WIND LOADS ON CANOPY ROOFS

Abstract

Canopy roofs are supported by columns and no walls. Being the most exposed part of the building, wind action is directly exerted both on the upper and lower surfaces. Therefore, these roofs seem more vulnerable to wind actions than those of enclosed buildings. In practice, such roofs often experience damage during windstorms. Canopy roofs are used for many structures, such as agricultural facilities (barns, etc.), bus and railway stations, carports, and modern, lightweight, tension membrane structures. Since wind flow around canopy roofs is rather complicated, the wind forces on the roofs are influenced by many factors, such as roof shape, roof pitch, obstruction under the roof and wind direction. Although few researchers have made studies on this subject, the number of the studies is rather limited, compared with that of enclosed buildings. According to most of the building codes, the pressure coefficients (Cp) for two types of canopy roofs namely pitched free roofs and troughed free roofs are available only for few wind incidence angles and only for few roof slopes. There are no values of pressure coefficient available for different kind of interference effect. Further, available information does not include wind pressure coefficients (Cp) on corners where high pressure or suction is expected especially in case of skew angles. Therefore, wind tunnel studies become important to measure wind loads on the models of canopy roof buildings with different roof slopes, blockages and interference. The canopy roof buildings selected for the present study are uniform in plan dimension. The length, width and eaves height of the buildings are taken as 12 m, 6 m and 3 m respectively. Only roof slope is changed 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 Indian Standard on Wind Loads [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 1:40 scale models of canopy roof buildings are fabricated using Perspex sheet. Total four models with varying roof slopes i.e. 0°, 10°, 20° and 30° are fabricated to study the effect ofroof slopes. Many numbers ofpressure points are created on the upper and lower surfaces of the models depending upon the requirement. Surface pressures on different building models are measured by the help of these pressure points. Canopy roof models with different parameters (i.e. with change in roof slope, change in blockage or change in the interference) are also tested in the closed circuit wind tunnel at free stream wind velocity of15 m/sec by changing different wind incident angles. Roughness grid is used to meet the wind tunnel simulation requirements and for the development ofturbulent flow for generating the Atmospheric Surface Layer in the wind tunnel. 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 upper and lower surfaces ofthe canopy roof models are expressed in the form ofnon-dimensional pressure coefficients (Cp). Mean, RMS, Max. and Min. wind pressures on the upper surface and the lower surface of roof models are measured in order to study the effect of varying wind incident angles namely 0°, 15°, 30°, 45°, 60°, 75° and 90° etc. onwind pressure distribution. Blockage and combination ofthe blockages on the side or sides ofthe canopy roof are considered as (a) single side blockages as 25%, 50%, 75% and 100%, (b) two sides having blockages as 25%+25%, 50%+50%, 75%+75%, 100%+100%, (c) two side lengthwise with 100%+100% and one side widthwise with 100% blockage and (d) two side widthwise with 100%+100% and one side lengthwise with 100% blockage. For the study of interference effect, interfering building is placed in different positions i.e. at 0°, 45° and 90° with respect to the wind incident angle and with change in the distance with object building. With the help of experimentally available data, contour plot, different graphical comparisons and ANN analysis are made to correlate the parameter with one another. The results obtained from the present study are also compared with the Indian Standard recommendations [IS: 875 (Part-3), 1987] for different parameters. According to Indian Standard IS: 875 (part-3), 1987 the pressure coefficients (Cp) for pitched free roofs are available only for few wind incidence angles and for few roof slopes. The pressure coefficients are not available for other wind incidence angles and roof slopes. Further, available information does not include wind pressure coefficients (Cp) on corners where high pressure or suction is expected especially in case of skew angles. The present study has yielded significant results for each individual parameter. It is found from the study that the pressure value widely varies on roof surfaces of the building with the changing wind incidence angle for different combination of parameter considered in the models. There is significant variation observed around the corner ofthe roof due to separation ofwind flow for all the combination ofparameter considered in the present study. in Computational Fluid Dynamics (CFD) offers a very powerful alternative to predict the wind related phenomena on buildings or different kind of structures. A software package, CFX-11 (ANSYS 11.0 SP1 2007) is used for CFD analysis. CFX-11 is a general CFD code based on the finite volume method and an algebraic multigrid coupled solver. By CFD analysis considering the same parameter as used in the experimental study, the wind flow pattern and pressure values on the roof surface are compared with experimentally obtained wind loads in phase one. Analysis by CFD shows that at the corner portion and area near the ridge line, flow separation occurs and because of that the pressure values at those points are higher than that of the other portion of the roof surface. Therefore, one needs to be more careful while designing the structural members at these areas of the roof. The third phase of the present study is to carry out analytical study to obtain the pressure values for interference effect with different wind incident angles by ANN. It is seen from the analytically obtained results through ANN that the pressure values are very close to the corresponding experimental values. IV