INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 4, No 3, 2014

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1 INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 4, No 3, 2014 Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0 Research article ISSN Wind loads on structurally coupled through single bridge tall buildings 1 1- Associate Professor, Department of Civil Engineering, PEC University of Technology, Chandigarh, , India. skverma5724@yahoo.com doi: /ijcser ABSTRACT Present paper describes the wind tunnel study carried out on rigid models of rectangular shape high rise buildings. The models are tested in the closed circuit wind tunnel having a cross section of 1.3m (width) x0.85m (height), at Civil Engineering Department, Indian Institute of Technology Roorkee, Roorkee. Perspex sheet 5mm in thickness is used for making two building models each with square in plan having size of 50mm 50mm 300mm.The two models are connected by a sky bridge,the connecting bridge is also made of Perspex sheet and is of size 15mmx15mm.and is located at 0.25H,0.5H&0.75H from bottom Pressure points are made on the models to measure the pressure distribution. The study is conducted for 3 incidence angles i.e. 0, 45, and 90. The spacing between the two building models is kept at 50mm.For comparison purposes, an isolated building model size 50mm 50mm 300mm is also tested in the BLWT. Wind pressure coefficients are calculated from the measured values of mean pressure on the model.the length scale of the model is 1:600. The results of these studies are presented in the present paper. It is observed from the study that negative pressure on opposite faces gets increased considerably when the models are close to each other. Keyword: Tall Building, Mean Wind Pressure, Sky Bridge, Boundary Layer Wind Tunnel 1. Introduction All the structures should be designed in such a manner that these can carry all the loads safely and with minimum cost.therefore, structural engineer must be in a position to anticipate the loads which the structure is likely to encounter during its lifetime. Broadly speaking, the civil engineering structures carry two types of loads namely permanent loads and occasional loads. Loads of permanent nature can be calculated with ease but for assessing the occasional loads it requires ingenuity on the part of the structural engineer. Due to limited land availability and population explosion, the present trend is towards high rise buildings. In the design of high rise buildings lateral forces play a major role. Two types of lateral loads are normally encountered in the design of civil engineering structures viz earthquake loads and wind loads. For estimating the seismic loads IS is used, while wind loads are calculated using IS875 (part-3)-1987, which gives values for pressure and force coefficients for certain structures for some specified conditions. Wind loads are generally calculated using these coefficients. Wind characteristics and hence wind loads are influenced by many factors. One of the major factors is the presence of the surrounding structures around the object structure. As normally high-rise buildings are not constructed in isolation but in clusters, therefore, wind characteristics are altered and hence resulting wind loads are modified. The surrounding buildings may provide shelter but on the other hand they Received on February, 2014 Published on May

2 may alter the air flow which can result in wind loads that are severer than would have occurred on the building in isolation. As no guidelines are available in the code/literature in this regard. In order to make a fair assessment of the wind loads experimental studies are conducted for such cases. One of the methods available for assessing wind loading is to carry out testing in Boundary Layer Wind Tunnel (BLWT). Therefore, the present study is undertaken to investigate the interference between two high rise buildings, which are connected through a skybridge. The models are tested in the closed circuit wind tunnel at Civil Engineering Department, Indian Institute of Technology Roorkee, Roorkee. 2. Experimental program 2.1 Details of Models Two models of rectangular plan building are made with Perspex sheet at a model scale of 1:600. The Perspex sheet of 5mm thickness is used for making the models. The plan size of each building model is kept as 50mmx50 mm and height is kept as 300 mm. The line diagrams of the models are shown in Figure 1. Figure 1: Structurally coupled buildings The models are named building model 1 and building model 2. There are 21 pressure points on 3 faces of each building model while on the fourth face there are 42 pressure points on each of building model. The pressure tappings are made of steel tube of 1mm diameter and about mm in length. Pressure tapping posi are shown in Fig

3 2.2 Flow characteristics Figure 2: Schematic sketch of pressure tapping positions All the models are tested in the closed circuit wind tunnel of Civil Engineering Department, Indian Institute of Technology Roorkee, India. The cross section of wind tunnel is 1.30m (width) 0.85m (height) and length of the test section is 8.20m.The power-law index(α) inside the wind tunnel is A reference pitot tube is placed at a distance of 3.5m from the grid, which is at a height of 500mm above the floor of wind tunnel to measure the free stream velocity during testing. Although tunnel is constructed for uniform flow only but in the present study the models are tested in boundary layer flow. 471

4 Figure 3: Position of model in wind tunnel The flow of the wind tunnel is made boundary layer by placing a grid of hollow circular tubes at the upstream end of the test section [Ahuja, 1989]. All the models are tested in boundary layer flow for three wind incidence angles i.e. the angle of attack of wind is kept 0, 45 and Measurement Technique First of all, velocity profile is measured at the test section i.e. at a distance of 5.95m from the grid (without model) with a free stream velocity of 15m/sec. For this purpose use is made of a second pitot tube. After this the test model is tested which is placed at a distance of 6.1m from the upstream edge of the test section (Fig 3) and pressure measurement is carried out for the entire surface of the test model using pressure transducer and data acquisition system by changing wind incidence angle. 2.4 Processing of results The pressure on the model surface is measured at each pressure point using Baratron pressure guage. The reading recorded from Baratron pressure guage is used to get the pressure at the point in N/m 2 by applying the following expression: Pressure = multiplying factor x Baratron range x Baratron reading.(1) The multiplying factor for the Baratron pressure guage is Eight different ranges of 1, 0.3, 0.1, 0.03, 0.01, and are available, and hence the sensitivity can be selected according to the requirements. Baratron reading is the reading recorded from the Baratron pressure guage using data logger. Here positive pressure (or pressure) means above ambient pressure and negative pressure (or suction) means below ambient pressure. From this calculated value of pressure, non dimensional coefficients are calculated. Wind pressures measured on the surfaces of the building model are expressed in the form of non-dimensional pressure coefficients defined as follow: Pmean P o Mean pressure coefficient= Cp,mean = 2 1/ 2 V (2) 472

5 3. Results and discussion Wind loads on structurally coupled through single bridge tall buildings As already indicated above, the present study has been carried out to study the wind load on structurally coupled tall buildings with square plan. The models are tested for 3 wind directions and the values of mean wind pressure coefficients are evaluated from the measured data. Mean pressure coefficients are drawn at different heights to understand the effect of structural coupling between two high rise buildings. These variations are shown in Figure 4 to Figure 7 for all the 3 wind incidence angles, for isolated case and two buildings connected through single sky bridge. It is observed that for some positions there is beneficial effect as well i.e the values reduce as compared to the isolated case. It is also observed that there is more variation on a horizontal face as compared to isolated case on D face. 4. Conclusions The following conclusions are drawn from the present study. i. When one of building model is shielded by another building model the pressure gets reduced on the shielded model. ii. The opposing faces i.e. face D experiences more fluctuations on a horizontal section as compared to other faces. iii. For some wind incidence angles, there is positive as well as negative pressure on the building. iv. Maximum value of positive pressure coefficient is in vicinity of 1.0 and this value is not affected much due presence of other building model or sky bridge. v. For most of the wind incidence angles there is considerable suction, this necessitates precaution while designing the cladding of tall buildings. 5. References 1. Ahuja A.K. (1989), Wind effects on cylindrical cable roofs Ph.D Thesis Department of Civil Engineering University of Roorkee, Roorkee India. 2. Ahuja, A.K., Pathak, S.K. and Mir, S.A. (1991), "Interference effect on wind loads on tall buildings, Evaluation and Rehabilitation of Concrete Structures and Innovations in Design- Proc. of ACI Int. Conf. Hong Kong, SP-128, Vol. II, pp Ahuja A.K.,Mir S.A.,Parsad J and Gairola Ajay, (2005),, Interference effects on wind loads on square plan shape low rise buildings Journal of Wind and Engineering, 2(1), pp Amin, J.A. (2008), Effect of plan shape on wind induced response of tall buildings, Ph.D. Thesis, Deptt. of Civil Engg., I.I.T. Roorkee. 5. ASCE: Minimum Design Loads for Buildings and other Structures. Structural Engg. Institute of the American Society of Civil Engineers, Reston. 6. IS: 875 (part 3)(1987), Code of practice for the design loads (other than earthquake) for buildings and structures (part 3, wind loads), New Delhi,India. 473

6 7. Liu Henry (1991), Wind Engineering.A Handbook for Structural Engineers, Prentice Hall, Englewood Cliffs, New Jersey. 8. Mac Donald A.J. (1975), Wind loading on buildings, Applied science publishers Ltd London. 9. Verma, S.K.,(2009), Wind Effects on Structurally Coupled Tall Buildings, Ph.D Thesis, Indian Institute of Technology, Roorkee. 10. Verma et.al, Distribution of wind pressure coefficients on square plan tall buildings along height and width International Journal of Construction Matrerials and Structures, 1(2), pp Figure 4: Variation of mean pressure co efficient along a case of building-0 o incidence angle Figure 5: Variation of mean pressure co efficient along a case of building-45 o incidence angle 474

7 Figure 6: Variation of mean pressure co efficient along a case of building-90 o incidence angle Figure 6: Variation of mean wind pressure alng height of building for single bridge for SB(0.2H 5 mm case) various angles 475

8 Figure 6: Variation of mean wind pressure alng height of building for single bridge for SB(0.2H 50mm case) various angles Figure 6: Variation of mean wind pressure alng height of building for single bridge for SB(0.7 H 50 mm case) various angles 476