Estimation of Wind Load on Tall Buildings

Abstract

Any Tall building can vibrate in both the directions of Along wind and Across wind caused by the flow of wind. Modern Tall buildings designed to satisfy lateral drift requirements, still may oscillate excessively during wind storm. These oscillations can cause some threats to the Tall building as buildings with more and more height becomes more vulnerable to oscillate at high speed winds. Sometimes these oscillations may even cause discomfort to the occupants even if it is not in a threatening position for the structural damage. So an accurate assessment of building motion is an essential prerequisite for serviceability. There are few approaches to find out the Response of the Tall buildings to the Wind loads. An Analytical approach given by Davenport and mentioned in the IS 875: part 3-1987 is used which is only applicable to a regular shape building but for an irregular shaped building and but for its accurate response and behavior under the high wind speeds is provided from a prototype measurements in a wind tunnel. A simulated wind tunnel experiment on an appropriate model of the building yields results which give a deeper insight into the phenomenon and provides more precise information, overcomes the shortcomings of the analytical formulation. In the wind tunnel testing the two methods are used for determining the response of any Tall building with an irregular or regular shape under the high wind speeds, one is the Dynamic Analysis method used to determine the wind loads on the buildings in which Base Forces can be economically obtained from HFFB test while the other one is Pressure measurement studies which are used for the safe designs of Individual structural elements as roofs and walls, and Individual cladding units including glazing and their fixing. This report deals with the wind tunnel studies carried out on a scaled down model of proposed tall building named Signature towers from Unitech company going to be build in Greater Noida, India which is 150m in height, carried out at the Boundary Layer Wind Tunnel, IIT Roorkee. The experimental results have been projected to estimate the full scale values using appropriate scaling laws. These have also been compared with the prototype responses computed analytically. The analytical values in the along wind direction have been obtained using the Davenport's `Gust Factor Approach'(1967). Pressure Studies are also done on the building for their claddings design and to know the pressure distribution on the building faces. The building models have been tested in a boundary layer flow corresponding to terrain category-III, as defined in IS: 875-part-3, 1987, ( = 0.18 as per IS: 4998) at a wind speed of 10.78 m/s at model top for stand-alone and interfering situations. Forces and moments (mean, maximum, minimum and R.M.S.) in along-wind and across-wind directions at the base of the building and torsional moment about vertical axis and coefficients of pressures (mean, maximum, minimum and R.M.S.) have been obtained for different wind incidence angles (00 and 3600) for stand-alone and interference condition. The results have been presented in the form of plots and tables and design values have been deduced. Peak accelerations at the top of the building have been obtained on the basis of the measured base moments and compared to those as computed from the formulation given in the IS:875. Story wise lateral design forces are computed by analytical method and check of Base Forces obtained from Pressure Studies with that obtained from Dynamic Analysis is discussed in Conclusion. Comparison of various design parameters is also done and interesting results have been obtained. It is found that the gap between the analytical estimates and wind tunnel test results depends upon the plan aspect ratio and shape of building. In the present case, experimental test results are higher than those of analytical estimates.