Structural Engineering Question: How does the British Airways i360 become the most slender tower in the world?
Awarded Guinness World Record for most slender tower in the world, the British Airways i360, designed by Marks Barfield Architects, the same team who designed the London Eye, has a height to width ratio of 41.15 to 1, with a height of 160m. Due to its slenderness and height, the material that makes up the i360 needs to be carefully designed and manufactured.
The i360 is made out of prefabricated cylindrical steel sections (that are 4m in diameter) by a Dutch steelwork specialist called Hollandia. A great deal of craftsmanship was needed to manufacture these steel circular tubes that are 6m to 12m in length since only low tolerances can be handled. These 17 tower steel sections need to be fabricated into perfect curves, have flanges at the ends shaped as a ring, fastened by 68 steel bolts at the flanges, and angles cut off at the sides for weld connections.
The i360 is made out of prefabricated cylindrical steel sections (that are 4m in diameter) by a Dutch steelwork specialist called Hollandia. A great deal of craftsmanship was needed to manufacture these steel circular tubes that are 6m to 12m in length since only low tolerances can be handled. These 17 tower steel sections need to be fabricated into perfect curves, have flanges at the ends shaped as a ring, fastened by 68 steel bolts at the flanges, and angles cut off at the sides for weld connections.
i360 Brighton Source: Tripadvisor.com
Other interesting features of the structure includes the tallest moving doughnut-shaped observation tower, made out of double glazed laminated glass sheets glued together. The moving viewing platform provides occupancy for 200 people and has 360 degree views!
Structural Design Challenges
It can be a disaster to design and construct this tower if coordination and planning are lacking. There are several types of challenges that the engineering team encountered. These included:
It can be a disaster to design and construct this tower if coordination and planning are lacking. There are several types of challenges that the engineering team encountered. These included:
- Live loading of people for the viewing platform
The viewing platform has to support around 200 people, so the design team had to consider large live loads and the flexural buckling of the tower induced by the axial compression force of the live load into the design of the tower. Moreover, the design team went above and beyond by considering “the worst case scenario”, which considers the unbalanced live loads where all 200 people are located at one side of the viewing platform. We need to take into account all scenarios, identify the worst case scenarios, and then design the structures according to those scenarios for conservative purposes.
i360 British Airways Viewing Platform Source: Businesstraveller.com
- Dynamic behaviour of wind loading
Another challenge was designing the tower to resist 50% chance of 3 seconds of extreme gusts within a 50 year period, which depends on the wind speed, wind direction, and the position of the platform. The tower also has to resist the effects of vortex shedding. Vortex shedding occurs when wind separates around the perimeter of the structure and flows and whirls around it. This then creates vibrations and additional lateral loads. The consultants came up with having 76 internal ‘sloshing liquid dampers’ to resist the lateral deflection, vibrations, and loading. These liquid dampers function by having water in a solid tank or container connected to the top of the tower and using the sloshing movement and energy of water to offset the tower deflection and to dissipate energy. Perforated aluminium cladding around the perimeter of the tower is also designed to resist wind flow.
Source: Vortex Shedding Vibrationconsultants.com
- Erection of tower
Another challenge was the construction of the tower. The top down jacking method had to be used. A jacking system was used to push each of the tubes up and fit the tubes together.
Prefabrciated Cylindrical Steel sections by Hollandia Source: https://pbs.twimg.com/
- Weak and difficult ground conditions
6 meters of uniformed marine deposits overlaid the chalk rock formation in the site.
Soil removal and replacement were used as methods for soil improvement for construction of the foundation. 7,200 tonnes of gravel and chalk were removed and were replaced with 3 meter thick reinforced concrete foundation. This increases soil friction angle, density, stiffness, and bearing capacity.
To reach down to the chalk formation for support, the consultants had to design a foundation that is 6.25 meters deep. The foundation used two types of piling; bearing piles and secant piles. Bearing piles were placed and driven 13 to 20 meter to the chalk strata, then pile caps and beams were constructed on top. Secant piles (shown below) were constructed with reinforced concrete pile shafts lined up and overlapped to create 10 meter deep basement walls. Alternative piles were constructed first by leaving a space for intermediate piles to fit in between.
Secant Wall Example Source: morrisshea.com
Removal of underground parts, such as electrical cables and Victorian sewer systems, would require permission from the English Heritage and the government. Careful ground investigation would have to be done to identify these parts and to apply for consent.
Overall, it does take lot of creativity, investigation procedures, and knowledge of clear specifics of structural engineering techniques to make this marvellous project a reality
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