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  1. 1. Products & Services <Search <Archives <eBulletin November 20, 2012 Wind and high-rise buildings by Windtech High-rise buildings are particularly prone to a range of wind effects which are dependent on their height and form. In fact, the taller the building, the more wind loads become the single most important factor that will impact on the design of a tall building. This article sheds some light on the workings behind the scene during the design process to address the implications of wind loads. This also furthermore looks into other issues that are brought about by wind which can be of potential nuisance to the owner/occupants and in the extreme case, cause catastrophic effects. The taller the building structure, the more significant its form should be shaped due to the forces of the localised winds. Two high-rise buildings situated at the same location and equal height and volume, can experience very different wind forces depending on their design and orientation. This is due to the fact that high- rise buildings are in fact dynamic and not static structures. Their motion is compounded by the way wind traverses around the structure, applying varying pressures over the building surface. Pressure loads experienced on a buildings’ surface varies due to a number of factors, including the design of the building, its local surrounds and local wind climate. Wind tunnel testing is used to accurately establish the effect of these parameters on the actual wind forces experienced. The measured wind loads on a building can vary significantly due to different design that may be employed. If this effect is not considered during the design process, the cost of building the structure may become excessive, and the project rendered unfeasible. For this reason, some architects are considering and also modelling the effects due to wind for a number of variations of the buildings concept, optimising the form and orientation of the building for their specific site. The form of the building does not only impact the forces on the structural framing that supports the building, but also on the amount of motion that the structure exhibits. If the motion is excessive then it can lead to a feeling of tiredness amongst occupants in certain areas. A lot of research on this subject has been undertaken over the past few years, particularly in Hong Kong. It has been established that for a given level of acceleration, the stiffer the building structure, the more likely that occupants are to feel that same level of acceleration. This is largely due to the fact that our bodies tend to respond to faster oscillations of up to 1 to 2 cycles per second. This potential for Shanghai World Financial Center, Pudong, Shanghai - Height 492 meters, designed by KPF
  2. 2. occupant discomfort due to building motion can be accurately predicted through detailed wind tunnel testing. In cases where the oscillations become excessive, and there is little that can be done to the structure’s design to reduce this, the installation of a mechanical device called a damper can be considered. A tuned damper is a device used to reduce the amplitude of the buildings oscillating motion under wind loads. When installed, dampers are located near the top of the building, and are often the most effective method of reducing the discomfort due to the building’s motion. All high-rise buildings require lifts to transport occupants throughout the building. Lift shafts however, not only provide a void for lifts, but also act as a vent which allows airflow throughout the height of the tower. Very tall lift shafts or vents can generate an effect known as the stack effect. This effect occurs as a result of the difference in temperature of the internal and external air and openings connected to the shaft. This effect can be further driven by internal cooling/heating in more extreme environments, even in the case where the shaft seems to be sealed to the outside. Small leakages such as from air intakes or exhausts can expose the building to the effects of potentially high temperature differences between the inside and outside air. In hot humid climates, the difference in temperature between the inside and outside of the building will tend to cause the heavier cooled air at the top of the lift shaft to fall towards the base of the shaft. This can generate a significant amount of air flow and pressure on the lower level lift doors, resulting in discomfort for occupants in lift lobbies, and in some cases can cause the lift doors to jam resulting in significant disruption. Windtech have inspected such situations on existing buildings, and have provided solutions to
  3. 3. mitigate this effect. Another effect which cause lift doors to malfunction is the issue of wind entry and pressurisation of entry/lift lobbies. This is caused as a result of external openings located in different pressure zones, directly connected to the lobby or lift shaft. There are a number of different ways to eliminate this problem, with this effect able to be recognised during the design phase through wind tunnel testing. Mitigation can be achieved by providing equalisation of the pressure across the lift door or to design effective air-locks for the entries, which are generally the main source of the high pressures in lobby spaces. Wind-noise is another phenomenon known to occur in some high-rise buildings. This effect can be generated from a range of possible sources including facade detail and articulations. Windtech Consultants have had extensive experience in both identifying elements on a building façade or a facade detail that can potentially generate excessive wind noise. It should be noted that, although not common, it is also possible for internal details to be a source for wind noise generation. These are just some examples of wind effects that need to be considered when designing a high-rise building. Examples of other wind related effects include pedestrian comfort under strong winds, wind pressures on the façade cladding and façade attachments, dispersion of exhausts on air quality, natural ventilation and wind-driven rain. (By Tony Rofail, Director, Windtech Consultants, Sydney) Contact Windtech Consultants Website www.windtech.com.au Copyright © Building.hk All rights reserved.

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