WIND LOADS ON GABLE ROOF BUILDINGS WITH ATTACHED CANOPIES

Abstract

Severe wind storms, including tropical cyclones, are known to cause considerable damage to life and property all over the world. The coastal zones ofIndia, in particular the East Coast, are regularly hit by cyclones while the inland regions are prone to high intensity windstorms. During recent years, wind loading on low-rise buildings has been an area of active investigation due to increasing public concern towards severe damage caused by windstorms. In the assessment ofwind loads on low rise buildings, the effect of attached canopies is sometimes disregarded. However, the windward canopies may be loaded severely due to wind, since the deflected flow on separation from the windward wall, gives rise to a pressure on the canopy upper and lower surface. Unfortunately, only very limited information is available on the subject. This is probably the reason why most Standards and Codes of Practice have poor and often conflicting documentation of provisions related to wind loads on canopies. The review of literature on the subject of wind pressures on canopies / eaves/ roof projections/ open verandahs has revealed inadequate information and much variation in the Codal provisions. The provisions suggested by codes are irrespective of the openings under the canopies, which is quite prevalent in India and the entire world. Therefore, there is need to carry out detailed study on the buildings with attached canopies. Design wind pressure coefficients is mainly obtained from different Codes and Standards, wherein the major source of information on which specifications are based, is wind tunnel testing of scaled rigid models under simulated flow. Recent studies on full-scale / model-scale comparisons have shown the importance of flow simulation and measurement techniques for better results and to enable improvement in the existing information. In this direction, an attempt has been made in the present work to establish firstly the flow simulation criteria. Further, under the simulated wind flow conditions, detailed study on gable roof buildings with attached canopies has been carried out. Most of the building codes specify wind loading on the basis of wind tunnel tests carried out on building models attached with canopies, but regardless of different canopy parameters which may affect the wind loads of buildings. In actual practice, the canopies used may be of different slope, length, width, location etc. The flow fields around the building with attached canopy changes with the type of canopy used/attached, thus creating a wind field, which is different in comparison to that for other type of attached canopy to the building. The openings under the canopies are also a factor which highly influences the wind field around the 11 lower surface of canopies. But unfortunately this is also not being considered in the recommendations ofthe codes. The effects ofcanopy attached to the building in a given situation depend vefy much on the direction ofwind flow and the upstream terrain conditions, leading to shieldingor amplificationeffect. In the present study, roughness grid have been used to meet the wind tunnel simulation requirements and for the development of turbulent flow for generating the Atmospheric Surface Layer in the 1.15m x 0.82m and 8.25m long Wind Tunnel. The building selected for the study is a small industrial gable (pitched) roof building with attached canopy, with the openings considered to be closed and open one by one at the time ofa windstorm. The length, width and eaves height of the building are 15m, 7.5m and 7.5m respectively. Roof slope selected for this study is 20°. The prototype is considered to be situated in a sub-urban terrain with well scattered objects having height between 1.5m and 10m, defined as Terrain Category 2 in IS: 875 (Part-3) 1987. For this terrain type, the variation of hourly mean wind speed with height is assumed to follow a power law with coefficient a = 0.157 and low turbulence intensity i.e. 1% at gradient height and 5% near the ground. The 1:50 scale Perspex models of the gable roof Building with attached canopies were fabricated. The study is divided in four parts i.e. influence ofslope ofcanopy, influence of length ofcanopy, influence ofmultiple canopies and influence ofopening under canopies. So depending upon the study, different models were fabricated. For influence ofslope ofcanopy study four models were fabricated having canopy pitch 0°, 10°, 20° and 30°. For influence of length ofcanopy study four models were fabricated having length ofcanopy as 75mm, 150mm, 250mm and 300mm. The horizontal canopies were fabricated for length ofcanopy studies. For influence of multiple canopy study three models were fabricated having single canopy, two canopies and three canopies. For influence of openings under canopies the same models were tested as that of multiple canopy study, but with different combination of openings under the canopies. So a total of nine models were fabricated as one model was common in all the parameters. The canopies have been fixed at the mid of eave height in all the models. The numbers ofpressure points were created on the upper and lower surface ofcanopy. In addition, pressure points were created on the roofsurfaces, front and back wall surface and canopy walls (end walls) ofthe building model. On canopy surface, the number ofpressure taps varies depending upon the type ofcanopy. in Surface pressures on the roof of the building models have been measured by connecting steel taps of 1.0 mm internal diameter, which are flushed to model surface connecting Vinyl tubing of 1.2mm internal diameter. The pressure taps was further connected to Baratron pressure gauge with 700 mm long and 1.2 mm internal diameter Vinyl tube. The Baratron pressure gauge from MKS Corporation,USA has been used to measure surface pressures. This pressure gauge gave the surface pressure in the form of a reading on analog scale. The same reading was converted in digital form by solid state integrator. The reading further recorded directly into the computer through the automatic data logger instrument from CCS, Roorkee. The reading was converted into the surface pressure in N/m by multiplying the Baratron reading and Baratron multiplying factor (i.e. Baratron Range). Wind pressures measured on the surfaces ofthe buildingmodels have been expressedin the form ofnon-dimensional pressurecoefficients Cp. The study has yielded significant results for each individual study. It has been foundfrom the study that the suction value is widely varied with the variation of slope of canopy. The variation in suction is around 100% from the minimum value. The canopy having 20° slope experiences minimum suction on upper and lower surface. It has been found that IS 875 (part-3) 1987 provides suction pressure coefficient on upper surface of canopy for wind angle 0° while the present study reveals a pressure coefficient value for the same parameters. Further IS code suggest only one coefficient value for all the canopy pitches between 0° to 30°, which means the canopy pitch does not have any effect on coefficient value on upper surface of canopy. These recommendations of code verified by the present study to some extent, as the change of canopy pitch affect minutely the pressure coefficient value. The present study confirms that the coefficient values are positive in nature on upper surface of canopy for all the canopy pitches. So the code suggested coefficient value is not holds good. The code suggests only two wind direction i.e. 0° and 180°. The experimental study shows that the wind angle influences the pressure coefficients to a large extent. The pressure coefficient value reduces at 45° wind incidence as compare to the 0° wind incidence. At 90° wind flow the pressure coefficients nature changes from positive to negative i.e. pressure become suction at this wind angle. The suction value increases with the increase in length of canopy to some extent, and then it decreases with increase in length. The Pressure on canopy surfaces changes slightly with the increase in canopy length. The upper surface of canopy is more affected by openings under the canopies as compared to the lower surface.