WIND PRESSURES AND INTERFERENCE EFFECTS ON TYPICAL LOW-RISE BUILDINGS

Abstract

Severe wind storms, including tropical cyclones, are known to cause considerable damage to life and property all over the world. The coastal zones of India, in particular the East Coast, are regularly hit by cyclones while the inland regions are prone to high intensity windstorms. During the 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 of wind loads on roofs, the effect of eaves (also called canopies or overhangs) is sometimes disregarded. However, the windward eaves may be loaded severely due to wind, since the deflected flow on separation from the windward wall, gives rise to a pressure on the lower eave surface, which reinforces the high suction on the upper eave 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 eaves. The review of literature on the subject of wind pressures on roof projections / eaves / canopies / open verandahs has revealed inadequate information and much variation in the Codal Provisions. Though the Canadian Code (1995) contains roof extensions all round the perimeter of a building, not much study is made on the two-side overhangs/eaves, quite prevalent in India. Therefore, there is need to carry out detailed study on the roofs with extended overhangs. Design wind loading 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 improvements in the existing information. In this direction, an attempt has been made in the present work to establish firstly the flow simulation criteria on the basis of Texas Tech University (TTU) building model study. Further, under the simulated wind flow conditions, detailed study on gable roof with extended overhang/eave has been carried out on isolated building as well as under interference from one/two similar building(s). Most of the building codes specify wind loading on the basis of wind tunnel tests carried out on isolated building models, though in practice, this is seldom the case. The flow fields of buildings placed in a group interfere with each other, thus creating a wind field, which is different in comparison to that for the isolated building. The effects of interference in 111 a given situation depend very much on the relative position of these structures, their orientation with respect to the direction of wind flow and the upstream terrain conditions, leading to shielding or amplification effect. 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 2.1mx2.0m Boundary Layer Wind Tunnel (BLWT). Five Counihan type vortex generators, a barrier wall and cubical blocks of 15.0cm, 10.0cm and 7.0cm .size have been used as roughness devices. Instantaneous velocity fluctuations have been recorded using hot-wire probe at a sampling frequency of 4 KHz for a duration of approximately 4 seconds, viz, a total of 16,384 samples are recorded at each point for flow characteristic measurement. The building selected for the study is a single story gable (pitched) roof building with overhangs on either side with the openings considered to be closed at the time of a windstorm (Fig. 1). The length, width and eaves height of the building are 7.1m, 3.9m and 3.9m respectively. Roof slopes selected for this study are 10°, 20° and 25°. 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 = 0.176. The turbulence intensity at the eave height is 20%, which is close to the mean full-scale value of TTU Mode M04 data. Integral length scale of turbulence Lux, is obtained as 0.45m. Small-scale turbulence parameterof the incident flow is evaluated at frequency n (=10U/ L) at model eave height. Average value of the small-scale parameter S has been obtained as 101 which is quite high and found to be appropriate for the model study. Wind tunnel studies so for have been made with smaller values ofSexcepting one reported by Tieleman. The flow characteristics obtained with the present set of roughness devices closely agreed with the simulation # 8 of Tieleman et al (1999), which gave the best duplication of mean and peak pressure coefficients comparing model/full-scale results, establishing adequacy of the 'S' value achieved. A 1:25 scale Perspex model of the TTU Building and three models of a gable roof building with overhangs and having 10°, 20° and 25° roof pitches were fabricated. Overhangs have been made as extension of the roof surface. Steel tubes of 1.0 mm inner diameter were inserted into the 2.0mm gap between two Perspex sheets making upper and lower surfaces of IV the overhangs, the technology for which was also developed. In all 66 pressure points were created on the upper surface of overhangs and 40 pressure points were created on the lower surface ofthe overhangs. In addition, 88 pressure points were created on remaining the surface ofthe roof. Pressure taps have been placed at a distance of0.6 mm from the edge ofthe roof projection (eave) and 0.4 mm from the ridge as well as the walls. 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 1.2 mm internal diameter. Athree stage tubing system is used for pressure measurement. Total length oftubing system used was 250 mm in length. Arestrictor of 60 mm in length and 0.4 mm in diameter was connected at 150 mm from pressure tap. Restrictor was connected to Scanivalve pressure scanner with 40 mm long and 1.2 mm internal diameter Vinyl tube. With this arrangement, an almost flat amplitude frequency response of the tubing system could be achieved and linear phase characteristics up to frequency 200 Hz. A32 channel ZOC12 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 was developed to acquire the voltage signal from Scanivalve through thePCL206 ADC Card. Data has been recorded at a sampling frequency of 400 samples/sec/channel. 12,000 samples of pressure data from each channel could be recorded, over 30 seconds duration of the record. Wind pressures measured on the roofof 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 the BLWT with flow conditions 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. The angle of wind attack was varied from 0° to 360° at an increment of 15°. This exercise helped in establishing the necessary flow conditions, in particular to simulate the flow parameters, 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 thereby ensuring achievement of proper simulation of surface winds in the experimental work. It thus established that it is possible to realize the necessary flow conditions for simulation of the atmospheric surface layer, which affects the wind pressures in low-rise, and particularly the single storey buildings. Isolated Building Model Study Building models with roof pitch of 10°, 20° and 25° have been tested for angle ofwind incidence varying from 0° to 90° at an increment of 15°. The total roof area was divided into nine different zones (Fig 1). Time history ofpressure fluctuations over each pressure point on the building roof was recorded. Wind pressures measured on the roof ofbuilding models have been expressed in the form ofpressure coefficients. The mean wind velocity at model eaves height has been used as the reference wind velocity. The flow was maintained so as to get this velocity as 9.68 m/s. Area averaged pressure coefficients for each zone on the upper roof surface (including overhang portion) and lower eave surface of the building have been calculated bytaking the average of all the pressure points existing in the particular zone of the roof. The variations of mean, rms, peak and design pressure coefficients for different zones of the building have been plotted for different angles of wind incidence. Thus the critical wind directions have been identified for different zones of the roof. Further, spatial distribution of the pressure coefficients for different roof slopes and for wind directions 0° and 90° have been presented in the form of contours. Worst values ofthe peak pressure coefficient amongst all wind directions have also been presented in the form ofcontours for each roof slope. Interference Study The building model with extended overhang having 25° roof slope has been selected to carry out the study on interference effect. Interference effects have been studied in two phases. In the first phase of the interference study, interference effects due to the presence of single similar building has been studied whereas in the second phase, interference effects due to a pairof similar buildings have been studied. Flow simulation for interference studies have been maintained same as for the isolated model study. The pressure coefficients obtained for the interfering cases are normalized by those for the isolated case thus obtaining the Interference Factors (IF). The observed values of area averaged mean, rms, peak, and design pressure coefficients obtained in the presence ofinterfering building(s) have been compared with those for the isolated one for the respective zones of the building roof. vi Application of Artificial Neural Network Artificial neural network (ANN) representations are capable of developing functional relationship from discrete values of input-output quantities obtained from computational approaches or experimental results. This generalization makes it possible to train a network on a representative set of input-output examples and get good results for a new input without training the network on all possible input-output examples. Data obtained from wind tunnel tests for interference effects expressed by Interference Factor (IF) on gable roof building has been used for training the network. The interference factor for the worst design pressure coefficients, independent of wind direction for each zone of the roof on the building for single building interference has been taken as output parameter of the neural network. Locations of the interfering building have been considered as input parameter. Training of the neural network is carried out by inputting the data sets, which consist of some selected locations of the interfering building and the corresponding values of IF for Cpq. These were determined independently for each zone of the roof. The trained network is then expected to predict the IF for Cpq for locations of interfering building not covered in the training data set. Correlation plots between ANN predicted values and the experimental values of IF for Cpq for different zones of the roof and also the contours for the two values (predicted and experimental) have been prepared, which indicate remarkable closeness. Conclusions The study has yielded significant results for the isolated building as well as for the interference conditions. The main conclusions of the study are as follows: • Contours show that the mean pressure coefficients follow same trend of increase from windward edge to windward wall for all roof pitches. • There is a little effect of roof pitch on the mean roof pressures for 90° angle of wind incidence (Fig. 1). • The rms wind pressures for windward eave are invariably higher than for the leeward eave for all roof pitches. • With increase in roof pitch from 10° to 25°, the mean suction (Cpmean) significantly decreases on the windward roof slope. VI1 • Wind directions parallel and perpendicular to the ridge are critical for most zones of the roof. However, oblique wind directions are also found to be critical for some zones of the roof. • Worst peaks on the upper roof surface of eave decreases with increase in roof pitch, whereas, for comer zones its value increases. • Peaks on the eave surface (upper and lower) are close to the values reported by Stathopoulos(1984). • Experimentally obtained values ofthe design pressure coefficients, Cpq are generally in good agreement with those given in the Indian Standard Code (IS: 875 Part-3 1987). However, some significant variations also exist between the codal and the experimental values, particularly for zones A, D, G and H. • The experimentally obtained peak value on eaves is found to be comparable with the value in the National Building Code of Canada (NBCC -1995). However, some significant deviations also exit. • Maximum increase in the design wind pressure coefficients for interference with a single similar building is observed as 48% and occurs at the comer zones. In case of interference from a pair of similar buildings the maximum increase is 54% and it occurs in the overhang's interior zone (B2). • The wind pressures due to interference from a pair ofbuildings are found to be higher (25% -54% increase over isolated case) than those due to interference by a single building (12% - 48% increase overisolated case) for different zones. • Correlation plots and contours show that most ofthe ANN predicted values are very close to the corresponding experimental values. • 50% reduction in the experimental work can be achieved for the case studied by using the neural network modeling for interference studies in low buildings, without sacrificing any accuracy. vin 15.6 END ELEVATION All dimensions are in cm >>r^ c ! a £ D3 E D G F H 28.0 PLAN 0 -Angle of wind incidence Fig. 1 Location of Different Zones on Roof of Building Model Studied. IX