WIND PRESSURES ON LOW-RISE HIP ROOF BUILDINGS

Abstract

Majority of the structures the world over are low-rise buildings used for commercial, industrial, residential and other purposes. The buildings are constructed in different types of terrain and topography with various plan forms. Wind loads generally govern the lateral strength of the buildings in areas other than the high seismic zones and this aspect is more evident in zones of severe wind such as the coastal regions, open terrain and hill slopes. The study of wind effects on a building consists of two parts, namely, (i) evaluation of wind loads, and (ii) estimation of its effect on the building. Wind loading on low-rise buildings has been an area of active investigation due to increasing public concern towards severe damage caused by wind storms, tornadoes or hurricanes in the recent years. Design wind loads are mainly obtained from different codes of practice, which are based on wind tunnel testing of scaled rigid models under simulated flow on accepted principles. Recent studies on full-scale/model-scale comparisons have shown the importance of flow simulation and measurement techniques for better results and to enable improvements in the existing information. In this direction, an attempt has been made in the present work to establish at the first instant the flow simulation criteria on the basis of Texas Tech University (TTU) building model study. Finally, under the simulated flow conditions, detailed study on hip roofbuildings has been carried out. In carrying out such a study, it has been recognized that wind loads are significantly affected by building roofpitch, overhangs, aspect ratio and the angle of wind incidence. Besides study on the hip roof buildings in the 'stand-alone' situation, effect of interference by similar buildings has been studied. An effort has also been made to evaluate design wind pressure coefficients for different zones of the pitched roof. The need for this study was considered to bemore relevant since the Indian Standard Code IS: 875 (Part-3) does not contain any information on hip roofs. Firstly, a model of the TTU building, made to a geometric scale of 1:50 was tested in the Boundary Layer Wind Tunnel (of 2.1m x 2.0m section) with flow conditions in similar to those in the prototype situation and measurements were made to obtain the mean, rms and peak pressure coefficients for comparison with the prototype values. This exercise helped in establishing the necessary flow conditions, in particular to simulate the effect of governing parameters, like scale, intensity of turbulence and the small-scale parameter, thereby ensuring accuracy of the results thus obtained. The measured values were found to be very close to those reported for the prototype TTU building thus establishing proper simulation of surface winds achieved in the experimental work. It thus ensured that it is possible to realise the necessary flow conditions for simulation of the atmospheric surface layer which affect the wind pressure in low-rise, particularly single storey buildings. Subsequently, a detailed investigation of wind pressure distribution on hip roof (with overhangs) of single storey buildings was taken up. Towards this end, seven hip roof building models of same plan dimensions (280mm x 140mm with 58mm eave height) with variable roofpitch of 10°, 15°, 20°, 25°, 30°, 35° and 40° were fabricated and tested in the same boundary layer wind tunnel. The wind incidence angle was varied at 15° interval to cover all possible wind directions. The 30° roof pitch has also been tested for three different overhang values of 0.5m, 0.75m and 1.1m and three aspect ratios (eave height to width ratio, the latter having a constant value of 140mm) of 0.4, 0.5 and 0.6 in order to observe the effect of overhang and aspect ratio on the roof pressures. The wind pressures measured on hip roofs have been compared with the results of other investigators wherever available. Besides, a comparison with gable roof has also been made to bring out the essential differences between the two types of roofs. Finally, the effect of interference due to a single as well as three buildings, all similar to the test building, has been investigated. Results of interference study have been presented in the form of'Interference Factor' (IF) which is defined as: Response (Pressure) in Interference Configuration IF Response (Pressure) in Stand-alone Configuration Interference factor contours have been plotted for mean, rms, peak and design pressure coefficients for all zones of the building roof for the case of interfering with a IV similar building. This is followed by IF values of mean, rms, peak and design pressure coefficients in the tabular form for a set of three interfering buildings placed variously. The main conclusions of the study are given below: Isolated Building Models Study TTU Building Model Cpmean, Cprms and Cpmin match closely for the roof and wall taps, except Cpmin for the roof corner taps for wind azimuths between 180° and 270° which were almost half those of the prototype values. Wind azimuth is seen to have a significant effect in shifting the location of the maximum pressure/suctions. Hip RoofBuilding Models Oblique wind directions are critical for most of the areas (zones) on the roof surface. However, wind directions parallel and perpendicular to the ridge are also observed to be critical for some areas of the roof. As the roof slope increases from 15° to 40°, the worst peak suction is seen to increase. The 40° hip roof experienced the highest peak suction at roof corners among the seven roofs tested. Compared to the gable roofs, the worst peak suctions were much smaller on hip roofs with 15° and 20° roofpitches. The maximum value of the mean, rms, peak (negative) and design pressure coefficients for local pressure zones, irrespective of wind direction, decreases sharply with change in the roof slope from 10° to 20° and increases gradually with change in the roof slope from 20° to 40°. Experimentally obtained values of the design pressure coefficients are found to be of the same order as given in the Building Research Establishment (BRE) Digest No.346 of November 1989. However, some variation exits between the BRE values and the values observed in the present study. Variation of overhang of the roof has been found to influence the hip roof pressures moderately. The critical wind direction remains the same for all values of the overhang. Effect of aspect ratio (height/width) is found to be significant for roof pressures. Here also the critical wind direction remains unchanged. Interference with a Similar Building Both shielding and enhancement for roof pressures have been observed for different zones for different positions of the interfering building. Maximum enhancement has been observed in the rms value of the pressures while maximum shielding has been noticed in the mean value. Maximum increase in the value of design pressure coefficients has been found to be 56% occurring in zone 5 of the roof and maximum shielding for the same has been obtained as 33% which occurs in zone 2 of the roof. Maximum enhancement in the peak (negative) pressures has been found to be 52%o occurring in zone 7 whereas maximum shielding for the same is 11% which occurs in zone 2 of the roof. Maximum amplification in the rms pressure coefficient has been found to be 66%> occurring for zone 5 and maximum shielding for the same is 32% which occurs in zone 2 of the roof. Maximum enhancement in the mean pressure coefficients is found to be 41% occurring in zone 1 whereas maximum shielding for the same is 34% which occurs in zone 2 of the roof. Interference with Three Similar Buildings Like single building interference, both shielding and enhancement in the pressure coefficients are also found to occur here. However, for most of the interfering building locations shielding has been observed. Maximum enhancement in Cpq is found to be 53%> occurring in zone 8 and the maximum shielding is 50% which is found to occur in zone 1 of the roof. Maximum amplification in Cpmin is observed to be 73% occurring in zone 8 and the maximum shielding is noticed as 37%which occurs in zone 1 of the roof. Maximum enhancement in Cprms is 61% occurring in zone 6 and the maximum shielding is 25% which occurs in zones 1 and 2 of the roof. VI Maximum amplification in Cpmean is 64%o occurring in zone 8 and the maximum shielding is 69%> which occurs in zone 1 of the roof. Various zones on the hip roof Vll