EFFECTS OF CROSS-SECTIONAL SHAPES ON RESPONSE OF TALL BUILDINGS UNDER WIND LOADS

Abstract

Tall buildings are replacing low-rise buildings in almost all the cities. The purpose of construction of high-rise buildings is to accommodate many flats and offices on a small plot area. Wind is one of the important loads considered at the time of analysis while designing both low-rise and high-rise buildings. Although architects generally deign multistoried apartment / office buildings in square or rectangular plan, they design the buildings with irregular cross-sections also at times for aesthetic reasons. For buildings with regular cross-sectional shapes such as square and rectangular shape, wind pressure and force coefficients are available in relevant standards on wind loads. However, the available information is not sufficient to design such type of tall buildings with irregular cross-sectional shapes. The flow field around tall buildings changes due to the change in cross-sectional shape, thus creating a wind field which is different in comparison to that for uniform plan shape buildings. It results in different wind loads than those which are available in the relevant standards on wind loads of various countries and research publications. Therefore it has become necessary to test the model of such a tall building with irregular plan shape in the wind tunnel to measure the wind loads acting on it every time it is designed. Further, the information about wind pressure coefficients and force coefficients available in relevant codes of practices on wind loads on tall buildings with regular shapes is for stand-alone or isolated condition only. Pattern and quantum of change in wind loads on a tall building due to the presence of other buildings of same or different cross-sectional shape and height are not reported in standards on wind loads. Wind tunnel tests are the only possible solution as on today to evaluate the wind loads on a tall building under interference condition. Aim of the present study, therefore, is to test the models of tall buildings with varying cross-sectional shapes in the wind tunnel to measure wind loads both under isolated and interference condition. In present study, eight types of cross-sectional shapes are considered. The cross-sectional shapes are so chosen that floor area remains same in all the cases. The present study is carried out under three major heads namely (i) experimental study-force measurements, (ii) experimental study-pressure measurements and (iii) wind response analysis. In the experimental study, the rigid models of tall buildings with different cross-sectional shapes are tested in an open circuit boundary layer wind tunnel having test section of length 15m and 2 m x 2 m cross-section. The prototype buildings are considered to be situated in a sub urban terrain with well scattered objects having height between 1.5 m to 10 m, defined as Terrain Category 2, Zone-V in IS: 875 (Part-3) 1987. Vortex generators, barrier v wall and small blocks are used as obstructions to flow to meet the wind tunnel simulation requirements and for the development of turbulent flow for generating the atmospheric surface layer in the wind tunnel. The prototype buildings are assumed to have ground floor plus 18 storeys with total height of buildings as 60 m and floor area 400 m2. Models for wind tunnel tests are made at a scale of 1:100. Two types of models are fabricated, wooden models for force measurements and Perspex sheet models for pressure measurements. Therefore, 8 no. wooden models and 7 no. Perspex sheet models i.e. 2 models for each 8 cross-sectional shapes except model-E, are made. Many numbers of pressure points are created on the surfaces of Perspex sheet models depending on the requirement. In first part of the study, base shear, base moments and twisting moments are measured by placing wooden models one by one on force balance under isolated as well as interference conditions. Force measurements are carried out at free stream velocities of 6, 8 and 10 m/sec approximately. In second part, mean, rms, maximum and minimum wind pressures are obtained at all pressure points on Perspex sheet models at free stream velocity of 10 m/sec. Models are divided into two categories i.e. (i) models having symmetry about both axis and (ii) models having symmetry about one axis. Models having symmetry about both axis are tested in the wind tunnel under 4 wind incidence angles namely 00, 300, 600 and 900. Models having symmetry about only one axis are tested in the wind tunnel under 7 wind incidence angles from 00 to 1800 at an interval of 300. Wind pressures measured on the surface of each models are expressed in form of non-dimensional pressure coefficients (Cp). Models are tested under isolated as well as interference conditions. The third part of the work presented in the thesis is to carry out analytical study to obtain response of tall buildings with different plan shapes under wind using wind loads obtained experimentally in part two. Prototype buildings are assumed to be made of R.C.C. beams and columns with gird size as 5 m x 5 m and storey height as 3.75 m for lowest storey and 3.125 m for remaining storeys. The buildings are analyzed using readily available software package STAAD.Pro. Static response including moments about both axis, shear forces, twisting moments and displacements in all columns at every storey are obtained under various wind incidence angles. These values are compared with one another to understand the effects of building cross-sectional shapes and wind incidence angles on the response of tall buildings under wind loads. At the end, conclusions are drawn as to which of 8 cross-sectional shapes considered in the present study is subjected to minimum wind loads under isolated condition and which vi one under interference condition. Similarly the cross-sectional shape which results in optimum response under wind loads is also identified. Results presented in the thesis can be made used by the architects and structural designers to design tall buildings of same or similar cross-sectional shapes studied and reported in this thesis.