Analysis the Behavior of Building with Different Soft Story

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1 IJSTE - International Journal of Science Technology & Engineering Volume 2 Issue 12 June 2016 ISSN (online): X Analysis the Behavior of Building with Different Soft Story Avinash Mishra Department of Civil Engineering Government Engineering College Azamghar Mrinank Pandey Department of Civil Engineering Madan Mohan Malaviya University of Technology, Gorakhpur Abstract Recent earthquakes have shown that the soft storey in building possess adverse effect on stability of structure. Heavy destruction of buildings with soft storey during the earthquakes has prompted research and analysis of the effects and stability of such buildings under earthquake condition.to analyze the effect of soft storey in seismic condition for multi-storeyed reinforced concrete building frame, building models (12 storey s) with identical building plan were analyzed. Soft storey level was changed from stilt floor to top floor for each model including a bare frame and a full infill. Equivalent static analysis was carried away using STADD-PRO Analysis. Results shows that the presence of soft storey at the top level does not significantly affect the structural performance of fully infill frame. While the presence of soft storey at the lower level greatly affects the structural performance. Keywords: Earthquakes, Soft story, High-rise building, STADD-PRO I. INTRODUCTION Due to urbanization and less availability of space multistory buildings in India have open first storey (soft storey). According to Indian seismic a soft story is one whose lateral stiffness is less than 50% of the story above or below. During EQ total seismic base shear imposed on a building is depend upon its natural period. The mass along the height and stiffness distribution is the main factor to affect the seismic force distribution. In soft first storey buildings the upper storey s experienced smaller inter-story drift in comparison of first storey due to stiffness. Due to maximum shear force at soft storey level the strength requirement at soft storey level is maximum. The soft storeys are especially dangerous in earthquakes because they cannot cope with the lateral forces due to sway mechanism of building. The study suggests that the presence of soft storey at the first storey level is most undesirable as it attracts large lateral forces that cannot be resisted alone by soft storey columns II. PRELIMINARIES A soft story is characterized by vertical discontinuity in stiffiness. When an individual storey in a building (often the ground level story) is made taller and more open in construction it is called soft storey. The beam and column of soft storey are designed to withstand two and half times the storey shears and moments calculated for specified seismic loads. In case of tall, relatively open ground floor is necessary, any of the following additional arrangement may be provided to reduce the effect of soft storey. 1) Some of the open bays of the buildings may be braced. 2) The building plan periphery may be kept open while the interior frames may be braced. 3) The numbers of ground columns are increased. 4) The ground floor columns may be made of the shape of frustum of cone. Fig. 1: Soft storey behavior of a building structure under lateral loading All rights reserved by 47

2 Analysis the Behavior of Building with Different Soft Story III. DESCRIPTION OF STRUCTURAL MODELS The research work so far is focused on the behavior of building with soft storey. it is observed that the designed forces in the buildings with open ground storey has not been has not been addressed adequately in the existing design codes. The amplification factor for the design forces in the open ground storey depends upon the number of stories, type and number of infill walls. They proposed an amplification factor to consider the nonlinear effects, for the design base shear based on liner analysis. The presence of soft storey either in ground story or in the upper storey, may lead to a dangerous sway mechanism in the soft story due to the formation of plastic hinges at the top and bottom end of the columns as these columns are subjected to relatively large cyclic deformation. The Indian standard recommend that the dynamic analysis of the structure is to be carried out considering the strength and stiffness contribution of infill frames, the inelastic deformation of the members, particularly those in the soft story and the members, particularly those in the soft storey and the members designed accordingly IV. BUILDING DESCRIPTION Here the model is of a twelve story moment resisting framed building having the plan dimensions of 15Mx20m with the bay of length of 5m in both direction and floor height of 3.2m is considered in the study. The structure is modeled as 3D frame using Staad Pro, and the masonry infill is modeled as quadrilateral shell element of uniform thickness of 0.23mm.The 14 models of 12 storied RC frame structure are prepared, the bare frame model considering the mass of the brick infill but neglecting its stiffness contribution of the brick infill are the basic models considered in the study.the sectional properties of building elements are given. Size of Elements Size of column Size of Beam Thickness of slab 600mmX600mm 300mmX600mm 125mm Properties of material Load Consideration Modulus of Elasticity Poisson s Ratio Grade Concrete MPa 0.2 M25 Steel MPa 0.3 Fe 415 Masonry 3500 MPa Seismic Zone Importance Factor Soil Type Live Load KN/m 2 STADD- Pro Designing BARE FRAME FULL INFLL 1 ST Storey Soft 2 nd Story Soft All rights reserved by 48

3 3 rd Storey Soft 4 Storey Soft Analysis the Behavior of Building with Different Soft Story 5 Storey Soft 6 Storey Soft 7 Storey Soft 8 Storey Soft 9 Storey Soft 10 Storey Soft 11 Storey Soft 12 Storey Soft V. ANALYSIS AND RESULTS Table 6 Maximum Bending Moment in Columns Storey Bare Frame Infill Different storey level All rights reserved by 49

4 Analysis the Behavior of Building with Different Soft Story Table 7 Lateral Displacement (mm) at Storey Level Storey Bare Frame Infill Different storey level Table 8 Lateral Force (KN) At the Storey Level Storey Bare Frame Infill Different storey level The following observations are made, based on the results presented in Table 1to 3 that in MI-RC frame in comparison to the bare frame indicates large increase in lateral stiffness due to substantial stiffness contribution of masonry infill. VI. GRAPHICAL PRESENTATION All rights reserved by 50

5 Fig. 2: Maximum Bending Moment in Columns (KN/M) At Storey Level Analysis the Behavior of Building with Different Soft Story Fig. 3: Lateral Displacements (In Mm) At The Storey Level Fig. 4: Lateral Forces (In KN) At the Storey Level VII. CONCLUSIONS The presence of soft storey at the lower levels of the building structure, especially at the first storey level is most undesirable, as it attracts larger storey force with greater storey drift in comparison to fully infill frame The stiffness contribution of masonry infill in the lower stories (specially the first storey) is very large compared to the stiffness contribution of the upper storey. The amplification factor of the seismic action effects in the vertical structural elements of soft storey needs to be scaled down, factor provided depends upon storey height, that s the soft storey is in lower portion or in upper portion. The presence of soft storey at the top level does not significantly affect the structural performance of fully infill frame. While the presence of soft storey at the lower level greatly affects the structural performance. All of the building structures, especially buildings having soft stories containing only frames as lateral load resisting systems are vulnerable during the earthquakes ACKNOWLEDGMENT This work has been carried out in civil engineering department of Madan Mohan Malaviya University of Technology, Gorakhpur, and Government engineering college Azamghar India. The authors present its heartiest gratitude towards the entire faculty members for their constant encouragements, guidance and supports. All rights reserved by 51

6 Analysis the Behavior of Building with Different Soft Story REFERENCES [1] ACI Committee 318, ACI, Structural Building Code and Commentary, American Concrete Institute, [2] Applied Technology Council, ATC 40, Seismic Evaluation and Retrofit of Concrete Buildings, Volume 1-2, California, [3] Army Corps of Engineers, Seismic Evaluation and Rehabilitation for Buildings, US. Washington, [4] Association of Bay Area Governments, ABAG Earthquake and Hazards Program-Mitigation Policy Review, [5] Athanassiadou C.J., Seismic Performance of RC Plane Frames Irregular in Elevation, Engineering Structures, doi: /j.engstruct , [6] Attard T. and Fafitis A., Modeling of Higher-Mode Effects Using an OptimalMulti-Modal Pushover Analysis, Earthquake Resistant Engineering Structures V, [7] Chintanapakdee C. and Chopra A.K., Evaluation of Modal Pushover Analysis Using Generic Frames, Earthquake Engineering and Structural Dynamics, Vol. 32, ( ), All rights reserved by 52