Steel Buildings in Europe

Part 3: Actions 3 - 25 The external and internal forces result in pressures perpendicular to the walls (vertical walls, roofs, etc.). By convention, pressure directed towards the surface is taken as positive, and suction, directed away from the surface as negative (Figure 8.1). q < 0 q > 0 Figure 8.1 Sign convention for the pressure As stated in EN 1991-1-4 § 5.3(2), the resulting wind force F w acting on a structure, or a structural component, can be determined by the vector summation of F w,e , F w,i and F fr . It can be globally expressed as follows: F w = c s c d c f q p ( z e ) A ref where: c s c d is the structural factor (for buildings with a height less than 15 m, it may be taken as 1) Note: the mean wind velocity v m ( z ) is necessary to calculate the structural factor c s c d . c f is the force coefficient for the structure (or structural element) A ref is the reference area of the structure (or structural element). Here it can be defined as the area of the projection of the structure or the structural component, on a vertical plan perpendicular to the wind direction. Practical approach In practice, the designer needs to evaluate the resulting pressure on the walls in order to determine the actions on the structural members. The resulting pressure can be expressed as follows: F w / A ref = c s c d w e – w i where: w e is the wind pressure acting on the external surface (see 7.2.1.2), w i is the wind pressure acting on the internal surface (see 7.2.1.3). In addition the effects of the friction forces (see 7.2.1.4) have to be considered when necessary.