At first wind loads were ignored because the weight of the construction materials and structural systems used in the first skyscrapers made vertical loads more critical than lateral loads, but over time wind loads became important, as the strength to weight ratio of construction materials and the ratio of floor area to structural weight in
structural systems increased and the total weight and rigidity of structures decreased. The effect of wind on tall buildings is explained in Chapter 5. Wind speed and pressure increase parabolically according to height, and therefore wind loads affecting tall buildings become important as the height of the building increases. In general, structural design begins to be controlled by wind loads in buildings of more than 40 storeys (ACI SP-97, 1989). Today, thanks to developments in structural systems and to high-strength materials, tall buildings have increased in their height to weight ratio but on the other hand reduced in stiffness compared with their precursors, and so have become greatly affected by wind. With the reduced stiffness, the sensitivity to lateral drift, and hence the sway under wind loads, increases. The sway, which cannot be observed outside the building or at the lower floors, can cause discomfort to occupants at the higher floors of a building. Architectural, structural, and mechanical design approaches (Chapter 6) are used to control lateral drift in tall buildings. In the design of tall buildings, for buildings below 40 storeys with height to width ratio (the ratio of the structural height of a building to the narrowest structural width at the ground floor plan, also termed aspect ratio) below 6, the values predicted in the building design codes can be used to determine wind loads. Because wind loads can change quickly or even suddenly, unlike live and dead loads, in order to estimate the wind load in buildings of more than 40 storeys, or that have an aspect ratio of 6 or higher (slender and flexible buildings), or that have unusual forms, dynamic effect of the wind and dynamic building response must be taken into account. In this context, dynamic calculation methods, or else wind tunnel tests, are recommended for estimating the wind loads on such buildings (Section 5.2).