Comparision Between Symmetric And Unsymmetric Building Considering Seismic Load

Transcription

1 Comparision Between Symmetric And Unsymmetric Building Considering Seismic Load Sayantika Saha Lecturer, Civil Engineering Department, W.B.S.C.T.E, Hooghly, West Bengal, India, Kolkata Abstract- At present scenario many buildings are symmetric in plan or in elevation based on the distribution of mass and stiffness along each storey throughout the height of the building.buildings with a symmetric distribution of stiffness and strength in plan undergo coupled lateral and torsional motions during earthquakes. Most recent earthquakes have shown that the irregular distribution of mass, stiffness and strengths may cause serious damage in structural systems. The buildings have un-symmetrical distribution of stiffness in building plan and frame elements. In this paper the effort is made to study the effect of eccentricity between centre of mass (CM) and centre of stiffness (CR) and the effect of stiffness of shear walls on the performance of the buildings. Two building models are considered for study, which are constructed on medium soil in seismic zone V of India (as per IS: ), one symmetric and one unsymmetric in stiffness distribution. Static analysis (for gravity and lateral loads) and non-linear analysis (assigning the hinge properties to beams and column sections) were performed. The stiffness characteristics control the dynamic response of the building structure.. From this project it is concluded that symmetric buildings perform better than un-symmetric buildings when subjected to earthquake forces. Index Terms symmetric; un-symmetric; time history I. INTRODUCTION The selection of a suitable procedure to evaluate performance of structures under seismic loads is one of the most sensitive issues that structural engineers face. This would be especially important when dealing with irregular structures since the wrong choice of a procedure would lead to results that are far away from the correct solution. One of the most common types of irregularities that found in most buildings is the plan irregularities. The existence of a symmetry in the plan is usually leading to an increase in stresses of certain elements that consequently results in a significant destruction.to evaluate the seismic behavior of complex tall asymmetric buildings with significant higher mode effects, the nonlinear dynamic analysis methods generally provide more realistic models of structural response and, thereby, provide more reliable assessment of earthquake performance than other methods.earthquakes are one of the most devastating natural hazards that cause great loss of life and livelihood. Most recent earthquakes have shown that the irregular distribution of mass, stiffness in buildings, such buildings undergo torsional motions. An ideal moment-resisting multistory building designed to resist lateral loads due to earthquake would consist of only symmetric distribution of mass and stiffness in plan at every storey and a uniform distribution along height of the building. Such a building would respond only laterally and is considered as torsionally balanced (TB) building. But it is very difficult to achieve such a condition because of restrictions such as architectural requirement and functional needs. The structures, whose performances were evaluated in this study, are designed as per codal provision. The nonlinear dynamic analysis of any structure or facility requires earthquake scenario in the form of acceleration time-histories). Seismic hazard analysis (SHA) involves the quantitative estimation of -shaking hazards at a particular site (Kramer, 1996). Safe design of a structure or facility is possible with the knowledge of the seismic hazard at a particular site or the region. Equivalent static force method of determining earthquake force is limited to the structures having height of less than 40 meters. Hence this study deals with medium rise buildings. The purpose of the paper is to summarize the basic differences in forces and moments and arising due to symmetric and un-symmetric IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 41

2 configurations in building plan and difference in stiffness of frame elements in plan. The motion is predicted using the theory using the available time history. II. OBJECTIVE OF WORK Two buildings of G+9 storey of same external dimension given in Fig -1 are modeled and analyzed using SAP with one symmetrical and another un-symmetrical in nature. One building is modeled with a plan symmetric about x-axis and y- axis and having the dimensions of all the columns being same in all directions. Another building with plan un-symmetric is modeled with larger column dimensions in the left and smaller column dimensions in the right portion of plan, thus the stiffness is varied. The forces and moments on individual members in both the cases are calculated using SAP Now, based on the maximum force, moment and torsion result the members are designed and the differences in the result of the design summary are to be compared. A. Performance study of symmetrical structure Fig.1. Plan of symmetric building Fig.2. 3-D model of the building in SAP The plan of the ten storied symmetrical building is shown with 5 bays of 3m span each in x- direction and 3 bays each of 3m span in y-direction. The lift well is situated at the centre to give it a symmetrical loading as well as stiffness to the system. The beam size is taken as 250mmX400mm for all the floors and the column dimension is taken as 450mmX450mm throughout the length of the building. The thickness of the slab is taken as 150mm.The structural members i.e. the columns, beams and slab are taken as reinforced concrete member and the walls are taken as masonary brick walls and the wall around the lift well is taken as shear walls of reinforced concrete. The thickness of the walls including shear wall is defined as 250mm. The building assume6r43d to be fixed at base. The dead load includes the self weight of the frame members, self weight of the slab, wall loads on the floor beams and parapeet wall load on the roof beams taken as uniformly distributed load. The self weight of the frame members are taken in SAP from the predefined self weights of the materials according to codal provisions. The slab weight is defined as area load in the direction of gravity. The wall weights are calculated and placed on beams as uniformly distributed load. IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 42

3 Fig.3 Wall Load According to codal provisions live load on the lift well area is taken as 6 KN/m2 and 3 KN/m2 on the other part of the floors and 1.5 KN/m2 on the roof. of time history i.e. time vs acceleration is taken as input file in SAP The building is subjected to dynamic load at an interval of 0.02 seconds. The mode of analysis is Eigen vectors in a Modal Load case type with maximum no. of modes being 12 and minimum 1,having a modal damping coefficient of 0.05 Fig.4 Live Load In case of wind load the external walls are defined as areas. According to codal provisions Wind direction Angle = 0 Windward coefficient Cp = 0.8 Leeward coefficient Cp = 0.5 Basic wind speed = 50 m/s Terrain category = 2 Risk coefficient k1 = 1 Topography factor k3 = 1 Fig.6 Time History According to codal provisions Seismic Zone Factor = 0.36 (zone V) Soil Type = II Importance Factor = 1 Response Reduction factor = 5 Fig.5 Wind Load The earthquake force is determined using non linear dynamic analysis by time history method. The graph IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 43

4 B. Performance study of un-symmetrical structure Fig.9 3-D model of the building in SAP Fig.7 Plan of unsymmtric building The plan of the ten storiedunsymmetrical building is shown with 4 bays with spans of 3m,5m, 2.5m,4.5m in x-direction and 3 bays of spans 3m,6m in y-direction. The spans are taken unequal to get an unsymmetrical plan about the central axis. The lift well is situated at the left-front corner to give it an unsymmetrical loading system. The beam size is taken as 250mmX400mm, C2 column dimension is taken as 450mmX450mm& C1 column dimension is taken as 450mmX600mm. The thickness of the slab is taken as 150mm. The structural members i.e. the columns, beams and slab are taken as reinforced concrete member and the walls are taken as masonary brick walls and the wall around the lift well is taken as shear walls of reinforced concrete. The thickness of the walls including shear wall is defined as 250mm. The building assumed to be fixed at base.the dead load, live load, wind load & earthquake load intensities & definitions are the same as in case of symmetrical buildings. Fig.8 Wall load Fig.10 Live Load IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 44

5 TABLE III SLAB RESULTS Building Type V13(KN) M11(KN-m) Symmetric Unsymmetric III. Fig.11 Wind Load COMPARISION OF ANALYSED RESULTS OF THE SYMMETRIC AND UNSYMMETRIC BUILDING TABLE I BEAM RESULTS Building Type V2(KN) T(KN-m) M3(KN-m) Symmetric Unsymmetric Building Type Symmetric unsymmetric TABLE II COLUMN RESULT Column P(KN) M2(KNmm) M3(KN- type Around Around Other columns Other columns C C C C C around C around TABLE IV RAFT FOUNDATION RESULTS Building Type M11 M22 VMax Symmetric Unsymmetric IV. CONCLUSION In case of unsymmetric buildings the center of stiffness and the center of mass does not coincide with each other. Therefore torsional moments arise when the structure is subjected to dynamic earthquake loads. Therefore it is not safe for buildings to be unsymmetric in nature.this view is also confirmed from the comparison between the analyzed results of the symmetric and the unsymmetric building frames and also the design results from the confirms the same view.the moments and all other forces are much higher in unsymmetric buildings, as a result the design dimensions of the members are higher in case of unsymmetric buildings.also the reinforcement percentage from ductility considerations and normal RCC design considerations are much higher in case of unsymmetric buildings. Therefore unsymmetric buildings are also uneconomical as compared to symmetric buildings. REFERENCES [1] WEI HONG, ALLEN YANG YANG, KUN HUANG AND YI MA, On Symmetry and Multiple-View Geometry:Structure, Pose, and Calibration from a Single Image, International Journal of Computer Vision 60(3), , 2004_c 2004 Kluwer Academic Publishers. Manufactured in The Netherlands, [2] Shivani Pyasi1, Nita Rajvaidya2, SEISMIC ANALYSIS OF UNSYMMETRICAL (G+10) MULTI-STOREY RC BUILDING WITH TWO SOFT STOREY AT VARYING FLOORS IN MEDIUM SOIL, International Research Journal of Engineering and Technology (IRJET) e-issn: Volume: 02 Issue: 07 Oct-2015 [3] Pralobh S. Gaikwad * and Kanhaiya K. Tolani, Review Paper on Dynamic Analysis of Building, International Journal of Current Engineering and Technology E-ISSN , P-ISSN IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 45