Wind load on building is a complex phenomenon because of the many flow situations arising from the interaction of wind and structures. Wind is composed of a varying sizes of eddies and rotational characteristics carried along in a general stream of air moving relative to eart’s surface. These eddies give wind its gusty or turbulent character. The gustiness of strong winds in the lower levels of the atmosphere largely arises from interaction with surface features. Near the earth surface, the motion is opposed, and the wind speed reduce by the surface friction. At the surface the wind speed reduce to zero and then begins to increase with height and at some height , So it could be at a certain altitude conditions, seismic forces acting on the tall building does not become dominant compared to the influence of the wind. Wind load regulation in Indonesia adopted a regulation of ASCE 7-02 (American Society of Civil Engineers Minimum Design Loads for Buildings and Other Structures). ASCE 7-02 provides three procedures for calculating wind loads for buildings and other structures, including the main wind-force-resisting systems and all components thereof. The designer can use method 1, the simplified procedure, to select wind pressures directly without calculation when the building is less than 18.28 m in height and meets all requirements given in Section 6.4 of the standard. method 2 can be used for buildings and structures of any height that are regular in shape, provided the buildings are not sensitive to across-wind loading, vortex shedding, or instability due to galloping or flutter; or do not have a site for which channeling effects warrant special consideration. method 3 is a wind-tunnel test procedure that can be used in lieu of methods 1 and 2 for any building or structure. method 3 is recommended for buildings that possess any of the following characteristics :
- Have nonuniform shapes.
- Are flexible with natural frequencies less than 1 Hz.
- Are subject to significant buffeting by the wake of upwind buildings or other
- Are subject to accelerated flow of wind by channeling or local topographic features.
Wind tunnel studies is the good way to analysis wind load on building compare to analytical study. Wind tunnel studies are recommended for situations when wake buffeting may exist due to significant upwind obstructions such as hills or significant aerodynamic term that describes a turbulent fluid region on the downstream side of a body, where strong eddies are generated which may impose critical fluctuating wind loads on structures downstream especially if the frequency content of turbulence excites the resonant frequency of the downstream structure. This wake buffeting can be very important for slender towers and other dynamically sensitive structures where overall structural wind loads can be increased dramatically. Wind tunnel studies are also recommended for structure whose site location makes them subject to channeling effect caused by topographical or neighboring tall buiding such as in the city center or central business. In that situation the wind velocity from certain wind directions will be locally accelerated as the flow is squeezed between the upwind obstructions causing increased wind loading on the nearby structures.