RESPONSE OF TALL BUILDING WITH DIFFERENT SIDE RATIO UNDER WIND LOAD

Abstract

High Rise Buildings are cantilever structures that are susceptible to both static and dynamic loadings. Such structures are subjected to significant lateral loads and must account for deflections and accelerations caused by horizontal loading, which is most commonly caused due to unanticipated deflections, wind, or earthquakes. When a building is much higher than neighboring buildings or its proportions are narrow enough to give the appearance of a tall building, it is classified as high-rise buildings. Occupant comfort along with serviceability is the dominant criteria along with safety of structure in design of such structures. In Aerodynamics Optimization, shape of buildings is extensively known concept that greatly determines the response of the high structures under wind loading. Optimizing the geometry of supertall structures for aerodynamics during the design stage is an excellent technique to reduce wind response. Tall building shapes geometry can optimize a fluid-based aerodynamic response. Studies have shown that softening corners, setbacks, changing cross-sectional form, adding spoilers and porosity, and apertures in the building elevation of tall buildings all reduce across-wind reactions. Wind is complicated phenomenon and is a random time-dependent load composed of a mean plus a fluctuating component. Due to this fluctuating component, all structures experiences dynamic oscillations. Motion of wind is so unpredictable that one need to be compute the statistical distribution of velocity rather than just simple averages. The mean component of wind speed produces a static force on a structure. The time-varying component that is the fluctuating component too, which is created by the gusty nature of the wind, is overlaid on the static component and has many frequencies distributed across a large band. Turbulence intensity, which is the ratio of the standard deviation to the mean wind speed and is given in percentages, is a common way to quantify variable velocity. When a wind load acts on a structure, it creates a positive pressure on the windward side and a suction (negative) pressure on the leeward side. The net wind force is computed as the summation of windward pressure and leeward suction but each of these two have their own local impact. Due to the roughness of earth surface, there acts a drag force on wind flow near the ground. This effect gradually decreases as the height increases and at a certain gradient level (around 400m), this drag-force becomes negligible. The degree of surface roughness and drag caused by surrounding projections that oppose wind flow determines the vertical profile of wind speed. Gradient height is the height at which the drag effects become zero, while gradient velocity is the corresponding velocity that do not show variation above this height. The atmospheric boundary layer is the height up to which terrain and topography influences the wind speed. In low rise buildings, only static effects are sufficient to be considered whereas in tall buildings, the aerodynamic and dynamic effects are to be analyzed along with the static effects. High Rise structures are subjected to along with as well as across wind effects. The along wind effect are caused primarily due to buffeting phenomenon caused due to gust effects whereas across wind induced effects are due to vortex shedding. Other dynamic wind induced phenomenon need to be evaluated that are due to increase in amplitude of oscillation with increase in wind speed. Galloping phenomenon are more susceptible to structural elements that are not circular, which is due to transverse oscillations of structures due to wind response that are in phase with motion due to the development of aerodynamic forces. Flutter is another unsteady oscillatory motion induced by the interaction of aerodynamic force and structural elastic deformation. The lateral stability and gravity system for the superstructure, as well as the foundation design, are the most important design concerns for tall buildings. The basic goal of tall building design is to offer enough stiffness to resist lateral or gravity loads.