Using wind tunnel testing to guide high-rise design in an area prone to typhoons.
The global trend toward urbanization has led to construction of more and taller high-rise buildings in recent years, especially in growing centers of commerce like Shenzhen, China. Located just north of Hong Kong, Shenzhen’s commercial development only began in the 1990s. Since then, development has continued at a rapid pace, and now includes the world’s fourth tallest building, the Ping An International Financial Center, scheduled to open late in 2016.
Its location and height made the wind load a primary factor in designing the Ping An International Financial Center. Nearly 600 meters tall, it is the world’s tallest building located in a typhoon prone area. That means the building will regularly be exposed to the high winds accompanying those fierce tropical storms.
Cross wind effects.
The influence of wind on super high-rise buildings is significant, mainly because of the crosswind effect. As wind passes the building, it creates periodic pressure changes on the sides of the building through the phenomenon of vortex shedding. The resulting strong force moves back and forth from one side of the building to the other, causing a significant lateral dynamic response due to resonant effects. These forces can result in large structural loads and potentially significant motion, especially in tall, slender buildings.
Although resisting extreme wind loads was clearly the primary challenge on this project, there was also a related engineering challenge that needed to be solved first: determining appropriate wind loads to use for the design in the absence of solid historical weather data.
Design Wind Speeds.
Because building codes generally do not provide all the wind loads needed to design super tall buildings, we typically model the building and its surroundings to test how it performs under a variety of wind loading conditions. We ordinarily transform historical weather data collected at anemometers at regional airports using a statistical model to identify the expected wind speeds for design, but could not do so in this case for several reasons.
The historical weather data available for Shenzhen lacked detail and did not cover a long enough period of time to reliably represent the area’s long-term weather patterns. Several aspects of how the weather data had been collected over the years were also problematic, including that the exposure of the weather station had changed significantly over the period of the records due to the rapid urbanization of Shenzhen. As a result, it was impossible to establish reliable baseline meteorological data for the area based on local weather records. In addition, there were uncertainties associated with relating measured wind speed records during historical typhoons to the expected wind speeds for design.
Because of its exposure to extreme climates around the world, KPF, the building architect, was anxious to use wind-resisting design concepts to develop an efficient, high performing building. In the concept phase of the structural design, our experts met with KPF to discuss relevant concepts and a plan for structural aerodynamic optimization.
This concept of aerodynamic optimization of a building’s appearance has been widely studied and has been used successfully by RWDI on numerous super high-rise buildings. One specific technique is including cross-section changes with height, which was used on the Burj Khalifa, the tallest building in the world, and the Shanghai Center. Another effective technique is modifying building corners through such things as chamfering, stepping, and so on. We used this approach on the Tapei 101 building.
Both of these methods were used on the Ping An International Finance Center at the stage of architectural schematic design. The section area of the building shrinks with the increment of height at first, and a detail that mitigates vortex shedding is applied on each corner of the building, optimizing the aerodynamic properties of the building and effectively controlling the crosswind effect.
Wind Climate Model.
An accurate wind climate model and appropriate wind loading are the foundations of a good wind engineering design. In lieu of adequate historical weather data for this project, we turned to our partner Applied Research Associates, Inc (ARA), to obtain the most reasonable expected local wind speed data and wind direction data for typhoons for different return periods. Based on well-respected models, the ARA typhoon simulation uses the Monte Carlo Technique to simulate 100,000 years of typhoons and provides reliable predictions of wind speed with respect to direction and return period. Combining typhoon simulation results with the meteorological data we did have available yielded scientific and reasonable wind speed and wind direction results.
With this loading data, we conducted comprehensive, advanced wind tunnel tests to validate the design and building performance. These included high-frequency force-balance (HFFB), aeroelastic model, cladding pressure, pedestrian level wind (PLW), and other methods. We then used state-of-the-art statistical analysis techniques and upcrossing methods to combine the wind tunnel test results, and the optimized final results were applied in the design.
One of the most important factors in making the Ping An International Financial Center project a success was the wind engineering, which provided the basis of structure and cladding design. We included a pedestrian wind study with this project to verify that the wind environment near the building would satisfy requirements. In addition to supporting this particular project, wind engineering studies for the Shenzhen Ping An International Financial Center provide a comprehensive, thorough, and practical engineering reference that can inform the design of future super tall building projects.