Complex wind engineering for a unique cluster of towers linked by a skybridge
Chongqing Raffles City is among the world’s most ambitious development projects. The 8.8 million sq. ft. mixed-use development comprises eight tall towers and is integrated into an urban hub that includes a metro station, a ferry terminal and other transport links. Situated at the confluence of the Yangtze and Jialing rivers, the Moshe Safdie-designed towers curve gently outward, evoking a fleet of sailing ships.
Chongqing Raffles City includes two 360-meter towers and six 250-meter towers. Four of the shorter towers are linked by a 300-meter long skybridge. Optimizing structural design to ensure safety, comfort and resilience to wind is a priority for any tall tower. Developments with multiple towers present additional complexities, since each tower can affect adjacent ones. This development, featuring not only a cluster of tall towers but curved structures connected by a cantilevered bridge, demanded especially sophisticated wind engineering analysis.
Because the design of this development was so complex, our team determined that we’d have to undertake multiple types of wind-tunnel tests to gain a robust understanding of the loads the structure would experience.
We needed to design a methodology that would capture and quantify a number of distinct effects and the correlations between them. First, as always, each individual tower would experience wind effects. Second, the towers, being clustered together, would interact with each other; this is a complex aerodynamic phenomenon with the potential to amplify wind forces. Third, the horizontal skybridge, 30 meters wide and at an elevation of about 250 meters, would experience wind loading directly. And finally, as the towers connected to the skybridge moved in response to the wind, the differences in their movements would subject the skybridge to additional shearing forces.
The project’s structural engineers, Arup, had engaged us to help them refine their structural design in the face of this exceptional complexity. We determined that delivering the intelligence they needed meant conducting multiple analyses – including, among other tests, high-frequency pressure integration analysis and our first-ever high-frequency force balance test of six towers simultaneously – and combining their results.
Our team’s deep experience with structural wind load studies for unusual structures – such as the CCTV building in Beijing, which also featured a cantilevered bridge element – was an important asset on this project. Not only did our engineers have the specialized capabilities needed to analyze the wind effects on this unique development, but the demands of earlier projects had spurred us to develop proprietary software for complex wind studies that let us effectively combine data from diverse testing approaches. Drawing on all these internal assets, we were able to deliver clear, concise results to Arup.
The results of our study, as well as resulting design guidance, helped Arup to optimize the structural design of this landmark project. We believe the approach we employed for this project will serve as an important benchmark for structural wind load studies for tall buildings with skybridge links. The Chongqing Raffles City development broke ground in 2012 and is currently being constructed. The project is on schedule for phased completion beginning in 2018.