Full Paper Proc. of Int. Conf. on Advances in Civil Engineering 2012

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1 Behaviour of R.C.C. Tall Buildings Having Different Shapes Subjected to Wind Load Prof. Sarita Singla 1, Taranjeet Kaur 2, Megha Kalra 3 and Sanket Sharma 4 1 ssaritasingla@yahoo.com 2 Baddi University/Civil Engineering Department, Baddi(HP), India taran_madaan@yahoo.co.in 3 Duskmk@gmail.com 4 sanket414@gmail.com Abstract Buildings are defined as structures utilized by the people as shelter for living, working or storage. As now a days there is shortage of land for building, the vertical construction is given due importance. A designer is interested in storey wise horizontal forces for analysis and design of structural frames. Hence, emphasis is given to compute the storey wise lateral forces due to wind on building. In the present study, a 35 storeyed building of different shapes- Square, Hexagonal and Octagonal, having equal plan area and equal stiffness of the columns has been analysed. Based upon the study, it is concluded that shape of the structure plays an important role in resisting wind loads. Octagonal shaped building performed the best followed by hexagonal shaped and square shaped building. Index Terms wind load, tall building, gust factor, square, hexagonal, octagonal I. INTRODUCTION Tall buildings are critically affected by wind loads. Wind exerts forces and moments on the structure and its cladding and also it distributes the air in and around the building mainly termed as wind pressure. Sometimes because of unpredictable nature of wind it takes so devastating form that it can upset the internal ventilation system when it passes into the building. For these reasons the study of air flow is becoming integral with the planning of a building and its environment. Tall buildings are flexible and are susceptible to vibrate at high wind speeds in all the three directions (x, y, and z) and even the building codes do not incorporate the expected maximum wind speed for the life of the building and does not consider the high local suctions which cause the first damage. Due to all these facts the wind load estimation for tall buildings are very much important. A. Importance of Wind Loads on the Tall Buildings Wind is a phenomenon of great complexity because of many flow situations arising from the interaction of wind with structures. Wind is composed of multitude of eddies of varying sizes and rotational characteristics carried along in a general stream of air moving relative to the earth s surface. These eddies give wind its gusty or turbulent character. The gustiness of strong winds in the lower levels of the atmosphere largely arises from interaction with surface 156 features. The average wind speed over a time period of the order of ten minutes or more tends to increase with height, while the gustiness tends to decrease with height. (Fig. 1) B. Effects of wind load Fig. 1. Generation of eddies A mean wind force acts on a building. This mean wind force is derived from the mean wind speed and the fluctuating wind force produced by the fluctuating flow field. The effect of the fluctuating wind force on the building or part thereof depends not only on the characteristics of the fluctuating wind force but also on the size and vibration characteristics of the building or part thereof. Therefore, in order to estimate the design wind load, it is necessary to evaluate the characteristics of fluctuating wind forces and the dynamic characteristics of the building. The factors generally considered in determining the fluctuating wind force are: 1) Wind turbulence (temporal and spatial fluctuation of wind 2) Vortex generation in wake of building 3) Interaction between building vibration and surrounding air flow For most buildings, the effect of fluctuating wind force generated by wind turbulence is predominant. In this case, horizontal wind load on structural frames in the along-wind direction is important. However, for relatively flexible buildings with a large aspect ratio, horizontal wind loads on structural frames in the across-wind and torsional directions should not be ignored. C. Hourly Mean Wind Speed (V Z ) The basic wind speed ( ) for any site shall be obtained from Fig 1(IS: 875(Part 3)-1987) and shall be modified to include

2 the following effects to get design wind velocity at any height ( ) for the chosen structure: a) Risk level; b) Terrain roughness, height and size of structure; and c) Local topography. = V b.k 2.k 3 = hourly mean wind speed in m/s, at height z V b = regional basic wind speed in m/s = probability factor (risk coefficient)(clause of IS: 875(Part 3)-1987)[4] k 2 = Terrain and height factor (Clause of IS: 875(Part 3)-1987) k 3 = topography factor (Clause of IS: 875(Part 3)-1987) Isyumov[1]overviews the action of wind on tall buildings and structures with emphasis on the overall wind-induced structural loads and responses also discussed the local wind pressures on components of the exterior envelope and the effects of buildings on winds in pedestrian areas.ahsan Kareem[2] pays tribute to the father of wind engineering, Jack E. Cermak, for his many valuable and pioneering contributions to the subject, followed by a reflection on the recent developments in wind effects on structures and an outlook for the future. This discussion encompasses modeling of wind field; structural aerodynamics; computational methods; dynamics of long -period structures; model to full-scale monitoring; codes/standards and design tools; damping and motion control devices. Davenport [3] attempts to trace the involution of a satisfactory to the loading of structures by gusts. It is suggested that a statistical approach based on the concepts of the stationary random series appears to offer a promising solution. Some experiments to determine the aerodynamic response of structures to fluctuating turbulent flow are described. Example are given of the application statistical approach to estimate the wind loading on a variety of structures, in noting including long span cables, suspension bridge, towers and skyscrapers. analysis of the building had been done by using STAAD Pro 2007 software and the performance was analyzed by varying the shape of structure. 1. Height of the building considered was 105 m/35 storeyed with reduced plan area after 20 storeys 2. Different shapes of the building studied were: a). Square b). Hexagonal c). Octagonal IV. PARAMETERS OF THE BUILDING 1. Different cases of the building analysed were as under: a). 35 storeyed Square framed building with reduced plan area after 20 storeys.(fig. 2) b). 35 storeyed Hexagonal framed building with reduced plan area after 20 storeys. (Fig. 3) c). 35 storeyed Octagonal framed building with reduced plan area after 20 storeys (Fig. 4) Various parameters of the buildings adopted were as under: Total Height = 105 m Plan Area up to height of 60 m = 900 m 2 Plan Area above height of 60 m = 324 m 2 Grid Size = 6 m x 6 m Size of Columns (Up to 60 m height)=600 mm x 600 mm Size of Columns (Above 60 m height)=450 mm x450 mm Size of Beams at each floor = 450mm x 450 mm Grade of Concrete in Columns = M60 Grade of Concrete in Beams = M25 Grade of steel = Fe 415 All supports were assumed to be fixed. II. OBJECTIVES OF THE PRESENT STUDY 1. To Study the behavior of tall structures when subjected to along wind loads. 2. To study the effect of shape of the building in plan on the behavior of the structure. 3. To determine the effect of wind load on various parameters like storey drifts, lateral displacements in the building. III. SCOPE OF THE PRESENT STUDY The scope of the present work included the study of the wind load estimation on tall buildings for the structural design purpose with the analytical approach given by Davenport s Gust Factor Approach in IS 875: part [4] and the Fig. 2. Plan and Elevation of Square Building 157

3 Fig. 3. Plan and Elevation of Hexagonal Building Fig. 4. Plan and Elevation of Octagonal Building A.Loadings Considered Dead Loads: The loads of the beams and columns had been taken in account by STAAD using the command of Self weight. Dead load of slab at each floor was taken as 6 kn/m 2. The Brick wall load of inner 4.5" thick wall was taken as 8 kn/ m and of outer 9" thick wall with glazing was taken as 10 kn/ m. Live Load The live loads had been taken as 3.00 kn/m 2 at all floors and 1.5 kn/m 2 at roof. Seismic Loads As per IS [5], seismic analysis of the structure was performed. The design horizontal seismic coefficient, A h for the structure had been computed using the following: 1. Zone factor, Z =0.24 (Zone IV) 2. Importance factor, I = Response Reduction factor, R =5 4. Soil type = Hard Soil 5. Damping Coefficient = 0.05 Wind Loads As per IS-875 part 3, wind analysis of the structure was performed. The horizontal wind force of structure had been computed using the Gust factor method approach given in IS 875(part 3)-1987). B. Wind Load Calculations Since early 1960 s, when Davenport s (1961) explained statistical concepts of the stationary time series for the determination of the response of simple structures to a turbulent gusty wind, efforts had been made to express peak stresses, accelerations, etc., in terms of the mean wind velocity, the spectrum of the gustiness and the mechanical and aerodynamic properties of the structure. Still today Davenport s (1967) gust loading factor approach forms the most acceptable approach for prediction of mean and fluctuating response of slender structures. C. Wind load Calculations Based Upon the Codal Provisions: Square building Terrain Category I Plan length = 30 m; Plan width = 30 m Height of building = 105 m Face width = 30 m; Face depth = 105 m Interval = 5 m Natural period = T 1.73 sec Frequency = 0.58 = 1.07; k 2 (at 105 m) = 1.00 k 3 = 1.00; V b = 47 m/s = Vb x x k 2 x k 3 =50.29 m/s P z (at the top) = N/m 2 C f =

4 Calculation for Gust Factor, (G) From Figure 8-11 (IS 875(part 3)-1987) gfr = 0.75; L (h) = 2200; B = 0.62 S = 0.09; E = 0.045; β = Ø = 0.15; Cy = 10; Cz = 12 H (height of structure) = 105m B (width of structure) = 30 m 0.24; f 0 = 0.58; F 0 = = 1.87 TABLE I. WIND INTENSITIESAT DIFFERENT HEIGHT OF THE BUILDING were also increasing and the wind intensities were decreasing with the variation of shapes from square to octagonal. The variation of wind intensity with height is shown in Fig 5. C. Load Combinations Load combinations were considered as per IS 875(part 5) VII. ANALYSIS OF RESULTS In the succeeding sections, the behavior of different buildings when subjected to wind load have been discussed. A. Effect of the Shape of the Building on Storey Drifts The Storey drifts for square, hexagonal and octagonal building are compared in Table II and Fig.6. TABLE II. STOREY DRIFTS AT DIFFERENT HEIGHTS Fig. 5. Variation of Wind intensities with height Where Ae = effective frontal height at any height z Vz = hourly mean wind speed in m/s, at height z = Vb x x k 2 x k 3 Pz = 0.6 Vz 2 Fz=Cf.Ae.Pz.G By using the Davenport Gust Factor Approach given in the code IS 875: part and with the data provided, wind intensity was determined. It was seen from Table 1 that withthe increase in the height of the building the wind intensities 159 Fig. 6. Variation of Storey drift From Table 2 it is evident that with the change in shape of building from square to octagonal the storey drifts of the building decreases. It is seen that the percentage reduction in octagonal building is more as compared to in hexagonal building. The storey drifts in the bottom most storeys are reduced by 5-12% in case of hexagonal building and 12-15% in case of octagonal building as compared to storey drifts in square building. Peak Storey drift in Square building is mm, in hexagonal building is mm and in octagonal building is mm. The percentage reduction in peak storey drift in hexagonal building is 11.59% and in octagonal building is 13.27% as compared to peak storey drift in square building. From Figure 6 it is clear that the storey drifts are reduced with the increase in number of sides of the building with the same column stiffness and grid size

5 due to reduction in effective area (Ae) of wind load application. B. Effect of the Shape of The Building on Lateral Displacements TABLE III. COMPARISON OF LATERAL DISPLACEMENTS Peak lateral displacement in square building is mm, in hexagonal building is mm and in octagonal building is mm.the percentage reduction in peak displacement in hexagonal building is 4.55% and in octagonal building is % as compared to peak lateral displacement in square Building. CONCLUSIONS The lateral joint displacements in square, hexagonal and octagonal buildings at different heights are compared in Table 3 and Figure 7.From the observations it is evident that with the change in shape of building from square to octagonal the lateral displacements of the building decreases. A 35 storeyed building of different shapes- Square, Hexagonal and Octagonal, having equal plan area and equal stiffness of the columns at each storey has been analysed. With the change in shape of building from square to octagonal the storey drifts and the lateral displacements of the building decreased. The storey drifts in the bottom most storeys were reduced by 5-12% in case of hexagonal building and 12-15% in case of octagonal building as compared to storey drifts in square building. The percentage reduction in peak displacement in hexagonal building was 4.55% and in octagonal building was % as compared to peak lateral displacement in square building. Based upon the above results, it is concluded that shape of the structure plays an important role in resisting wind loads. Octagonal shaped building has lesser storey drifts, lesser lateral displacements at the joints as compared to hexagonal and square shaped building. REFERENCES Fig. 7. Variation of Lateral Displacements with height [1] Isyumov,N.(1999). Overview of Wind Action on Tall Building and Structures. Wind Engineering into the 21st Century, Larsen, Larose &Livesey (eds -1999) Balkema, [2] Kareem,A.(1992). Dynamic Response of High Rise Buildings to Stochastic Wind Loads. J. W.E. & I.A.(Proc. VIII Int. Conf. on Wind Engrg., Ontario, Canada), 41-44, [3] Davenport,A.G.(1967). Gust Loading Factors. J. Struct. Engg., ASCE, 93(ST3), [4] BIS 875: (1987). Indian Standards Code of Practice for Design Loads (Other than Earthquake) for Buildings and Structures pt.3 - Wind Loads. Bureau of Indian Standards, India. [5] BIS [5], Indian Standard criteria of earthquake resistant design of structure. Bureau of Indian Standards, India. 160