Wind has generally six components: 3 forces and 3 corresponding moments. In structural engineering, the force and moment corresponding to the vertical axis (lift and yawing moment) are not so critical for the building. Therefore, along wind (drag forces) and transverse wind (crosswind) are of major significance. While the maximum lateral wind-loading and deflection is in parallel with the wind (along wind direction), the maximum acceleration which leads to a human perception of motion is in the axis perpendicular to the wind excitation (crosswind) (Figure 46). "There are three main reasons that a building responds in such a way in its' perpendicular to the wind direction: 1) the biaxial displacement induced in the structure because of either asymmetry in geometry or in applied wind loading; 2) the turbulence of wind; and 3) the negative-pressure wake or trail on the building sides" [33]. Usually, the transverse impulses are of the half frequency compared to the along-wind impulses. Those transverse impulses are applied alternatively to the left and to the right, causing a phenomenal so called "vortex shedding". A simple formula that is very widely used to calculate the frequency of the vortex shedding is: f= (V x S) / D; where f= the frequency of vortex shedding, V= the mean wind speed at the top of the building, S= the Strouhal number (a dimensionless parameter for the shape); D=the diameter of the building. Apart from vortex shedding, phenomena that amplify the dynamic response of a building due to wind are buffeting, galloping and flutter. Buffeting occurs mostly in slender high-rise, in the alongwind direction due to turbulence. Flutter and galloping are usually not critical for common building structures.