INFLUENCE OF HORIZONTAL OFFSETS OF OPENINGS IN SINGLE STORY BRICK MASONRY BUILDINGS UNDER SEISMIC FORCES

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1 INFLUENCE OF HORIZONTAL OFFSETS OF OPENINGS IN SINGLE STORY BRICK MASONRY BUILDINGS UNDER SEISMIC FORCES Ankitha George. 1, Dr. Rajesh K. N. 2 1 M. Tech scholar, Dept. of Civil Engg, Govt. College of Engineering Kannur, Kerala, (India) 2 Associate Professor, Dept. of Civil Engg, Govt. College of Engineering Kannur, Kerala, (India) ABSTRACT Masonry structures are good in resisting gravity loads, but do not perform well when subjected to lateral loads, such as seismic loads. Its behavior under the effect of earthquake ground motions has not been identified clearly because of its complex material nature. Unreinforced masonry buildings have load-bearing walls with an irregular arrangement of openings. In this paper, the influence of horizontal offset of an opening in a masonry wall of a single room one storey building was studied with respect to their behaviour under seismic activities. The building was modelled in MIDAS Gen15 software using finite element technique and then its seismic evaluation was carried out using time history analysis. Keywords: Finite Element Modelling, Horizontal offset, Midas Gen15, Time History Analysis, Unreinforced Masonry Building. I. INTRODUCTION In many seismically active areas of the world, major part of the buildings are older structures made of unreinforced masonry. Masonry is a heavy, brittle material with low tensile strength and exhibits little ductility under seismic loads. Unreinforced masonry is highly susceptible to earthquake damage than various other types of construction materials. They are usually characterized by sudden and dramatic collapse. This indicates a significant risk during an earthquake. The possibility of combining elements of masonry such as mortar and brick with different qualities and geometry give masonry a wide range of alternatives of mechanical behaviour and structural performance. The presence of openings in the masonry walls leads to significant uncertainty due to the variability of size and position of the openings [6]. In general, the presence of openings results in a reduction of stiffness as well as strength [7, 8]. A typical masonry wall when subjected to earthquake load, usually initiate shear cracks at the corners of openings and extends towards the middle. In general, the crack pattern depends on the position and size of the opening. This paper is aimed to assess the effects of horizontal offset irregularities of an opening in a single storey masonry building on the seismic capacity of unreinforced masonry walls with openings. Buildings were modelled using finite element technique which was found as the most accurate [5]. 6 P a g e

2 II. FINITE ELEMENT MODELLING Finite element modelling can be done in two ways such as by considering masonry as a single phase material to very complex method such as considering each element and joint separately. This method has been widely used and become a well-accepted tool. The two methods of modelling are macro-modelling and micro-modelling. Masonry has orthotropic material properties due to the presence of themortar joints. In this study macro modelling concept was used for the modelling of masonry as if it is composed of a single homogeneous material. Finite element modelling of the load bearing masonry buildings were done using 8 noded solid elements in MIDAS Gen Properties of Materials The properties used for modelling the buildings [3] are given below: Modulus of elasticity normal-to-bed-joints (Ez) -600 Mpa Modulus of elasticity parallel-to-bed-joints (Ex ) Mpa Modulus of rigidity (Gxy) Mpa Poisson s ratio (v) Density of masonry Kg/m 3 Concrete compressive strength -25 MPa 2.2 Details of the Building The dimensions of the buildings analyzed were 6m x 3m x 3m (L x B x H). Buildings were provided with openings one door of size 1.0m x 2.1m and one window of size 1.0m x 0.9m on one longer wall and two windows on the shorter walls. No opening was provided in the back wall. The plan view and the finite element model of the building is shown in Fig 1(a) and Fig. 1(b). The walls of the building were assumed to be fixed at their bases all along their lengths. In order to study the effects of horizontal offset of an opening under seismic load, the window W1 in the front wall was moved from its original positionto distances 0.3m,0.6m,0.9m and 1.2m horizontally. Thus, six buildings were modelled in Midas Gen15 software for each horizontal offsets from the initial position. Fig.1 Plan view of the building Fig. 2 Brick masonry building model in Midas Gen 7 P a g e

3 III. FREE VIBRATION ANALYSIS Free vibration analysis is essential to understand the response of the building under dynamic loads.free Vibration Analysis of the buildings were carried out using Midas Gen 15 software and the natural frequencies of the first mode of vibration in each case is given in Table 1. It was found that the natural frequency value is decreasing as the offset distance increases. It indicates the reduction in stiffness as the offset distance increases. TABLE 1 Natural Frequencies Horizontal offset of window W1 from initial position(m) Natural Frequency (Hertz) IV. TIME HISTORY ANALYSIS Time-history analysis is a technique used to determine the dynamic response of the structure under the action of any set of time-dependent loads. The peak stresses for the ground motion input were obtained by carrying out a time history analysis. The walls of a masonry building offer resistance against lateral dynamic loads by developing flexural and shear stresses, during earthquake ground motions. Time history analysis was carried out for all the six types of models created.the equations of motion, expressed in general matrix, notation is: [M]{u}+ [c]{u}+ [K]{u} = -[M]{u g(t)} (1) Where, [M] = Structural Mass matrix, [K] = Structural stiffness matrix, [C] = Structural stiffness matrix {} u = nodal displacement vector, {} u = nodal velocity vector {} u = nodal acceleration vector {u g(t)} = ground acceleration vector V. RESPONSE OF BRICK MASONRY BUILDING TO EL-CENTRO ACCELERATION The best way to evaluate seismic performance of structures is by monitoring structural behaviour under real earthquake records. Time history analysis is a part of structural analysis and is the calculation of the response of a structure to any earthquake. Here a linear transient analysis was carried out with El-Centro earthquake 8 P a g e

4 acceleration record as input. A part of the ground motion acceleration record recorded at El-Centro site, 270 degree, during California earthquake of May 18,1940 is shown in Fig. 2. The maximum peak of the input is g and the duration is seconds Fig. 2 Acceleration Time History of El-Centro All the six types of box-type brick masonry structures discussed earlier were analyzed to find their responses to El-Centro, 270degree acceleration. Maximum displacement and maximum shear stress was noted for all the six cases. 5.1 Maximum Displacement The roof displacement noted for each case of offset distance is tabulated in Table 2.It was found that maximum displacement of the masonry building varies with the change in the position of openings. The value of maximum displacement increased as the window opening, W1 was moved horizontally from the initial position. When the opening W1 is moved horizontally to 0.3m, 0.6m, 0.9m, and 1.2m respectively, maximum roof displacement value was found to be increased by 10%, 31%, and 76%, and 300% of that of initial condition. TABLE 2 Maximum Displacement Horizontal offset of window W1 from initial position(m) Maximum displacement of building (mm) P a g e

5 5.2 Maximum Shear Stress Maximum shear stress is an important parameter to evaluate the performance of masonry buildings. It was found that maximum shear stress developed in a masonry building is greatly influenced by the position of openings. Fig. 4 shows the maximum shear stress developed when no horizontal offset was given. The locations of occurrence of maximum shear stress was mainly at the corners of openings. Maximum shear stress noted in each case of offset is tabulated in Table 3. It was found that the value of maximum shear stress increases as the window opening is moved horizontally from the initial position. Fig. 5 and Fig. 6 represents the variation of maximum displacement and maximum shear stress graphically. TABLE 3 Maximum Shear Stress Horizontal offset of window W1 from initial position(m) Maximum shear stress(n/mm 2 ) Fig. 4 Maximum shear stress (0 offset) Fig.3 Maximum Displacement Variation 10 P a g e

6 Fig. 4Maximum Shear Stress Variation VI. CONCLUSION Box type single story brick masonry building was modelled in Midas Gen 15 software using finite element technique. In order to study the effects of horizontal offset of an opening under seismic load, the window W1 in the front wall was moved from its original position to distances 0.3m, 0.6m, 0.9m and 1.2m horizontally. Free vibration studies were conducted to find the natural frequencies of buildings under each offsets. Then time history analysis was carried out for all the six types of models created. El Centro earthquake was given as input for time history analysis. Maximum displacement and maximum shear stress were compared for all the six cases. It was observed from the present study that the position of an opening in a masonry wall has considerable influence on stiffness, maximum displacement and maximum shear stress. The natural frequency value was found to be decreasing as the horizontal offset distance increases. This reduction in natural frequency shows a reduction in stiffness. So it can be concluded that the position of openings in a masonry building influences the overall stiffness considerably. The maximum displacement under the given seismic force was found to be increasing with an increase in the horizontal offset. It indicates the influence of position of offset on maximum displacement. The location of maximum shear stress was found around the corners of the openings and it was observed that the maximum shear stress value increases as the window opening was moved horizontally. REFERENCES [1] A. Giordiano, E. Mele, A. De Luca, Modelling of Historical Masonry Structures: Comparison of different approaches through a Case study. Engineering Structures, 24, 2002, [2] J. Andreas, G. Gregory Penelis, and G. Christos Drakopoulos, Evaluation of Simplified models for Load Lateral Analysis of Unreinforced Masonry Buildings. Journal of Structural Engineering, 128, 2002, [3] K.S. Nanjunda Rao, S. Raghunath, and Jagadish, Containment reinforcement for earthquake resistant masonry buildings, 13 th World Conference on Earthquake Engineering, Vancouver, B.C., Canada, Paper No. 1968, P a g e

7 [4] B. Saikia, K.S. Vinod, S. Raghunath and K.S. Jagadish, Effect of reinforced concrete bands on dynamic behaviour of masonry buildings, The Indian Concrete Journal, October 2006, [5] Y. Tianyi, L. Franklin, T. Roberto Leon, and F. Lawrence Kahn, Analyses of a Two-Storeyed Unreinforced Masonry Building. Journal of Structural Engineering,ASCE, 132, 2006, [6] M. Shariq, H. Abbas, H. Irtaza, and M. Qamaruddin, Influence of openings on seismic performance of masonry building walls, Building and Environment, 43, 2008, [7] L.D. Decanini, L. Liberatore and F. Mollaioli, The influence of openings on the seismic behaviour of infilled framed structures, 15 th World Conference on Earthquake Engineering, [8] M. Mohammadi and FarzadNikfar,Strength and Stiffness of Masonry-Infilled Frames with Central Openings Based on Experimental Results, Journal of Structural Engineering, ASCE,139, [9] P.G. Asteris, M.P. Chronopoulos, C.Z. Chrysostomou, H. Varum, V. Plevris, N. Kyriakides, V. Silva, Seismic vulnerability assessment of historical masonry structural system. Engineering Structures, 62, 2014, P a g e