Paper cv-105-performance of masonry infill in rc frame structures
2nd International Conference on Emerging Trends in Engineering & Technology, April 12-13, 2013College of Engineering, Teerthanker Mahaveer University1Performance of Masonry Infill in RC Frame StructuresMohd Danish1, Shoeb Masood2, Zaid Mohammad1The weakness in structures is exposed by earthquake event. An earthquake force is a very peculiar force andbehaves quite differently than other types of loads, such as gravity and wind loads. It strikes the weakest spot in thewhole three dimensional building. This should be an eye opener for designers and builders. Due to ignorance indesign and poor quality of constructions, results many weaknesses in the structure to cause serious damage to lifeand property. Masonry infill are used to fill the gap between the vertical and horizontal resisting elements ofbuilding frames, assuming that these infill will not take part in resisting any kind of load either axial or lateral.Hence, its significance in the analysis of frames is generally neglected. In fact, an infill wall considerablyenhances the rigidity and strength of the frame structure. It has been observed through various researches, that theframe considering no infill has comparatively lesser stiffness and strength than the infill frame and therefore theirignorance cause failure of many multi-storey buildings when subjected to seismic loads.In the present study, the finite element analysis of 6-Storey R.C frames with and without infill (i.e. bare frame) hasbeen carried out. Linear analysis and design of all RC frame structures has been performed as per IS: 1893 (Part 1)- 2002 and IS: 456 - 2000. In this study only in-plane stiffness of masonry has been considered. Infill panels havebeen modelled as equivalent diagonal strut elements. The behaviour of buildings has been studied with the help ofResponse Spectrum Analysis (SRSS Method) using FEM based software. The behaviour of these buildings underGravity and Seismic loads has been observed. A significant reduction in storey drift has been observed when infillis considered. Strength and Rigidity of RC frame structures considering infill has been observed to be increased.Keywords: Masonry infill, Equivalent Diagonal Strut Elements, Seismic loads.1. IntroductionAn earthquake force is a very peculiar force andbehaves quite differently than other types of loads, suchas, gravity and wind loads. It strikes the weakest spot inthe whole three dimensional building. This should bean eye opener for designers and builders. Ignorance indesign and poor quality of construction results manyweaknesses in the structure to cause serious damage tolife and property. One of the examples, which shookthe country on 26th January 2001, is Bhuj Earthquake,which caused thousands of casualties with over300,000 buildings collapsed.Masonry in-fill is the integral part of R.C.C framestructure and some steel structures. Masonry in-fill arefrequently used to fill the void between the vertical andhorizontal resisting elements of the building frameswith the assumption that these in-fill will not take partin resisting any kind of load either axial or lateral.Hence, its significance in the analysis of framegenerally neglected.1. M. Tech. Student, Department of Civil Engineering,A.M.U Aligarh, India. Email: firstname.lastname@example.org ,mob: +91-9634982278, Email: email@example.com. M. Tech. Student, Department of Civil Engineering, NIT-Hamirpur, India, Email: firstname.lastname@example.org, mob:+91-9736161475As recent studies have shown a properly designed in-filled frames can be superior to a bare frame in terms ofstiffness, strength and energy dissipation. Fromstructural point of view, the composite action betweenin-fill panels and frames give more lateral resistanceand in-plane stiffness. As a result, total and inter storeydrift is reduced.In non-linear range, in-fill acts as a good damper bydissipating energy through cracking. Subsequent tocracking of in-fill, the centre of stiffness gets shiftedtowards the stiffer portion of the building and theeccentricity between the centre of stiffness and thecentre of mass get increased, thus, torsion dominatesthe structural behaviour of the building and extra shearstress get induced in frame elements. It is also beenobserved that for many buildings, ground storey is keptopen by removing all in-fill for parking. The removalof in-fill leads to more ductility demand in the openground storey. All the inelasticity gets concentrated inthe open ground storey and it can damage severely.Past studies also carried out on the behaviour of R.Cframe with in-fills and the modelling & analysis of theR.C frame with and without in-fills. Smith [1, 2, 3]used an elastic theory to proposed the effective widthof the equivalent strut and concluded that this widthshould be a function of the stiffness of the in-fill with
2nd International Conference on Emerging Trends in Engineering & Technology, April 12-13, 2013College of Engineering, Teerthanker Mahaveer University2respect to that of bounding frame. By analogy to abeam on elastic foundation, he defined thedimensionless relative parameters to determine thedegree of frame in-fill interaction and thereby, theeffective width of the strut. Singh  found in hisresearch that in the dynamic analysis of a completebuilding system, the inclusion of the effect of in-fill isessential for a realistic prediction of the behaviour; hefurther concluded that there is very limited literatureavailable on dynamic response of 3-D in-filledreinforcement concrete frames. Bell and Davidson found that a review of international research andguidelines indicate that in-fill panels, where present ina regular arrangement, have a significant beneficialinfluence on the behaviour of RC buildings. Thesecontrasts with New Zealand guidelines, which can givean impression that in-fill masonry panels, have adetrimental influence on the behaviour of buildings dueto soft storey effects. The reviewed sources indicatethat due to stiffness, strength, and damping effects ofin-fill panels, deformations are below that required fora soft storey mechanism. A review of internationalresearch and guidelines indicate that in-fill panels,where present in a regular arrangement, have asignificant beneficial influence on the behaviour of RCbuildings. These contrasts with New Zealandguidelines, which can give an impression that in-fillmasonry panels have a detrimental influence on thebehaviour of buildings due to soft storey effects. Thereviewed sources indicate that due to stiffness, strength,and damping effects of in-fill panels, deformations arebelow that required for a soft storey mechanism. Dasand Murty  carried out non-linear pushover analysison five RC frame buildings with brick masonry in-fills,designed for the same seismic hazard as per Euro-code,Nepal Building Code and Indian and the equivalentbraced frame method given in literature. In-fills arefound to increase the strength and stiffness of thestructure, and reduce the drift capacity and structuraldamage. In-fills reduce the overall structure ductility,but increase the overall strength. Building designed bythe equivalent braced frame method showed betteroverall performance. Amato et al.  discussed themechanical behaviour of single storey-single bay in-filled frames and generalised analytical proceduresavailable in the literature for the identification of a pin-jointed strut equivalent to the in-fill to take theinfluence of vertical loads into account. Detailednumerical investigation on in-filled meshes has provedthat in the presence of vertical loads it is possible that astrong correlation between the dimension of theequivalent diagonal strut model and a single parameter,which depends on the characteristics of the system. Afamily of curves has obtained for different values ofvertical load. Baran and Sevi  have found throughvarious analytical and experimental studies that hollowbrick in-fills could not only increased both strength andstiffness of RC frames but also adequately be modelledby diagonal compression struts. Asteris et al. conducted quasi-static experiments on frames withmasonry in-fill panels with openings that revealimportant insights regarding the global as well as thelocal response of the tested in-fill frames. In particular,the experimental results indicate that the failure modesof the in-filled frames classified into distinct modes.Such a classification of the failure modes (crackpatterns) enhances considerably the understanding ofthe earthquake resistant behaviour of in-filled framesand leads to improved comprehension of theirmodelling, analysis and design. Mohan and Prabha concluded that Equivalent Static Method can be usedeffectively for symmetric buildings up to 25m height.For higher and unsymmetrical buildings, responsespectrum method shall used. For important structures,time history analysis shall performed as it predicts thestructural response more accurately in comparison withother two methods since it incorporates P-Δ effects andmaterial non-linearity, which is true in real structures.Therefore, the presence of in-fill influence thebehaviour of moment resisting frame and thecharacteristic configuration of the in-fill panels canalter the predominant mode of structural actionparticularly when the frames subjected to lateral loads.2. Objectives of the Present StudyThe objectives of the present study are as follows:i. To study the effect of in-fill on 6 storeyed RCframe buildings.ii. To compare the seismic response of thebuilding in terms of base shear, storey drift,mode participation factor and time-period ofvibration.Linear analysis and design of all RC frame structureshas been performed as per IS: 1893  and IS: 456. The behaviour of buildings studied with the helpof Response spectrum analysis (SRSS method) usingFinite element method (FEM) based software.3 Methods and Analysis3.1 Parametric Study and ModellingIn present study, effect of in-fill is considered in theanalysis of RC frame to take the maximum advantageof in-fill and to relate the results with the practicalfield. To study the stiffness change in RCC frame
2nd International Conference on Emerging Trends in Engineering & Technology, April 12-13, 2013College of Engineering, Teerthanker Mahaveer University3building three types of frames have been considered:first, a bare frame; second, a frame with in-fill at allstoreys; third, a frame with no in-fill at first storey. Thein-fills are modelled as equivalent diagonal struts andthe analysis has been carried out using FEM basedsoftware.A symmetric 6 storey building has been consideredwith the plan as shown in Fig. 1 and elevations ofdifferent geometric models are shown in Fig. 2(a) to2(c). The overall plan dimensions of the RC framestructures of 14.4 m × 24.4 m are measured along thecentral line of the columns. This building is assumed tobe fixed at ground level and all storey heights are takento be 3.35 m each. A solid RCC slab of 110 mmthickness has been considered. Dead Load and LiveLoad intensities for roof and floor are given in Table 1and Table 2 respectively. The section details for beams,columns and in-fill walls are given in Table 3. Thethickness of exterior and interior brick masonry in-fillhas been considered as 250 mm and 150 mmrespectively and only in-plane stiffness of infill wall isconsidered. No reduction in weight of in-fill due toopenings is considered. Design consideration forstaircase is not taken into account, however its deadload and location are considered in modelling. A 3Dview of building skeleton is shown in Fig 3.The comparative study between the bare framebuildings and the buildings with in-fill has been carriedout with respect to dynamic characteristics that arefundamental time periods, mass participation factorsand storey drifts with the help of Square root ofsummation of squares (SRSS) method.Fig. 1 Plan of the BuildingFig. 2(a) Analytical Model of Bare FrameFig. 2(b) Frame with In-Fill at all StoreysFig. 2(c) Frame with no In-Fill at First StoreyFig. 3 Three-Dimensional view of the Building
2nd International Conference on Emerging Trends in Engineering & Technology, April 12-13, 2013College of Engineering, Teerthanker Mahaveer University4Table 1 Dead LoadsS. No. Load Type Intensity (KN/m2)1 Terrace Water Proofing 2.52 Floor Finish 1.03Sanitary Blocks includingfilling2.5Table 2 Live LoadsS. No. Load Type Intensity (KN/m2)1 Roof 1.52 Library 103 Assembly Hall 54 Sanitary Blocks 35 Office Floors 46 Officer‟s Chamber 37 Stairs 58 Corridor 5Table 3 Section DetailsS. No. Member Size (mm)1 Beams (Transverse*) 500 × 3002 Roof Beams (Longitudinal**) 300 × 3003 Corridor Beams(Longitudinal) 300 × 3004 External Beams (Longitudinal) 350 × 3005 Columns 600 × 5006 External Walls 2507 Internal Walls 1508 Slab 110*along z-direction; **along x-direction3.2 Equivalent Strut ModelThe in-fill walls considered without opening in thepresent study are modelled as equivalent diagonal strutas proposed by Smith [1, 2, 3]. The use of EquivalentStrut Model is attractive from practical point of view.The properties required for defining the strut modeldepend on type of analysis. For linear type of analysis(as in present study), only the area, length of the strutand modulus of elasticity are required to calculate theelastic stiffness of in-fill strut. The followingexpressions have been used to determine theparameters required for modelling the diagonal strut (asshown in Fig. 4).42sin42 tEhIEmcfh 42sin42 tELIEmbfL 2221Lhw Where, Em is Elastic Modulus of masonry wall, Ef isElastic Modulus of masonry of frame material, t isThickness of the in-fill wall, h is Height of the in-fillwall, L is Length of the in-fill wall, Ic is Moment ofInertia of the column of the frame, Ib is Moment ofInertia of the beam of the frame, θ is tan-1(h/L) and Wis Width of the Equivalent Strut.The calculations for width of equivalent diagonal strutsusing above expressions are given in Table 4.Table 4 Calculations for width of EquivalentDiagonal Strut for both external and internalmasonry in-fill**The equivalent strut shall have the same thickness and modulus ofelasticity as the in-fill panel it represent.Where, H is storey height in m, L is length of memberin m, x is Tan-1(H/L) in radian, Ef is modulus ofelasticity of concrete in MPa, Em is modulus ofelasticity of masonry in MPa, Ic is moment of inertia ofcolumn in m4, Ib is moment of inertia of beam in m4, tis thickness of masonry walls in m, w is width ofequivalent diagonal strut in m and w is width ofequivalent diagonal strut adopted for analysis in m.
2nd International Conference on Emerging Trends in Engineering & Technology, April 12-13, 2013College of Engineering, Teerthanker Mahaveer University5Fig. 4 Equivalent Diagonal Strut3.3 Response Spectrum AnalysisModal analysis based on response spectrum has beenadopted to dynamically analyse the structure with thehelp of FEM based software. The following ResponseSpectrum given in IS 1893 (Part 1):2002 for hard soil(for 5% damping) has been used for the analysis:Response Spectra Curve for finding base shear isshown in Fig 5.0.44.0140.010.0,50.210.000.0,151TTTTTgSaFig. 5 Spectra Curve for finding Base Shear fromFundamental Time PeriodA comparison of the dynamic characteristics of thebare frame building; frame with in-fill at all storeys andframe with no in-fill at first storey is observed. whereinthe time period, mass participation factor (%), designbase shear and storey drift obtained from the analysisresults corresponding to mode 1, mode 2 and mode 3 asgiven by FEM software are observed.Zone factor, Z = 0.16 (Zone III), Importance factor, I =1.5, Soil site type = hard soil, Response reductionfactor, R = 3 and Damping is assumed to be 5 %.4 Discussion of Results4.1 Time Period and Fundamental Time PeriodWhen a building is subjected to dynamic action itdevelops a vibratory motion in the building due to itselastic properties and mass. The vibration is similar tothe vibration of a violin string, which consists of afundamental tone and the additional contribution ofvarious harmonics. Similarly, the vibration of abuilding consists of a fundamental mode of vibrationand the additional contribution of various modes,which vibrates at higher frequencies. On the basis oftime period the building may be classified as Rigid (T< 0.3 sec), Semi-Rigid (0.3 sec < T < 1 sec), andFlexible Structure (T > 1). Fundamental period ofvibration can be determined by code base empiricalformula.The time period obtained from dynamic analysis of 6storey building in Z-direction of seismic force for firstthree modes are given in Table 5. The fundamentaltime periods for the building, estimated by using theempirical expression given in IS 1893 (Part 1): 2002are given in Table 6, which shows decrease in timeperiods with the inclusion of in-fill.Table 5 Time periods obtained from dynamicanalysis of RC buildings in Z-directionFrameConfigurationTime Periods (Sec)Mode 1 Mode 2 Mode 3Bare frame 0.632 0.505 0.198With in-fill 0.257 0.147 0.085With no in-fill at1ststorey0.341 0.246 0.100Table 6 Fundamental Time Period (Sec) for RCbuilding in Z-directionFrame Configuration Fundamental Time Period (Sec)Bare frame 0.712With in-fill 0.4774.2 Mass Participation FactorThe effective modal mass provides a means for judgingthe significance of a particular mode of vibration in thedynamic analysis. It has been observed during thisstudy that with the different frame configurations (i.e.bare frames, frames with in-fill at all storeys andframes with no in-fill at first storey), the massparticipation factors (in Z-direction) for the first mode
2nd International Conference on Emerging Trends in Engineering & Technology, April 12-13, 2013College of Engineering, Teerthanker Mahaveer University6gets increased when effect of in-fill is considered asgiven in Table 7.Table 7 Mass Participation Factor for RCBuildingsFrame ConfigurationMass Participation Factor (%)Mode 1 Mode 2 Mode 3Bare frame 80.72 0.00 11.27With in-fill 87.84 0.55 9.38With no in-fill at 1ststorey 96.29 0.08 3.244.3 Design Base ShearThe design base shear „VB‟ given in Table 8determined as per code IS 1893 (Part 1): 2002 with thefollowing conditions.Table 8 Design Base ShearFrameConfigurationBase Shear (kN) Vb/ VbBare frame 794.08 1.112with in-fill 1728.25 -No in-fill at 1ststorey 1734.52 -The design base shear Vb as per IS: 1893  shall becalculated by the following formula:WAV hb Where,RgZISA ah2 W is Seismic Weight of the buildingIf „VB‟ is less than „Vb‟, all the response quantities shallbe multiplied by the ratio Vb/VB.4.4 Storey DriftThe inter storey drift is restricted so that the minimumdamage would take place during earthquake and posingless psychological effect in the mind of people. TheIndian Seismic Code IS 1893 (Part 1): 2002recommends that “The storey drift in any storey due tothe minimum specified designed lateral force, withpartial load factor of 1.0, shall not exceed 0.004 timesthe storey height.” The variation of storey drift withheight as observed during space frame analysis isshown in Fig. 6 (for different frame configurations asFig. 6 Relationship between Drift and Storey Heightduring Space Frame Analysismentioned above). It has been observed that the storeydrifts are considerably reduced when the effect of in-fill are considered. All the drifts are found to be withinpermissible limit i.e. 1.34 cm.5 Concluding RemarksThe following conclusions are drawn from the presentstudy:i. The natural period of vibration of the buildingframe depends upon its mass and lateral stiffness.Masonry in-fill panels increases both the mass andstiffness of the building, though the contributionof the latter is more significant.ii. Time period of frames obtained after plane frameanalysis gets substantially reduced by theinclusion of infills.iii. Fundamental time periods as estimated by usingempirical expression given in IS: 1893 (Part 1):2002 has been found to be decreasing with theinclusion of infill.iv. The mass participation factor increases with theinclusion of infills and it decreases considerablywith the increase in number of modes.v. It has been observed that the storey drifts areconsiderably reduced when the effect of infill areconsidered. All the drifts are found to be withinpermissible limit, i.e. 1.34 cm.vi. When there are no infills at the ground storey, thestorey drifts is found to be considerably greaterthan that observed when the effect of in-fill at
2nd International Conference on Emerging Trends in Engineering & Technology, April 12-13, 2013College of Engineering, Teerthanker Mahaveer University7ground storey is considered. Hence, stilt buildingsare more vulnerable to collapse due to soft storeyformation.AcknowledgementWe wish to express our deep sense of regard and sincerethanks to Prof. Amjad Masood, Department of CivilEngineering, and Dr. Mohd Shariq, Assistant Professor, CivilEngineering Section, University Polytechnic, A.M.U.Aligarh for their expert guidance, helpful suggestionsthroughout the work presented in this work.References SMITH B S, The Composite Behaviour ofInfilled Frames. In Proceedings of a Symposiumon Tall Buildings with Particular Reference toShear Wall Structures, University ofSouthampton, Department of Civil Engineering.(Oxford: Pergamon Press), 1966. SMITH B S, Lateral stiffness of infilled frames,Journal of Structural division, ASCE, 88 (ST6),pp. 183-199, 1962. SMITH B S, Behaviour of square infilledframes, Journal of Structural division, ASCE, 92(ST1), pp. 381-403, 1966. SINGH H, Response of Reinforced ConcreteFrames with Infilled Panels under EarthquakeExcitation, PhD Thesis, Department of CivilEngineering, Thapar Institute of Engineering &Technology, March 1995. BELL D K AND DAVIDSON B J, Evaluationof Earthquake Risk Buildings with MasonryInfill Panels, NZSEE Conference, PaperNo.4.02.01, 2001. DAS D AND MURTY C V R, Brick MasonryInfills in Seismic Design of RC FrameBuildings: Part 2- Behaviour, The IndianConcrete Journal, August 2004. AMATO G, CAVALERI L, FOSSETTI M,AND PAPIA M, Infilled Frames: Influence ofVertical Load on The Equivalent Diagonal StrutModel, The 14thWorld Conference onEarthquake Engineering, Beijing, China, 2008. BARAN M AND SEVI T, Analytical andexperimental studies on infilled RC frames,International Journal of the Physical Sciences,Vol. 5(13), pp. 1981-1998, 2010. ASTERIS P G, KAKALETSIS D J,CHRYSOSTOMOU C.Z., SMYROU E.E.,Failure Modes of In-filled Frames, ElectronicJournal of Structural Engineering 11(1), 2011. MOHAN R AND PRABHA C, DynamicAnalysis of RCC Buildings with Shear Wall,International Journal of Earth Sciences andEngineering ,, ISSN 0974-5904, Vol. 04, No 06,pp 659-662, 2011. IS 1893 (Part 1), Indian Standard: Criteria forEarthquake Resistance Design of Structures,New Delhi, 2002. IS 456, Indian Standard: Plain and ReinforcedConcrete- Code of Practice, New Delhi, 2000.