Wind tunnel testing high-rise buildings

Throughout the world, and the Middle East is no exception, increasingly complex high-rise buildings are being developed. The emergence of unusual, exotic and ever taller structures has lead to an increasing need for the services of wind engineers who can test the building design in wind tunnels and assist in optimising the building’s shape and wind resistance in the process.

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By  Colin Foreman Published  July 15, 2004

Wind tunnel testing high-rise buildings |~|Aeroelastic DSCN1408body.jpg|~||~|Throughout the world, and the Middle East is no exception, increasingly complex high-rise buildings are being developed. The emergence of unusual, exotic and ever taller structures has lead to an increasing need for the services of wind engineers who can test the building design in wind tunnels and assist in optimising the building’s shape and wind resistance in the process. Most codes and standards recognise that wind tunnel tests can produce more reliable loading estimates and incorporate provisions for this. In some locales it is commonly understood that the code provisions are very conservative and significant savings in cost of structure and cladding can be obtained through carrying out a wind tunnel test. boundary The type of wind tunnel used is called a “boundary layer wind tunnel” and it incorporates a long section upwind of where the model is placed in which floor roughness and turbulence generators are present. This is to simulate the natural drag of the Earth’s surface (ground, vegetation, terrain, man-made objects) on the wind flowing over it. Great care is taken to produce the correct change in wind speed and gustiness with height for each type of terrain, over which the wind approaches your project. In situations where the wind approaches your project site over mountainous terrain, a parallel study, using a smaller scale model of the topography to evaluate its effect on the approaching wind, might be needed. The wind tunnel test facility needs to have appropriate instrumentation and data acquisition in order to obtain the huge amounts of information required from the test in a reasonable length of time. This is usually in the form of simultaneous surface pressure measurements (for cladding design), high-frequency force-balances for measurement of overall structural loading, and hot-wire anemometry for measurement of wind speeds and turbulence. Test should be done when a building is: more than 10 storeys in hurricane areas; 22 to 25 storeys in non-hurricane areas; unusual shapes; complex surroundings (terrain or other structures); want to optimise cost and safety of project. The tests themselves should be carried out by a suitably experienced wind consultant. For the tests a consultant will need: Preliminary info on project including information on the structure and the cladding. Other issue such as pedestrian wind and exhaust dispersion should also be discussed. RWDI constructs the actual study model in its own shop, generally using a plastic shell with several hundred or more surface pressure taps installed at points on the surface (where experience with wind flows dictates that the important wind pressures for design are likely to occur). For structural loads, a lightweight model is mounted on a stiff force balance which measures the overall fluctuating wind loads on the entire structure. These lightweight models can generally be produced quickly so that the most urgently needed loads can be acquired as a priority task. Models of the surrounding buildings, usually within a radius of 460m to 760 m of your site, are generally constructed from plastic foam. These are placed on a circular disk and set on a turntable in the wind tunnel. Rotating the turntable effectively simulates wind approaching from any direction. Aeroelastic models, which are constructed to flex in the wind tunnel like the real structure does in response to ambient wind, are used only for exceptionally tall buildings or flexible structures, such as bridges. The wind tunnel is capable of measuring a very large quantity of data. Hundreds of pressure locations can be measured hundreds of times each second and the data stored. The data are then processed to provide design information on peak loading of cladding elements, instantaneous pressure differences across parapets, canopies and across corner mullions. Both pressures and suctions must be evaluated. Still, this sounds (and is) complicated. Structural loading tests use fluctuating forces and moments measured in the tunnel and combine them with the structural dynamics of the building. Furthermore measurements are generally made for 36 wind directions in 10° increments and all of this is combined with statistical models of the local wind climate, which in the Middle East would include the summer Shamal. An experienced wind consultant can boil all this down to a simple set of recommended loads, which plugs into your structural design as if you used the code. The typical form of these recommendations is a set of block diagrams of design wind pressures is showing pressure zones mapped on the surface of the building (to be included in a bid package) or a set of loads to apply along the height of the structure so the structural engineer can evaluate the sizing of structural members. Are there any other advantages to doing a wind tunnel test? Once a scale model is constructed, there are many optional tests and analyses that can be of great benefit to the design, such as: parametric analysis of structural loads (changes in stiffness, mass, damping), differential loads on adjacent elements, loading of secondary structural members, effects of other buildings in phased projects, loads during construction phases, pedestrian wind environment, snow loading, roof paver lift off, gravel scour, flow visualisation and measurement of exhaust re-entrainment, sliding snow and ice and rain infiltration under canopies. More detailed studies such as aeroelastic studies, which may be necessary for bridges, large span roofs, spires and cantilevered features where vortex shedding or instabilities are possible, can also be undertaken. While complicated to carry out, initiating a wind tunnel test and using the results are not rocket science for a structural designer. Even projects which, at first examination, look entirely “code-like”, can benefit greatly from incorporating the physics of the situation through a wind tunnel test. Dr. Peter Irwin and Dr. Anton Davies are principals of Rowan Williams Davies & Irwin (RWDI) based in the firm’s Guelph, Ontario, Canada wind tunnel laboratory. They recently presented the first Dubai workshop on wind engineering for modern high rise buildings at the American University of Sharjah, UAE, co-organised by the Dubai Municipality Building Permits and Structural Engineering Department.||**||

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