PUSHOVER ANALYSIS ON EXISTING RC BUILDING

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1 PUSHOVER ANALYSIS ON EXISTING RC BUILDING S.T.Jadhav P.G. Student, Department of Civil Engineering S.B.Patil Assistant Professor, Department of Civil Engineering Dr. H.S. Chore Professor and Head, Department of Civil Engineering Abstract The nonlinear static analytical procedure (Pushover analysis) as introduced by ATC-40 is applied for the evaluation of existing design of a reinforced concrete bare frame, frame with infill in outer periphery and frame with open ground storey. In order to examine the performance of these models, the pushover analysis for seismic evaluation of existing building is performed. After performing the analysis pushover curve are obtained. It is concluded that presence of percentage of infill in RC frame considerably reduces the fundamental natural time period of the buildings under consideration. These increase the stiffness which may cause an increase or decrease of base shear, depending on both the frame analyzed and characteristics of ground motion. Keywords Pushover analysis, Fundamental natural time period, Storey displacement, Bare frame, Soft storey, Existing RC building. I. INTRODUCTION Amongst the natural hazards, earthquakes have the potential for causing the greatest damages to engineered structures. Since earthquake forces are random in nature & unpredictable, the engineering tools needs to be sharpened for analyzing structures under the action of these forces. India has a number of the world s greatest earthquakes in the last century. In fact, more than fifty percent area in the country is considered prone to damaging earthquakes. The northeastern region of the country as well as the entire Himalayan belt is susceptible to great earthquakes of magnitude more than 8.0. During the last century, 4 great earthquakes struck different parts of the country: (1) Great Assam ea rthquake (1897), (2) Kangra earthquake (1905), (3) Bihar Nepal earthquake (1934) and (4) Assam earthquake (1950). In recent times, damaging earthquakes experienced in our country include (1) Bihar Nepal earthquake (1988), (2) Uttarkashi earthquake (1991), (3) Killari earthquake (1993), (4) Jabalpur earthquake (1997), (5) Chamoli earthquake (1999) and (6) Bhuj earthquake (2001) and recently occurred (7) West Bengal earthquake (2011). In all of these earthquakes there is huge loss of life and very large destruction of existing reinforced concrete (RC) buildings. Most recent constructions in the urban areas consist of poorly designed and constructed buildings. The older buildings, even if constructed in compliance with prevailing standards, may not comply with the more stringent specifications of the latest standards of IS 1893( Part 1):2002, IS 4326:1993 and IS 13920: The existing buildings can become seismically deficient since design code requirements are constantly upgraded due to advancement in engineering knowledge. Therefore, an accurate estimation of the performance of structure during an earthquake is crucial for estimating the actual effects of that earthquake on the existing RC structures. The primary objective of this work is to study the seismic response of RC frame building using performance based seismic evaluation. The effect of earthquake forces on (G + 4) storey building with and without the effect of brick infill for various parameters is proposed to be carried out with the help of pushover analysis as introduced by ATC-40 [3]. The various parameters are percentage of infill only in outer periphery and effect of soft storey mechanism on building. The major objectives of the research work are as follows: To study performance of frame without the influence of masonry infill wall panel in Reinforced Concrete frame structure in terms of deformation. To study the behavior of frame with brick masonry infill by modeling masonry infill as a diagonal strut [4]. Determination of performance point of both models. Comparison of fundamental natural time period and storey displacement for bare frame and frame with infill idealizations. II. REVIEW OF LITERATURE A)Past studies on seismic evaluation of existing RC buildings without brick masonry infill. Agrawal and Chourasia [1] presented the methodology for estimation of seismic vulnerability of Indian City base on I-1

2 qualitative and quantitative approach. Based on these approaches they have only explained the method for computation of demand capacities ratios for masonry as well as for RC buildings. Sinha and Goyal [5] presented the preanalysis step before going in for detailed analysis, i.e. Rapid Visual Screening. In this procedure, many building aspects were utilized for predicting the approximate vulnerability of existing buildings based on score assignment. Mwafy and Elnashai [6] presented the validity and the applicability of this technique, inelastic static pushover analysis are assessed by comparison with dynamic pushover idealised envelopes obtained from incremental dynamic collapse analysis. Lakshmanan [7] reported the pushover analysis performed on the representative building using displacement coefficient method included in FEMA-356. After performing the pushover analysis number of hinge in beam and column were noted for each performance range and to these, weightage factor was assigned and vulnerability index were found out from which good or poor performance of building elements during earthquake were indicated which could be useful for further retrofitting strategy. Lang [8] proposed an evaluation method developed for seismic risk assessment, within the scope of the earthquake scenario project for the city of Basel, Switzerland, in order to determine the seismic vulnerability of existing buildings. B) Past studies on seismic evaluation of existing RC buildings with brick masonry infill. Singh and Das [9] analyzed three sets of RC buildings- 4 storied, 8 storied and 16 storied, with the effect of brick masonry infill. The URM masonry was modeled as a diagonal strut using the expression given by FEMA 356. based nonlinear analysis procedure was adopted using SAP2000 nonlinear analysis software. Mahmud et al. [10] analyzed the ten storied building with brick infill panel. Finite Element package ANSYS 5.6 was used for the development of the model. In complete modeling, BEAM44 3-D Elastic Beam element was used to represent beams and columns. Korkmaz et al. [11] studied a 3-storied R/C frame structure with different amount of masonry infill walls, to investigate the effect of infill walls on earthquake response of such types of structures. The diagonal strut approach was adopted for modeling masonry infill walls. Also, it can be interpreted from the results that application of infill increases the axial force carrying capacity of frames. Das and Murty [12] analyzed five RC frames with brick masonry infill and designed using EC (8), NBC and Indian seismic code. The modeling concept of brick infill was developed and the same concept was utilised in modeling the infill panel in their companion study. The various concepts of different codes were also discussed. SAP2000 software was used for analysis purpose. Kaushik et al. [13] carried out investigation on improving the strength and stiffness of open first ground storey. A typical four storey RC frame was designed and analysis was carried out using SAP2000 software. Several additional strengthening schemes were suggested to improve the lateral load capacity of such buildings. III. NUMERICAL PROBLEM A building is assumed for seismic analysis that consists of a 5 storied R.C.C. Residential building. The plan of the building is regular in nature as it has all columns at unequal spacing. The building is located in Seismic Zone III and is assume on hard type soil. The building is 17.0 m in height 37.0 m in length and 13.0m in width. The important features of this building are shown in Table 1. TABLE 1: Salient features of the building 1. Type of Structure Multi-storey frame 2. Zone III 3. Layout As shown in Figure no 1 4. Number of stories 5(G + 4) 5. Ground storey height 3.0m 6. Floor-to-floor height 3.0 m 7. External walls 250 mm thick including plaster 8. Internal walls 250 mm thick including plaster 9. Live load 3.0 kn/m Materials M 20 and Fe Seismic analysis Pushover analysis(nonlinear static analysis) 12. Design Philosophy Limit state method conforming to IS 456 : Size of exterior 230 x 450 mm column 14. Size of interior column 230x 450 mm 15. Size of beams in 230 x 450 mm longitudinal and transverse direction 16. Total thickness of slab 125 mm IV.PUSHOVER ANALYSIS Pushover analysis is an approximate analysis method in which the structure is subjected to monotonically increasing lateral forces with an invariant height-wise distribution until a target displacement is reached. Pushover analysis consists of a series of sequential elastic analyses, superimposed to approximate a force-displacement curve of the overall structure. A two or three dimensional model which includes bilinear or trilinear load-deformation diagrams of all lateral force resisting elements is first created and gravity loads are applied initially. A predefined lateral load pattern which is distributed along the building height is then applied. The lateral forces are increased until some members yield. The structural model is modified to account for the reduced stiffness of yielded members and lateral forces are again increased until additional members yield. The process is continued until a control displacement at the top of building reaches a certain level of deformation or structure becomes unstable. The roof displacement is plotted with base shear to get the global capacity curve as shown in figure 1. Pushover analysis can be performed as forcecontrolled or displacement controlled [7]. 2

3 vii. Analysis is then run and pushover curves are obtained. FIGURE 1.Global Capacity (Pushover) Curve of a Structure Generally, pushover analysis is performed as displacement-controlled to overcome these problems. In displacement-controlled procedure, specified drifts are sought (as in seismic loading) where the magnitude of applied load is not known in advance. The magnitude of load combination is increased or decreased as necessary until the control displacement reaches a specified value. In displacement control method the target displacement is considered to be 4% [14]. The internal forces and deformations computed at the target displacement are used as estimates of inelastic strength and deformation demands that have to be compared with available capacities for a performance check. As graphically presented in Fig.2, the nonlinear static analysis procedure requires determination of three primary elements: capacity, demand and performance. The demand spectrum curve is normally estimated by reducing the standard elastic 5% damped. FIGURE 3: Model 1 (Bare Frame) FIGURE 4. Model 2 (Frame with Masonry Infill) FIGURE 5: Model 3 (Frame with soft storey) V.RESULTS AND DISCUSSION FIGURE 2. Nonlinear Analysis Procedure. Push over analysis in SAP2000: The step by step procedure for buildings with rigid floor diaphragm is as follows: i. A three dimensional computer model was generated. ii. Response spectrum analysis was performed for specified response spectrum. iii. The calculated the control displacement as 4% of height of the building. iv. The rigid floor condition is given to the floors at different levels. v. The primary elements are identified and plastic hinges are assigned. vi. Pushover analysis cases are then defined. The first case is for dead starting from zero initial conditions (unstressed state). The three models of the building are analyzed using SAP 2000, the effect of masonry infill in seismic evaluation of bare frame, frame with infill in outer periphery only and frame with soft storey is studied. The results obtained from the analysis performed on 5 storey building for various parameters are discussed below. A) Fundamental natural time period : Fundamental natural time period as per IS 1893(Part-I): 2002 and as per analysis using SAP 2000 software for various models are as shown in Table 2. It is observed from Table 2 that the analytical natural period do not tally with the natural periods obtained from the empirical expression of the code. Introduction of infill panels in the RC frame reduces the time period of model 2 and model 3 thus enhances the stiffness of the structure. Bare frame idealization leads to overestimation of natural periods and under estimation of the design lateral forces. It has been found that in Outer infill configuration 3

4 (Model 2) there was 40% reduction in time period compared to the bare frame (Model 1) and in ground soft storey model (Model 3) 26% reduction in natural period was observed compared to bare frame model (Model 1). Model type Using IS 1893: 2002 Using software TABLE 2: Fundamental time period Model 1 Model 2 Model 3 X Y X Y X Y Thus this indicated that presence of percentage of infill in RC frame considerably reduces the fundamental natural time period of the buildings under consideration. These increase the stiffness which may cause an increase or decrease of base shear, depending on both the frame analyzed and characteristics of ground motion. Also it is observed from model 3 that the fundamental time period increases as compare to model 2; this phenomenon takes place due open ground storey effect. Thus it is concluded that the fundamental period increases with the decrease in percentage of infill as expected, due to reduction in stiffness of models under consideration. The code formula for fundamental time period considers the infill, but it does not consider the infill property or percentage opening in infill. Thus the empirical formula given in the code may not be applicable to all kind of structures. As the opening percentage increases, the fundamental period of structure approaches the natural period of bare frame as expected, and shown in Fig. 6. Storey No. TABLE 3: Roof displacements for various models Model 1 Model 2 Model is plotted for all the model buildings and shown in Fig. 7 below. Infill act as equivalent diagonal strut in RC frame, and is responsible for increasing the storey stiffness. From Fig. 7, it is clear that the inclusion of effect of infill drastically decreases the storey displacements as seen from model 2 and model 3 as compare to model 1. In case of model 1, the whole building sways like an inverted pendulum with maximum sway concentrated in the ground storey. The ground storey columns of model 1 acts as the pendulum rod while the rest of the building acts as a rigid pendulum mass. As a consequence, large movements occur locally in the ground storey alone in case of model 1 and model 3. Due to this phenomenon large damages in ground storey columns of model 1 and model 3 are expected during the earthquake. Thus the ground storey columns should be strengthened appropriately so that the structure can withstand the earthquake effects. FIGURE 7. Variations of storey displacement FIGURE 6.Variations of fundamental time period B) Effect of variation of infill amount on storey displacement: The storey displacement for the five storey building for various models at various floor levels is presented in Table 3. It is observed that by considering the infill, there is a decrease in the roof displacement of model 2 and model 3 upto 75 % and 47 % as compare to model 1 respectively. Also the roof displacement of model 2 is decreased by 53% as compared to model 3. VI.SUMMARY Although from all of the above stated procedures, the nonlinear static analysis procedure is most convergent procedure for determining the performance of the building. Using the capacity spectrum method, the pushover analysis traces the actual failure behavior in the form of capacity curve. The method is useful in predicting the seismic performance of the buildings under consideration. 4

5 VII. REFERENCES [1] S.Agrawal and A. Chourasia. Methodology for Seismic Vulnerability Assessment of Building Stock in Mega Cities, A Workshop on Microzonation Interline Publishing, Bangalore. [2] U. Yazgan. The Use of Post-Earthquake Residual s as A Performance Indicator in Seismic Assessment, Ph.D. Thesis, Department of Civil Engineering, Middle East Technical University (METU), [3] ATC. Seismic evaluation and retrofit of concrete buildings. ATC-40 Report, Applied Technology Council, Redwood City, California, 1, [4] Federal Emergency Management Agency (FEMA 356). Prestandard and commentary for the seismic rehabilitation of buildings, A Report, Washington, D.C., [5] R. Sinha and A. Goyal. A National Policy for Seismic Vulnerability Assessment of Buildings and Procedure for Rapid Visual Screening of Buildings for Potential Seismic Vulnerability. Department of Civil Engineering Indian Institute of Technology Bombay, Implications, The Indian Concrete Journal, August 2004, pp [13] H.Kaushik, D. Rai and S.Jain, Effectiveness of Some Strengthening Options for Masonry-Infilled RC Frames with Open First Story, Journal of Structural Engineering, 135(8), August 1, 2009,pp [14] G.Mulgund and A. Kulkarni. Seismic Assessment of RC Frame Buildings with Brick Masonry Infill, International Journal of Advanced Engineering Sciences and Technologies, 2(2), 2011, pp [15] R. Thapaliya. Assessing building vulnerability for earthquake using field survey and development control data: A case study in lalitpur sub-metropolitan city. Ph. D. thesis, Nepal, March [16] A. Meher Prasad. Seismic Vulnerability of Existing RC Buildings in India. 13th World Conference on Earthquake Engineering Vancouver, B.C., Canada, 1207, August 1-6, [6] A. Mwafy, A. Elnashai. Static pushover versus dynamic collapse analysis of RC buildings, Department of Civil and Environmental Engineering, Imperial College, Imperial College Road, London SW7 2BU, UK,26 May [7] N. Lakshmanan. Seismic Evaluation and Retrofitting Of Buildings and Structures. ISET Journal of Earthquake Technology, Paper No. 469, Vol. 43, No. 1-2, March-June 2006, pp [8] K. Lang. Seismic vulnerability of existing buildings, Swiss Federal Institute of Technology Zurich, University of London, England, [9] Y. Singh and D.Das. Effect of URM Infill s on Seismic Performance of RC Frame Buildings. 4 th International Conference on Earthquake Engineering Taipei, Taiwan, October 12-13, [10] K. Mahmud, R. Islam and Al-Amin. Study the Reinforced Concrete Frame with Brick Masonry Infill due to Lateral Loads, International Journal of Civil & Environmental Engineering IJCEE-IJENS, 10(4), [11] K.A.Korkmaz, F. Demir and M. Sivri., Earthquake Assessment of R/C Structures with Masonry Infill Walls, International Journal of Science & Technology, 2(2), 2007, pp [12] D.Das and C.V.R. Murty. Brick Masonry Infill in Seismic Design of RC Framed Buildings: Part I- Cost 5