Wind tunnel testing and shape modifications facilitate a much taller structure
Steeped in superlatives, the Burj Khalifa has redefined the 21st-century meaning of skyscraper as the world’s tallest building. Located on the southeast coast of the Persian Gulf in Dubai, the most populous city in the United Arab Emirates (UAE), the 2,716.5-foot (828-meter) building has quickly become one of the most recognizable structures in the world. With a style reminiscent of spiral minarets that taper as they reach skyward, Burj Khalifa is a mixed-use high rise, featuring more than 900 private residential apartments, a 304-room hotel, and numerous corporate suites on the tower’s upper floors.
Because wind loading and wind-induced motions were expected to be so critical to the structural design of this tall, slender building, we were brought in to consult early in the design process. Designed at its core as a bundled tube, an approach selected for its inherent strength and stability, Burj Khalifa features a Y-shaped plan that was adopted specifically to “confuse the wind.”
In addition to the movement-related concerns typically associated with very tall buildings, the original design had been based on a building that was not nearly so tall, similar in concept to the final product but approximately 984 feet (300 meters) shorter. Construction had already begun on foundations based on the original design when the project goal changed. We were tasked with finding ways to reduce the wind load effects on the building foundation to enable construction to a much greater height.
Early wind testing using a high-frequency-force-balance provided initial wind load information for the initial design that accounted for the crosswind effects of vortex shedding on the building, providing insights we shared with the design team that led to the shaping of the tower. We followed early studies in the wind tunnel with additional rounds of wind tunnel testing as the tower geometry evolved. This testing enabled us to quickly evaluate the effectiveness of the team’s reshaping efforts as we worked to minimize wind effects and accommodate the client’s changes. By changing the number and spacing of the setbacks, among other shape modifications, it became possible to substantially reduce wind forces on the tower by encouraging vortex shedding that is not synchronized over the height of the tower.
Accounting for the Wind Climate
We also thoroughly analyzed how changing the building’s orientation would alter wind effects. This involved conducting studies to characterize the statistics of extreme winds based on historical records collected at local weather stations. In addition, we conduced computationally intense simulations using state-of-the-art MM5 weather modeling to gain an improved understanding of upper level winds.
Overlaying the findings of our wind climate analyses with the aerodynamic data collected from the wind tunnel studies led us to recommend reorienting the tower such that a “nose” rather than the fork of the “Y” faces the direction from which strong winds most frequently blow.
Extensive Wind Tunnel Testing Program
As the design progressed, we conducted an extensive program of wind tunnel testing that was carried out at our Guelph, Ontario, facility using mostly 1:500 scale models. The test regimen included rigid-model force balance tests, full multi-degree-of-freedom aeroelastic model studies, measurements of localized pressures, and pedestrian wind environment studies. We also built and tested at high speeds a large, 1:50 scale model of the top third of the tower, which was used to evaluate the extent of possible Reynolds number effects and to refine estimates of the calculated wind loads on the structure and pressure on the surface of the building.
Working closely with the other members of the design team to design the Burj Khalifa was an experience that established new standards of what close collaboration with our partners means for us. This structure truly is one of the wonders of the modern world and its successful completion established a new benchmark for the integrated practice of wind engineering and architecture. We are proud of our contribution to a project that truly exemplifies what it means to push the boundaries of the possible.