EFFECT OF ARCHITECTURAL FEATURES ON WIND LOAD IN BUILDINGS

Abstract

Millions of the buildings built all over the world can be categorized as low-rise and are used for residential, commercial, industrial and other purposes. Wind load for such type of low-rise buildings constitutes an important part in design. Various architectural features of these buildings, such as aspect ratio, roof angle, overhang and boundary wall can affect the surface pressures both mean and fluctuating components acting on the roof and wall surfaces, as they are the result of an integral effect of velocity fluctuations interacting with these features at different wind incidence angles. It is difficult to describe fully the wind loading on low-rise buildings with different architectural features on the basis of the prescriptive wind codes and standards. Therefore, effect of different architectural features on wind load variations on roof and walls of gable buildings has been studied and is the focus of this thesis. Design wind loading is mainly obtained from different Codes and Standards, wherein 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 improvements in the existing information. In this direction, an attempt has been made in the present work to establish the flow simulation criteria based on Texas Tech University (TTU) building model study. Further, under the simulated wind flow conditions, detailed study on gable roof building models with different configurations like roof angle, aspect ratio, without and with overhang has been carried out for isolated as well as with interference of boundary wall. In the present study, different roughness devices have been used to meet the wind tunnel simulation requirements and for the development of a highly turbulent flow for generating the Atmospheric Surface Layer in the 2.1 m x 2.0m Boundary Layer Wind Tunnel (BLWT). Five Counihan type vortex generators, a barrier wall and cubical blocks of 10.0 cm, 7.0 cm and 3.5 cm size have been used as roughness devices. Instantaneous velocity fluctuations have been recorded using hot-wire probe for flow characteristic measurement. Commonly available single story gable roof buildings without any openings have been considered for present study. Buildings with two different roof angle, 25° and 10°, with and without overhang have been chosen. Height (H) and width (B) of gable buildings are 3.25 m and 3.75 mand length (L) of the buildings 7.5 mand 22.5 mhave been considered for aspect ratios (L/B) of the building is 2 and 6 respectively. Overhangs are extended on longitudinal side of building up to 1.5 m from building wall. In addition, boundary wall of height 1.65 m has been considered to find the interference effect on wind loads. The prototype is considered to be situated in an open terrain with well scattered obstructions having heights generally between 1.5 to 10 m, defined as Terrain Category 2 in IS: 875 (Part-3) 1987. For this terrain type the velocity profile has a power law exponent a = 0.14. The turbulence intensity at the eave height is 18.62%, which is close to the mean full-scale value of TTU building data. Integral length scale of turbulence Lux, is obtained as 0.45 m. A 1:25 scale Perspex model of the TTU Building and eight gable roof building models of different configuration are fabricated for wind tunnel testing. Overhangs have been made as extension of the roof surface. Steel tubes of 1.0 mm inner diameter are inserted into the 2.0 mm gap between two Perspex sheets making upper and lower surfaces of the overhangs. Pressure taps have been placed at a distance of 0.6mm from the edge of the overhangand 0.4 mm from the ridge as well as the walls. Surface pressures on the roof and walls of the building models have been measured by connecting steel taps (10 mm long, 1mm internal diameter) of stainless steel tube which are fixed through holes drilled in to the Perspex sheet. One end of the tap is flushed with the roof and wall surfaces connecting Vinyl tubing of 1.2 mm internal diameter. Tubing for measuring surface pressure consists of 300 mm vinyl tube with a 20 mm long restrictor placed at 200 mm from the pressure point. With this arrangement, approximately flat amplitude frequency response of the tubing system could be achieved and linear phase characteristics up to frequency 200 Hz. A 32-channel ZOC23B pressure scanner from Scanivalve Corporation Ltd. has been used to measure surface pressures. The output signal in the form of voltage from pressure scanner has been recorded using PCL206 ADC Card. A computer programme has been developed to acquire the voltage signal from Scanivalve through the PCL206 ADC Card. Data have been recorded at a sampling rate of 500 samples/sec/channel for a sample duration of 16 seconds. Wind pressures measured on different surfaces of the building models have been expressed in the form of non-dimensional pressure coefficients. Study ofTTU Building Model TTU building model has been tested in two conditions (isolated condition and with boundary-wall at varying distances) in the BLWT to obtain the mean, peak pressure and standard deviation of pressure coefficients. For validation of flow simulation with full-scale VI data, experiment has been performed for the angles of wind incidence of 0° to 360° at 15° interval for isolated condition. However, test for building with boundary-wall has been performed for 0°, 30°, 60° and 90° angles ofwind incidence. The measured values ofpressure coefficients for isolated condition are found to be very close to those reported for the prototype TTU building thereby ensuring achievement of proper simulation of surface winds in the experimental work. It, thus establishes, that it is possible to realize the necessary flow conditions for simulation of the atmospheric surface layer, which affect the wind pressures in low-rise, and particularly the single storey buildings. In the presence of the boundary-wall at different distances, it has been observed that the pressure values on the roofand wall reduce significantly. Maximum reduction in the pressure values have been found on the roof taps when the boundary wall position is at three times the height of the building wall. For short and long building walls, the maximum reduction for positive pressure has been found when boundary wall is at 1.5 times the height of building wall and negative pressure values for the wall surface of building reduced significantly when wall position is at three times the height of building wall. Study of Gable Roof Building Models Buildings models with roof slope of 25° and 10° have been tested in two conditions (isolated condition and with boundary wall at different distances). For isolated condition, experiment has been performed for the angles of wind incidence 0° to 90° at an increment of 15°. However, for building with boundary wall wind directions 0° to 90° at an interval of 30° have beentaken. Theflow has been maintained as in the testing of TTUbuilding model. Area averaged pressure coefficients for each zone on different surfaces (overhang, roof and wall surface) of the building have been calculated by taking the average of all the pressure points existing in the particular zone. The variations of mean, peak, standard deviation and design pressure coefficients for different zones of the building have been plotted for different angles of wind incidence for isolated as well as for interference condition of boundary wall at varying distances. Thus, the critical wind directions have been identified for different zones of the building surfaces. Results of gable building models have been presented separately for overhang, roof and wall surface: A. Study of overhang Significant differences have been found on pressure coefficients on different zones of overhang due to change in wind incidence angle and aspect ratio of building. The concept of Vll simultaneous and non-simultaneous pressure coefficients have been illustrated and discussed for overhang. Significant changes have been observed in pressure variation on the overhang in the presence of boundary wall. It has been observed that with an increase of distance between building and boundary wall, there is first a significant decrease in the magnitude of negative mean pressure coefficients (suction) on overhang, and shows maximum positive values when boundary wall is at two times the height of building wall; further increase in the distance tapers off the effect of boundary wall. Experimental values have been compared and discussed with design values of pressure coefficients recommended by codes and standards. B. Study of roof surface It is found that, for Isolated condition the value of design pressure coefficients (suction) increases to some extent for buildings with L/B = 6 as compared to that of the building with L/B = 2. The experimental values of design pressure coefficient for different zones of the roof have been compared with various Codes. Experimental values for all cases of roof zones are less as compared to codal values i.e. codal values are conservative. In presence of boundary wall, suction on different zones of roof decreases maximum when boundary wall distance is three times the heights of building walls. Further increase in the distance of boundary wall from the building tapers off the effect on pressure variation. C. Study of wall surface of building The comparison, of the experimental results of wind load on walls of gable-roof buildings for isolated condition with different codes and standards, has indicated that pressure coefficients obtained from the experimental studies for different zones of the wall are of the same order as given in different codes. It has been observed that the design wind pressure on wall surface of building with overhang increases considerably compared to that of the building without overhang. In the presence of the boundary wall, significant reductions in both positive and negative values of pressure coefficients have been found on the wall surface of building. The maximum reduction for positive pressure is found when the spacing of boundary wall is 1H to 1.5H of building wall. Negative pressure values for the building wall reduce significantly when wall position is at three times the height of building wall. Vlll