PERFORMANCE ASSESSMENT OF DESIGNED OMRF & SMRF FRAME

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1 e-issn Volume 2 Issue 6, June 2016 pp Scientific Journal Impact Factor : PERFORMANCE ASSESSMENT OF DESIGNED OMRF & SMRF FRAME SONALI C. PATIL 1, SANJAY BHADKE 2 1 PG Student, 2 Assistant Professor, 1,2 Department of Civil Engineering TGPCET, Nagpur, India Abstract This paper investigates seismic performance and vulnerability analysis of 4-storey and 6-storey code-conforming (IS: , Indian standard for plain and reinforced concrete code and IS: , Indian standard criteria for earthquake resistant design of structures) reinforced concrete (RC) buildings. The buildings are designed for two different cases such as ordinary moment resisting frame (OMRF) and special moment resisting frame (SMRF). The nonlinear static analysis (pushover analysis) is used to capture initial yielding and gradual progressive plastic behavior of elements and overall building response under seismic excitations. The deformation characteristics of structural elements are essential to simulate the plastic hinge formation in the process of generation of capacity curve during the pushover analysis. An analytical procedure is developed to evaluate the yield, plastic and ultimate rotation capacities of beams and columns along with different plastic hinge lengths. In the present study, user defined plastic hinge properties of beams and columns are modeled using analytical expressions developed based on Eurocode 8 and incorporated the same in pushover analysis using SAP2000. Keywords Plastic hinge length, Seismic performance, nonlinear static analysis, Pushover analysis, SMRF, OMRF I. INTRODUCTION The nonlinear static analysis, to evaluate the seismic performance of buildings, represents the current trend in structural engineering and promises a reasonable prediction of structural behavior. The analysis provides adequate information on seismic demands imposed by the design ground motion on the structural system and its components. Earthquake is a phenomenon related to violent shaking that takes place underneath the earth. Massive strain energy discharged at the time of an earthquake and travels as unstable waves called as seismic waves in every directions through the Earth s layers, which refracting and reflecting at every interface. The destruction to structures because of earthquake depends on the stuff that the structure is formed out of, the sort of earthquake wave (motion) that is distressing the structure, and also the ground on that the structure is constructed. Therefore the dynamic loading which acts on the structure throughout an earthquake is not only external loading, but also inertial effect caused by motion of support. The different factor that causes damage to the structure throughout earthquake is mass irregularity, vertical irregularities, torsional irregularity, irregularity in strength and stiffness, etc. In multi-storied RC framed buildings, destruction from earthquake ground motion usually starts at locations of structural weaknesses there in buildings. In some of the cases, these weaknesses are also developed by discontinuities in stiffness, strength or mass between adjacent stories. Over the past decades it has been recognized that destruction control has become a more specific design consideration which will also be carried out most effectively, by the way of introducing some kind of nonlinear analysis into the seismic design methodology. Following this pushover analysis has been developed during past years and has end up with the preferred method of analysis for performance-based seismic design (PBSD). It is the approach by which the ultimate strength and the limit state can be quite simply investigated after yielding, which has been researched and utilized in practice for earthquake engineering and seismic All rights Reserved 397

2 II. BUILDING PERFORMANCE LEVEL Building Performance Level A performance level describes a limiting damage condition which may be considered satisfactory for a given building and a given ground motion. The limiting condition is described by the physical damage within the building, the threat to life safety of the building s occupants created by the damage, and the post-earthquake serviceability of the building. ATC-40 describes standard performance levels for structural performance as: a. Operational b. Immediate Occupancy (IO): very limited structural damage has occurred. The risk of lifethreatening injury from structural failure is negligible, and the building should be safe for unlimited egress, ingress, and occupancy. c. Damage Control: a range of IO and Life Safety (LS). It limits the structural damage beyond the Life Safety level, but occupancy is not the issue. E.g. the protection of significant architectural features of historic buildings or valuable contents d. Life Safety (LS): the injuries during the earthquake may occur; the risk of life-threatening injury from structural damage is very low. e. Limited Safety: a range of LS and Collapse Prevention (CP), Structural Stability. Some critical structural deficiencies are mitigated. f. Structural Stability or Collapse Prevention (CP): Substantial damage to the structure has occurred, including stiffness and strength of the lateral force resisting system. However, all significant components of the gravity load resisting system continue to carry their gravity demands. g. Not considered: The performance level of a building is determined based upon its function and importance. Public building is expected to have a performance level of operational or immediate occupancy. A residential building must have a performance level of damage control or life safety. For the temporary structure is 16 under the structural stability or not considered. The force deformation relationship as well as the structural performance levels is given in Figure 1. Where: A = the origin B = yielding IO = immediate occupancy LS = life safety CP = collapse prevention C = ultimate capacity D = residual strength E = total failure Fig 1: Force-deformation curve Five points labeled A, B, C, D, and E are used to define the force deflection behavior of the hinge. Three points labeled IO, LS, and CP are used to define the acceptance criteria for the All rights Reserved 398

3 III. OBJECTIVE OF THE WORK The main objectives of this work includes the following, 1) To determine the response of 7 storey RC frame structure i.e., base shear and lateral displacement by Equivalent static lateral force method and performance point by pushover analysis. Modeling and analysis are achieved using SAP ) Equivalent static lateral force method is conducted for zone-ii and zone-v according to IS (Part 1) for soft soil type (type III). 3) All the models are studied and analyzed using pushover analysis. IV. PARAMETRIC STUDY A reinforced concrete frame with 7(G+6) storey of dimension 25mx12m, has been taken for seismic analysis. a) Using equivalent static lateral force method for zone-iv for soil type-iii (soft soil) as per IS 1893(part 1):2002. b) Using Pushover analysis. V. METHODOFANALYSIS The study undertakes the following analysis Equivalent Static Lateral Force Method (ESLM). Pushover Analysis. VI. Structure type Plan dimension Storey height Height of building Grade of concrete DESCRIPTIONS OF BUILDING Description of building Ordinary Moment Resisting Frame[OMRF] & Special Moment Resisting Frame[SMRF] 12x25m 3m G+6=7 storeys M25(beams and slabs) & M25 (columns) Grade of steel Fe415 Beam sizes B1-230 mm X 350 mm, B2-230 mm X 300 mm column sizes C1-230 mm X 400 mm C2-400 mm X 400 mm slab thickness 150mm Live load 2.0kN/m2 Floor finish 1.0kN/m2 Zone factor IV Soil type Soft soil Importance factor 1 Response reduction factor 3.0(OMRF) and All rights Reserved 399

4 VII. Fig 2: plan of building NONLINEAR STATIC PUSHOVER ANALYSIS Analysis methods are broadly classified as linear static, linear dynamic, nonlinear static and nonlinear dynamic analysis. In these the first two is suitable only when the structural loads are small and at no point the load will reach to collapse load. During earthquake loads the structural loading will reach to collapse load and the material stresses will be above yield stresses. So in this case material nonlinearity and geometrical nonlinearity should be incorporated into the analysis to get better result. Nonlinear static pushover analysis or Push-over analysis is a technique by which a computer model of the building is subjected to a lateral load of a certain shape (i.e., parabolic, triangular or uniform). The intensity of the lateral load is slowly increased and the sequence of cracks, yielding, plastic hinge formations, and failure of various structural components is recorded. In the structural design process a series of iterations are usually required during which, the structural deficiencies observed in iteration is rectified and followed by another. This iterative analysis and design procedure continues until the design satisfies pre-established performance criteria. In the other hand, static pushover analysis evaluates the real strength of the structure so that it will be useful and effective for performance based design. This method is considered as a step forward from the use of linear analysis, because they are based on a more accurate estimate of the distributed yielding within a structure, rather than an assumed, uniform ductility. The generation of the pushover curve also provides the nonlinear behavior of a structure under lateral load. However, it is important to remember that pushover methods have no rigorous theoretical basis, and may be inaccurate if the assumed load distribution is incorrect. For example, the use of a load pattern based on the fundamental mode shape may be inaccurate if higher modes are significant, and the use of any fixed load pattern may be unrealistic if yielding is not uniformly distributed, so that the stiffness profile changes as the structural All rights Reserved 400

5 This analysis provides data on the strength and ductility of the structure which otherwise cannot be predicted. Base shear versus top displacement curve of the structure, called pushover curves, are essential outcomes of pushover analysis. These curves are useful in ascertaining whether a structure is capable of sustaining certain level of seismic load. The basic steps of POA are: 1. Assume the nonlinear force-displacement relationship of individual elements of structure (including yield strength, post yield stiffness and stiffness degradation, etc) 2. Calculate the target displacement of structure 3. Select a reasonable lateral load pattern, and pushing the structure under this load pattern which is monotonically increasing step by step, when a structural member yields, then its stiffness is modified, until the roof displacement of structure is up to the target displacement or the structure collapses. At this time, the evaluation of seismic performance of structure is obtained. The main output of a pushover analysis is in term of response demand versus capacity as shown in Figure Demand and capacity is mutually dependent. As displacements increase, the period of the structure lengthens. This is reflected directly in the capacity spectrum. Inelastic displacements increase damping and reduce demand. The capacity spectrum method reduces demand to find an intersection with the capacity spectrum where the displacement is consistent with the implied damping. The intersection between capacity and demand curve develop the performance point. At the performance point, capacity and demand are equal. The displacement of the performance point is the target displacement. Fig 3: Demand vs Capacity Curve Target Displacement and Lateral Load Pattern The target displacement and lateral load pattern is very important for POA to evaluate the seismic performance of structures. The target displacement is intended to represent the maximum displacement likely to be experienced during the design earthquake. The load patterns are intended to represent and bound the distribution of inertia forces in a design earthquake. One procedure for evaluating the target displacement as per FEMA 356 is given by the following All rights Reserved 401

6 BaseForce KN International Journal of Current Trends in Engineering & Research (IJCTER) VIII. RESULTS & DISCUSSIONS BEHAVIOR OF OMRF & SMRF STRUCTURAL SYSTEM The behavior of OMRF & SMRF is taken as a basic study on the structures. The later forces resisting system is done for each building categorized based on lateral loads, lateral drifts, orientation of the shear wall & material quantity in terms of steel reinforcement alone. The modeled frame is a multi storied structure with a 12 m x 25 m (rectangular plan) and area of 300 sqm which have a bay of 3m x 6 m.lateral forces considered in seismic area Lateral drift/deflections are checked against the requirements of clause of IS i.e. under transient seismic load. Deflections are discussed below for the OMRF &SMRF structural system. PLASTIC HINGE LENGTH Plastic hinges form at the maximum moment regions of RC members. The accurate assessment of plastic hinge length is important in relating the structural level response to member level response. The length of plastic hinge depends on many factors. The following is a list of important factors that influence the length of a plastic hinge 1) level of axial load 2) moment gradient 3) level of shear stress in the plastic hinge region 4) mechanical properties of longitudinal and transverse reinforcement 5) concrete strength and 6) level of confinement and its effectiveness in the potential hinge region. The roof displacement obtained in this study obviously show that the demands of 4-storey buildings are higher than those of 6-storey ones. Therefore, it is difficult to precisely estimate which building group is more vulnerable during a seismic event. However SMRF building shows higher capacity compared to OMRF. The study also reveals that the amount of transverse reinforcement plays an important role in seismic performance of buildings, as the amount of transverse reinforcement increases the sustained damage decreases. A profound variation in capacity and displacement are brought out by varying the plastic hinge length and designing the building as OMRF and SMRF. Table 1 shows the inelastic response displacements of the frame. It is observed that inelastic displacement of all the frames are within collapse prevention Displacement m Fig.4 Capacity curves of six storey All rights Reserved 402

7 Base Shear KN International Journal of Current Trends in Engineering & Research (IJCTER) Displacement m Fig.4 Capacity curves of six storey SMRF OMRF SMRF Item Value Item Value C C C1 1 C1 1 C2 1 C2 1 Sa Sa Te Te Ti Ti Ki Ki Ke Ke Alpha Alpha R R Vy Vy Dy Dy Weight Weight Cm 1 Cm 1 IX. CONCLUSIONS This study has illustrated the nonlinear static analysis responses of OMRF and SMRF building frames under designed ground motions. The capacity against demand is observed significantly higher for SMRF building frames compared to OMRF. The user defined hinge definition and development methodology is also described. The user- defined hinges takes into account the orientation and axial load level of the columns compared to the default hinge. The influence of plastic hinge on capacity curve is brought out by deploying five cases of plastic hinge length. The study reveals that plastic hinge length has considerable effects on the displacement capacity of frames. Based on the analysis results it is observed that inelastic displacement of the modern code-conforming building frames are within collapse prevention level. The vulnerability index which is a measure of damage is estimated for both SMRF and OMRF are presented for 6-storey buildings. From the study it is apparent that, the OMRF framed buildings are more vulnerable than SMRF. The vulnerability index of the building quantitatively express the vulnerability of the building as such, where as storey vulnerability All rights Reserved 403

8 assist to locate the columns in the particular storey in which significant, slight or moderate level of damages have taken place. REFERENCES [1]. Dr. Mohd. Hamraj Performance Based Pushover Analysis of R.C.C Frames for Plan Irregularity International Journal of Science, Engineering and Technology Volume 2, Issue 7, Sep-Oct [2]. Gayathri.H, Dr.H.Eramma, C.M.RaviKumar, and Madhukaran A Comparative Study On Seismic Performance Evaluation Of Irregular Buildings With Moment Resisting Frames And Dual Systems International Journal of Advanced Technology in Engineering and Science, Volume No.02, Issue No. 09, September [3]. Mr. Gururaj B. Katti and Dr. Basavraj S. Balapgol Seismic Analysis of Multistoried RCC Buildings Due to Mass Irregularity by Time History Analysis International Journal of Engineering Research & Technology (IJERT) ISSN: Vol. 3 Issue 7, July [4]. Santhosh.D Pushover analysis of RC frame structure using ETABS IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE) e-issn: , p-issn: X, Volume 11, Issue 1 Ver. V (Feb. 2014), PP [5]. MohommedAnwaruddinMd. Akberuddin and Mohd. ZameeruddinMohd. Saleemuddin Pushover Analysis of Medium Rise Multi-Story RCC Frame With and Without Vertical Irregularity Int. Journal of Engineering Research and Applications Vol. 3, Issue 5, Sep-Oct 2013, pp [6]. N.K. ManjulaPraveen Nagarajan and T.M. MadhavanPillai A Comparison of Basic Pushover Methods International Refereed Journal of Engineering and Science (IRJES) ISSN (Online) X, (Print) ,Volume 2, Issue 5(May 2013), PP [7]. Ankesh Sharma and BiswobhanuBhadra (2013) seismic analysis and design of vertically irregular rc building frames [8]. C.M. Ravi Kumar, Babu Narayan, M.H. Prashanth, H.B Manjunatha and D. Venkat Reddy Seismic Performance Evaluation OfRc Buildings With Vertical Irregularity ISET GOLDEN JUBILEE SYMPOSIUM Indian Society of Earthquake Technology Department of Earthquake Engineering Building IIT Roorkee, RoorkeeOctober 20-21, 2012, Paper No. All rights Reserved 404