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Chapter 1
Ambitiously tall
The fascination of building begins as a child with stacking wooden blocks to create a tower. The thrill and excitement of those early days is amplified for structural engineers designing tall buildings where the need to reach greater heights (and make sure the blocks do not tumble down) is played out on a much larger scale.
A growing trend for more and more unconventional architectural forms is creating an equal mix of opportunity and constraint for engineering designers where the most challenging schemes are not just extremely tall, but may also hold other complex structures aloft. Working closely with architects to develop structural solutions, these framing systems can be bold and profoundly influence a building’s identity, or be completely concealed from view.
The tall buildings described in this book make a statement on the landscape: symbolic of power for a city and pride for its people. In the role of structural engineer, Arup uses many different methods to allow these urban icons to reach their summits safely and efficiently, often involving specialist computer analysis as well as good, old-fashioned human ingenuity. Structural systems made up of outriggers and bearings, belt trusses and bracing, mega columns and pioneering vertical stressing lift an architect’s dream off the drawing board and into (steel and) concrete reality.
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Case study 1a
Raffles City Chongqing
Chongqing, China
CONTRIBUTORS: ANDREW LUONG, PENNY CHEUNG + JIE LIU
| Client: | CapitaLand Ltd |
| Design architect: | Moshe Safdie and Associates; P & T Group, Chongqing Architecture and Design Institute |
| Arup’s role: | Structural, civil and fire safety engineering plus sustainability services for all design stages |
| Year of completion: | 2018 |
| Height: | Eight towers: |
| T1 & T6 ∼250m |
| T2, T3S, T4S & T5 – ∼270m (including conservatory) |
| T3N & T4N – 350m |
| Number of storeys: | 46 to 79 floors above ground with 3 floors basement |
| Gross floor area: | 1,129,423m2 |
| Use: | Mixed |
| Main contractor: | Tender A for China State Construction Engineering Corporation (CSCEC) Third Bureau. Tender B for CSCEC Eighth Bureau |
Chongqing is a rapidly growing city in western China at the confluence of the Yangtze and Jialing rivers. An important trading centre connecting the east and west of China, the municipality (under direct central government control since 1997) is undergoing expansion and regeneration.
Celebrating prosperity and heralding the city’s future successes is the iconic Raffles City Chongqing megasculpture – an eight-tower mixed-use development made up of curved, slender buildings which mimic the billowing sails of ancient ships which once passed along its great rivers.
The tallest two towers to the north of the development are 355m tall and form the front of the “ship”. They are connected to a bank of four 265m tall towers by two high-level link bridges (a 20m wide pedestrian bridge and a 5m wide fire escape route). Two further 235m tall freestanding towers complete the mixed-use development, which sits on a nine-storey retail podium.
On top of the bank of four towers is a 280m long dazzling steel and glass conservatory that houses the hotel lobby, an observatory, restaurant, pool and spa. So many aspects of this development – its curves, height and slenderness – push the boundaries of tall building design, but none more so than the conservatory. While much engineering ingenuity went into each tower at Raffles City Chongqing, particularly in developing the structural system for the tallest north towers, the conservatory presented the greatest complexities.
1a1 Raffles City Chongqing: an eight-tower megasculpture with stunning rooftop conservatory
Engineers have been designing tall buildings and long span structures for a long time. But designing a combination of the two – a tall, inhabited structure and a long spanning one at high level – is a new proposition and a unique approach was needed to ensure Raffles City Chongqing would be structurally and economically feasible.
Understanding how the four towers and the conservatory would interact with each other was the main point to consider – should the conservatory be physically connected to the towers and move when they moved under high winds, seismic activity, settlement or thermal response? Or should it be isolated from the towers so that the conservatory would be unaffected by any tower movement? And in this isolated scenario, was it possible for a conservatory to float 250m high above the ground?
At the start of the design process, there was no obvious solution, but engineers considered that, since Chongqing is an area of comparatively low seismic activity, the most economic design should be based on a “normal” rather than “extreme” loading scenario. In addition, the design would need to incorporate some means of withstanding the most extreme conditions arising from an earthquake with the probability of occurring once every 1,600 to 2,500 years.
Although the two tallest towers are linked to the conservatory towers via two bridges, it was considered early on that these connections should each incorporate a 1.8m flexible zone adjacent to the north towers to absorb the substantial differential movement which would occur across the towers. This meant that the four towers and conservatory could work as a separate structural system to the north towers.
1a2 Raffles City Chongqing: a mixed-use development
How to make a conservatory float
Three scenarios were considered to connect the conservatory to the towers. For comparison, a simple base case for the scheme with a monolithic arrangement was initially explored (see Figure 1a3). Here, with the conservatory rigidly connected to the towers, the forces developed in the structure would be large, requiring large structural components and bulky detailing which would have significant cost implications and encroach on lettable space.
The second scenario considered splitting the conservatory into sections so that each tower supported one portion. Movement joints in the conservatory would allow each portion to move relative to one another while still being fixed to the tower. Further analysis of this option revealed that provision for up to 3m of relative movement would be needed for this system to work. A movement joint which could accommodate this amount of movement would be unsightly in the sleek conservatory, as well as impose too many restrictions on how the space could be used. Constructing the hammerhead towers would also take longer and, hence, be more expensive, so this solution was also ruled out.
1a3 Exploring the connectivity between the conservatory and towers
The third scenario explored the use of bearings, which would have the effect of reducing the magnitude of the forces transferred to the conservatory from the towers during earthquakes. Connecting the conservatory and four towers dynamically would lead to a lighter structure, which appealed to the architect and would have the added benefit of being much less expensive than the base case. The impact of using bearings on all towers or just two and allowing the other two towers to be rigidly connected to the conservatory was investigated to determine where effic...
