ARCHIVED - Bracing against the wind
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May 08, 2008— Ottawa, Ontario
Ever stood at the top of a very tall building and felt a tinge of panic when it swayed? You can put your insecurity away if NRC's wind engineers were on the job before the building went up.
The NRC Institute for Aerospace Research (NRC-IAR) helps to determine the wind loading and aeroelastic stability of tall buildings and long bridges, before they're built. These studies are especially important when the planned structure will be extremely long or tall, and built on complex terrain or an exposed site.
Working with precise scale models of planned structures, NRC provides design engineers with exhaustive information about the potential effect of wind on the structures. "It's an inexpensive investment when you think of the builder's peace of mind," says Dr. Guy Larose, senior research officer and wind engineer. During his eight years with NRC, Dr. Larose has managed comprehensive wind effects studies on several well-known buildings and bridges of exceptional scale.
"The latest ultra-tall building that NRC has worked on is the Burj Dubai in the United Arab Emirates," says Dr. Larose. "It won't be completed until 2009, but it is already the world's tallest building and free-standing tower."
The Reynolds number expresses the ratio of the inertial forces to the viscous forces of a fluid on a surface. It is used to identify and predict different boundary layer flow regimes, such as laminar (smooth) or turbulent. Laminar boundary layer flow occurs at low Reynolds numbers, where viscous forces dominate. It is characterized by smooth, constant fluid motion. At high Reynolds numbers, the inertial forces dominate and the boundary layer is turbulent, affecting the aerodynamics.
According to Dr. Larose, NRC became involved in this project through its close working relationship with RWDI (Rowan Williams Davies and Irwin Inc.), an Ontario consulting firm that studies wind effects on structures. One of its leaders is Dr. Peter Irwin, a former employee of NRC-IAR and one of Canada's best wind engineering consultants. "RWDI has its own wind tunnels ― on a smaller scale than ours ― in Guelph, Ontario," says Dr. Larose. "This company got the contract for the aerodynamic studies of the Burj Dubai, as it did for the Tacoma Narrows Bridge, which we also worked on. When a project requires tests that exceed RWDI's own capabilities, Dr. Irwin partners with NRC."
"NRC has the only Canadian facility capable of performing wind tunnel tests at high Reynolds numbers on large-scale models," explains Dr. Larose. These tests are done in NRC's 9m by 9m wind tunnel. "We can reproduce conditions approaching what a tall building would encounter at full scale," he adds.
Modelling the Burj Dubai Tower
Designed by Skidmore, Owings and Merrill, the spire atop the tower is planned to reach a height that exceeds 750 metres (2,460 feet), and the tower will have 164 floors.
For its own tests, RWDI had built a model of the entire tower at a scale of 1:500, and performed the tests at a relatively low Reynolds number. For the NRC tests, RWDI provided a 1:50 scale model of the top third of the tower made of balsa wood and aircraft plywood, with all the external detail carefully crafted. "We then fitted it with surface pressure transducers," says Dr. Larose. The tests were conducted at high wind speed, yielding a high Reynolds number.
|1:50 scale model of top of Burj Dubai building, in NRC's 9m x 9m wind tunnel.|
NRC has a wide range of measurement techniques and instruments to collect information about the loads on a structure and how it will respond to various wind conditions. NRC researchers can generate the wind conditions that are relevant to each structure, taking its proportions, design and surrounding terrain into account. "For the Burj Dubai project, we simulated the turbulent flow conditions that can occur at high elevation in the Dubai region," says Dr. Larose. "Wind at the top of a tall building will be much stronger than at ground level, but less turbulent."
How do design engineers use this information? "RWDI used the data to determine the extent of Reynolds number effects, calculate wind loads on the structure, and pressure on the surface of the building," says Dr. Larose. "The results from the NRC and RWDI tests together helped the design engineers define the required size and strength of the foundations, what cladding is required on the building, and many other construction decisions right up to the thickness of the glass at the top of the tower."
According to Dr. Larose, building codes do not provide all the information needed to ensure the safety of structures with exceptional proportions and design. In cases like this, engineers must perform wind tests before making critical decisions on the materials and dimensions of the foundation. One of the things they need to be able to predict is "acceleration" – that is, the rate of change of the back-and-forth movement at the top of the building.
Although a building has to be flexible to stay upright in the wind, if there is too much acceleration, people can get seasick. If wind tunnel tests show the acceleration is too high, the builder must add "damping" to the structure, like shock absorbers in cars, to reduce vibrations and swaying. Large damping devices ― such as a mass rolling on rails ― can be installed at the top of the building where it counters any back-and-forth movement and absorbs energy.
"Although construction of the Burj Dubai Tower is not complete, we have finished our tests," says Dr. Larose. "RWDI engineers got confirmation that their pressure distribution predictions were safe and reliable, which gave them peace of mind." Given the estimated US$4.1 billion construction budget, peace of mind is no small thing.
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