Specification of the design wind load—A critical review of code concepts

In a simplified approach, the design wind load can be specified based on an appropriate small target value of the exceedance probability. For the ultimate limit state, the reasonable reference period is the projected design working life of the structure; for the serviceability limit state a suitable reference period is one year. Basically, at least the extreme wind speeds and the extremes of the aerodynamic coefficients have to be understood as random variables. Further random variables are the duration of a single storm and the relative intensity over the length of the storm. Neglecting these two parameters may lead to underestimations of the design wind load. The design values of the wind speeds are specified in codes with mainly two different concepts: either in terms of a product of the characteristic wind speed and a partial factor or directly as design value. The variable wind speed is represented in codes by gust wind speeds, by 10-min mean wind speeds or by hourly mean wind speeds. For the design value of the aerodynamic coefficient, mainly two concepts are used in codes: the mean value of the extremes or the 78%-fractile value, the latter known as ‘Cook–Mayne’ coefficient. The paper tries to sort out the differences between these approaches and tries to comment on one or the other shortcoming. Additionally, the complexity of the codification task is discussed when different wind climates have to be covered.