Building Aerodynamics is the art and science of determining wind loads on the main wind force resisting system and on components & cladding of buildings and structures. Pedestrian level winds (wind comfort) may also be of interest. Other testing encompasses smoke, smell or pollutant transport. Testing may be done in two boundary layer wind tunnels and in an acoustic wind tunnel.
The economic design of buildings and structures can be enhanced by extreme value analysis of long term wind speed data which is available from meteorological stations in proximity of the site. A usual outcome of such analysis is a 10 to 30% decrease of peak velocity pressure qp compared with EN 1991-1-4 and corresponding National Annexes. For wind from the east, the reduction may be even more pronounced and is most efficiently used in structural design if combined with wind direction dependent pressure or force coefficients. The wind rose shows qp according to the code (solid line in blue), 50% of qp (dashed line in blue) and qp based on wind speed data analysis from a weather station (solid line in red).
The wind loading of main wind force resisting systems (overall wind loads) is typically determined using extreme value analysis based on time series of relevant load effects. For example, the foundation design of high-rise buildings is largely governed by the peak bending moments and by peak torsion. Using suitable methods such as the load-response-correlation method (LRC), the pressure field causing the load effect in question can be reconstructed. It is typically condensed on a per floor basis for critical wind directions. Resonant dynamic wind loading that may be caused by vortex shedding or gust buffeting is included in this calculation. If static and dynamic wind loads are combined to an equivalent static wind load (ESWL), the structure is ready to be designed. The photograph shows a wind tunnel model of three high-rise buildings and the proximity model.
The wind design of façade and roof elements is largely governed by peak suction acting on small tributary areas. Using extreme value analysis (method by Cook and Mayne) of pressure-time-series combined with correlation of pressure taps, the design peak suction or peak pressure coefficient is calculated. As a first approximation, it is typically assumed that the building skin is airtight, i.e. external pressure coefficients need to be determined.
Local wind loads are typically provided for tributary areas of 1m2 and 10 m2, and are condensed for the envelope of all wind directions to simplify their application in structural design. Dynamic wind loading from vibrations due to turbulence in resonance with the structure may also be of concern for components and cladding if the natural frequency is below 5Hz. The figure shows roof wind loading for tributary areas of 1m2and 10 m2, applicable zoning, and a building plan view.
Cladding and components, double skin façades, and other elements of the building envelope that are back vented benefit from pressure equalization. In a best case scenario, the wind loading of the outer skin is reduced by 2/3 compared with the airtight inner skin wind loading. The pressure differential between the outer and inner skin is significantly influenced by the through flow and by the back flow resistances. The cavity flow is driven by the pressure differential. Generally speaking, the cavity pressure is more constant than the external pressure. In addition, large pressure gradients occur in the vicinity to building corners. Therefore, it is recommended to prevent communication of the cavity air flow of two adjacent building faces by sealing the gap along the vertical edge at the corner. The graph shows an unblocked cavity at the building corner with through flow and gap flow.
Wind loads on attachments to buildings such as parapets, balconies, solar panels sunshades, canopies, solid or open signs, and rooftop equipment can be determined using either wind tunnel testing or recognized literature. The choice of the appropriate method depends on the design task.
Wind resistance of loosely-laid pavers against uplift or overturning is a critical design aspect. Gravel scour on roofs is another issue that needs to be addressed. The question arises which gravel size is needed to prevent scouring of loosely-laid roof aggregate at the design wind speed. Sometimes additional measures need to be undertaken to increase the critical wind speed at which gravel motion occurs. Loosely-laid roof pavers need to have sufficient weight to be resistant against wind uplift which in turn depends on pressure equalization, roof zone, building size and a number of other parameters. Use of analytical models from recognized literature can be a valuable wind design tool.
A wind comfort study analyzes and quantifies the change of wind speed in proximity to the ground caused by the displacement effect of present and planned surroundings taking into account site-specific wind statistics. As a result, the local wind speed-up or slow-down is determined which forms the basis for the evaluation of the usage of outdoor areas for different purposes. A common comfort criterion is that for frequent outdoor sitting use, e.g. restaurant and café, frequent sitting comfort is required. Occasional sitting is acceptable for occasional outdoor seating, e.g. general public outdoor spaces, balconies and terraces intended for occasional use, etc. Standing is acceptable for entrances, bus stops, covered walkways or passageways beneath buildings. Walking is acceptable for external pavements and walkways. Furthermore, a separate safety criterion applies to pedestrians and cyclists.
In some cities wind microclimate studies are required as part of the planning applications of new development proposals.
Wind comfort studies need to be carried out in a boundary layer wind tunnel modeling the surroundings of the building site. As a first step, the sand erosion technique may be applied. Further insights into wind microclimate may be gained by measurement of local wind speed and turbulence for critical wind directions. Through analysis of areas with local wind speed-up, measures to improve pedestrian level wind conditions can be undertaken. Such mitigation measures may include geometrical changes of buildings, addition of vegetation or canopies.
Sand erosion studies require recordings shifted in time of a sand layer which previously was deposited on the turntable on which the model sits. Local wind speed-up acts to scour the sand. Typically, sand erosion studies are conducted in wind direction increments of 30 degrees. The figure shows the wind comfort in an outdoor area between high-rise buildings as determined from a sand erosion study.
To ascertain the habitability of a high-rise building, horizontal peak accelerations need to be determined and evaluated using a suitable criterion. A number of criteria to assess human response to wind-induced motions in buildings are available in the published literature. Generally speaking, human perception is sensitive to amplitudes and frequencies at which building motions occur.
Based on a combination of local wind speed data, empirical assessment outlined in standards, and of measured wind tunnel data, peak accelerations occurring on the upper level of a high-rise building are calculated. Using the one-year return period as defined in ISO 10137:2007, wind induced vibrations are evaluated with regard to habitability depending on the occupancy, i.e. regular office or residential occupancy.
• A central control is based on one or more sensors that measure the wind speed and direction at locations where the approach wind flow is only slightly perturbed. Typically, such locations are optimized based on wind tunnel testing and are likely to be found on the building roof. Hence, sunshade control is based on wind speed-up factors versus wind direction for the different building faces. With the maximum allowable wind speed of the sunshade system given by the manufacturer, threshold values are set at which the sunshades, e.g. awnings or blinds, go to a safe stow position.
• Decentralized control of sunshades involves placement of wind sensors on all façades based on a suitable zoning. While many anemometers need to be placed across the façades, the advantage of such methodology is that no conversion of wind speed from a rooftop measurement to a local wind speed is necessary. Another advantage is that decentralized sunshade control will adapt to changing surroundings, i.e. newly built environment which is not the case for centralized sunshade control.
Monday - Friday: 08:00–17:00
Saturday - Sunday: closed
info [at] ifi-ac.de