Seismic Response of Existing RC Building Under Revised Zone Classification Pushover SEISMIC RESPONSE OF EXISTING RCSeismic BUILDING UNDERUsing REVISED Analysis, V.Panneer Selvam, K.Nagamani , Journal Impact Factor (2015): 9.1215 (Calculated by GISI) SEISMIC ZONE CLASSIFICATION USING PUSHOVER ANALYSIS www.jifactor.com V.Panneer Selvam#, K.Nagamani* Volume 6, Issue 6, June (2015), Pp. 23-32 Article Id: 20320150606003 International Journal of Civil Engineering and Technology (IJCIET) © IAEME: www.iaeme.com/Ijciet.asp ISSN 0976 – 6308 (Print) ISSN 0976 – 6316(Online)

#

IJCIET ©IAEME

Research Scholar, College of Engineering, Anna University, Chennai * Professor, College of Engineering, Anna University, Chennai

ABSTRACT Existing Reinforced Concrete (RC) buildings constructed before two decades typically have the design details which are considered to be highly inadequate under the present revised seismic code of practice. Many of these structures have suffered significant structural damage during recent earthquakes. Significant research effort has been devoted to the development of behavioral models and modeling techniques to predict the behavior of these buildings. However there are no models that have been shown to predict observed response with a high level of accuracy and precision. In this paper, a seven storey RC building is considered to investigate the structural seismic response. This RC building is analysed and designed for the gravity and lateral loads using the codal provisions followed a decade ago (previous version of the code). Then the designed structure is evaluated for the seismic performance under the old and the revised code of practice using Pushover Analysis. The Displacement controlled Pushover Analysis was carried out and the Pushover Curve was obtained for the building in both X and Y directions. The Capacity Spectrum, Demand Spectrum and Performance point of the building, under old and the revised seismic zone classification was found in both the direction using the analysis carried out in SAP 2000. From the analysis it is understood that, the frame is capable of withstanding the presumed seismic force with some significant yielding at several beams. The results obtained in terms of demand, capacity and plastic hinges gave an insight into the real behaviour of structure. Keywords: Pushover Analysis, Old RC Building, Capacity Curve, Performance Point, Plastic Hinge. 1.0 INTRODUCTION The recent earthquakes in India have led to an increase in the seismic zoning factor over many parts of the country. Also, ductility has become an issue for all those buildings that were designed and detailed using earlier versions of the codes. Many concrete structures have been collapsed or severely damaged during these earthquakes. This indicates the need for evaluating the seismic adequacy of existing buildings. India’s four recent upgraded seismic zones emphasize this for more than 60% of the land. Under such circumstances, seismic qualification of existing buildings under revised codes has become extremely important. In particular, the seismic rehabilitation of older concrete structures in high seismicity areas is a matter of growing concern, since structures www.iaeme.com/ijciet.asp

23

[email protected]

Seismic Response of Existing RC Building Under Revised Seismic Zone Classification Using Pushover Analysis, V.Panneer Selvam, K.Nagamani , Journal Impact Factor (2015): 9.1215 (Calculated by GISI) www.jifactor.com

venerable to damage must be identified and an acceptable level of safety must be determined. To make such assessment, simplified linear-elastic methods are not adequate. One of the emerging fields in seismic design of structures is the Performance Based Design. Nonlinear static analysis or push over analysis has been developed over the past years and has been a preferred procedure for seismic performance evaluation of several structures. Basically, a pushover analysis is a series of incremental static analysis carried out to develop a capacity curve for the building. Based on the capacity curve, a target displacement which is an estimate of the displacement that the design earthquake will produce on the building is determined. Peter Fajfar and M.Eeri [1] carried out studies on a nonlinear analysis method for performance based seismic design. In this paper, the author has presented a simple non linear method for seismic analysis of structures. It combines the push over analysis of multi degree of freedom model with response spectrum analysis of an equal single degree of freedom system. The method is formulated in acceleration-displacement format, which enables the visual interpretation of the procedure and of relations between basic quantities controlling seismic response. Inelastic spectra, rather than elastic spectra with equivalent damping and period are applied. This feature represents the major difference with respect to the capacity spectrum method. Moreover demand quantities are obtained without iterations. Generally, the results of the N2 method are reasonably accurate, provided that the structure oscillates predominantly in the first mode. Some additional limitations apply. M. Seifi, J. Noorzaei, M. S. Jaafar, E. YazdanPanah [2] presents the state of the art development in nonlinear static pushover analysis in earthquake engineering. In this paper, the authors compared nonlinear static pushover (NSP) analysis to nonlinear dynamic time-history analysis. Conceptually, NSP method relies on pushing the structure with incremental static lateral load by considering material inelasticity and geometric nonlinearity. Pushover procedure for evaluating the seismic adequacy of reinforced concrete frames was presented by A. Shuraim, A. Charif [3]. In this paper the author has utilized nonlinear static analytical procedure (Pushover) as introduced by ATC-40 for the evaluation of existing design of a reinforced concrete frame, in order to examine the applicability of the pushover for evaluating design of new buildings. In the first approach, the potential deficiencies were determined by redesigning under one selected seismic combination in order to show which members would require additional reinforcement. In the second approach, a pushover analysis was conducted to assess the seismic performance of the frame and detect the locations of the plastic hinges. The paper shows that vulnerability locations revealed from the two procedures are significantly different, where the latter procedure tends to overestimate column strength, consequently, concealing earlier detection of column weaknesses. The paper provides rational explanations for the apparent discrepancy that can be taken into consideration in order to make pushover methodology applicable when designing or evaluating existing design of new buildings. Rahul Rana, Limin Jin and AtilaZekioglu [4] carried out push over analysis of a 19 storey concrete shear wall building”.In this paper, author has done Pushover analysis on a nineteen story, slender concrete tower building located in San Francisco with a gross area of 430,000 square feet. NieJiaguo, Qin Kai, Xiao Yan [5] carried out studies on push-over analysis of the seismic behavior of a concrete-filled rectangular tubular frame structure. In this paper, in order to investigate the seismic behavior of concrete-filled rectangular steel tube (CFRT) structures, push-over analysis of a 10-story moment resisting frame (MRF) composed of CFRT columns and steel beams was conducted. The results show that push-over analysis is sensitive to the lateral load patterns, so the use of at least two load patterns that are expected to bound the inertia force distributions is recommended. Oguz, Sermin [6] carried out evaluation of pushover analysis procedures for frame structures. Modal Pushover Analysis on reinforced concrete and steel moment resisting frames covering a broad range of fundamental periods was carried out. Certain response parameters predicted by each pushover procedure were compared with the 'exact' results obtained from www.iaeme.com/ijciet.asp

24

[email protected]

Seismic Response of Existing RC Building Under Revised Seismic Zone Classification Using Pushover Analysis, V.Panneer Selvam, K.Nagamani , Journal Impact Factor (2015): 9.1215 (Calculated by GISI) www.jifactor.com

nonlinear dynamic analysis. A. Ismail [7] carried out non linear static analysis of a retrofitted reinforced concrete building. In this study, the author made an attempt for investigating the seismic behavior of a typical existing building in Cairo by performing static pushover analysis before and after retrofitting the columns by reinforced concrete, steel sections or carbon fiber reinforced polymer (CFRP) composite jackets. In this paper the author has carried out pushover analysis for evaluation of seismic performance of existing reinforced concrete structure under revised seismic zone classification. The present study is to evaluate the behavior of existing seven storey reinforced concrete structure located in Chennai region, which was designed as per the earlier version of the seismic code (IS: 1893 – 1984), to comply with the provision mentioned in the revised version of seismic code (IS 1893 (Part 1) : 2002). 2.0 PUSHOVER ANALYSIS Pushover analysis (Fig. 1) is a nonlinear static analysis for a reinforced concrete (RC) framed structure subjected to lateral loading.This lateral loads represents the inertial forces which the structure would be experienced when subjected to ground shaking.First gravity loads are applied, and then the lateral load is applied incrementally at the end of the gravity push. Building is displaced till the ‘control node’ reaches ‘target displacement’ or building collapses.The sequence of cracking, plastic hinging and failure of the structural components throughout the procedure is observed.Using a pushover analysis, a characteristic non linear force-displacement relationship (Base shear & control node displacement) can be determined. Pushover analysis is a static, nonlinear procedure in which the magnitude of the lateral force is incrementally increased, maintaining the predefined distribution pattern along the height of the building. Pushover analysis can determine the strength (weak links and failure modes), and drift capacity and the seismic demand for this structure subjected to selected earthquake. Local Nonlinear effects are modeled and the structure is pushed until a collapse mechanism gets developed. At each step, the base shear and the roof displacement can be plotted to generate the pushover curve. It gives an idea of the maximum base shear that the structure was capable of resisting at the time of the earthquake. For regular buildings, it can also give a rough idea about the global stiffness of the building. The responses obtained from pushover analysis are:

www.iaeme.com/ijciet.asp

25

[email protected]

Seismic Response of Existing RC Building Under Revised Seismic Zone Classification Using Pushover Analysis, V.Panneer Selvam, K.Nagamani , Journal Impact Factor (2015): 9.1215 (Calculated by GISI) www.jifactor.com

Fig. 1 Schematic Representation of Pushover Analysis Procedure Estimates of force and displacement capacities of the structure. Estimates of force (axial, shear and moment) demands on potentially brittle elements and deformation demands on ductile elements. Estimates of global displacement demand, corresponding inter-storey drifts and damages on structural and non-structural elements expected under the earthquake ground motion considered. Sequences of the failure of elements and the consequent effect on the overall structural stability. Identification of the critical regions, where the inelastic deformations are expected to be high and identification of strength irregularities (in plan or in elevation) of the building. In pushover analysis the building is pushed with a specific load distribution pattern along the height of the building. The magnitude of the total force is increased but the pattern of the loading remains same till the end of the process. Pushover analysis results (i.e., pushover curve, sequence of member yielding, building capacity and seismic demand) are very sensitive to the load pattern. The lateral load patterns should approximate the inertial forces expected in the building during an earthquake. The distribution of lateral inertial forces determines relative magnitudes of shears, moments, and deformations within the structure. The building has to be modeled to carry out nonlinear static pushover analysis. This requires the development of the force - deformation curve for the critical sections of beams, columns. The force deformation curves in flexure were obtained from the reinforcement details and were assigned for all the beams and columns.User-defined PMM (PM-M hinges are assigned at the ends of column members which are subjected to axial force and bending moments) and M3 (M3 hinges are assigned at the ends of beam members which are subjected to bending moments) curves are developed using the rotation capacities of members. Target displacement is the displacement demand for the building at the control node subjected to the ground motion under consideration. This is a very important parameter in pushover analysis because the global and component responses (forces and displacement) of the building at the target displacement are compared with the desired performance limit state to know the building performance. There are two approaches to calculate target displacement: Displacement Coefficient Method (DCM) of FEMA 356 and Capacity Spectrum Method (CSM) of ATC 40. Both of these approaches use pushover curve to calculate global displacement demand on the building from the response of an equivalent single-degree-of-freedom (SDOF) system. www.iaeme.com/ijciet.asp

26

[email protected]

Seismic Response of Existing RC Building Under Revised Seismic Zone Classification Using Pushover Analysis, V.Panneer Selvam, K.Nagamani , Journal Impact Factor (2015): 9.1215 (Calculated by GISI) www.jifactor.com

3.0

DESCRIPTION OF THE FRAMED STRUCTURE

A residential building having stilt + 6 floors located in Chennai (Fig. 2) is considered for the analysis. Each floor contains 6 apartments (total 48 apartments). The building was analysed and designed for the combination of Dead, Live, Wind and Seismic load. The seismic analysis of the building was carried out using IS: 1893 – 1984. Geometrical details are, Length – 42.7m Width – 18.3m Height – 21.2m Dead load due to brick wall, finishes, beam, column & slab are calculated based on the unit weight of material. Brick load are calculated for height of wall. Live load on floor is 2 kN/m2& on Roof is 1.5 kN/m2. Wind load on the building is calculated as per IS 875 (Part-III). Seismic load is calculated as per IS: 1893 – 1984. Seismic Zone =2 Basic horizontal seismic coeff., αo = 0.02 αh = β I αo Coefficient, β =1 Importance Factor, I =1 Design Hor. Seismic Co-efficient, αh = 0.02 (αh= β I αo) Time period = 0.47 s (x-dir) & 0.30 s (y-dir) Co-eff based on flexibility, C = 0.7 (x-dir) & 1.0 (y-dir) Design Base Shear, Vb = 0.014W (x-dir) & 0.02W (y-dir) Seismic load is calculated as per IS: 1893 (Part 1) – 2002. Seismic Zone =2 Seismic Intensity = 0.1 Importance Factor =1 Response Reduction Factor =3 Time period = 0.47 s (x-Dir) = 0.30 s (y-dir) Spectral Acceleration, Sa/g = 2.5 (both x & y -Dir) Design Hor. Seismic Co-efficient = 0.04167 Design Base Shear, Vb = 0.04167W Seismic load is calculated as per IS: 1893 (Part 1) – 2002. Seismic Zone =3 Seismic Intensity = 0.16 Importance Factor =1 Response Reduction Factor =3 Time period = 0.47 s (x-Dir) = 0.30 s (y-dir) Spectral Acceleration, Sa/g = 2.5 (both x & y -Dir) Design Hor. Seismic Co-efficient = 0.0667 Design Base Shear, Vb = 0.0667W Where, W = total dead load + appropriate amount of live load

www.iaeme.com/ijciet.asp

27

[email protected]

Seismic Response of Existing RC Building Under Revised Seismic Zone Classification Using Pushover Analysis, V.Panneer Selvam, K.Nagamani , Journal Impact Factor (2015): 9.1215 (Calculated by GISI) www.jifactor.com

Fig. 2 3D View of the Existing Reinforced Building The pushover analysis of the building is carried out in SAP. Initially the basic model of the building is created in SAP as shown in Fig. 3.

Fig. 3 3D Building Model created in SAP The properties and acceptance criteria for the pushover hinges are defined for the model. Several built-in default hinge properties are available for concrete and steel in SAP. Then the pushover hinges are located in the model by selecting one or more frame members and assigning them hinge properties. The pushover load cases are defined in the software. First basic static analysis and dynamic analysis are carried out on the model. Then static non linear pushover analysis is carried out. The capacity curve of the building along x direction is shown in Fig. 4. The plastic hinge pattern in the building along one typical duirection (x direction) is given in Table 1.

Fig. 4 Capacity Curve of the Building www.iaeme.com/ijciet.asp

28

[email protected]

Seismic Response of Existing RC Building Under Revised Seismic Zone Classification Using Pushover Analysis, V.Panneer Selvam, K.Nagamani , Journal Impact Factor (2015): 9.1215 (Calculated by GISI) www.jifactor.com

The performance levels are the discrete damage states identified from a continuous spectrum of possible damage states (Fig. 5). The structural performance levels based on the roof drifts are labeled as A, B, C, D and E are used to define the force deflection behavior of the hinge. The performance levels (IO, LS, and CP) of a structural element are represented in the load versus deformation curve as shown below, 1) A to B – Elastic state, i) Point ‘A’ corresponds to the unloaded condition. ii) Point ‘B’ corresponds to the onset of yielding. 2) B to IO- below immediate occupancy, 3) IO to LS – between immediate occupancy & life safety, 4) LS to CP- between life safety to collapse prevention, 5) CP to C – between collapse prevention and ultimate capacity, i) Point ‘C’ corresponds to the ultimate strength 6) C to D- between C and residual strength, i) Point ‘D’ corresponds to the residual strength 7) D to E- between D and collapse i) Point ‘E’ corresponds to the collapse.

Fig. 5 Performance Level of a Structure Table 1 Plastic hinge pattern for X Direction

The capacity spectrum of structure obtained for x direction for zone II and Zone III is shown in Fig.6a and 6b. The plastic hinge pattern for the building is shown in Fig. 7a & 7b. www.iaeme.com/ijciet.asp

29

[email protected]

Seismic Response of Existing RC Building Under Revised Seismic Zone Classification Using Pushover Analysis, V.Panneer Selvam, K.Nagamani , Journal Impact Factor (2015): 9.1215 (Calculated by GISI) www.jifactor.com

Table 1b Plastic hinge pattern for Y Direction

The capacity spectrum of structure obtained for x direction for zone II and Zone III is shown in Fig.6a and 6b. The plastic hinge inge pattern for the building is shown in Fig. 7a & 7b.

Fig. 6a Capacity Spectrum for Zone II

Fig. 6b Capacity Spectrum for Zone III

www.iaeme.com/ijciet.asp

30

[email protected]

Seismic Response of Existing RC Building Under Revised Seismic Zone Classification Using Pushover Analysis, V.Panneer Selvam, K.Nagamani , Journal Impact Factor (2015): 9.1215 (Calculated by GISI) www.jifactor.com

Fig. 7a Plastic Hinge Formation X Direction

Fig. 7b Plastic Hinge Formation Y Direction 4.0

SUMMARY AND DISCUSSIONS

The existing, 12 year old, 6-storey residential building located in Chennai is analyzed and designed for different combinations of dead, live, wind and seismic code provisions. After construction, the seismic code WAS revised with upgradation of seismic zones over many parts of the country. Hence the qualification of the existing structure under the revised seismic zone has to be studied. The nonlinear static analysis (pushover analysis) is a relatively simple way to explore the nonlinear behaviour of buildings. Analytical model was created, representing the existing building, using elastic beam and column members as elastic elements with plastic hinges at their ends. Analytical models are incorporated to represent inelastic material behaviour and inelastic member deformations for simulating numerically the post yield behaviour of the structure under expected seismic load. Pushover analysis is performed on the existing building for both zones (II & III). Target displacement of the building was 80 mm but the building is analysed for the displacement upto 200 mm. Pushover parameters were evaluated and compared for both zones. 5.0

CONCLUSIONS

From the analysis it is understood that, the frame is capable of withstanding the presumed seismic force with some significant yielding at several beams. The results obtained in terms of demand, capacity and plastic hinges gave an insight into the real behaviour of structure. All the plastic hinges formed in the beams, columns are within the acceptance criteria of plastic hinge. Lateral deformations at the performance point are within the target displacement of the structure. Maximum total drift, maximum inelastic drift, and structural stability does not exceed the limitations of the performance level, therefore the present building is considered safe against the revised seismic provisions. www.iaeme.com/ijciet.asp

31

[email protected]

Seismic Response of Existing RC Building Under Revised Seismic Zone Classification Using Pushover Analysis, V.Panneer Selvam, K.Nagamani , Journal Impact Factor (2015): 9.1215 (Calculated by GISI) www.jifactor.com

REFERENCES 1. 2. 3. 4.

5.

6. 7. 8.

9.

10.

Peter Fajfar, Eeri, M (2000); “A nonlinear analysis method for performance based seismic design”. Earthquake spectra, vol.16, no: 3, pp-573 to 592. M. Seifi, J. Noorzaei, M. S. Jaafar, E. YazdanPanah (2008); “Nonlinear Static Pushover Analysis in Earthquake Engineering: State of Development”.ICCBT2008. A. Shuraim, A. Charif;”Performance of pushover procedure in evaluating the seismic adequacy of reinforced concrete frames”. King Saud University. Rahul RANA, Limin JIN and Atila ZEKIOGLU (2004); “Push over analysis of a 19 story concrete shear wall building”.13th World Conference on Earthquake Engineering Vancouver, B.C., Canada August 1-6, 2004 Paper No. 133. NIE Jiaguo, QIN Kai, XIAO Yan (2006); “Push-Over Analysis of the Seismic Behavior of a Concrete-Filled Rectangular Tubular Frame Structure”. TSINGHUA SCIENCE AND TECHNOLOGY ISSN 1007-0214 20/21 pp124-130, Volume 11, Number 1, February 2006. Oğuz, Sermin(2005); “Evaluation of pushover analysis procedures for frame structures”.. Graduate School of Natural and Applied Sciences. A. Ismail (2014) “Non linear static analysis of a retrofitted reinforced concrete building” HBRC Journal (2014) 10, 100–107. Dr. Suchita Hirde and Ms. Dhanshri Bhoite, “Effect of Modeling of Infill Walls on Performance of Multi Story RC Building” International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 4, 2013, pp. 243 - 250, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316. Turkia Haithem and Lahbari Noureddine, “Seismic Response Model Including SSI of RC Buildings on Isolated and Raft Foundations” International Journal of Civil Engineering & Technology (IJCIET), Volume 6, Issue 3, 2015, pp. 118 - 131, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316. Shaikh Zahoor Khalid & S.B. Shinde, “Seismic Response of FRP Strengthened RC Frame” International Journal of Civil Engineering & Technology (IJCIET), Volume 3, Issue 2, 2012, pp. 305 - 321, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.

www.iaeme.com/ijciet.asp

32

[email protected]