European Journal of Scientific Research ISSN 1450-216X Vol.71 No.2 (2012), pp. 195-202 © EuroJournals Publishing, Inc. 2012 http://www.europeanjournalofscientificresearch.com
Pushover Analysis of Existing Reinforced Concrete Framed Structures A. Vijayakumar Assistant Professor, Department of Civil Engineering Jansons Institute of Technology, Coimbatore, Tamilnadu, India E-mail:
[email protected] D. L. Venkatesh Babu Principal, United Institute of Technology Coimbatore, Tamilnadu, India Abstract The existing building can become seismically deficient since seismic code requirements are constantly upgraded and advancement in engineering knowledge. Further Indian buildings built over past two decades are seismically deficient because of lack of awareness regarding seismic behavior of structures. This paper aims to evaluate the zone – III selected existing reinforced concrete building to conduct the non-linear static analysis (Pushover Analysis). The pushover analysis shows the pushover curves, capacity spectrum, plastic hinges and performance level of the existing building. The non-linear static analysis gives better understanding and more accurate seismic performance of buildings as progression of damage or failure can be traced. Keywords: Existing Reinforced Concrete building, Seismic zone, Pushover Analysis, capacity spectrum.
1. Introduction Most of the existing buildings are in seismically active zones and are designed for gravity loads only. A large number of existing buildings in zone-III is need seismic evaluation due to various reasons such as, noncompliance with the codal requirements, updating of codes, design practice and change the use of the building. However, the existing structure in the earthquake region Zone III has to be provided by some rehabilitation to sustain the expected performance level. Before rehabilitation work, it is necessary to understand the capacity of the existing building to check if it meets the intended performance level. The analytical techniques proposed in [1, 2] simplified Nonlinear analysis procedure (pushover analysis) to determine the displacements demand imposed on the building expected to yield. The Nonlinear static procedure in these documents is based on the capacity spectrum method, and assumes that the lateral force distribution for the pushover analysis and the conversion of the results of the capacity diagram is based only on the fundamental vibration mode of the elastic structure. This paper [3] described SAP2000 is used in performing a pushover analysis of a simple three dimensional building. SAP2000 is a state-of-the-art, general purpose, and three dimensional structural analysis programs. SAP2000 has static pushover analysis capabilities which are fully integrated into the
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program; allow quick and easy implementation of the pushover procedures for both two and three dimensional buildings. It [4, 6, 7] presented a comparative study for storey level vs. Lateral load, storey shear, and storey moment for five storey buildings located at various zones. The result showed various types of plastic hinges separately in all beams, columns and tabulated along with the graph. It can be concluded that the square shape of column cross section improves the behavior of a structure especially under earthquake forces. This paper pointed out the [8, 9] effect of plastic hinges in Nonlinear analysis and inelastic time history analysis of different storey of concrete buildings. The results are shown in terms of base shear capacity, displacement capacity and formation of hinges. This paper described [10, 11, 12] with different amount of masonry infill walls are considered to investigate the effect of infill walls on the earthquake response of these types of structures. Pushover curves are obtained for the structures using nonlinear analysis for SAP 2000 and ETABS. From the pushover curves, storey displacement, relative storey displacement, maximum plastic rotations is determined. The building was assumed to be placed [13, 15] in various zones of India. Response spectrum analyses are carried out for all seismic zones in India considering with and without infill stiffness. Pushover analysis is carried out to produce a pushover curve consists of capacity spectrum, demand spectrum and performance point. Pushover analysis shows that performance load of the building components and also the maximum base shear carrying capacity of the structures for various zones. Response spectrum analysis shows that the failure is more in zone IV and V and the structure must be retrofitted.
2. Research Significance The present study is to evaluate the behavior of G+2 reinforced concrete bare frame subjected to earthquake forces in zone III. The reinforced concrete structures are analyzed by nonlinear static analysis (Pushover Analysis) using SAP2000 software. It shows the performance levels, behavior of the components and failure mechanism in a building. It also shows the types of hinge formation. The strength and capacity of the weakest components are then increased by retrofitting techniques.
3. Rapid Visual Screening, Date Collection and Preliminary Evaluation The Rapid visual screening involves a quick assessment of a building based on visual inspection alone. It is kind of statistical guideline to the inspectors to identify and inventory the vulnerable buildings. In order to facilitate seismic evaluation, it is necessary to collect relevant date of a building as much as possible through drawings, enquiry, design calculation, soil report, inspection report reports of previous investigation, previous repair work, any complaints by the occupants etc. A site visit is essential for data collection. The purpose of preliminary evaluation is to identify the areas of seismic deficiencies in an engineered building before a detailed evolution is undertaken. It checks the code compliance for seismic design and detailing. Table 1and 2 shows the building survey data sheet of an existing building and the parameters required for seismic analysis. Table 1: S.No
1
2
Building survey data sheet: General Data Description Address of the Building ● Name of the building ● Plot Number ● Locality / Township ● District ● state Name and type of owner / tenant
Information Building Description Hostel Zone-III Coimbatore Tamilnadu Private
Notes
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Table 1:
Building survey data sheet: General Data - continued Coimbatore Consultants Architects & Engineers
3
Name of builder
4 5 6
Name of architect Name of engineer Use of building Hostel Year of construction and subsequent 2000 remodeling, if any Plan size (approximate) 24.384 m X 16.855m Building height 8.778 Number of storey’s above ground level 2 (Each floor 2.926m) Number of basements below ground level 0 Type of structure ● Load bearing wall ● RC frame RC frame ● RC frame and shear wall ● Steel frame Open ground storey No Roof – top water tank, heavy machinery or Yes any other type of large mass Architectural features -Expansion / Separation joints No Photograph / sketch Attached with sheet Survey Visited building site Yes Structural drawing available Yes Architectural drawing available Yes Geotechnical report available No Construction specifications available Yes Designer contacted Yes Exposure condition Environment Hot / Wet Deterioration noticed No Geotechnical and geological data Type of soil Hard rock Type of foundation Isolated footing Seismic zone III IS 1893;2002,Figure History of past earthquakes 1990 Feb08, Magni-6 Variables for analysis Dead loads (unit weights) ● Masonry ● Concrete 20 kN/m3 IS 875;1987 (part I) ● Steel 25kN/ m3 ● Other materials Imposed (live) loads ● Floor loads 2 kN/m3 IS 875;1987 (part II) 1.5 kN/m3 ● Roof loads Wind loads Not consider IS 875;1987 (part III) Snow loads Not consider IS 875;1987 (part IV) Safe bearing capacity Not available Importance factor, I 1.5 IS 1893;2002, Table No 6 Seismic zone factor, Z 0.16 IS 1893;2002, Table No 2 Response reduction factor , R (ordinary 3 IS 1893;2002, Table No 7 moment resisting frame) 0.75 Ta= 0.075h and Ta= 0.009h/√d without and with infill Fundamental natural period, T stiffness
7 8 9 10 11 12 13 14 15 16 17 1 2 3 4 5 6 1 2 1 2 3 4
1
2 3 4 5 6 7 8 9
Pushover Analysis of Existing Reinforced Concrete Framed Structures Table 2: S.No
1
2
3 4 5 6
198
Building survey data sheet: Building Data (moment resisting frame) Description Type of building ● Regular frames ● Regular frames with shear wall ● Irregular frames ● Irregular frames with shear wall ● Open ground storey Horizontal floor system ● Beams and slabs ● Waffle slab ● Ribbed floor ● Flat slab with drops ● Flat plate without drops System of interconnection foundations ● Plinth beams ● Tie beams Grades of concrete/steel used in different parts of building Method of analysis Computer software used
Information
Notes
Regular frames
Beams and Slabs
No interconnection M15, Fe415 ---
The structures are designed according to the Indian code IS456-1978 and analyzed as per previous seismic code IS-1893-1984. Table 3 shows the beams and columns dimensions. Table 3:
Beam dimensions S.No 1 2 3 4 5 6 7 8
Beam and Column Notations B1 B1a B2 B2a B3 B4 C1 C2
Size (M) 0.273X0.412 0.273x0.312 0.280x0.60 0.280x0.701 0.280x0.60 0.23x0.114 0.40X0.228 0.490x0.228
Nos 16 1 10 1 6 15 12 20
The required material properties like weight density, modulus of elasticity, grade of concrete and grade of steel used are acceptance default values. Beam and column members have been defined as frame elements with the appropriate dimensions and reinforcements. Slabs are defined as an area element having the properties of shell elements with required thickness. Slabs have been modeled as rigid diaphragms. Soil structure interaction has not been considered and columns have been restrained in all six degrees of freedom at the base. The 3D modeling of the bare frame is shown in Figure 1. Figure 1: Building Frame
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After modeled the structural components the following load cases are assigned. Gravity loads on the structure include the self weight of beams, columns, slabs, walls and other permanent members. The self weight of beams, columns (Frame members), slabs (Area section) and brick wall (Diagonal strut) are automatically considered by the program itself. The live load has been assigned as uniform area loads on the slab elements as per IS 1893 (part-1) 2002. The live load is considered for seismic weight calculation as per table – 8, IS 1893 (Part-1) 2002 [19]. In this case 25% of the imposed load has been considered as seismic weight.
4. Pushover Methodology 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. With the increase in the magnitude of the loads, weak links and failure modes of the building are found. Pushover analysis can determine the behavior of a building, including the ultimate load and the maximum inelastic deflection. 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. 4.1. Nonlinear Plastic Hinges Properties 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 and brick masonry by using the guidelines [5] as mentioned above. The force deformation curves in flexure were obtained from the reinforcement details and were assigned for all the beams and columns. The Nonlinear properties of beams and columns have been evaluated using the section designer and have been assigned to the computer model in SAP2000. The flexural default hinges (M3) and shear hinges (V2) were assigned to the beams at two ends. The interacting (P-M2-M3) frame hinges type a coupled hinge property was also assigned for all the columns at upper and lower ends. The axial hinges (P) were assigned to the brick masonry strut element.
5. Nonlinear Static Pushover Analysis The material model used in the static Nonlinear pushover analysis is based on the procedures proposed by the [1, 2] documents, defining force – deformation criteria for the hinges used in the pushover analysis. Figure 2 describes the typical force-deformation relation proposed by those documents. Five points labeled A, B, C, D and E are used to define the force deflection behavior of the hinge and these points labeled A to B – Elastic state, B to IO- below immediate occupancy, IO to LS – between immediate occupancy and life safety, LS to CP- between life safety to collapse prevention, CP to C – between collapse prevention and ultimate capacity, C to D- between C and residual strength, D to E- between D and collapse >E – collapse. Figure 2: Force-Deformation for Pushover Analysis
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In this study, the seismic response of bare frame is evaluated using the design level earthquakes of zone III as specified in the Indian Code. The building is assumed to be located on medium soil site. The pushover hinges are assigned for beams and columns and the lateral forces are applied at each floor level at the design center of mass. Pushover analysis should be performed separately for the two orthogonal directions in order to study the performance of the buildings in both directions. There are therefore three pushover cases for the evaluation of buildings. Gravity push, which is used to apply gravity load, (D.L.+0.25LL), Push 1- is the lateral push in X direction, starting at the end of gravity push, Push2- is the lateral push in Y direction starting at the end of gravity push.
6. Result and Discussion 6.1. General A Three storied hostel reinforced concrete bare frame was taken for the investigation. The frame was subjected to design earthquake forces as specified in the IS code for zone III along X and Y directions. The responses of the frames are discussed in the below. 6.2. Pushover Curve Bare frame pushover curves for the building in X and Y directions as shown in Figure 3. These curves depict the global behavior of the frame in terms of its stiffness and ductility. For bare frame average base shear from pushover analysis is 1024.721 KN and average displacement of 0.171m in X direction, and average base shear from pushover analysis is 2483.06 KN and average displacement of 0.0234m in Y direction. The stiffness and ductility of the frames are more in Y direction as compared to Xdirection. Figure 3: Bare Frame pushover Curve in X and Y direction 1200
800
Base Shear in KN
Base Shear in KN
1000
600
400
200
0 0.00
0.05
0.10
0.15
Displacement in m
0.20
3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600 400 200 0 0.00
0.01
0.02
0.03
0.04
Displacement in m
6.3. Capacity Spectrum Capacity spectrum is the capacity curve transformed from base shear Vs roof displacement coordinates into spectral acceleration Vs spectral displacement (Sa Vs Sd) co-ordinates. The performance point is obtained by superimposing demand spectrum on capacity curve transformed into spectral coordinates. To have desired performance, every structure has to be designed for the spectral acceleration corresponding to the performance point. The performance point is obtained at a base shear level of 987.274KN and displacement of 0.015m in the X direction. In this performance point the large hinges are immediate occupancy level. The performance point is obtained at a base shear level of
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1331.687KN and displacement of 0.0044m in the Y- direction. In this performance point the large hinges are collapsing prevention level. 6.4. Plastic Hinges In X direction plastic hinges formation of the building mechanisms have been obtained at different displacements levels. Plastic hinge formations starts with beam ends and later proceeds to base columns of lower stories, then propagates to upper stories and continue with the yielding of interior intermediate beams. But since yielding occurs at designing events B (yielding), IO (Immediate occupancy), LS (Life safety) and last hinge CP (collapse prevention) respectively, the amount of damage in this direction will be limited. The first hinge formation of base shear and displacements are 659.038KN and 0.0073m. In Y direction Plastic hinge formation starts with beam ends and base columns of lower stories, then propagates to upper stories and continues with yielding of interior intermediate beams. But since yielding occurs in the order B (yielding), IO (Immediate occupancy) is minimized, LS (Life safety) and CP (collapse prevention). The CP is higher as compared to X direction and three hinges of beams have reached C (Ultimate moment) respectively, the amount of damage is larger as compared to X direction. The first hinge formation of base shear and displacements are 598.366KN and 0.0016m.
7. Conclusion The pushover analysis is a simple way to explore the nonlinear behavior of the buildings. The results obtained in terms of pushover demand, capacity spectrum and plastic hinges gave an insight into the real behavior of structures. The existing building in a seismic zone –III is designed and constructed using IS-456-1978 and analyzed as per previous seismic code IS-1893-1984 is found inadequate for revised code IS-1893-2000 provisions. Most of the hinges have developed in the beams in the form of Immediate occupancy, Life safety, Collapse prevention and few in the columns. The column hinges have limited the damage. Some of the beams have reached an ultimate moment in Y direction, hence it cannot be accepted. Therefore beams must be strengthened and improved by the performance of the structures.
References [1] [2] [3] [4]
[5] [6]
ATC 40, (1996), Seismic evaluation and retrofit of concrete buildings Applied Technology Council. FEMA, (1997), NEHRP – Guidelines for the Seismic rehabilitation of buildings, FEMA 273, NEHRP – Commentary on the guidelines for the seismic rehabilitation of buildings, FEMA 274, Federal Emergency Management Agency Washington. D.C. Ashraf Habibullah, S.E., and Stephen S.E. (1998), ‘Practical three dimensional nonlinear static pushover analyses’, Published in structure magazine winter. Charkha, S.D. (2008), ‘Comparative study of static and dynamic analysis of five storey building situated in zone II’, International conference on innovations in building materials, Structural designs and construction practices, Bannari Amman Institute of Technology, Coimbatore, pp. 252. FEMA 356, (2000), Prestandard and commentary for the seismic rehabilitation of buildings, American society of civil engineers, Reston VA. Jaswant, N., Arlekar, and Sudhir Jain, K., Murty C.V.R. (1997), ‘Seismic Response of RC Frame Buildings with Soft First Storeys’, Proceeding of the CBRI Golden Jubilee Conference on National Hazards in Urban Habtat, New Delhi.
Pushover Analysis of Existing Reinforced Concrete Framed Structures [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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Shah. B.A., and Patel, P.V. (2003), ‘Seismic evaluation of RCC framed structures using static pushover analysis’, International conference on recent trends in concrete technology and structures, Kumaraguru College of technology, Coimbatore, pp. 658-665. Kadid, A., and Boumrkik, A. (2008), ‘Pushover analysis of reinforced concrete frame structures’, Asian Journal of Civil Engineering (building and housing), 9, pp.75-83. Mehmet Inel, and Hayri Baytan Ozmen. (2006), ‘Effects of plastic hinge properties in nonlinear analysis of reinforced concrete buildings’, Engineering structures, 28, pp. 1494-1502. Kasim Armagan Korkmaz, Fvat Demir, and Mustafa sivri. (2007), ‘Earthquake assessment of R/C structures with masonry infill walls’, International Journal of Science and Technology, 2(2), pp. 155-164. Khaja Aboul Sattar, Dyamuanal, S.S., and Sathish, A. Annigeri. (2006), ‘Seismic evaluation of asymmetric multistory buildings by pushover analysis’, 13th symposium on earthquake engineering IITR, pp. 715-726. Konuralp Girgin, and Kutlu Darlinmaz. ‘Seismic response of infilled framed buildings using pushover analysis’, The bulletin of the Istanbul Technical University, 54(5). Ramkumar, P., and Baskar, K. (2008), ‘Structural evaluation of RC framed building under various seismic zones’, National conference on failure studies and remedial measures to concrete structures, K.S.R. College of Engineering, Tiruchengode, pp. 124-135. A. Mehar Prasad. (2008), Structural Analysis for Seismic Retrofit, Handbook on Seismic Retrofit of buildings, IITM, pp. 8.1-8.27. Santoshkumar, Dyamuanal, S.S, and Sathish, A., Annigeri. (2006), ‘Performance based Evaluation of symmetric multistoried buildings’, 13th symposium on earthquake engineering IITR, pp. 704-714. IS: 456, (2000), Plain and Reinforced Concrete- code of Practice, Bureau of Indian standards, Manak Bhavan, 9, Bahadur Shah Zafer Marg, New delhi 110002, India. IS: 875, (1987), Code of practice for design loads (other than earthquake) for building and structures-Part 2, Imposed loads Indian Standards, New Delhi. IS: 1893 (Part I), (2002), Criteria for earthquake resistant design of structures (Fifth Revision) Indian Standards, New Delhi.
