Managing wind and earthquakes, the skybridge concept floats to lofty heights in Kolkata, India.
The capital of West Bengal, boasts more than 14M people in its metropolitan area, making it the third-most populous in India.
Eighty of its residents of will be paying about 100M Rupees (£1.2M) for a four-to-six bedroom apartment in Atmosphere, twin residential towers in a new development area.
The purchase allows residents access to the Deya – meaning “cloud” in Bengali – a 7,800t, three-level bridge suspended 100m above ground.
As the centrepiece of the £74.8M, 39-storey high-rise luxury apartment project, the Deya seems to have taken inspriration from the Marina Bay Sands resort, Singapore, whose swimming pool spans three towers.
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But the three-level Deya will also house a gym, spa, squash and badminton courts, a jogging track, mini-theatre, golf driving range, open-air party deck and amphitheatre, along with multiple lounges and spaces for gatherings. The exterior will be lit by LED lamps at night and will have a “silver lining” with 15,000 kinetic discs shimmering on its surface.
Situated on the outskirts of the city, in a new development area between the heart of old Kolkata and the airport, the structure has been built within a category 3 earthquake zone.
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One of the challenges for engineers was to come up with a way of accommodating movement in the twin towers at either end of the bridge. “After a number of optional studies we decided to take the bridge… on four bearings, allowing the buildings to move without the bridge moving,” says Web Structures director Hossein Rezai. “The alternative to this – we could have made the Deya monolithic with the two towers and create a torsion frame which would have meant the structure would have been significantly heavier.”
The Deya, now finished, can move freely up to a maximum of 250mm in extreme conditions.
“More frequent wind and ground tremors would allow the building to move 75mm to 100mm. But most of the movement would happen in the first 50mm – 75mm,” says Rezai.
Popular base isolation systems include elastomeric isolators, and friction pendulum bearings, but these were deemed unviable due to the high loads, and lack of available space in which to install them.
For the Deya, the customised isolation bearing system from designer Mageba and fabricators RJ Watson uses disc bearings, “not springs”, and a PTFE/stainless steel interface.
“There is a system built into the bearing assembly where the compressible layers, not springs, but long cylindrical “baguettes” sit horizontally. As the top of the bearing moves, they get compressed at one end, building energy and pushing the bearing back to the centre. But essentially it’s a relatively simple compressible material,” says Rezai.
deya bearing finished
A scale model and full scale prototype were tested in Alden, New York State, with the results confirming their viability.
“Each bearing receives a fairly large vertical load, (up towards 2,500t),” says Rezai. “And the lateral movements of about 200mm – these are large numbers to be factored for bearings. Even the largest ’off the shelf’ bearings are not equipped for such large movements and deformations. So a lot of care was taken in the study and design, both by us and the bearing manufacturers.”
The size required of the bearings was another challenge – larger than a standard shipping container, they were shipped from the US on flat rack containers.
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Each 14.5t bearing assembly sits on a concrete plinth and 200mm thick reinforced concrete slab over 2.5m deep structural steel bearing support girders which in turn disperse the loads into the structure of each tower.
The basis of the bridge’s design is that two longitudinal trusses, enhanced with an arch structure and stiffened with a diagonal grid system span from one tower to the other. The decks are then supported on transverse steel trusses. Together these take all of the dead and live loads back to the bearings.
With the bearings designed, raising the bridge 100m and into position was another major challenge.
It’s a solution that we think could certainly apply elsewhere, where we have supertall buildings and there’s a bearing sandwiched between the top half and the bottom half.
Web Structures director Hossein Rezai
One option for construction would have been to build the Deya in-situ. But this would have required incredibly complex temporary works and supports.
The next obvious option would have been to pre-assemble the structure, then use a temporary crane arrangement which would lift it into place. “So you would pre-assemble about 1,000t cranage and remove it later. But then, we thought ‘the weight of the temporary works is almost the same as the weight of the permanent works’. Plus, the temporary works cannot be reused readily, it has been formed and forged to conform with the shape of this building. So you actually use twice the steel, and only use half of it.”
The third and final option was to build the two ends of the Deya, then use them as bases from which strand jacks lifting the central section into place. “Thereby we effectively have no permanent steelwork. A little bit of temporary works, maybe 10% of total. But this is something that reduces the carbon footprint of the build and we think it’s a world first.”
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Once lifted there was a 1m gap between the mid-section and two end segments. These were welded and bolted together with additional structural steel sections. This process took about 20 days, after the lift was completed in about 12 hours at 2mm to 3mm/second.
Rezaisays using bearings above 100m in towers could prove useful in future megatall skyscrapers.
“It’s a solution that we think could certainly apply elsewhere, where we have supertall buildings and there’s a bearing sandwiched between the top half and the bottom half. And that way we are relieving the top half of a lot of lateral stresses. It could be a new genre, of splitting supertall or megatall buildings into two halves, designed as if they are two 200m-300m buildings sitting on top of each other, with an isolator in between. The bearings in such a situation would be doing the same thing they are doing now (for the Deya).”
- Deya total length – 97.5m
- Distance between towers, where midsection is being lifted through – 48m
- Width of midsection – 22m
- Weight of midsection lift – 650t
- Height at the centre of midsection from ground level – 110.5m
- Total weight of skybridge (including concrete slabs, landscaping, swimming pool etc) – 7,800t
- Permanent structural steel: approximately 2340t
- Total number of bolts – 30,000
- Thickness of steel plate – ranges from 10mm to 150mm
- Four strand jacks lifted the midsection: two 300t, two 500t strand jacks
- Each strand – 15.7mm diameter
- Total number of strands – 120
- Façade system is made of perforated aluminium sheets. All façade nodes and structural members were made in Germany, painted in Singapore and shipped to site for installation