Assessment of the most adverse wind form-factor for single storey buildings

Abstract

A major consideration in the design of single storey buildings is the wind load imposed on the walls and roof of the structure. This wind load is mainly (by definition) the result of the wind velocity at the area of the building under consideration.In Australia and New Zealand the Standard AS/NZS 1170.2:2011 is the main technical standard governing the assessment of wind loads on buildings, and hence a major influence on their design.In AS/NZS 1170.2:2011 (p.14) there is an implied assumption that the wind, on the scale of buildings, has a straight or uniform front, where the velocity is identical from one end of the building to the other. And that if the wind is hitting the building at an angle, that the passage of the impacting wind front will be linear. In other words, that any gusts will be of the same magnitude as they pass various parts of the building.Furthermore, on the same page (p.14), there is reference to a "3 second gust wind data", where the maximum wind velocity is maintained for 3 seconds on to the building.This thesis shows that: a) The wind does not impinge on to a building as a uniform, ruler-straight front, and b) The wind does not maintain a maximum velocity for 3 seconds at any part of a building.The thesis demonstrates that the wind has, at any interval of even a few seconds, a constantly fluctuating irregular front. Also, that the wind at a particular part of the building will vary in speed and direction from one fraction of a second to the next.The implication for building design is that the structure cannot experience the full wind load even in the most adverse conditions. That only a part of the structure can suffer the maximum wind speed at a time, and even then for less than one second at a time.Thus the total wind load that must be resisted by extended portions of a building issignificantly less than assumed by AS/NZS 1170.2:2011.The thesis proposes an equation that defines the "shape" of the wind front as a number which shall be called the "wind form factor". Although the behavior of air streams has been investigated in detail in respect of boundary layers and gradients varying with height, at levels of less than a metre, in micro seconds, and also at the very large scale of kilometres, and over hours, there is very little information as to the behavior of the wind at intermediate distances of a few metres and in time periods of seconds.It is hoped that this thesis will go some way towards addressing the lack of data.