EVALUATION OF WIND FORCES ON TALL BUILDINGS DUE TO INTERFERENCE

Abstract

In the last century, development in the analysis and design of structures, advancement in construction technology, and improvement in making of light weight materials with high strength has led to construction of tall slender buildings. With the buildings becoming progressively taller, lighter and more flexible, their aerodynamic stability under wind become a major concern for the Structural Engineer. Wind induced response of tall rectangular buildings is a function of many parameters. These include the geometric and dynamic characteristics of the building as well as the turbulence characteristic of approach flow. The flow approaching a tall building inherits the combined effect of wake of obstacles it has overcome. These may be natural features like forests, hills, etc, and man-made structures, both low and high rise buildings. Tall buildings are expensive structures. However, the extra cost involved in the prediction of their response under wind loading from wind tunnel measurements is relatively small while being very useful for arriving at a safe and economical design with acceptable levels of vibration induced by wind. Wind tunnel investigations are needed due to shortcomings and limitations of the analytical procedures and also because very little is available on the effect of interference from adjoining tall structures .A tall building which is interfered by other in its vicinity will behave differently under wind action as compared to when it is in isolation due to modified wind flow around it. A simulated wind tunnel experiment on an appropriate model of the building yields results which give a deeper insight into the phenomena and iii provides more preciSe information, overcoming the shortcomings of the analytical formulation. A building of width:depth:height proportions as 1:2:10 has been chosen for the study. The building dimensions are 24m x 48m x 240m. In the first phase of the study wind tunnel experiments have been performed in a boundary layer on a 'stick' type aeroelastic model which simulates the fundamental mode shape of vibration of the building in either directions. Displacements were observed for different value of reduced velocity. The building being rectangular, it has been studied along both of its principle directions, i.e., in long and short afterbody orientations. When a building is located in an urban environment, it is exposed to wind of altogether different characteristics than wind over an open region. This is on account of the turbulent, 'wake region' created behind an upstream building. The interactions with the upstream building(s) can produce significant changes in the responses of a tall building. In the second phase of my study, effect of interference on the same building due to other similar buildings, but of varying aspect ratio in plan (1:1, 2:1), have been studied. For this purpose, the aeroelastic model of the building has been used while interfering models are rigid. The interfering models have been referred to as 'narrow' and 'medium' width models. To cover a wide range, the interfering model was placed at various locations on the upstream as well as downstream side of the aeroelastic model. iv Results of the isolated buildings are presented as its response related to reduced velocity. The experimental results have been projected to estimate the full-scale values using appropriate scaling laws. Results of the interference study are presented in the form of Buffeting Factors' (BF) defined as Response in interference configuration BF — Response in stand-alone configuration For the interfering building study, contours of BF are plotted to cover the entire region around the building. The BF contours have been prepared for both of the interfering models, and, for the various wind speeds. These contours show graphically the variation of BF for a specific reduced velocity as an interfering model occupies different positions. It was found from the experiments that BF values change with the geometry and position of the interfering model. For square interfering model the BF values are maximum when interfering model is in line with that of aeroelastic model. But for the rectangular model BF values are maximum at the lateral offset BF values are more at lower reduced velocity. The results obtained have been compared with those on similar models for terrain category 3 and 4 reported by Abhay Gupta. In general the interference effects are smaller both in along-wind as well as across-wind as seen for the rms responses..