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J. Acad. Indus. Res. Vol. 1(12) May 2013 801
©Youth Education and Research Trust (YERT) Pandey et al., 2013
ISSN: 2278-5213
Influence of proximity on wind loads on Tall Buildings
Sushil C. Pandey*, Ashok K. Ahuja and Anupam Chakrabarti Dept. of Civil Engg., Indian Institute of Technology Roorkee, Roorkee, India
[email protected]*; [email protected]; [email protected]; +91 7500030856 ______________________________________________________________________________________________
Abstract For evaluation of wind loads on tall buildings, the designers generally refer the relevant standards to pick the wind pressure coefficients and wind force coefficients. However, these standards suggest the design pressure coefficients and force coefficients for isolated buildings only. No information is available in the standards regarding wind loads on tall buildings under interference condition. An experimental study is thus carried out on the models of square plan tall buildings to study the influence of proximity of nearby buildings on wind loads on tall buildings, with special reference to base shear and base moments. The base shear (Fx), base moment (My) and torsional moment (Mz) acting on the instrumented building model is recorded for various wind incidence angles. The wind incidence angles varied from 0o to 180o in the intervals of 45o while spacing is varied from 0 mm to 1000 mm. Fx was maximum at 45° and 135° wind incidence angles and minimum at 0°, 90° and 180°. It was observed that torsional or twisting moment was maximum in case of isolated model as compared to interfering cases and thus, interference of buildings actually reduces the torsional moment irrespective of spacing.
Keywords: Proximity, wind loads, tall buildings, base shear, base moment, torsional moment.
Introduction The accurate determination of design wind load on structures is very important from safety point of view and economy. In modern day, tall buildings are being constructed close to one another due to shortage of space. The amount and distribution of wind loads on a tall building under isolated condition is different as compared to when it is placed in interfering environment. Neighbouring structures may either decrease or increase the flow induced forces on a building, depending mainly on the geometry and arrangement of these structures, their orientation with respect to direction of flow. Surrounding buildings may give shelter in one case but in another case it may channel the air flow that may increase the wind load when compared to building in isolation. Most of the information available about the wind loads on buildings is mainly for an isolated building. Wind loading standards (AS/NZS: 1170.2-2002, ASCE: 7-02-2002, BS: 63699-1995, EN: 1991-1-4-2005, IS: 875 (Part-3) 1987) offer little guidance to the designers for assessing the effects of interference. Very little information is available regarding wind loads on buildings in a group. Scruton and Newberry (1983) conducted wind tunnel tests for two prismatic buildings located close to each other and found an increase in suction values by two to three times of those for isolated building. Elizabeth (1990) studied on shielding effect of upstream buildings on downstream buildings. Shielding is found to be maximum when wind incident normally. The author also noticed that lower and narrower obstructions at greater separation distance causes less shielding while taller and wider obstructions at lesser separation distances increased the effect of shielding.
Mir (1988) conducted wind tunnel tests on two sets of similar buildings to study interference effects. The results showed that the neighbouring building changes the mean static wind loads on another building appreciably. Yahyai (1990) in a study on interference between two similar size buildings showed that an upstream interfering building causes shielding effect on a downstream building significantly. Since available information is not enough for the structural designers to make use of it while designing tall buildings for wind loads with varying arrangement, an experimental study has been carried out on the models of tall buildings keeping them in different patterns as detailed in this study. Materials and methods Wind tunnel specifications: The models of tall buildings are tested under boundary layer flow in an open circuit wind tunnel with a cross-section of 2 m (width) x 2 m (height) and the length of the test section as 15 m. Floor roughing devices namely vortex generators, barrier wall, cubical blocks are used on the upstream end of the test section to achieve the mean wind velocity profile corresponding to terrain category 2 as per Indian Standard on wind loads. The model of a square plan tall building is placed on force balance at a distance of 10.5 m from the upstream edge of the test section, with and without nearby building blocks of same shape and dimensions. The experiments are carried out at free stream wind velocity of 9.78 m/sec (Approx. 10 m/sec).
RESEARCH ARTICLE
J. Acad. Indus. Res. Vol. 1(12) May 2013 802
©Youth Education and Research Trust (YERT) Pandey et al., 2013
Model description: The instrumented building and other interfering buildings are assumed to have floor area of 200 m2 each with a height of 100 m. Rigid models to represent the prototype are made of plywood at a scale of 1: 200. The height of each model is 500 mm. Other dimensions of these models can be seen in Fig. 1. Hatched building represents the instrumented building while the others represent interfering buildings.
Fig. 1. Arrangement of models with plan dimensions.
Following cases are considered to study the effect of interference on the instrumented building (Fig. 2). Case 1: Variation of Fx, My and Mz on single instrumented building model with wind incidence angles (Fig. 2a). Case 2: Variation of Fx, My and Mz on instrumented building model with wind incidence angles (Fig. 2b). Case 3: Variation of Fx, My and Mz on instrumented building model with wind incidence angles and spacing of interfering buildings (Fig. 2c).
Fig. 2. Wind directions for different setups
Measurement technique: Rigid model of a tall building is placed on five component load cell. The model is first placed in isolated condition (Fig. 3a) and later with two models with same size and shape on upstream side without gap (Fig. 3b) and with gap (Fig. 3c). The base shear (Fx), base moment (My) and torsional moment (Mz) acting on the instrumented building model is recorded for various wind incidence angles. In cases 1 and 2, only wind incidence angle is varied whereas in case 3, spacing between instrumented building model and interfering models (‘S’ in Fig. 2c) is also varied.
Fig. 3. Typical arrangement of models in wind tunnel.
a. Isolated condition b. Interference condition without gap c. Interference condition with gap
a b
a b c
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J. Acad. Indus. Res. Vol. 1(12) May 2013 803
©Youth Education and Research Trust (YERT) Pandey et al., 2013
The wind incidence angles are varied from 0o to 180o in the intervals of 45o while spacing is varied from 0 mm to 1000 mm. Base shear, base moment and torsional moment values are recorded for 1 min at an interval of 1 sec in a data logger and average values are calculated. Results and discussion Variation of base shear, base moment and torsional moment measured on the instrumented model in various setups as discussed in Case 1, Case 2 and Case 3 as a function of (i) wind incidence angle and (ii) spacing between instrumented building and interfering buildings are shown in Fig. 4 to 6. It is seen from Fig. 4 that Fx is maximum at 45° and 135° wind incidence angles and is minimum at 0°, 90° and 180° wind incidence angles. It is due to maximum exposed areas at 45° and 135°. Further maximum value is almost 1.25 times the minimum values. When interfering building blocks are placed without any gap (i.e. S=0), Fx values are almost same as isolated case at 0° and 90° wind incidence angles. At other angles, it is different from the corresponding values of isolated case. Fx is less than isolated case for wind incidence angles between 0° to 90°, but greater in case of 90° to 180°. This indicates that whereas interference effects are beneficial for 0° to 90° angles, it is harmful for angles 90° to 180°. Maximum values of Fx in this interference condition is almost 3.8 times the minimum value. Further maximum value of Fx in this case is almost 1.5 times the maximum value of Fx in isolated case. As spacing between instrumented building block to interfering blocks increases, maximum value of Fx decreases and for a gap of 500 mm and above (i.e. 5 times the lateral dimension of the interfering building block in the direction of wind), variation of Fx with wind incidence angle becomes quite identical to that of isolated case. Figure 5 show that interfering buildings mainly affect the base moment than any other parameter. All the values of My are greater than those obtained for isolated model though the peak for all cases occur at 135° up to a spacing of 150 mm. It is observed from Fig. 6 that torsional or twisting moment is maximum in case of isolated model as compared to interfering cases. Thus, interference of buildings actually reduces the torsional moment irrespective of spacing. Conclusion Following are the conclusive points made from the study: 1. There is large influence of nearby building blocks on
wind loads acting on building under consideration. 2. Building blocks in near proximity are not always
beneficial one. They cause adverse effect too. 3. Wind loads get increased on the building under
considertaion almost 1.5 times due to presence of nearby building blocks.
4. Wind incidence angles also effects wind loads on square plan tall buildings. Ratio of maximum to minimum wind load due to variation in wind angle is around 1.25 times that in isolated case. However, this ratio is around 4 in interference case.
Fig. 4. Variation of base shear with wind incidence angles
and spacing between blocks.
Fig. 5. Variation of base moment with wind incidence angles and spacing between blocks.
Fig. 6. Variation of twisting moment with wind incidence angles
and spacing between blocks.
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J. Acad. Indus. Res. Vol. 1(12) May 2013 804
©Youth Education and Research Trust (YERT) Pandey et al., 2013
Acknowledgements The work presented in this paper is part of the research work being done by the first author for his M. Tech. degree under the supervision of remaining authors. References 1. BS: 63699-1995. Loading for buildings: Part 2-code of
practice for wind loads. 2. Elizabeth, C.E. 1990. Shielding factors from wind tunnel
studies on prismatic structures. J. Wind Engg. Aerodynam. 36: 611-619.
3. IS: 875 (Part-3)-1987. Code of practice for design of wind load (other than earthquake) for buildings and structures. BIS, New Delhi.
4. Mir, S.A. 1988. Interference effects between two prismatic buildings. M.E. Thesis, University of Roorkee, Roorkee.
5. Scruton, C. and Newberry, C.W. 1983. Estimation of
wind loads for buildings and structural design. Proc. Institution of Civil Engineers, London. 25: 97-126.
6. Yahyai, M., Pande, P.K., Krishna, P. and Kumar, K., 1990. Interference effects on tall rectangular buildings. Proc. Int. Sym. Wind loads on Civil Engg. Structures, New Delhi.
7. AS/NZS: 1170.2-2002. Structural design actions, Part-2: Wind action.
8. ASCE: 7-02-2002. Minimum design loads for buildings and other structures.
9. EN: 1991-1-4-2005. Euro code 1: Actions on structures wind actions.
