Analysis and wind-tunnel simulation of pedestrian-level winds in San Francisco

The city of San Francisco enacted a pedestrian-level wind ordinance as part of its city and county municipal code in 1985. The purpose of this paper is to present in engineering detail the wind tunnel testing procedure and associated analysis which allows scaling of the windtunnel measurements. This technique uses site specific meteorological weather data, coupled with the wind-tunnel data, to predicted full-scale wind speeds. The full-scale speeds are presented in terms of 10% exceeded equivalent wind speeds as required by the wind ordinance. Also, a description of the wind-tunnel facility in which many of the pedestrian-level wind studies are carried out is presented. 1. I N T R O D U C T I O N The San Francisco Downtown Plan, adopted in 1985 by the city of San Francisco, established separate criteria for comfort in areas of substantial pedestrian use, for comfort in public seating areas, and for wind safety. The wind intensity is defined in terms of equivalent wind speed. This term denotes wind speed averaged over an hour (hourly mean windspeed), modified to include the level of gustiness, or turbulence, expected on site. The equivalent wind speed as calculated assumes an unaltered wind with an inherent turbulence intensity of 15% of the hourly mean windspeed value. The turbulence intensity is defined as the root mean square (rms) of the instantaneous deviations from the value of the mean velocity, divided by the mean velocity value. When turbulence intensity at a measurement point is greater than 15%, the mean velocity is multiplied by two times the turbulence intensity plus 0.7 to create its equivalent windspeed. This equation follows from relationships developed by Hunt et al. (1976) and Jackson (1978) in which winds with different turbulence intensities were compared to each other for their effects on pedestrians. The comfort criterion for seating areas is 7 mph (3.1 m/s) equivalent wind to be exceeded not more than 10% of the time year round between the hours of 7 a.m. and 6 p.m. The wind speed criterion is based on wind effects summarized in Penwarden (1973), Melbourne (1978), and Arens (1981). The time interval, chosen by the Department of City Planning (DCP), represents the period when most of the population is exposed to the wind; the 10% figure also was chosen by DCP. It is essentially an environmental quality decision, based on Penwarden's study (1973) of wind complaints in shopping centers. Penwarden found that substantial complaints occurred when the limit of comfortable wind speed was exceeded more than 10% of the time. The comfort criterion for pedestrian areas is an 11 mph (4.9 m/s) equivalent wind to be exceeded not more than 10% of the time year round between the hours of 7 a.m. and 6 p.m. The wind speed part of this criterion is based on the original 11 mph (4.9 m/s) limit used by Penwarden (1973); Hunt, et al. (1976); Melbourne (1978); and others. The time interval and the 10% exceedance figure are based on the same logic as the criterion for seating areas. 0167-6105/92/S05.(X) © 1992 Elscvicr Scicncc Publishcrs B.V. All rights rescrvcd.