International Journal of Advanced Engineering Technology E-ISSN

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1 International Journal of Advanced Engineering Technology E-ISSN Research Article WIND INTERFERENCE ON SINGLE SIMILAR GABLE ROOF BUILDING WITH OVERHANGS Narayan K a *, Gairola A b Address for Correspondence * a Associate Professor, Department of Civil Engineering Institute of Engineering & Technology Lucknow, India b Professor, Department of Civil Engineering Indian Institute of Technology, Roorkee, India ABSTRACT The building model with extended overhang having 25 º roofs slope which is widely used in coastal zones in India has been selected to carry out the study on interference effect. An interference effect due to the presence of single similar building has been studied. The design pressure coefficients obtained for the interfering cases are normalized by those for the isolated case thus obtaining the Interference Factor (IF). The observed values of area averaged design pressure coefficients obtained in the presence of interfering building(s) have been compared with those for the isolated one for the respective zones of the building roof. Maximum increase in the design wind pressure coefficients for interference with a single similar building is observed as 48% and occurs at the corner zones. KEYWORDS Gable roof building, overhangs, design pressure coefficient, interference factor INTRODUCTION Severe windstorms, including tropical cyclones, 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 whereas the inland regions are prone to high intensity windstorms. In 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 every year. In the assessment of wind loads on roofs, the effect of eaves (also called canopies or overhangs) is sometimes disregarded. However, 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. Wind Engineering Standards and Codes offer little guidance to the designer for assessing the effect of interference. In a study on the evaluation of wind loads acting on low-rise buildings in the presence of a large nearby building by Stathopoulos (1), comparisons with the National Building Code of Canada and ANSI Standard show underestimation (upto 46%) or overestimation (upto 525%) of the code specifications, which are generally for isolated prismatic buildings. These results indicate that code recommendations may be significantly low (unsafe) or uneconomically conservative; therefore, the effect of adjacent structures on wind loads should be evaluated properly for realistic wind load design of buildings. The Australian standard for Minimum Design Loads on Structures (2) has incorporated a brief guideline on interference, but only as a general warning note: The flow around any structure in a group will usually differ from that around a similar isolated structure leading to different forces... Interference effects are prevalent in structures located less than 10b apart, where b is the dimension of the structure normal to wind Three main reasons appear to explain the lack of a comprehensive and generalized set of guidelines for wind load modifications caused by adjacent buildings. First, the complex nature of the problem even for a single additional building, since there are a large number of variables involved including the size and shape of buildings, their relative positions, wind direction and topographical conditions; second, the scarcity of adequate experimental data; and third, the widely held notion that wind loads on a building are expected to be generally less severe if surrounded by other structures than if it is isolated. This last reason, though quite applicable to the building surrounded by a large number of similar structures, becomes debatable where only two or three buildings interact, since several studies have shown quite adverse effects depending on the relative location of these buildings. Maximum studies on low-rise buildings in recent years have been focused on buildings free of obstructions in the surroundings. However, most of the low buildings are situated in urban areas in close vicinity of neighboring buildings and other structures. Peterka and Cermak (3) studied adverse wind loading induced by adjacent buildings. Changes in mean flow patterns due to the presence of other buildings were reviewed and possible reasons for the enhanced wind pressures were explained. The wake produced by upwind buildings and surface roughness of the object building decides the separation position for air stream and changes the pressure distribution. The subject of interference gained importance after the collapse of the three (out of eight) cooling towers at the Ferry bridge Power Plant on 1 st Nov, 1965 (Sachs, 4). The cause was determined that the increased loading on the towers was due to the presence of adjacent towers.walker and Roy (5) performed an experimental study of wind loads on houses in urban

2 environment and compared the case with an isolated building in open terrain. Ho et al (6) studied effects of the surroundings on wind loads on flat roof lowrise buildings. They concluded that with the increased surrounding obstruction, the mean wind pressure acting on the building decreased, while the unsteady pressure increased. Holmes and Best (7), Holmes (8) studied the effect of grouping of houses in characteristic suburban street patterns. It was found that significant increase in the magnitude of the negative roof pressure occurs when one extra half row of houses is added to each side of an isolated low set house. The shielding effects of upwind buildings are dependent strongly on the ratio of building spacing to height. Kumar (9) studied the interference effect on a gable pitched roof building by a similar building and found that the effect of interference is maximum when the spacing between the two buildings is between 0.25 and 0.50 of the width of the building. Surry and Lin (10) studied the characteristic of high suction on flat roof buildings in the presence of surrounding buildings. A typical low-rise building grouping configuration was used. It was found that the presence of the surrounding buildings generally reduces the suctions on the roof. Near the corner, the surroundings lead to a reduction in magnitude of 50% to 65% for the configuration investigated. Khanduri et al (11) in their state-of-the-art paper have discussed enormous possible surroundings for interference effects in low as well as high rise structures. Shakeel (12) studied interference effect on 30º pitched hip roof building model with single similar as well as three similar interfering buildings in a regular pattern. He observed both shielding and enhancement of roof pressures on different zones of the test building. Ahuja et al (13) presented an interference study on rigid model of a low-rise building with a high-rise building. Wind pressures on high-rise building were adversely affected. This influence is quite distinct for the spacing values up to 5 times the height of the low-rise building model. Santiago Pindado et al (14) studied the effect of an upstream building on the suction forces on the flat roof of a low-rise building placed in the wake of the former. Experimental results reveal that the wind load increases as the relative height of the upstream building increases, the wind load being highest for intermediate distances between buildings, when a passage between them is formed. Wonsul Kim et al (15) studied the local peak pressure coefficients between two buildings using wind tunnel experiments for various locations, different height ratios of interfering building and wind directions. The results show that highest peak suctions on a principal building increased with increase in height ratios of the interfering building. Alok David John et al (16) studied the pressure variations on overhang, roof and wall surface of gable roof building due to interference effect of a boundary wall. It has been observed that pressure values reduce significantly due to presence of boundary wall. Since numerous situations are possible amongst the surroundings for a building or group thereof, generalization of results for the effect of interference is difficult, and has not even been attempted. The best that can be expected from more studies is a widened database with possibly some kind of generalization for different categories of situations. INTERFERENCE MECHANISM There are many parameters which affect the manner in which one building modifies the forces on another building in its vicinity. These are: size and shape of the building, wind velocity and direction, type of approach terrain and above all, the location and proximity of neighboring buildings. With the inclusion of another building in the vicinity, the loading pattern becomes quite complex. The buildings may experience increased or reduced wind loads depending upon their geometries, spacing as well as the characteristics of wind flow and the upstream terrain. In this case the wake of the upstream building is considerably distributed by the downstream building and a part of the shear layer is greatly accelerated around the inner side wall of the downstream building. This results in an increase in the negative pressure (suction) on the inner side wall of the downstream building and the generation of a net inward lift. Sakamoto et al (17), Taniike (18) and Gowda and Sitheeq (19) have performed flowvisualization around a pair of buildings to study the flow pattern generated due to interaction effects. It was observed that when the clear spacing between the models is small, the downstream model is completely submerged by the shear layer emanating from the upstream model, thus undergoing high suction on all its exposed faces. With an increase in the spacing between the two models, these shear layers direct towards the front face of the model, resulting in an increase in the pressure distribution on the front face of the downstream model. At a further increase in the clear spacing between the two models, the influence of the wake of the upstream model on the front face of the downstream model tends to diminish and the downstream model approaches the behaviour of an isolated free-standing model. The arrangement of buildings, their relative size and the direction of wind determine the extent of interaction. When an upstream building blocks another building, it increases or decreases the forces

3 on the downstream building by modifying the structure of wind in its wake. Mean along-wind forces on a downstream building are reduced due to shielding by the upstream building. This shielding clearly decreases as the separation between the buildings increases. The increased turbulence intensity in the wake of an upstream building tends to increase the dynamic loading on the downstream building. For large separation distances, the vortices get enough time and space to become well organized before they hit the downstream building, thus increasing the vertical correlation of wind loading which is responsible for higher dynamic loads. For smaller separation distances, the downstream building interferes with the steady vortex shedding and disrupts its frequency, thus destroying the vortex shedding mechanism and resulting in a small increase in dynamic loads. A downstream interfering building has very little effect on the loads and response of an upstream building for most locations. However, for locations of close proximity, a downstream building can significantly alter the wake characteristics of the upstream building thus resulting in high dynamic loads on it. Maximum interference effects can be expected for the open-terrain exposure, steadily reducing for the suburban and reaching a minimum for the urban terrain. This is because for the open terrain, low turbulence intensity promotes an organized wake behind an upstream building with high energy content. The high energy vortices in the wake of the upstream building excite the downstream building and lead to high interference effects, i.e. increased dynamic loads on the building. An urban approach terrain on the other hand creates turbulence which disturbs the organized vortices and reduces the strength of vortex shedding by redistributing energy to a wide band of frequencies. This leads to lower levels of excitation for the downstream building, resulting in smaller interference effects as compared to the open terrain. Tall buildings upstream may produce adverse effects on a downstream building. This phenomenon has been explained by considering the boundary layer flow around high-rise buildings. The pressure on the front face of a building decreases downwards due to the decreasing velocity in the boundary layer; consequently, the pressure gradient induces a downward draft of air which can result in substantial velocities (and pressures) at lower levels. Thus, smaller structures in the immediate vicinity of tall buildings would be subjected to higher wind loads. Larger cross-sections upstream are expected to produce higher interference effects on the downstream building because of an increase in the size of the upstream wake and, therefore, higher dynamic wind loads are expected; mean loads, however, would be reduced due to greater shielding. Wind loads for design of various kinds of structures are invariably obtained from design Codes and Standards. 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 much different from that for the isolated building. The effect of interference in a given situation depends very much on the relative position of these structures, their orientation with respect to the direction of wind flow and the upstream terrain conditions, which may lead to either shielding or amplification effect. INTERFERENCE FROM A SINGLE BUILDING The building model with extended overhang having 25º roofs slope which is widely used in coastal zones in India has been selected to carry out the study on interference effect (Fig.1). A building similar to the test building has been used as an interfering building. The test building is placed in such a way that wind direction at 0º is normal to the ridge. Due to limitation of the size of the turntable, fifteen locations for the interfering building model could be selected. For every location of the interfering building, angle of wind incidence has been varied from 0º to 90º with increments of 30º, and surface pressures over the building roof measured in each case. The observed values of area averaged design pressure coefficients (Cpq) for each zone of the building have been normalized by the corresponding value for the standalone (isolated) case and termed as Interference Factor (IF). The coordinates of the interfering building (x, y) are mentioned in terms of distance of the center of the interfering building from the center of test building. The x and y axes have been considered along the longitudinal and transverse directions of the test building respectively with the corresponding building dimensions denoted as 2a and 2b. General layout for the case of single similar interfering building is shown in Fig 2(a). The entire roof surface is divided into 9 zones to evaluate the design wind pressures as shown in Fig 2(b). Zones B1, B2 and C are located on eave (overhang) portion. These zones are treated as eave zones. Tables 1(a) and 1(b) give the maximum values of IF along with the corresponding location of interfering building. Contours of worst values of Interference Factor, independent of wind directions have been presented in Figs 3 to 5 for upper and lower zones of eave.

4 Fig. 1 Plan, Elevation and Isometric view of the Building Studied Fig. 2(a) Location of Test Building and of Interfering Building Fig. 2(b) Location of different zones on building roof

5 Table 1(a) Maximum IF for Cpq, all Azimuths and 25º Pitch Roof for Upper Zones Table 1(b) Maximum IF for Cpq, all Azimuths and 25º Pitch Roof for Lower Eave Zones Zone Interference Factor (IF) Location of Interfering Building, (x, y) A 1.20 (0, 4b) B (4a, 2b) B (0, 8b) C 1.48 (3a, 2b) D 1.35 (0, 8b) E 1.35 (0, 8b) F 1.47 (0, 10b) G 1.44 (0, 8b) H 1.43 (0, 8b) Pressure Coefficient Cpq Zone Interference Factor (IF) Location of Interfering Building, (x, y) B (2a, 4b) B (2a, 8b) C 1.51 (4a, 0) Fig. 3 Interference Factor Contours for Cp Values for Zones A, B1, B2 and C due to Change in Position of Single Interfering Building (All Azimuths)

6 Fig. 4 Interference Factor Contours for Cp Values for Zones D, E, F and G due to Change in Position of Single Interfering Building (All Azimuths)

7 Fig 5 Interference Factor Contours for Cp Values for Zone H, Lower Eave Zones B1, B2 and C due to Change in Position of Single Interfering Building (All Azimuths) RESULTS AND DISCUSSION Interference Effects on Upper Roof Zones (including Eaves) In Zone A the maximum IF for Cpq is found to be 1.20 and occurs at (0, 4b) location of the interfering building. The value of IF lies between 1.00 and IF for Cpq decrease as the interfering building is moved away from test building in line to its longitudinal direction. In Zone B1 the Interfering building causes both amplification as well as shielding of pressures. The maximum value of IF for Cpq is observed to be 1.32 and occurs at (4a, 2b) position of the interfering building. The value of IF for Cpq lies between 0.90 and In Zone B2 the maximum IF for Cpq is found to be 1.38 at position (0, 8b). The value of IF decreases in longitudinal direction of the test building. Maximum shielding is observed at interfering position (3a, 6b). In Zone C the maximum value of IF for Cpq is found to be 1.48 at interfering building position (3a, 2b). The value of

8 IF for Cpq ranges form 1.1 to In Zone D the maximum value of IF for Cpq is found to be 1.35 at (0, 8b) position of interfering building. It lies between 0.94 and The values of IF for Cpq increases in the transverse direction and decreases in the longitudinal direction along the line of test building. In Zone E the contours of IF for Cpq follow the same pattern in line transverse to the test building. The maximum value of IF for Cpq is observed to be 1.35 at (0, 8b) as in the case of Zone D. The value of Cpq lies between 0.66 and At some location shielding is also observed. In Zone F the contours of IF for Cpq follow the same pattern. The maximum value of IF for Cpq is 1.47 being observed at (0, 10b) position. In Zone G the values of IF for Cpq near the test building are higher while away from the test building shielding is observed. The maximum value of IF for Cpq is found to be 1.44 at location (0, 8b). The value of IF for Cpq ranges from 0.78 to In Zone H the value of IF for Cpq increases in line transverse to the test building. The maximum value of IF for Cpq is observed as 1.43 at (0, 8b) and ranges from 1.00 to Interference Effects on Lower Eave Zones The maximum value of IF for Cpq is found to be 1.43 at (2a, 4b) position of the interfering building and its value lies between 0.86 and Insignificant amplification is observed in the line of transverse and longitudinal directions of the test building. In Zone B2 the maximum value of IF for Cpq is found to be 1.40 at (2a, 8b) position of the interfering building and its value lies between 0.86 and The value of IF for Cpq increases in the line of transverse direction of the test building. Insignificant variation of its value in longitudinal direction of the test building is observed. The maximum value of IF for Cpq is found to be 1.51 at (4a, 0) position of the interfering building in Zone C. Its value increases in both longitudinal and transverse directions. IF value for Cpq lie between 1.21 and CONCLUSION In case of interference from a single similar building for upper roof zones, the maximum enhancement in the design pressure coefficient, Cpq has been observed to be 47 48% for Zones C & F followed closely by Zones G & H as 43 44%. In general, the increase is between 35 48% (except small Zones A & B1), while minimum enhancement is 20% for Zone A. Maximum shielding due to interfering building for design pressure coefficients is observed to be 63% for Zone E. For lower eave zones, the maximum enhancement in Cpq has been observed to be 40 51% for Zones B1, B2 & C, against 32 48% for the upper eave zones. It has been observed that the presence of interfering building changes the pressure distribution over the building roof. Both amplification and shielding of pressures have been observed for different zones for different positions of the interfering buildings. The wake produced by the interfering building changes the wind flow separation points on the principal building, which leads to a changed pressure distribution on the roof, causing either shielding or amplification. ACKNOWLEDGEMENT The work presented in this paper is part of the research work of the first author carried out at the Department of Civil Engineering Indian Institute of Technology Roorkee, Roorkee, India. REFERENCES 1. Stathopoulos, T. (1984), Average wind loads on low buildings due to buffeting, Journal of Structural Engineering, ASCE, Vol. 110, No. pp AS/NZS :2002, Australian / New Zealand Standard Structural Design Actions, Part 2: Wind Actions. 3. Peterka, J.A. and Cermak, J.E. (1976), Adverse wind loading induced by adjacent buildings, Journal of Structural Division, ASCE, 102 (ST3), pp Sachs, P. (1978), Wind Forces in Engineering, Pergamon Press Ltd., USA. 5. Walker, G.R. and Roy, R.J. (1985), Wind loads on houses in urban environment, Proc. of Asia- Pacific Symposium on Wind Engineering, University of Roorkee, Roorkee, India. 6. Ho, T.C.E., Surry, D. and Davenport, A.G. (1991), Variability of low building wind loads due to surroundings, Journal of Wind Engineering and Industrial Aerodynamics, Vol. 38, pp Holmes, J.D. and Best R.J. (1979), A wind tunnel study of wind pressures on grouped tropical houses, Wind Engineering Report 5/79, James Cook University of North Queensland, Australia. 8. Holmes, J.D. (1994), Wind pressure on tropical housing, Journal of Wind Engineering and Industrial Aerodynamics, Vol. 53, pp Kumar, A. (1994), Wind interference amongst low-rise buildings, M.E. Thesis, University of Roorkee, Roorkee, India. 10. Surry, D. and Lin, J.X. 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