STUDY OF PEDESTRIAN LEVEL WIND ENVIRONMENT IN THE VICINITY OF TALL BUILDINGS

Abstract

Tall buildings cater to the need for increased workspace with a reduced footprint, enhance the skyline of major cities and also enhance a city's image and prestige. Tall buildings also tend to intercept upper level high velocity winds and deflect them to ground level giving rise to conditions that could be unpleasant or even dangerous to pedestrians. This problem gets aggravated by isolated tall buildings or buildings that rise above their surroundings. During the 1980s and 1990s many cities around the world, such as Auckland, Wellington, Boston, Toronto, and San Francisco instituted bylaws related to the pedestrian wind environment. Pedestrian wind comfort and safety in urban areas is an emerging area of research which requires inputs from wind engineers, urban designers, architects and urban planners. The focus of this study is on the pedestrian wind environment in the vicinity of tall buildings. India has seen an unprecedented growth of tall buildings in recent years. This growth has mainly centered in Mumbai, the National Capital Region (NCR) of Delhi namely Gurgaon, Ghaziabad and Greater Noida and in cities like Calcutta, Bangalore and Hyderabad. In Mumbai alone there are more than 1200 high rise buildings and more than 80 tall buildings having more than 30floors are presently under construction. The Indian Wind Code {IS: 875(Part 3) 1987} does not contain any references to pedestrian wind environment and there are no ordinances, urban design guidelines or criteria for assessing pedestrian level winds. Further a shared image of the wind environment between the architectural profession and the wind engineering community does not exist. The present study attempts to bridge this gap. Case studies of many tall buildings in Mumbai, Delhi, Gurgaon, and Ghaziabad have shown that there exists the potential• for adverse pedestrian wind effects in the vicinity of tall buildings in the country. Wind amplification, recorded at the base of several tall apartments and hotel buildings has strengthened the need for the present study. • The present study aims to make a comprehensive assessment of the pedestrian level wind environment in the vicinity of tall buildings through wind tunnel investigations. The broad objectives behind the tests are (1) to understand the various flow mechanisms that result from the complex interaction between wind and tall buildings (2) to identify areas of iii wind speed amplification and stagnation in the vicinity of tall buildings (3) to explore the role of architectural and landscape elements in ameliorating adverse pedestrian level winds. Techniques commonly used in wind tunnels for investigating pedestrian level winds are normally classified as Point Methods and Area Methods. Point methods include such techniques as hot-wire or hot film anemometry, pressure measurement using Irwin probes, and thermistors. These methods measure wind conditions only at discrete points around the model. The Scour or Erosion Technique is an area method used for making a continuous assessment of the wind environment around a test site covering large 'areas. The erosion technique enhanced with an image processing system developed by Eaddy, (Eaddy, 1999) at the University of Auckland has been used in the wind tunnel investigations. A 1: 400 scale wind tunnel model of a representative neighborhood comprising of nine clusters in the environs of Delhi has been used for the assessment of pedestrian level winds in the low speed section of the de Bray boundary layer wind tunnel located in the Aerodynamics laboratory at the University of Auckland. Parametric studies, carried out by varying the height of a tall building in the central cluster of the layout have identified zones of wind amplification and stagnation. Tests have been carried out to examine the impact of architectural elements such as podium, canopy and void in mitigating adverse vertical and horizontal wind flows. Further- tests have examined the role of landscaping as mitigation measures in areas found to be excessively windy. Monthly wind data from Delhi for a period of 13 years has been analyzed by fitting Weibull distributions (by obtaining appropriate values of the coefficients k, A and c) for all the eight predominant directions tested. These values, in combination with the wind tunnel data (which gives velocity ratios between pedestrian and reference locations have been incorporated into an image processing program system to yield probability of exceedance maps of different wind speeds at the test site. The assignment of pedestrian criteria requires the proportion of time that certain wind speeds are exceeded for all directions and this value is calculated by summing the proportion of time that wind speed exceeds a certain speed at each point for all directions. The resulting values are then compared against the pedestrian comfort criteria for a number of mean wind velocities for assigning various comfort categories. ry The wind tunnel investigations have helped in identifying different flow phenomena such as downwash, gap effect, the corner effect and speed up through passages, which cause adverse wind speeds at pedestrian level. The significance of building orientation and the proximity of surrounding buildings in modifying air flow patterns in the vicinity of tall building is also clearly indicated from the erosion patterns: The parametric tests have shown that pedestrian comfort and safety are affected with an increase in the height of tall buildings. The results of the tests have also brought out the effectiveness of architectural elements and landscape elements in mitigating adverse pedestrian level winds. Comfort criteria for pedestrian safety and comfort for mean wind speeds as well as gust equivalent mean wind speeds have been proposed for assessing pedestrian level winds. The purpose of the criteria is to avoid excessive wind velocity and turbulence in outdoor pedestrian spaces. The performance categories set tolerable wind levels for various pedestrian environments depending on the likely frequency and type of usage of those environments. The criteria have been formulated to ensure that comfortable and safe conditions prevail in the areas where significant pedestrian activity is expected to occur. n