Study on Behaviour of Rc Structure with Infill Walls Due to Seismic Loads

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    International Research Journal of Engineering and Technology   (IRJET)   e-ISSN: 2395 -0056   Volume: 04 Issue: 06 | June -2017 www.irjet.net p-ISSN: 2395-0072   © 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 2494 STUDY ON BEHAVIOUR OF RC STRUCTURE WITH INFILL WALLS DUE TO SEISMIC LOADS Yadunandan C 1 , Kiran Kuldeep K N 2   1 P.G. Student, Civil Engineering Department, Sri Jagadguru Balagangadharanatha Institute of Technology, Bengaluru - 560060, Karnataka, India    2  Assistant Professor, Civil Engineering Department, Sri Jagadguru Balagangadharanatha Institute of Technology, Bengaluru - 560060, Karnataka, India ---------------------------------------------------------------------***---------------------------------------------------------------------  Abstract –   Since long masonry infill are being used to fill up the voids between the horizontal and vertical structural elements such as beams and columns. They are treated as non-structural elements and they are not considered during the analysis and design of the structure. But, when laterally loaded they tends to interact with the RC frame, changing the structural behaviour. However, infill walls contribute to lateral stiffness and seismic resistance to the building. In this study, an attempt is being made to incorporate the masonry infill in the form of Equivalent diagonal strut whose width is calculated using the various relations proposed by the researches. A general review of the relations proposed by the researches in calculating the width of the Equivalent diagonal strut is being made and compared. The paper also focuses to study the behaviour of bare frame and infilled frame. The aim of this research work is to present a comparative study and analysis of G+3 story building with and without opening and soft story by performing linear dynamic analysis using ETABS software .Results for base shear, story drift, lateral loads, story displacement, column forces and time period are compared for different models .   Key Words :  Masonry infill  , Infill opening, Soft story, Equivalent diagonal strut Response spectrum analysis. 1. INTRODUCTION RC moment resisting frame buildings are the most preferred type of construction in developing countries like India. RC moment resisting frame buildings consist of moment resisting frame with masonry wall as Infill’s. These walls are considered as nonstructural elements in construction practices. In present day practice of building design, buildings are designed as framed structures while effect of infill masonry walls is ignored and considered as nonstructural elements. Due to the above reason, buildings behave in different manner with infill wall when compared with only moment resisting frames. In past four decades, through lots of analytical and experimental studies importance of brick infill has been recognized however its strength and stiffness contribution has been neglected by considering it as nonstructural elements. Another important aspect concerns the numerical simulation of the infilled frames. The structural model can be idealized by different techniques and can be divided into micro model and macro model. In the present paper the masonry infill wall is modeled has “Equivalent diagonal strut” considering the strength and stiffness of brick masonry infill .This strut is designed in such a manner that it only carries compression. 2. OBJECTIVES    To study the behaviour of RC frame with brick infill by modeling infill as a diagonal strut.    Understand the suitability of different macro models available for considering the infill effects in reinforced concrete infilled frames.    Investigate the contribution of masonry infill walls to lateral strength and lateral stiffness of the building.    To study the effect of opening and soft story on the performance of masonry infilled RC framed structures. 3. METHODOLOGY    In the present study, the RC members and masonry infill Walls are modeled using ETABS software.    The analytical macro models are modeled and analyzed for linear dynamic analysis.    Response spectrum method of analysis is adopted for the analysis of infilled frame with and without opening and soft story and the results are compared. 4. REVIEW OF MACRO MODELS 4.1 EQUIVALENT DIAGONAL STRUT MODEL The existence of infill influences the distribution of lateral loads on the framed structures due to the increase in stiffness. The investigation of interaction of infill with frames has been endeavored by utilizing many analyses like theory of elasticity or finite element analysis. Because of complexity and uncertainty in defining the interaction between infills and the frames, several approximate methods are being developed. A prominent among the most prevalent and known approaches is by replacing masonry infill by equivalent diagonal struts, the thickness of which is equal to the thickness of masonry infills. The primary issue with this    International Research Journal of Engineering and Technology   (IRJET)   e-ISSN: 2395 -0056   Volume: 04 Issue: 06 | June -2017 www.irjet.net p-ISSN: 2395-0072   © 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 2495 approach is to find the effective width. Numerous Scientists have proposed different techniques for determining the width of equivalent diagonal strut. Strut width leans on the length of contact between the columns and the wall ( αh) and between the beam and wall (αL).   Fig -1 : Equivalent diagonal strut model 4.1.1 Holmes Model Holmes (1963) took the idea from Polyakov (1956) and stated that infilled walls can be replaced by a equivalent diagonal strut which has the same thickness and material as the infill wall. b w = d w /3 Where, dw= Diagonal length of the panel 4.1.2 Stafford Smith and Carter model Stafford Smith and Carter (1969) have proposed a theoretical relationship for the width of the diagonal strut on the basis of relative stiffness of infill and frame. w = 0.58 ( (λ  h H inf  ) 0.335.dinf    λ  h  = t = Infill wall thickness, H inf   = Height of the infill, E inf   = Modulus of elasticity of the infill, E c   = Modulus of elasticity of the column, I c   = Moment of inertia of the columns , H = Total frame height, θ = Angle between diagonal of the horizontal and the infill,   λ  h = Dimensionless parameter. 4.1.3 Mainstone model Mainstone (1971) performed tests on frames with brick infill walls and gave equivalent diagonal strut model this approach takes into contribution of both Infilled Frame stiffness and its ultimate strength. w = 0.16 d inf    ((λ  h H inf  ) -0.3   λ  h  = 4.1.4 Paulay and Preistley model Paulay and Preistley (1992) stated that higher estimation of width(w) will effect in a stiffer structure and potentially superior seismic reaction. w =0.25 d inf   Where, d inf = Diagonal length of the infill 4.1.5 Hendry model Hendry (1998) has also presented equivalent strut width that would represent the masonry that actually contributes in resisting the lateral force in the composite structure w = 0.5   h  = L  = α h , α L = Contact length between wall and column at the time of initial failure of wall. I b  = Moment of inertia of the beam L inf = Length of the infill i.e. Clear distance between columns. 4.1.6 FEMA model FEMA (1998) proposed that infill wall thickness which is represented has equivalent strut can be obtained by w = 0.175 d inf    ((λ  h H inf  ) -0.4   λ  h  =    International Research Journal of Engineering and Technology   (IRJET)   e-ISSN: 2395 -0056   Volume: 04 Issue: 06 | June -2017 www.irjet.net p-ISSN: 2395-0072   © 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 2496 Table -1:  Equivalent diagonal strut Width  Sl No Model Equivalent strut width (m) 1 Holmes 1.73 2 Smith and Carter 4.91 3 Mainstone 0.54 4 Paulay and Preistley 1.32 5 Hendry 0.68 6 FEMA 273 0.52 With reference to various literature reviews, Mainstone's relation was widely used for most of the experimental and analytical works, as it predicted the value of the width of the Diagonal strut which was very near/close to the Romanian code and it was commonly adopted because of its simplicity. 4.2 PERFORMANCE OF INFILL FRAME WITH CENTRAL OPENING Asteris et al. (2011) presented the analytical results of the influence of opening size on the seismic response of masonry infilled frames with central opening. Fig. shows the variation of the ‘λ’ factor as a function of the opening % for the case of an opening on the compressed diagonal of the infill wall. Opening % (α w ) = Width of strut with opening = Stiffness Reduction factor as per Figure x w without opening Fig -2 : Stiffness reduction factor for Infill with opening Table -2: Stiffness reduction factor and width of strut for different percentage of opening   % of opening Stiffness reduction factor , λ   Width of strut, m 0 -  0.540 10 0.45 0.267 20 0.32 0.173 30 0.21 0.113 40 0.13 0.071 5. BUILDING DESCRIPTION Table -3:  Description of the model  Table -4: Parameters of G+ 3 storey Diagonal strut model      International Research Journal of Engineering and Technology   (IRJET)   e-ISSN: 2395 -0056   Volume: 04 Issue: 06 | June -2017 www.irjet.net p-ISSN: 2395-0072   © 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 2497 Fig -3 : Plan layout of G+3 story building model Fig -4 : Elevation of bare frame and infilled frame Fig -5 : Elevation of 20% and 40% opening infilled frame Fig -6 : Elevation of soft story at ground floor and third floor 6. RESULTS AND DISCUSSIONS 6.1. Comparision with bare frame, infilled frame and infilled frame with 20 and 40% opening 6.1.1 BASE SHEAR The Base shear is more in infilled frame than bare frame because it depends on the stiffness in the frame. Due to the presence of infill (strut) the stiffness of the frame is increased resulting in increased sesmic forces than bare frame. Chart -1 : Bar graph showing variation of base shear 6.1.2 STORY DRIFT Introduction of infill in the building structure reduces the seismic demands of the building both in terms of storey drift and the horizontal displacement. Story drift is more in bare frame than 20% and 40% infill.
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