Testing multiple scale models yields appropriate design loading for unique structure
Located in Singapore’s central business district, just one degree north of the equator, CapitaGreen is a 40-story commercial development that features lush vegetation in its rooftop forest and throughout the height of the building. Designed by Toyo Ito & Associates, the building is intended to be like a plant growing toward the sky. Its irregular hexagon shape nearly fills a triangular lot in among the city’s other tall buildings. The building is topped with a distinctive petal-like structure on its rooftop which is designed as a wind scoop to funnel fresh air into the building.
What perhaps makes the building most iconic is its unique façade system. This consists of inner and outer layers of glass separated by a 2 meter (6.5 foot) space, much of that planted with vegetation. The inner layer consists of a high-performance double-glazed unitized curtain wall system, but it is the outer layer that makes the facade so unique. It is a complex arrangement of flat glass panels and narrower glass fins which are spaced apart to create a permeable outer layer that allows air to circulate while sheltering the vegetation.
We were retained to determine wind loads for the design of the structure and the building’s exterior envelope. We also were tasked with conducting an assessment of the potential for the unique envelope of the building to generate noise or oscillate due to the wind and to study wind effects on pedestrians in the area.
Modeling the façade system and the wind scoop for wind tunnel testing were the most interesting and challenging aspects of this unique project.
We used traditional 1:400 scale wind tunnel testing for the overall cladding pressure study. Using this reduced scale allowed us to simulate the effect of surrounding buildings and the complex aerodynamics of the building. Our 1:400 scale model was built using SLA prototyping technology, which allowed for very fine and accurately modeling of the outer skin geometry, but we had concerns whether at this small scale the physics of the airflow was being accurately simulated. To address this, we also tested larger 1:100 scale sectional models of the two-layer facade system in the wind tunnel. Testing the larger scale models more accurately simulated the flow through the gaps in the exterior façade, which enabled us to confirm, and refine as necessary, the pressures identified during the 1:400 scale model tests.
Based on more simplified calculations, our assessment of the rooftop wind scoop indicated the individual petals might be susceptible to potentially catastrophic phenomena such as flutter, galloping and vortex induced oscillations. To verify the stability of these petals, we constructed and simulated in the wind tunnel a 1:120 scale aeroeleastic model of the upper portion of the CapitaGreen building, which incorporated physically how the two tallest and most vulnerable petals of the wind scoop vibrate and interact with the wind. If a structure is aerodynamically unstable it typically shows a sharp, large increase in deflections over a narrow band of wind speeds that is above and beyond what is expected due to turbulent buffeting. For all of the tested cases we found no such responses for the wind speeds that the building is expected to experience during its lifetime.
With a presence distinctly different from the surrounding built environment, CapitaGreen represents the best in sustainable design. CapitaGreen was named the Best Tall Building of 2015 for the Asia and Australasia region by the Council on Tall Buildings and Urban Habitat (CTBUH). It also was the winner of the 2016 Singapore Green Building Council-BCA Sustainability Leadership Award for Design & Performance – Commercial.
Our careful and studied analysis facilitated the design and construction of a unique building that has redefined the possible, raising the bar for similar endeavors as well as ensuring the CapitaGreen’s structural performance for years to come.