Design Wind Pressures (P_Z):

It depends upon the basic wind velocity (V_b), the height of the structure (z) above ground level, the terrain category, the local topography, the aspect ratio (i.e. ratio of the length and breadth of the building or structure), the slope of the structure and the solidity ratio or openings in the structures. This is obtained by the following formula

P_z  = 0.6 (V_z )² N / m²

where V_z  is the design wind speed in m/sec at height z. For determination of V_z and, therefore, P_z, you are referred to consult IS: 875 (Part III: Wind Loads). Normally, you will be provided with the values of P_z.

The effect of wind on steel roof structures is also to create either suction (negative pressure) or pressure (positive) depending on the angle of inclination or slope of the roof, and the direction of prevailing winds. In case of GCI or AC sheets the suction pressures may be quite high which require the anchorage of the sheets and their supporting elements against the possibilities of their flying off (Figure 3).

The maximum wind speed as given in IS : 875 (III) is 55 m/sec corresponding to which the basic wind pressure at a height of 10 m from ground level is about 1.8 kN/m². For the smallest indicated wind speed of 33 m/sec the corresponding value of wind pressure is 0.65 kN/m². For a particular region a suitable value is selected and prescribed for design purposes.