READING TIME: 5 MINUTES
(Source: Karlatornet)
Tall buildings are uncommon in Scandinavia, let alone in Sweden. The appetite to build for statement, though, has firmly gripped Gothenburg, a city located in southwestern Sweden. Developers, Serneke, have teamed up with global multidisciplinary firm, Skidmore, Owings & Merrill LLP (SOM), and Danish practice, Entasis, to create a 73 floor landmark for the city as it gears up to celebrate its 400th anniversary in 2021.
As the beacon of this regeneration project, stemming from industrial wasteland in the Lindholmen district of Gothenburg, the 245m Karlatornet (Karla Tower) will stand as the focal point. The process of transforming the wasteland, lying on abandoned docks and fisheries, into a new amenity-filled district affords the team with much more flexibility for space than on a smaller plot. As part of Forum Periti Inginiera’s series of engineering focussed lectures, SOM structural engineer, Dr Karl Micallef, took us through the challenges in designing Scandinavia’s soon-to-be tallest building.
These were my 5 takeaways:
1. Karlatornet forms part of the first phase of the Karlastaden complex. According to Dr Micallef, the (predominantly residential) tower’s concept revolves around ‘twisted ribbons of flowing fabric’. Although the building employs a simple square shaped form, the facade twists halfway up, as seen below, giving it its characteristic architectural form.
2. Due to its height and slenderness, Karlatornet adopts an outrigger structural system.
What is an outrigger structural system? It’s when the external columns at the periphery of the building are linked to the central core using horizontal structures, such as trusses, on multiple levels, to provide stability and minimise damage. Read more on outrigger systems here. SOM have a reputation for building tall buildings and 15 of the world’s current tallest have been engineered by the firm. It’s no surprise then that it takes great pride in creating specific structural systems for each unique project. In response to the geographically unstable region that Karlatornet is being built on, two double-storey-height outriggers, situated at the perimeter of the tower, connect the core to other columns in the building, thereby providing lateral stability to the entire structure.
3. Following wind tunnel tests, the building’s initial master plan height of 280m was lowered to 245m. It was back to the drawing board for the architects who realised just how much the building accelerated during strong winds. Its orientation towards the city centre also impacted its overall performance as that was where the wind was coming from. By scaling down the tower’s height and rotating it by 45°, acceleration* was lowered by 55% altogether. This helped reduce sway significantly.
4. The tower is set on 57 concrete piles extending up to 60m deep through Gothenburg’s clay, and socketed a further 3m into the granite-like bedrock. Gothenburg’s clay composition is a feature unique to the city and the ground doesn’t make it easy for buildings of this size to be built on it. Through the advice of geotechnical engineers, SOM managed to create these piles, measuring 2m in diameter, which take the actual total height of the building to nearly 300m.
5. An expensive boring process. Due to the tough nature of the granite, diamond-tipped drill bits, costing €600 each, were used to bore through the rock. They were also replaced every few hours due to being worn out quickly. You do the maths on costs when it comes to boring for 57 piles.
* acceleration – in the context of tall buildings, acceleration means how much a building sways under wind. Excessive acceleration, particularly during strong winds, can cause discomfort to occupants.
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