EFFECTS OF GEOMETRICAL SHAPES ON WIND LOADS ON BUILDINGS

Abstract

Due to population explosion, the demand for more built up space is ever increasing both for working and living purpose, and hence scarcity of land is being felt. This has led to spiraling land prices resulting in construction of a number of high-rise buildings. Better construction materials, new structural systems and better construction techniques have resulted in flexible and slender tall structures. Architects generally design low-rise human dwellings and multistoried apartment as well as office /commercial buildings in square or rectangular plan. Whereas low-rise buildings are generally provided with flat or sloping roof, high-rise buildings invariably have flat roofs. However, architects sometime make small changes in its cross-section or elevation in order to improve its esthetic. Simplest way is to either attach or remove the edges or corners in them. Some of the recently constructed eye-catching high-rise buildings include Burj Khalifa Tower in Dubai, Strata SE 1 High-Rise Building in London and Taipei 101 in Taiwan. Improvement in esthetic is not achieved free of cost. The alterations suggested by architects in basic form not only increase cost of labour and some time that of materials also, but also increase the cost of structural system as revised shape and form of the building attract increased design loads, especially wind loads. After architects finish with functional and esthetic design of buildings, whether low-rise or high-rise, it is the structural designers who have to start with designing and detailing of structural system. While carrying out structural analysis which is the pre-requisite of structural design, different types of loads are considered. Designers make use of relevant standard for evaluation of different types of-loads. In case of wind load calculation, IS:875 (Part-3) 1987 is made use in India. There are similar standards in USA, UK, Australia and Europe. All these standards give information about wind pressure and wind force coefficients on both low-rise and high-rise buildings of simple plan shape. In case of buildings with un-common shape in elevation or plan, designers are left with no other choice than going for wind tunnel testing. Due to non-availability of wind load information on such building shapes, architects sometime come out with very un-economical designs or the designs where the occupants of the building or the pedestrian moving around the building feel discomfort. i It is, therefore, necessary that information available in the standards on wind loads should be enhanced through wind tunnel studies so that both architects and structural designers should be able to come out with aesthetically pleasing designs of buildings which are both functionally and economically viable. However, review of literature indicates that very few researchers have so far carried out experimental investigations to study the influence of geometrical shapes on wind loads on low-rise and high-rise buildings. It was, therefore, proposed to carry out experimental study in the present research program on the models of low-rise and high-rise buildings with varying geometrical shapes. The following cases of study have been carried out in the present research programme and results are presented herein. (i) To measure wind pressure, distribution on a saw-tooth roof of low-rise building at varying wind incidence angles in order to study the effect of no. of span on wind pressure distribution and wind directions (ii) To measure wind pressure distribution on the surfaces of a square-plan high-rise building with rectangular corners, chamfered corners and cut corners with varying wind incidence angles to study the effect of corner configuration (iii) To measure wind pressure distribution on the surfaces of a square-plan high-rise building with horizontal projections on external surface to study the effect of surface roughness. (iv) To measure wind pressure distribution on the surfaces of a square-plan high-rise building with through passage and obstructed passage near the base, specially to measure the wind environment inside the passage (v) To measure base shear and moments by varying length-to-width ratios of a rectangular-plan building keeping the . cross-sectional area the same under different wind incidence angles to study the influence of length-to-width ratios on wind forces (vi) To measure the base shear and moments by varying block size of a L-shape building keeping the cross-section area the same under different wind incidence angles to study the influence of block dimension on wind forces In the present study, the building models are tested under boundary layer flow corresponding to terrain category — 2 as per IS Code in the Open Circuit Boundary Layer Wind Tunnel at a the Department of CIVILENGINEERING, Indian Institute of Technology Roorkee, Roorkee, India. The models for the measurement of wind pressure are made of Perspex sheet with numerous pressure points on the surfaces of the models. Models for wind force measurements are made of plywood. Pressure values are measured using Differential Pressure Transducer. Wind forces are measured by mounting the building models on 5-component load cell to measure base shear and moments values. Mean Pressure Coefficient values are obtained from the wind pressure values measured at all the pressure points and results are shown in the form of pressure coefficient contours for wind pressure distribution on roof surfaces in case of low-rise building models and wall surfaces in case of high-rise building models. Variation of base shear and moments with (i) wind incidence angle, and (ii) length-to-width ratios in case of (a) rectangular-plan high-rise buildings and (b) block size in case of L-shape buildings are shown in the form of x-y plots. It is concluded from the results obtained experimentally that the wind loads on both low-rise and high-rise buildings are highly influenced by the geometrical shapes of the buildings in addition to wind incidence angles. The results presented in the thesis will be of great use to the architects and structural designers. The experts responsible for revising the standards on wind loads from time to time will also be able to make use of these results. iv