WIND EFFECTS ON TALL BUILDINGS WITH PECULIAR SHAPES

Abstract

High-rise buildings are replacing low-rise ones especially in rural area of almost all the cities. The purpose is to accommodate many flats and offices on a small plot area. Construction of up to 100 storey high building today is as simply as constructing a twostorey building about a century ago. While designing such tall buildings, wind is the important horizontal load to be considered in analysis. For buildings with regular shapes such as square, octagonal, circular or rectangular, wind pressure or force coefficients are available in relevant Standards on Wind Loads. In the modern era, the esthetic of the building also plays an important role in designing high-rise buildings. The informations available in Wind Load Standards are not sufficient to design such tall buildings with peculiar shapes. Most of the building codes specify wind loading on the basis of wind tunnel tests carried out on building models with uniform cross-section along the height. But in actual practice, the composite plan shapes such as square and octagonal, square and circular, and square and hexagonal etc. can also be used. The flow field around the high-rise buildings change due to composite plan with varying height ratios, thus creating a wind field, which is different in comparison to that for uniform plan shape buildings. But unfortunately this is not included in the recommendations of the codes. Therefore, wind tunnel studies become important to measure wind loads on the models of such buildings. In the present study, three different building plan shapes namely square, octagonal and circular are considered. Also considered are the composite plan shapes namely square to octagonal and square to circular with different height ratios. The present study is carried out under two major heads namely (i) experimental study and (ii) analytical / response study. In the experimental study, the rigid models of tall buildings with uniform plans and composite plans are tested in the wind tunnel in order to measure mean and fluctuating pressures at various points on different surfaces. Models are tested in a closed circuit wind tunnel, in which roughness grid is used to meet the wind tunnel simulation requirements and for the development of turbulent flow for generating the Atmospheric Surface Layer in the 1.3 m x 0.85 m and 8.2 m long wind tunnel. The effects of various geometric and flow parameters such as uniform plan, composite plan, height ratio, wind incidence angle etc. on wind pressure distribution are also studied. ii The buildings selected for the study are uniform or composite plan tall buildings with glass cladding. The length, width and total height of the buildings are 30 m, 30 mand 150 m respectively. Only height ratio is changed for composite plan buildings keeping the above dimensions same. The prototype buildings are considered to be situated in a sub-urban terrain with well scattered objects having height between 1.5 m and 10 m, defined as Terrain Category 2, Zone-V in IS: 875 (Part-3) 1987. For this terrain type, the variation ofhourly mean wind speed with height is assumed to follow a power law index with coefficient a = 0.143 and low turbulence intensity i.e. 0.9% at gradient height and 2.5% near the ground. The 1:300 scale models of tall buildings with flat roof are fabricated using Perspex sheet. Total nine models are fabricated. Many numbers of pressure points are created on the surfaces of the models depending upon the requirement. Mean, rms, maximum and minimum pressures are measured at all pressure points on the entire surfaces of the building models at free stream wind velocity of 15 m/sec. Each model is tested in the wind tunnel under six wind incidence angles namely 0°, 15°, 30°, 45°, 60° and 75°. Surface pressures on different building models are measured by connecting steel taps of 1.0 mm internal diameter, which are flushed to model surface to Baratron pressure gauge with 700 mm long and 1.2 mm internal diameter Vinyl tube. Wind pressures measured on the surfaces of the building models are expressed in the form of non-dimensional pressure coefficients (Cp). The results obtained from the present study are compared with the Indian Standard recommendations [IS: 875 (Part-3), 1987] for uniform cross-section buildings. IS Code suggests pressure coefficients only for two wind directions namely 0° and 90°. But, there is no information about the skew angles. The present study has yielded significant results for each individual study. It is found from the study that the suction value widely varies on parallel side faces of uniform square and octagonal building with the changing wind incidence angle. The variation in suction is around 3 times from the minimum value at 15° wind angle. There is significant variation observed around the junction level of composite buildings. Comparatively greater pressure occurs just above the junction of the building model due to separation of wind flow. The second phase ofthe present study is to carry out analytical study to obtain response of tall buildings with uniform plans and composite plans under wind using experimentally in obtained wind loads in phase one. Prototype buildings are assumed to be made of R.C.C. beams and columns with grid size as 5m*5mand storey heights as 3.75 m(ground floor) and 3.25m (remaining floor). After obtaining wind pressures experimentally, the horizontal nodal forces due to wind at each nodal point on the entire skeleton ofthe buildings are evaluated. Then the buildings are analyzed by readily available software package STAAD Pro. Mean response including moments, shear forces and displacements are obtained under various wind angles in order to study the effects due to different combinations of shapes and wind directions. It is seen from the response study that twisting moment at the junction of out-side central column of the composite building is very large as compared to upper and lower value. Even it varies from 2 to 16 times. At junction level, horizontal structural member (beam) caries more shear force and moments compared to others. Therefore, one needs to be more careful which designing the structural members at junction level due to the high shear force created by large twisting moment. Further, optimum (minimum) tip displacement is noticed in 50% height square and 50% height circular plan shape building at 0° angle ofwind attack. IV