Pedestrian Wind Studies

A sheet of air moving over the earth's surface is reluctant to rise when it meets an obstacle such as a tall building. If the topography is suitable, it prefers to flow around the building rather than over it. Some examples are shown in Fig.1.30. There are good physical reasons for this tendency, the predominant one being that wind, if it has to pass an obstacle, will find the path of least resistance, i.e., a path that requires minimum expenditure of energy. As a rule, it requires less energy for wind to flow around an obstacle at the same level than for it to rise. Also, if wind has to go up or down, additional energy is required to compress the column of air above or below it. Generally, wind will try to seek a gap at the same level. However, during high winds when the air stream is blocked by the broadside of a tall, flat building, its tendency is to drift in a vertical direction rather than to go around the building at the same level; the circuitous path around the building would require expenditure of more energy. Thus, wind is driven in two directions. Some of it will be

Figure 1.28. High-frequency force balance model: (a, b) Close-up views of base; (c) high-rise model atop the base. (Photo courtesy of Dr. Peter Irwin, Rowan, Williams, Davis & Irwin, Inc.)

Figure 1.28. High-frequency force balance model: (a, b) Close-up views of base; (c) high-rise model atop the base. (Photo courtesy of Dr. Peter Irwin, Rowan, Williams, Davis & Irwin, Inc.)

deflected upward, but most of it will spiral to the ground, creating a so-called standing vortex or mini tornado at sidewalk level.

Buildings and their smooth walls are not the only victims of wind buffeting. Pedestrians who walk past tall, smooth-skinned skyscrapers may be subjected to what is called the Mary Poppins syndrome, referring to the tendency of the wind to lift the pedestrian

Figure 1.29. Schematic representation of five-component force balance model.

literally off his or her feet. Another effect, known as the Marilyn Monroe effect, refers to the billowing action of women's skirts in the turbulence of wind around and in the vicinity of a building. The point is that during windy days, even a simple activity such as crossing a plaza or taking an afternoon stroll becomes an extremely unpleasant experience to pedestrians, especially during winter months around buildings in cold climates. Walking may become irregular, and the only way to keep walking in the direction of the wind is to bend the upper body windward (see Fig.1.31).

Although one can get some idea of wind flow patterns from the preceding examples, analytically it is impossible to estimate pedestrian-level wind conditions in the outdoor areas of building complexes. This is because there are innumerable variations in building location, orientation, shape, and topography, making it impossible to formulate an analytical solution. Based on actual field experience and results of wind-tunnel studies, it is, however, possible to qualitatively recognize situations that adversely affect pedestrian comfort within a building complex.

Model studies can provide reliable estimates of pedestrian-level wind conditions based on considerations of both safety and comfort. From pedestrian-level wind speed measurements taken at specific locations of the model, acceptance criteria can be established in terms of how often wind speed occurrence is permitted to occur for various levels of activity. The criterion is given for both summer and winter seasons, with the acceptance criteria being more severe during the winter months. For example, the occurrence once a week of a mean speed of 15 mph (6.7 m/s) is considered acceptable for walking during the summer, whereas only 10 mph (4.47 m/s) is considered acceptable during winter months.

surroundings can experience high wind surroundings may be protected from large loads and concentrate pedestrian-level winds wind loads and concentrated pedestrian winds

from high winds, reducing wind loads and wind, resulting in higher wind loads and pedestrian level winds pedestrian level winds

Figure 1.30. Near wind climate.

A tall building concentrates wind at its base

Openings through a building at the base may induce high velocities in the opening

Multi-sided buildings may not permit full development ot local pressures, frame loads, or pedestrian-level winds

Circular buildings may reduce trame loads and pedestrian-level winds but increase local cladded loads at the points where the wind separates from the building

Setback all around the building may improve or worsen wind concentration, depending upon S and H

A low-pedestal building concentrates wind on the root, not at the base

Setback all around the building may improve or worsen wind concentration, depending upon S and H

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