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The wind factor
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The vertical growth of cities across the world has led to the development of wind engineering as a major factor in structural design.
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That sleek car beckoning visitors to the showroom is most likely to have been designed using wind tunnel studies to cut down air resistance and improve driveability. Wind tunnel testing is normally associated with the automobile industry where reducing air drag is integral to the design of vehicles with better profiles and improved fuel-efficiency.
At the global level, there is another sector that is increasingly recognising the importance of wind tunnel studies in design. The vertical growth of cities across the world has led to the development of wind engineering as a major factor in structural design. Wind tunnel testing is now widely used to reliably predict localised wind conditions on cladding or glazing, as well as the overall structural wind loads on the frames of high-rise buildings.
Employed in the early stage of design, it helps minimise wind effects by reshaping the structure, fine-tuning stiffness and mass distributions and orienting the structure properly. In a wind tunnel, the environment around a building is simulated to replicate the actual wind conditions. The design is then tweaked to achieve the desired effect.
In India, however, architects and builders involved in high-rise construction projects depend more on building codes and standards which have limitations in assessing the wind load factor. Many tall buildings are designed without detailed wind tunnel tests. As a result, they fail to assess the impact of various factors on the structure, such as geometry, surroundings, orientation, direction, complex structural interaction and cross wind response.
“Code analytical methods are helpful for preliminary design and for simple situations, but provide conservative wind loads in most cases and underestimation in others,” points out K. Suresh Kumar, director, RWDI Consulting Engineers (India), a Technopark-based firm here that offers wind engineering solutions. A Canadian company with research facilities in Canada, the U.S. and the U.K., RWDI has played consultant to prestigious projects such as the Petronas Towers, Kuala Lampur, the World Trade Centre, New York, the Manchester stadium, the Paris Theme Hotel and Casino, Las Vegas, the National Bank of Dubai and the Grand Mosque, Saudi Arabia. One of their recent projects is the Burj Dubai, which is claimed to be the tallest in the world when completed.
In India
“The general trend in India is to design a high-rise building based on the wind load provisions in the Indian Standard IS:875 specified in the Building Code. Most users are unaware that the standard is derived from a set of wind tunnel experiments of buildings with simple geometries.”
Mr. Kumar points out that Part 3 of IS:875 is specific on the limitations. “It says that the standard does not apply to buildings and structures with unconventional shapes, unusual locations and abnormal environment conditions. Special investigations and wind tunnel studies are needed in such cases to assess wind loads and their effects.”
According to Mr. Kumar, a wind engineered building is not all about precise loads for design. “One of the most important issues in a tall building is serviceability related to comfort. If the occupants experience motion sickness caused by base sway of the structure, they become uneasy and the building is considered substandard. Once completed, it is difficult to correct a structural problem using external damping devices,” he says.
Urban cluster
In an urban cluster, there are many parameters which affect the manner in which one building modifies the forces on another one in its vicinity. These are size and shape of the building, wind velocity and direction, approach terrain and the geometry and proximity of neighbouring buildings.
A high-rise building is subjected to different wind loads when it is located in a cluster of structures. Vortices from an upstream structure may cause a vortex-induced oscillation response in downstream buildings at frequencies determined by the geometry of the upstream building. On the other hand, a structure in the wake of another one will be subjected to reduced drag.
In other cases, complex structural designs like modal coupling could induce high wind response. Tweaking the structural internal core is the solution to eliminating such effects.
Cross-wind responses of buildings are mainly influenced by the geometry and immediate surroundings. Considering the numerous imponderables, the effects cannot be codified. Therefore site specific wind tunnel tests are the only solution to determine the loads on buildings in crowded urban locations.
“Wind tunnel tests often reveal that the building codes grossly overstate the necessary design load. Also it is common practice in India to complete the foundation before getting wind tunnel results.
T. NANDAKUMAR
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