Wind Pressure Analysis

The Suction of Wind
The John Hancock building in Boston is a classic example of failure due to unanticipated wind induced loading. In this case the glazing system was not designed to withstand the negative pressure experienced. Consequently, windows were literally “sucked” from the building. Tall buildings, irregularly shaped buildings, or buildings in close proximity create unique wind flow patterns. These may result in excessively high pressures that are difficult to anticipate based on experience alone. A better understanding of the wind’s interactions with buildings more than negates these concerns; it allows innovative building forms, along with optimized structural systems.

How Can You Anticipate Wind Pressure Problems?
A wind pressure analysis will determine the magnitude of the pressures required for design, to prevent dangerous failures and keep costs in line. These analyses can be applied to the entire structure or specific design features.

Site Model:
Wind pressure analysis requires construction of a detailed scale model of the proposed building and other significant structures in the vicinity. Pressure taps situated on the surface of the proposed structure(s) permit measurement of detailed surface pressure distribution.

Testing:
The model is tested in the University of Western Ontario's boundary layer wind tunnel under appropriate conditions for the site. Pressure readings are taken with a high speed computerized data acquisition system for the equivalent of one hour at full scale.

Analysis:
Wind pressure data provides peak, mean, minimum, and rms coefficients of exterior pressure at each tap location for 36 wind directions. These are combined with the statistical wind climate for the area to provide predictions of peak exterior pressures and suctions for various return periods. The resultant predicted peak exterior pressure and suctions are often presented as pressure contour diagrams produced to overlay the various building profiles. Interior pressures induced by air infiltration and mechanical systems are estimated and their effect added to the measured exterior pressures to provide maximum differential exterior pressures and suctions for the building cladding. The results are used to design wall and window systems, roof surfaces, ventilation intakes, exhausts, and accesses, eliminating what have proven to be potential problem areas on numerous occasions.

Mean Wind-Induced Forces:
The mean exterior pressure coefficients can be integrated over the facade of a building to provide mean shears, torque and bending moments at various heights along the building for each wind azimuth direction. Mean data is used because peak pressures do not occur simultaneously. This data provides a check against force balance data and is used to provide the mean load distribution required to determine the effective static load distribution.