WIND EFFECTS ON STRUCTURALLY COUPLED TALL BUILDINGS

Abstract

Due to population explosion, the demand for more built up space is ever increasing both for working and living, and hence scarcity of land is being felt. This has led to spiralling land prices resulting in construction of a number of high rise buildings. Better construction materials, new structural systems, better construction techniques coupled with more sophisticated and faster computing machines has resulted in flexible and slender tall structures, which are lighter in weight and generally low in damping. Such structures are susceptible to wind forces. Strong winds occur frequently in nature causing severe damage to high rise structures resulting in loss of lives and property. Therefore wind force is one of the prime considerations in design of such slender and flexible structures. So, it is desirable to assess wind loads and resulting effects to ensure safe and serviceable design of such structures with higher degree of refinement. The wind loads on the structures are computed using pressure coefficients or force coefficients, which are available in relevant wind load codes / literature. However the information about pressure coefficients and force coefficients are available for specific shapes and for some wind incidence angles only. Moreover these coefficients are specified for isolated buildings only. Wind characteristics and hence wind loads are influenced by many factors. One of the major factors is the presence of the surrounding structures around the object structure. Sometimes two closely spaced tall buildings are, structurally coupled through bridges at different levels for functional reasons. Scant information regarding wind loading on structurally coupled tall buildings is available, however, for economical, safe and serviceable design of such structures the estimation of wind loads is a must. As no guidelines are available in the code / literature in this regard, in order to make a fair assessment of the wind loads experimental studies are conducted for such cases. One of the methods available for assessing wind loading is to carry out testing in Boundary Layer Wind Tunnel (BLWT).Therefore, the present study is undertaken to study the interference between two high rise buildings and to study the effect of bridge position located at different heights for different spacing of models. The models are tested in the closed circuit wind tunnel at CIVILENGINEERING Department, Indian Institute of Technology Roorkee, Roorkee. The present study has been carried out in two phases, namely experimental study and analytical study. The purpose of the experimental study is to conduct wind tunnel testing of the different building models in order to assess the effect of adjoining building and connecting bridge on the pressure distribution on the faces of the building models. In order to use the results of experimental study for prototype, it is very important that the models are tested in similar conditions to which the prototypes are exposed. Hence the flow simulation and measurement techniques are very critical for obtaining accurate test results. Therefore the efforts have been made to simulate the flow conditions which are similar to terrain category -II. In this terrain category the obstructions are well scattered and have the height in the range of 1.5m to 10m. All the models have been tested in a closed circuit wind tunnel. The size of the wind tunnel is 1.30mX0.85mX8.25m. Present study is conducted for rigid models of isolated building model, and then pressure measurement is made for rigid models of unconnected building spaced 50mm, 100mm, and 150mm apart. Further to study the effect of bridge position two building model are spaced 50mm and 100mm apart and two building models are connected by single bridge located at different heights namely H/4, H/2, 3H/4 from bottom. Also the study is made by connecting the two models by two bridges located at different heights. The wind pressure measurements are made by varying wind incidence angle from 0° to 90° at an interval of 15° i.e. at 0°, 15°, 30°, 45°, 60°, 75° and 90° and pressure measurements are taken on all the faces of the models. Mean, maximum and RMS values of wind pressure coefficients are evaluated from the experimental data. Pressure coefficients so obtained are depicted by contours for all the faces of the buildings and also mean pressure coefficients are plotted on the periphery of the two buildings at selected heights. It is noticed from the experimental results that due to presence of adjoining building the suction on the building faces increased. While due to presence of connecting bridge the suction on the building decreased as compared to unconnected cases. However, near the connecting bridge location there are more pronounced variations on the opposing faces as compared to other regions on the building faces. As the shielding effect increases, the suction on the shielded building is observed to be lesser than on the isolated building. It is also noticed that mean pressure coefficients as specified by various codes for the leeward face of isolated building are very less than the values obtained experimentally for 0° wind incidence angle To study the effect of structural coupling on the structural response of two high rise buildings, linear static analysis is carried out using STAAD PRO2005 software. The structural system adopted for the building is tube in tube, each of the vi two buildings is square in plan with dimension as 30mX30mX180m. The wind loads considered for the analysis are obtained from experimental tests conducted in BLWT. The two buildings are connected by one skybridge / two skybridges, with dimension of each skybridge as 30mX6mX3m. The bridge is located at different levels. Effect of structural coupling has been studied for the following: • Maximum nodal displacement at top. • Nodal displacement at diagonally opposite nodes. • Diagonally opposite columns. • Beam at particular position i.e. B101 and B40101. • Maximum forces in beam / column at a level. From the analytical study it is noticed that nodal displacement is significantly influenced by wind angle. However, the effect of connecting bridge on the nodal displacement is observed when the wind blows parallel to connecting bridge; also the higher position of connecting bridge for this orientation is more effective. When wind blows at right angles to the connecting bridge, then, as the bridge is located at a higher location, the displacement on the building increased marginally as compared to lower position of connecting bridge. Also it is observed that whenever there are abrupt discontinuities in buildings due to belt truss, cross braces or connecting bridge etc, it increases the stiffness in abnormal proportion at such local regions. This results in sharp variation in forces / moments in the members in the vicinity of such locations. Hence the designer has to be extra cautious in the design of members which are in the vicinity of such locations. Followings are the major contributions / achievements of the present study: 1. The experimental data of wind pressure distribution on various faces of the building models have been generated for isolated and two buildings placed in vicinity of each other. This information has been generated for two buildings which are either unconnected or connected to each other by skybridge / skybridges placed at different heights. This information is useful for the designer when dealing with wind loads. 2. From the analytical results the effect of connecting bridge on nodal displacement / structural response of the adjoining buildings has been studied. 3. The designers have to be very cautious while designing the members/claddings at locations close to the connecting skybridge or at locations of abrupt discontinuities in stiffness caused due to presence of belt truss etc. VII