PERFORMANCE ASSESMENT OF RC BUILDING FRAMES BY
International Journal of Scientific & Engineering Research, Volume 7, Issue 10, October-2016ISSN 2229-5518234PERFORMANCE ASSESMENT OF RCBUILDING FRAMES BY NON LINEARANALYSISDain Thomas, Sruthi K ChandranAbstract:Recent earthquakes in which many concrete structures have been severely damaged or collapsed, haveindicated the need for evaluating the seismic adequacy of existing buildings. About 60% of the land area of our country issusceptible to damaging levels of seismic hazard. One of the procedures is the nonlinear static pushover analysis whichis becoming a popular tool for seismic performance evaluation of existing and new structures. By conducting thispushover analysis, we can know the weak zones in the structure and then we will decide whether the particular part is tobe retrofitted or rehabilitated according to the requirement. In the present study the push over analysis is performed onRC building frames by changing the footing, infill wall, aspect ratio and introduction of bracings on the SAP2000 (version14). These four features have the capacity to increase the seismic performance of the building and the bracings can beused for retrofitting for buildings which are structurally weakIndex Terms— ASPECT RATIO, PUSHOVER ANLAYSISS, BRACINGS, RC BUILDING FRAME—————————— ——————————its limitations, weaknesses and the accuracy of itspredictions in routine application should be identified byonlinear static analysis, or pushover analysis, hasstudying the factors affecting the pushover predictions. Inbeen developed over the past twenty years and hasother words, the applicability of pushover analysis inbecome the preferred analysis procedure for designpredicting seismic demands should be investigated forand seismic performance evaluation purposes as thelow, mid and high-rise structures by identifying certainprocedure is relatively simple and considers post elasticissues such as modeling nonlinear member behavior,behavior. However, the procedure involves certaincomputational scheme of the procedure, variations in theapproximations and simplifications that some amount ofpredictions of various lateralvariation is always expected to exist in Although, inliterature, pushover analysis has been shown to captureessential structural response characteristics under seismicload patterns utilized in traditional pushover analysis,action, the accuracy and the reliability of pushoverefficiency of invariant lateral load patterns in representinganalysis in predicting global and local seismic demandshigher mode effects and accurate estimation of targetfor all structures have been a subject of discussion anddisplacement at which seismic demand prediction ofimproved pushover procedures have been proposed topushover procedure is performed.overcome the certain limitations of traditional pushoverprocedures. However, the improved procedures areHorizontal strength has constantly been a mainmostly computationally demanding and conceptuallyissue of structures generally in the areas with highcomplex that use of such procedures are impractical inearthquake vulnerability, later this problem has beenengineering profession and codes.examined and eccentric, concentric and knee bracing1 �————— Dain Thomasis currently pursuing master of technology at SreepathyInstitute of Management and Technology, Vavanoor, India.PH09400682216 E-mail: dain1193@gmail.com Sruthi.K.Chandranis currently the associate professor at SreepathyInstitute of Management and Technology, Vavanoor, India PH09746709319 E-mail: sruthi.k@simat.ac.inAs traditional pushover analysis is widely usedfor design and seismic performance evaluation purposes,systems have been proposed and as a result these systemswere implemented by the civil engineers. Inelasticperformance is one of the main issues impelling the choiceof bracing systems. The bracing system that has a moreplastic distortion before collapse can consume moreenergy during the earthquake. The scope of this study isthat Special moment RC building frames are commonlyconstructed in earthquake prone countries like India sincethey provide much higher ductility. Failures observed inpast earthquakes show that the collapse of such buildingsis predominantly due to the formation of soft-storymechanism in the ground story columns. The study onlydeals with the RC framed buildings. The studies herecontains the two different types of support condition thatIJSER 2016http://www.ijser.org
International Journal of Scientific & Engineering Research, Volume 7, Issue 10, October-2016ISSN 2229-5518is fixed and hinged supports. The base of the column isfixed and the soil structure interaction is ignored. The bareframe, weak infill and strong infill wall are only usedhere. The steel bracings of ISMB100 is used as thebracings.Figure.1 - Construction of pushover curve2 LITERATURE REVIEW235to other model considered. When evaluating the postelastic behavior of the bare frame with infill frames, it wasfound that the infill frame is showing better performancein terms of resisting base shear but weak in exhibiting theductile behavior since the open ground floor exhibitingthe soft story effect.Helmut Krawinkleret al., [3] studied the pros andcons of Pushover analysis and suggested that elementbehavior cannot be evaluated in the context of presentlyemployed global system quality factors such as the R andRw factors used in present US seismic codes. They alsosuggested that a carefully performed pushover analysiswill provide insight into structural aspects that controlperformance during severe earthquakes. For structuresthat vibrate primarily in the fundamental mode, thepushover analysis will very likely provide good estimatesof global, as well as local inelastic, deformation demands.This analysis will also expose design weaknesses that mayremain hidden in an elastic analysis. Such weaknessesinclude story mechanisms, excessive deformationdemands, strength irregularities and overloads onpotentially brittle elements such as columns andconnections.Oğuz, Sermin [4], ascertained the effects and theaccuracy of invariant lateral load patterns utilized inpushover analysis to predict the behavior imposed on thestructure due to randomly Selected individual groundmotions causing elastic deformation by studying variouslevels of Nonlinear response. For this purpose, pushoveranalyses using various invariant lateral load patterns andModal Pushover Analysis were performed on reinforcedconcrete and steel moment resisting frames covering abroad range of fundamental periods. The accuracy ofapproximate Procedures utilized to estimate targetdisplacement was also studied on frame structures.Pushover analyses were performed by both DRAIN-2DXand SAP2000. The primary observations from the studyshowed that the accuracy of the pushover resultsdepended strongly. On the load path, the characteristics ofthe ground motion and the properties of the structure.IJSERThe current study by Howard H.M Hwang [1] to achievemore stringent acceptable risk level required for high riskand essential buildings the important factor is to increasestructural strength and stiffness. Therefore the seismicLFRD criteria developed in this study are applicable tothree categories of building in different zones of America.The proposed seismic LRFD criteria will produce riskconsistent structures under various design conditions,because seismic-load factors and importance factors aredetermined from optimization. Two types of limit states,first yielding and collapse of a structure are considered.The study concludes that the collapse limit state controlsthe design and evaluation of buildings. It implies that ifthe design satisfies the requirement for life safety in theevent of a large earthquake, it will also satisfy therequirement for no structural damage in the event of amoderate earthquake. This is especially true for buildingsin eastern United States, where large earthquakes areinfrequent.The study was conducted by Pranamya [2]shaped a comparative study on an existing RC framestructure with and without considering infill stiffness forthe 2D and 3D models in which the slabs were modelledas membrane with rigid diaphragms, membrane withsemi rigid diaphragms, shell element and without anydiaphragms using static nonlinear pushover analysis.Infill frames modelled based on equivalent strutapproach. Non-linear pushover analysis was performedconsidering moderate seismic zone (ZONE-III) of India.From the analysis result, it was found that the bare frameand the infill frame with slab modelled as a shell elementshowed better performance in resisting base shear butrelatively weak in showing the ductile behavior comparedMehmet et al., [5], explained that due to itssimplicity of Pushover analysis, the structural engineeringprofession has been using the nonlinear static procedureor pushover analysis. Pushover analysis is carried out fordifferent nonlinear hinge properties available in someprograms based on the FEMA-356 and ATC-40 guidelinesand he pointed out that Plastic hinge length (LP) hasconsiderable effects on the displacement capacity of theframes. The orientation and the axial load level of thecolumns cannot be taken into account properly by thedefault-hinge properties.Vijayakumar and Babu, [6] estimated thebehavior of G 2 reinforced concrete bare frame subjectedto earthquake forces in zone III. The reinforced concretestructures were analyzed by nonlinear static analysis(Pushover Analysis) using SAP2000 software. It wasfound that the pushover analysis is a simple way toIJSER 2016http://www.ijser.org
International Journal of Scientific & Engineering Research, Volume 7, Issue 10, October-2016ISSN 2229-5518236explore the nonlinear behavior of the buildings. Theresults obtained in terms of pushover demand, capacityspectrum and plastic hinges gave an insight into the realbehavior of structures. The existing building designed andconstructed using IS-456-1978 and analyzed as per IS1893-1984 and is found inadequate in code IS-1893-2000provisions. Most of the hinges have developed in thebeams in the form of immediate occupancy, Life safety,Collapse prevention and few in the columns. The columnhinges have limited the damage. Some of the beams havereached ultimate moments which have to be strengthenedand improved by the performance of the structures.3 PUSH OVER ANALYSISLinear elastic analysis gives a good indication of elasticcapacity of structures and indicates where the firstyielding will occur but it cannot predict failuremechanisms and accounts for redistribution of forces dueto progressive yielding. Among different approachesdescribed in ATC-40, Nonlinear Static Pushover analysisis very popular because of its simplicity and ability toestimate component and system level deformationdemands with acceptable accuracy without intensivecomputational and modeling effort as dynamic analysis.Pushover analysis is a static, nonlinear procedure inwhich the magnitude of the structural loading isincrementally increased in accordance with a certainpredefined pattern. Pushover analysis may be categorizedas displacement controlled pushover analysis when lateralmovement is executed on the building and its equilibriumdesignates the forces. In the same way, when lateral forcesare enforced, the analysis is termed as force-controlledpushover analysis. The target displacement or target forceis projected to signify the maximum displacement ormaximum force expected to be qualified by the structureduring the design earthquake. Response of structurebeyond full strength can be bent on only by displacementcontrolled pushover analysis. Hence, in the present study,displacement-controlled pushover method is used foranalysis of structural steel frames. A plot of the total baseshear versus top roof displacement in a building isattained by this analysis that would specify any earlyfailure or weakness. The analysis is performed up tofailure, thus it permits purpose of collapse load andductility capacity.Figure.2 - Performance Levels Described By PushoverAnalysisA typical pushover curve is shown in Figure 2. Forceversus displacement is plotted for gradually increasinglateral loads till failure. Beyond elastic limit, differentstates such as Immediate Occupancy (IO), Life Safety (LS),Collapse prevention (CP), E collapse are defined as perATC 40 and FEMA 356.Immediate occupancy IO: damage is relatively limited; thestructure retains a significant portion of its originalstiffness.Life safety level LS: substantial damage has occurred tothe structure, and it may have lost a significant amount ofits original stiffness. However, a substantial marginremains for additional lateral deformation before collapsewould occur.Collapse prevention CP: at this level the building hasexperienced extreme damage, if laterally deformedbeyond this point, the structure can experience instabilityand collapse.IJSER4 DESCRIPTION OF THE STRUCTUREIn the present study a three dimensional reinforcedconcrete building frames for the analysis of the footingsand the infill wall and a two dimensional reinforcedconcrete building frames are used for the aspect ratio andthe steel bracings. The building consists of G 9 stories.For simplicity all columns are assumed to be fixed at thebase. The height of each floor is 3.0m. The sizes used forbeam is 250 x 300, column is 300 x 300 and that of bracingis ISMB100. 2- Bay two dimensional steel frame structureswith and without bracing systems with different aspectratios ranging from 2.0 to 4.5 has been modelled andanalyzed using SAP2000. Bracings considered for thestudy are X braced, V bracings Inverted V bracings, ZXbracings, and Zipper bracings. Live load on floor is takenas 3kN/m2 and on roof is 1.5kN/m2. Floor finish on thefloor is 1kN/m2. Weathering course on roof is 2kN/m2.In the seismic weight calculation only 25% of floor liveload is considered. The building is steel moment resistingframe considered to be situated in seismic zone V. Themedium type of soil is considered in the analysis. . TheIJSER 2016http://www.ijser.org
International Journal of Scientific & Engineering Research, Volume 7, Issue 10, October-2016ISSN 2229-5518code used for assigning earth quake is IS 1893 2002. Thestructure is assumed to be at zone V to get the maximumresponse of the structure due to earth quake. Consideringall the aspect ratios.i. Bare Frameii. One Bay X Braced FrameIii.Two Bay X Braced FrameIv. One Bay V Braced Framev. Two bay V braced framevi. One bay ZX braced framevii. Two bay ZX braced frameviii. One bay inverted v bracingix. Two bay inverted v bracingx. One bay zipper bracingxi. Two bay zipper bracing237Figure.4–Push over curveFigure.5–Capacity curve5 RESULTS AND DISCUSSIONSThe results of this study can be mainly included threesections.SUPPORT CONDITIONIn this study we considered only two support conditionsthat’s fixed and hinged support condition.IJSERFigure.6–Capacity tableFigure.3 - Both support conditionHere in this analysis the fixed support and the hingesupport almost same values. On the pushover analysiscurve the performance point are all most same as 1.41 and1.38. Hence the two support effect must be negligiblewhile considering the effect on the RC building frames.INFILL WALLFor considering the effect of infill wall on the building thestructure must be undergone design. For that theprocedure as same as the pushover analysis. Then theframe subject to the design. We consider three types offrames of bare frame, weak infill wall and strong infillwall. Here in this analysis the strong infill wall can able toreduce 50% of the failure and the weak infill wall canreduce 10% of the failure than the bare frame.IJSER 2016http://www.ijser.org
International Journal of Scientific & Engineering Research, Volume 7, Issue 10, October-2016ISSN 2229-5518238bracings. About 9 type of the bracing styles are used inthis analysis.Figure.7–Bare frame resultFigure.11 – Roof displacementIJSERFigure.8–Weak infill wall resultFigure.12 – Base shear6SUMMARY AND CONCLUSIONSFrom my analysis I found that the support condition doesnot have any influence in the structure and the infill wallhas certain importance in the structure. On the bare framethe failure will be very high. But on using the infill wallthe percentage of the failure is reduced gradually. On theweak infill wall the 10% of reduction is done to the failureof the concrete frames. By the analysis of the strong infillwall there is a 50% reduction in the failure of the concreteframes. This shows the importance of the infill wall on theconcrete frame performance. Use of the bracings increasesthe structural adequacy of the building. It increases theperformance of building, significant change or reductionin roof displacement is shown when bracings are used.Usage of bracings increase the base shear capacity ofbuildings. The buildings which failed during seismicanalysis (bare frame) shown structural adequacy whenbracings are introduced. X and Zipper Bracings shownaround 80- 90% increase in stability of buildings.Normally zipper bracing are not used for retrofittingpurposes as it hard to find window or door openings forbuildings. X and V bracings are commonly used forretrofitting purposes.Figure.9 – Strong Infill WallFigure.10 – Failure7FUTURE SCOPE OF THE STUDYBRACED FRAME WITH ASPECT RATIOThe aspect ratio from the 2 to 4 is considered for theanalysis purpose. The ISMB100 is used as the steelThe scope of this study is that Special moment RCbuilding frames are commonly constructed in earthquakeprone countries like India since they provide much higherIJSER 2016http://www.ijser.org
International Journal of Scientific & Engineering Research, Volume 7, Issue 10, October-2016ISSN 2229-5518ductility. Failures observed in past earthquakes show thatthe collapse of such buildings is predominantly due to theformation of soft-story mechanism in the ground storycolumns. The study only deals with the RC framedbuildings. The studies here contains the two differenttypes of support condition that is fixed and hingedsupports. The base of the column is fixed and the soilstructure interaction is ignored. The bare frame, weakinfill and strong infill wall are only used here. The steelbracings of ISMB100 is used as the bracings.8 REFERENCES[1] IS 875 Part 1, 2, 3 and 4, (1987),”Indian Standard Codeof practice for Design loads for Buildings andstructures”, Bureau of Indian Standards New Delhi.[2]. IS: 456 (Fourth Revision),(2000),”Indian standard codefor practice for plain reinforced Concrete for generalbuilding construction”, Bureau of Indian StandardsNew Delhi.[3].IS 1893 Part 1, (2002),”Indian Standard Criteria forEarthquake Resistant Design of Structures”, Bureau ofIndian Standards New Delhi[4].Hazelton, C.B. and G.G. Deierlein (2007). “AssessingSeismic Collapse Safety of Modern ReinforcedConcrete Frame Buildings”, Blume EarthquakeEngineering Research Center Technical Report No. 156,Stanford University, 313 pp.[5].Deodhar, S. V. and A. N. Patel (1998) Ultimate strengthof masonry in filled steel frames under horizontal load”,Journal of Structural Engineering. StructuralEngineering Research Centre. 24.237-241.[6].Das, S. (2000),”Seismic design of vertically irregularreinforced concrete structures”, Ph.D. Thesis, NorthCarolina State University”, Raleigh. NCIJSERIJSER 2016http://www.ijser.org239
RC building frames by changing the footing, infill wall, aspect ratio and introduction of bracings on the SAP2000 (version 14). These four features have the capacity to increase the seismic performance of the building and the bracings can be . is predominantly due to the formation of soft-story mechanism in the ground story columns. The study .
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