrooms in some of the core space to improve the apartments,” says Moseley. “We took almost 30% out of the core concrete overall, reducing its stiffness significantly, but still had the capacity to add the extra storeys on top.