International Journal of Engineering Trends and Technology (IJETT) – Volume 10 Number 2 - Apr 2014 Non-Linear Static Pushover Analysis of Real Life Reinforced Concrete Frame with ATENA 3-D Program Beena Kumari 1 1 Research Scholar, Department of Civil Engineering, Thapar University, Patiala, India Abstract— The non-linear static analysis (push over analysis) provides good understanding of seismic behavior of structures. Pushover analysis produces pushover curve which consists of capacity spectrum. In the current paper, the non-linear response of four storeyed RC frame under the pushover loading (monotonically incremental load) has been obtained to study the response and load-carrying capacity of RC frame using non-linear finite element analysis. An analysis model for four storeyed RC frame using software ATENA, a computer program using stress analysis with finite element method, is presented. The results obtained from FE analysis are compared with the experimental data for four storeyed RC frame, at the same location as used in experimental test. The correctness of the finite element model is assessed by the comparison with experimental results which are to be in good agreement. The pushover curves (base shear versus displacement curves) obtained from finite element analysis agree with the experimental results in linear and nonlinear range. Keywords: Finite Element modeling, Finite Element Analysis, Reinforced concrete frame I. INTRODUCTION The concrete has been the most preferred construction material for many decades due to its wonderful properties There are a variety of factors which are responsible for the degradation of RC structures such as increasing load, deterioration of steel due to corrosion, seismic forces, environmental degradations and accidental impacts on the structure. The increase in height of modern structures is intimately related with the extent and impact of disaster in terms of human and economical loss in the occurrence of structural failure. The existing structures can also become seismically deficient since design code requirements are constantly upgraded due to advancement in engineering knowledge. In India most of the buildings constructed over past two decades are seismically inadequate due to lack of knowledge regarding seismic behavior of structures. Hence a careful and systematic structural safety analysis becomes compulsory. To evaluate RC structures against failure an accurate estimation of the ultimate load is necessary and the prediction of the load-deformation behavior of the structure throughout the range of linear and non-linear response is advantageous. To obtain a global mechanism for the structure ISSN: 2231-5381 with a ductile behavior the non-linear analysis is required to be carried out. Nowadays the statically nonlinear method called pushover method, is becoming a popular tool for seismic performance evaluation of new and existing structures. The pushover analysis is a nonlinear static analysis for a reinforced concrete framed structure subjected to lateral loading. The gravity loads are applied and then lateral loading is applied first in Xdirection starting at the end of the gravity push and next in ydirection again starting at the end of gravity push[1]. The pushover analysis provides ample information on seismic demands imposed by the design ground motion on the structural system and its components. The objective of pushover analysis is to evaluate the expected performance of structural systems by estimating performance of a structural system by estimating its strength and deformation demands in design earthquakes by means of static inelastic analysis, and comparing these demands to available capacities at the performance levels of interest. The inelastic static pushover analysis can be viewed as a method for predicting seismic force and deformation demands, which accounts in an approximate manner for the redistribution of internal forces that no longer can be resisted within the elastic range of structural behavior [2]. Fig. 1 Diagram depicting the development of an equivalent SDOF system for a pushover curve (FEMA 440, 2005). While a concrete element undergoes large deformations in the post-yield stage, it is assumed that all the deformation takes place at a point called “plastic hinge”, a typical response at a plastic hinge may be as shown in Fig.2. Here, Point A is the origin; B is the point of yielding; BC represents the strainhardening area; C is the point corresponding to the maximum force; and DE is the post-failure capacity region[3]. http://www.ijettjournal.org Page 97 International Journal of Engineering Trends and Technology (IJETT) – Volume 10 Number 2 - Apr 2014 correct procedure to evaluate the characteristics can be judged by comparing the analytical results with those of the experiments. A Finite Element Modeling in ATENA 3-D The fast development in computer technology has led to the development of computer programs that have implemented finite element analysis where the stresses and strains of the element are computed. Among them is ATENA 3-D.The program has a computing platform based on the graphical user interface through which all the inputs are entered. The computation is based on Lagrange formula. The software is designed for nonlinear analysis for solids, especially for reinforced concrete elements. The properties and the characteristics of the non-linear materials can be Fig. 2 Idealized load deformation curve [3] defined manually or can be selected from the common ones, Mathematically, nonlinear static analysis does not lead to a such as the standardized concrete and steel classes, the values unique solution. Small changes in properties or sequence of are predefined in the program [5]. The program system loading can lead to large variations in the nonlinear response. ATENA offers a variety of material models for different The pushover analysis may be carried out using force control materials and purposes. The most important material models or deformation control. In the first option, the structure is in ATENA for RCC structure are concrete and reinforcement. subjected to an incremental distribution of lateral force, and These highly developed models take into account all the incremental displacements are calculated. In the second option, important aspects of real material behavior in tension and the structure is subjected to a deformation profile, and lateral compression. The program has three main functions: forces needed to generate those displacements are computed. 1) Pre-processing: The geometry of the elements Since the deformation profile is unknown, the first option is including the spatial position of the reinforcement by commonly used. grid coordinates, the nonlinear characteristics for the II. FINITE ELEMENT MODELING To model the complex behavior of reinforced concrete analytically in its non-linear zone is difficult. Due to this engineers in the past had to rely on empirical formulas derived from several experiments for the design of reinforced concrete structures. The finite element method has thus become a foremost computational tool, which allows complex analyses of the nonlinear response of RC structures to be carried out in a usual fashion. It is able to model RC structure easily and is able to calculate the non-linear behavior of the structural members. The combination of the introduction of digital computers and the finite element method have led many efforts to develop analytical solutions which would evade the need for experiments undertaken by researchers. FEM is useful for obtaining the load deflection behavior in various loading conditions but the results of any analytical model have to be verified by comparing them with experiments in the lab or with full-scale structure as experimentation is an excellent tool for the verification purpose and to provide a firm basis for design equations, which are invaluable for design but at the same time it is very difficult and un-economical to perform experiment on full scale structures [4] because conducting these tests are timeconsuming and expensive and often do not simulate exactly the loading and support conditions of the actual structure. The ISSN: 2231-5381 materials, load assignment, bounding conditions for the macro-elements, finite element meshing and various other parameters like loading steps used for the analysis are defined. 2) Analysis: In this part the results can be monitored and accessed in real time, as the analysis enters from one step to another. 3) Post-processing: This function provides access to the results at each loading step expressed graphically and numerically. Modeling in ATENA 3-D is a little bit difficult unlike other programs , but a comprehensive virtual model can be achieved. The precise positioning of the reinforcement in the concrete is possible which gives an idea about the problems that may take place and permits to find solution in the design stage. The areas with a plastic potential are better described and analysed. The potential plastic areas can be characterized directly through stress development until it reaches the yield limit and through crack development for concrete. Based on stresses and non-linear properties of the materials, results for each material can be obtained. http://www.ijettjournal.org Page 98 International Journal of Engineering Trends and Technology (IJETT) – Volume 10 Number 2 - Apr 2014 B Modeling of Concrete In ATENA modeling of concrete is done using 3D solid brick element with 8 up to 20 nodes having three degree of freedom at each node: translations in the nodal x, y and z directions. This is an iso-parametric which is capable of plastic deformation, cracking in three orthogonal directions, and crushing. The most significant feature of this element is the treatment of non-linear material properties. Concrete exhibits a large number of micro-cracks at the interface between coarser aggregates and mortar, even before subjected to any load. The presence of these micro-cracks has a great effect on the behavior of concrete, since their propagation during loading contributes to the nonlinear behavior. Many of these micro-cracks are caused by segregation, shrinkage or thermal expansion of the mortar. Since the aggregate-mortar interface has a significantly lower tensile strength than mortar, it constitutes the weakest linkage in the composite system and is the main cause for the low tensile strength of concrete. The stress-strain relation of the constituent materials and the magnitude of stress mainly affects the response of a structure . The stress-strain relation in compression is of main concern as concrete is used normally in compression. 1) Equivalent uniaxial law: The complete equivalent uniaxial stress-strain diagram for concrete is shown in Fig. 3.The numbers of the diagram parts (Material state numbers) are used in the results of the analysis to indicate the state of damage of concrete. Unloading is a linear function to the origin. Thus, the relation between stress σcef and strain εeq is not unique and depends on a load history. A change from loading to unloading occurs, when the increment of the effective strain changes the sign. If subsequent reloading occurs the linear unloading path is followed until the last loading point U is reached again. Then, the loading function is resumed. The peak values of stress in compression fcef and in tension ftef are calculated according to the biaxial stress state. Thus, the equivalent uni-axial stress-strain law reflects the biaxial stress state[6]. 2) Behavior of cracked concrete: The progressive cracking results in nonlinear response of concrete which results in localized failure. The structural member is cracked at discrete locations where the concrete tensile strength is exceeded. As cracking is the major source of material nonlinearity, realistic cracking models need to be developed in order to accurately predict the load-deformation behavior of reinforced concrete members. The selection of a cracking model depends on the purpose of the finite element analysis. There are two crack models in ATENA-3D, smear crack model and discrete crack model. If overall load-deflection behavior is of primary interest, without much concern for crack patterns and estimation of local stresses, the "smeared" crack model is probably the best choice. If detailed local behavior is of interest, the adoption of a "discrete" crack model might be necessary. ISSN: 2231-5381 Fig. 3 Uniaxial stress-strain law for concrete [6] Fig. 4 Stages of crack openings [6] C Modeling of reinforcement Modeling of reinforcement in finite element is much simpler than the modeling of concrete. Reinforcement modeling could be discrete or smeared. In the present work, a discrete modeling reinforcement has been modeled using bar elements in ATENA. Reinforcement steel is a 3D bar element, which has three degrees of freedom at each node; translations in the nodal x, y and z direction. Bar element is a uniaxial tension-compression element. The stress is assumed to be uniform over the entire element. This element is capable of plastic deformation, also creep, swelling and large deflection capabilities are included in the element. 1) Stress-Strain laws for reinforcement: Reinforcement can be modeled in two different forms: discrete and smeared. Discrete reinforcement is in form of reinforcing bars. In present study discrete approach has been used. The smeared reinforcement is a component of composite material and can be considered either as a single material in the element under consideration or as one of the more such constituents. In both cases the state of uni-axial stress is assumed and the same formulation of stress-strain law is used in all types of reinforcement. The bilinear law, elastic-perfectly plastic, is assumed as shown in Fig. 5. The initial elastic part has the elastic modulus of steel Es. The second line represents the plasticity of the steel with hardening and its slope is the http://www.ijettjournal.org Page 99 International Journal of Engineering Trends and Technology (IJETT) – Volume 10 Number 2 - Apr 2014 hardening modulus Esh. In case of perfect plasticity Esh =0. Limit strain εL represents limited ductility of steel. . Fig. 6 Floor plan , roof plan, and loading pattern [7] Fig. 5 Bilinear stress-strain law for reinforcement [6] III CASE STUDY A full–scale four storeyed structure tested by Reactor Safety Division, Bhabha Atomic Research Centre under lateral monotonically increasing pushover loads at tower testing facility at Central Power Research Institute, Bangalore, is taken as case study. The test was conducted under steadily increasing monotonic lateral load in an inverted triangular pattern till failure. The main objectives of this work was to test a real- life structure under pushover loads. In order to keep the structure as close to reality as possible, no special design for the structure was performed. Fig. 7 (a) Experimental loading pattern and (b) loading, meshing and monitoring points in ATENA B. Summary of material Properties The portion of a real life existing office building which was un-symmetric in plan was intentionally selected so that it had certain eccentricities. Also the column sizes and sections were varied along the storey as in the case of original real life structure [7]. A complete data base of the experiment carried on a full scale real life structure mentioned above has been used for the for the finite element analysis for the validation of analytical results in the shape of Push-over curve, displacements at various floor levels and damage patterns at different loads. In the present study, non-linear response of RCC control frame using FE modeling under the incremental loading has been studied with the intention to investigate the relative importance of several factors in the non-linear finite element analysis of RCC frames using Atena3D. These include the variation in base-shear v/s displacement graph, crack patterns, propagation of the cracks and the crack width on the analytical results. Concrete, reinforcement steel and steel plates have been used to model the RCC frame. The specification and the properties of these materials are as under: 1) Concrete: In ATENA, concrete material is modeled as a 3D nonlinear cementitious2. The physical properties of 3D nonlinear cementitious2 material are given in Table 1. The values are calculated as per IS code 456:2000 and remaining are the default values. 2) Reinforcement Bars: HYSD steel of grade Fe-415 of 28mm, 25mm, 20mm, 16mm and 12mm diameter are used as main steel while 8mm and 10mm diameter bars are used as shear reinforcement. The properties of these bars are shown in Table 1. 3) Steel Plate: The function of the steel plate in the ATENA is for support and for loading. Here, the property of steel plate is same as the reinforcement bar except its yield strength. The HYSD steel of grade Fe-415 is used for steel plate. A. Model Details The structure analyzed is a four-storeyed moment-resisting frame of reinforced concrete with properties as specified above. The concrete floors are modeled as rigid. The plan of the building is shown in the Fig. 6. The load on the structure was applied in an inverted triangular fashion. The reinforcement and ratio of force at “1st floor: 2nd floor: 3rd floor: 4th floor” was kept as “1:2:3:4” as shown in Fig. 7. Properties Elastic Modulus Poisson Ratio Specific weight Compressive Strength ISSN: 2231-5381 Table 1 Material Properties of Concrete and Reinforcement http://www.ijettjournal.org Concrete 2,360 MPa Properties Elastic modulus 0.2 24 KN/M3 Poisson Ratio Specific weight 0.3 785 KN/M3 20 Yield Strength 415 MPa MPa Reinforcement 200000 MPa Page 100 International Journal of Engineering Trends and Technology (IJETT) – Volume 10 Number 2 - Apr 2014 curve is same as per other floor levels. The maximum deflection as base shear 950 KN has been found to be 156 mm which increases up to 345mm, even after without any increase of load. It can be observed from this figure at level 2 that the structure showed the same behavior as that of third and fourth floor level. The maximum deflection at base shear 950 KN has reached to the value of 265mm, after this it started increasing and has reached to the maximum value of 598mm.The displacements at various floor levels for different base shear have been tabulated in Table 2: TABLE 2 COMPARISON OF DISPLACEMENTS AT DIFFERENT BASE SHEAR AT VARIOUS FLOOR LEVELS Fig. 8 Modelling of reinforcement in ATENA Sr. No. IV RESULTS AND DISCUSSIONS All Finite element analysis of RCC frame under the static incremental loads has been performed using ATENA software. Subsequently these results are compared with experimental results of four storey full Scale reinforced concrete structure under monotonic Push-over Loads. This is followed by load deflection curve and the cracking behavior obtained from the analysis. A. Push Over curves at various floor levels The load on the structure has been gradually increased in the steps till failure. The behavior of frame at every step has been studied. Major observations made are discussed as given below: Floor level-4 It can be seen from Fig. 10 that the structure behaved linearly elastic up to the value of base shear around 350 KN. At this point the minor cracks started to get generated at top floor level. After this point there is a slight curvature in the plot and deflection started increasing with the load increments. When the base-shear reached to the value of 500 KN, the graph showed non-linearity in its behavior. It is clear from the figure. that at base shear 750 KN, increase in deflection is more with load increments. At 850 KN base shear deflection has been observed to be 150mm and a rapid increase in displacement is observed. After base shear value of 880 KN there is constant increase in deflections and plot has become almost flat. After the value of base-shear 925 KN deflection started increasing without any significant increment in load; it has reached to the value of 340 mm with the baseshear value 950 KN. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. Base Shear In (KN) 100 150 200 250 300 350 402 452 497 547 601 652 702 752 802 850 882 952 962 947 Deflection at 1th Floor (mm) 0.837 1.29 1.8 2.5 3.06 4.06 5.34 6.66 9.2 13.2 15.2 20.6 23.8 26.4 43.4 74.7 87.4 158 230 346 Deflection at 2th Floor (mm) 1.32 2.02 2.8 3.83 4.7 6.25 8.1 9.99 14.1 19.1 22.5 31.6 37.1 41 69.2 119 139 267 391 598 Deflection at 3th Floor (mm) 1.74 2.67 3.68 4.94 6.05 7.91 10.1 12.6 17.7 23.1 27.3 38.8 45.5 50.2 86.4 142 165 331 488 766 Deflection at 4th Floor (mm) 2.08 3.18 4.5 5.81 7.0 9.15 11.6 14.1 20.2 25.9 30.6 43.1 50.6 55.8 93.4 151 177 345 502 787 B. Comparison between the FE model and the experimental results of the RC frame The pushover curves (Base shear v/s Floor Displacement) as obtained from experimental results for extreme right side column are plotted in Fig. 9. As can be seen from the figure, the maximum displacement has been obtained as 765 mm. The comparison of base shear versus deflections of experimental & FE model results of RC frame are given in Table3. Floor level-3 has been depicted through Fig. 11. It can be observed from this figure that the load-deformation behavior is same as of fourth floor level. The maximum value of deflection has been observed to be 328 mm at the base shear of 950 KN. Floor level-1 and 2 has been depicted through Fig.12-13. It can be seen from the figure that floor level-1 has experienced the minimum deflection even though the damage experienced by this floor is maximum. The variation of load-deflection ISSN: 2231-5381 Fig. 9 Combined Pushover Curve from experimental data [7] http://www.ijettjournal.org Page 101 International Journal of Engineering Trends and Technology (IJETT) – Volume 10 Number 2 - Apr 2014 TABLE3. COMPARISON OF EXPERIMENTAL & FE M ODEL RESULTS AT TOP FLOOR LEVEL Sr. No. Experimental Results Base Shear Deflection FE model Results Base Shear Deflection (KN) (KN) (mm) (mm) 1. 196.2 7 196 4.32 2. 294.3 10 294 6.86 3. 392.4 20 392 10.6 5. 686.7 94 687 49.1 6. 784.8 160 787 84.3 7. 882.9 470 883 177 8. 882.9 765 922 208 9. 392.2 720 950 340 10. -- -- 947 787 The behavior of the frame has been observed to be linear up to the value of base shear around 360 KN and the first crack has been observed, whereas the structure has been found to be linear up to the value of base shear 300 KN in case of experiment. At this point the flexural tension cracks at the base of the columns depicting reduction in stiffness have been observed. At base shear value of approximately 500 KN, the cracks at the base of the columns have been found to open wider and failures at other location like beams and beam – column joints started. Whereas in case of experiment at a base shear value of approximately 500 KN, the cracks at the base of the columns have been observed and failures at beam – column joints start to show up. After reaching the base shear values of 750 KN, displacements have found to be increasing at fast rate whereas in case of experiment at the base shear values of 700 KN, the joints of the frame have found to be displaying rapid degradation and the inter storey drift increasing rapidly. After this stage, at further increase in the lateral load 880 KN, the FE model has been observed to display soft behavior and displacement increased for the same increase in the base shear but experimentally after reaching a base shear of 800 KN, the frame has been found undergoing increasing displacement at almost constant load. Maximum deflection has been found to be more than 900mm at 950 KN. Maximum deflection at fourth level at 880 KN is 770 mm experimentally which is 173 mm in case of FE model. Fig. 10 Pushover Curve at floor level-4 (Experimental & FE model) Fig. 11 Pushover Curve at floor level-3 (Experimental & FE model) Fig. 12 Pushover Curve at floor level-2 (Experimental & FE model) Variety of failures like beam-column joint failure, flexural failures and shear failures have been observed almost in the same way as seen in the case of experiment. Prominent failures shown by both models have been the joint failures. Severe damages have been observed at joints of lower floors, moderate damages at first and second floors and minor damage at top floor has been observed. Fig. 13 Pushover Curve at floor level-1 (Experimental & FE model) ISSN: 2231-5381 http://www.ijettjournal.org Page 102 International Journal of Engineering Trends and Technology (IJETT) – Volume 10 Number 2 - Apr 2014 C. Crack Patterns The first shear crack has been observed at value of base shear 300 KN, when frame is in linear zone. This crack appeared at left side beam-column joint of first storey. At value of base shear 350 KN shear cracks have been observed near beam column joints at1st and 2nd floor level. The beamcolumn joint failures at 1st floor level only along with few flexural cracks in beams at value of base shear 450 KN is observed. The beam- column joint failures can be observed in sides of the frame at 1st, 2nd and at some points of 3rd floor level along with some flexural cracks in beams at value of base shear 625 KN . Lower level floor slab is also showing cracks. At value of base shear 850 KN at 1st, 2nd ,3rd floor level and few points of 4th floor level have experienced the beam- column joint failures along with flexural cracks in beams. The elements of lower two storeys have experienced moderate damage whereas upper storeys experienced minor damage.At value of base shear 962 KN the beam- column joint failures at 1st, 2nd ,3rd and at few points of 4th floor level along with flexural cracks in almost all the flexural members can be visualised from different side views All floor slabs except top level floor slab are showing cracks. Major damage has been noticed in elements of lower two storeys whereas 3rd storey experienced moderate damage. The top storey experienced minor damage. IV CONCLUSIONS The main observations and conclusions drawn are summarized below: 1. The frame behaved linearly elastic initially. Then it depicted non-linearity in its behavior. After this, increase in deflection has been observed to be more with load increments. After reaching the maximum value of base-shear it has kept increasing even at decremented load steps. 2. The joints of the structure have displayed rapid degradation and the inter storey deflections have increased rapidly in non-linear zone. Severe damage has occurred at joints at lower floors whereas moderate damage has been observed in the first and second floors. Minor damage has been observed at roof level. 3. The frame has shown variety of failures like beamcolumn joint failure, flexural failures and shear failures. Prominent failures shown by FE model are joint failures. Flexural failures have been seen in beams due to X-directional loading. 4. The results of FE model of the control frame have found to be higher by 8% of the experimental results. So it can be concluded that FE model push over results holds fairly good with the experimental results though they are on slightly higher side. 5. After comparing experimental and FE Model baseshear v/s deflection curves of the frame at various floor levels the FE model results can be said to be reasonably in close agreement with the experimental data from the full-scale frame test. Fig. 14 Crack patterns in columns, beams and slab at various loads REFERENCES [1]. Valles, R.E., Reinhorn, A.M., Kunnath, S.K., Li, C. and Madan, A. “IDARC2D Version 4.0:A Computer Program for the Inelastic Analysis of Buildings”, Technical Report NCEER 96-0010,State University of New York at Buffalo, Buffalo, U.S.A, 1996. [2]. Habibullah, A., Pyle, S., “Practical Three Dimensional Non-linear Static Push-Over Analysis” Structure magazine, winter 1998. [3]. Lakshman N., “Seismic evaluation and retrofitting of buildings and structures” Journal of Earthquake Technology, Paper No. 469, Vol. 43, No. 1-2, pp. 31-48,2006. [4]. 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FEMA 440, ATC 55 project, 2005. ISSN: 2231-5381 http://www.ijettjournal.org Page 104