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- 1. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME 208 A COMPARATIVE STUDY OF THE EFFECT OF INFILL WALLS ON SEISMIC PERFORMANCE OF REINFORCED CONCRETE BUILDINGS 1 Prerna Nautiyal*, 2 Saurabh Singh* and 3 Geeta Batham 1,2 Student of M.E. Structural Engineering, Department of Civil Engineering, 3 Assistant Professor , Department of Civil Engineering, 1,2,3 University Institute of Technology, RGVP, Bhopal, Madhya Pradesh, INDIA. ABSTRACT In the building construction, framed structure is frequently used due to ease of construction and rapid progress of work, and generally these frames are filled by masonry infill panels or concrete blocks. This paper elaborates the effect of infill wall during the earthquake. The effect of masonry infill panel on the response of RC frames subjected to seismic action is widely recognized and has been subject of numerous experimental investigations. Infill behaves like compression strut between column and beam and compression forces are transferred from one node to another. The model uses an equivalent diagonal method to calculate the infill walls, as recommended in the literature. The results also show that infill walls reduce displacements, time period and increases base shear. So it is essential to consider the effect of masonry infill for the seismic evaluation of moment resisting RC Frame. Keywords: Infill wall, Effect of infill wall, Modeling of the infill wall, Soft storey, Seismic behavior of infilled structure, Diagonal strut method. 1. INTRODUCTION It has always been a human aspiration to create taller and taller structures. Development of metro cities in India there is increasing demand in High Rise Building. Column and girder framing of reinforced concrete, or sometimes steel, is in-filled by panel of brickwork, block work, cast in place or pre-cast concrete. Infill panel elements as the part of the building RC structure play a very important role on the seismic performance of the building structure. In general design practices in India, the strength and stiffness of infill walls are ignored with the assumption of conservative design. In actual, infill walls add considerably to the strength and rigidity of the structures and their negligence will cause failure of many of multi-storeyed buildings. The failure is basically due to stiffening effect of infill panels which is cause of i) unequal distribution of lateral forces in the INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND TECHNOLOGY (IJCIET) ISSN 0976 – 6308 (Print) ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), pp. 208-218 © IAEME: www.iaeme.com/ijciet.asp Journal Impact Factor (2013): 5.3277 (Calculated by GISI) www.jifactor.com IJCIET © IAEME
- 2. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME 209 different frames and overstressing of some of the building frames; ii) soft storey or weak storey; iii) short columns or captive column effect; iv) torsional forces ; v) cracking of the infill walls. Several researchers are unanimous in pointing out the benefits of associating frames with infill walls, which significantly increases the mechanical strength and rigidity of the infill frame. Analytical models based on the concept of the equivalent diagonal strut, considering the structure as an equivalent braced frame system with a diagonal compression strut replacing the infill, provide an accurate prediction of the behaviour of steel frames. Soft stories Many urban multistory buildings in India today have open first storey as an unavoidable feature. This is primarily being adopted to accommodate parking or reception lobbies in the first storey. The upper stories have brick infilled wall panels. The draft Indian seismic code classifies a soft storey as one whose lateral stiffness is less than 70% of the storey above or below [Draft IS:1893, 1997]. Interestingly, this classification renders most Indian buildings, with no masonry infill walls in the first storey, to be “buildings with soft first storey.” A soft story is illustrated above, as an apartment complex with a row of garages below the first level. This is garage level is sometimes called “tuck under parking.” Fig:1 Soft storey 2. BASIC CONCEPT[1][2][3] The building with soft story behaves differently as compared to a bare framed building (without any infill) or a fully infilled framed building under lateral load. A bare frame is much less stiff than a fully infilled frame; it resists the applied lateral load through frame action and shows well-distributed plastic hinges at failure. When this frame is fully infilled, truss action is introduced. A fully infilled frame shows less inter-storey drift, although it attracts higher base shear (due to increased stiffness). A fully infilled frame yields less force in the frame elements and dissipates greater energy through infill walls. The strength and stiffness of infill walls in infilled frame buildings are ignored in the structural modeling in conventional design practice. The design in such cases will generally be conservative in the case of fully infilled framed building. But things will be different for a soft story framed building. Soft story building is slightly stiffer than the bare frame, has larger drift (especially in the ground storey), and fails due to soft storey-mechanism at the ground floor. Therefore, it may be unconservative to ignore strength and stiffness of infill wall while designing soft story buildings. Inclusion of stiffness and strength of infill walls in the Soft story building frame decreases the fundamental time period compared to a bare frame and consequently increases the base shear demand and the design forces in the ground storey beams and columns. This increased design forces in the ground storey beams and columns of the Soft story buildings are not captured in the conventional bare frame analysis. An appropriate way to analyze the Soft story buildings is to model the strength and stiffness of infill walls. Unfortunately, no guidelines are given in IS 1893: 2002
- 3. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME 210 (Part-1) for modeling the infill walls. As an alternative a bare frame analysis is generally used that ignores the strength and stiffness of the infill walls. The total seismic base shear as experienced by a building during an earthquake is dependent on its natural period, the seismic force distribution is dependent on the distribution of stiffness and mass along the height. In buildings with soft first storey, the upper stories being stiff, undergo smaller inter-storey drifts. However, the inter-storey drift in the soft first storey is large. The strength demands on the columns in the first storey for third buildings is also large, as the shear in the first storey is maximum. For the upper stories, however, the forces in the columns are effectively reduced due to the presence of the Buildings with abrupt changes in storey stiffness have uneven lateral force distribution along the height, which is likely to locally induce stress concentration. This has adverse effect on the performance of buildings during ground shaking. Such buildings are required to be analyzed by the dynamic analysis and designed carefully. 3. MODELLING OF INFILL WALL [4][5] Most of the previous research model infill wall as an equivalent diagonal strut. This section summarises different approaches to model infill was as equivalent struts. Basically there are four approaches to model the equivalent strut found in literature. These approaches are explained below: 3.1 ELASTIC ANALYSIS APPROACH The modelling of infill wall as an equivalent diagonal compression member was introduced by Holmes (1961). The thickness of the equivalent diagonal strut was recommended as the thickness of the infill wall itself, and the width recommended as one-third of the diagonal length of infill panel. The width of the strut using Airy’s stress function was found to vary from d/4 to d/11 depending on the panel proportions. Later, a number of tests conducted by Smith (1966) proved that the equivalent strut width (w) is a function of relative stiffness (λh) of the frame and infill wall, strength of equivalent corner crushing mode of failure (Rc ) and instantaneous diagonal compression in the infill wall (Ri ). Fig:2 A typical panel of the infilled frame Fig:3 Behavior of typical panel subjected to lateral load
- 4. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME 211 In 1969, Smith and Carter combined all the previous works (Smith 1962, 1966) and developed an analysis approach based on the equivalent strut concept to predict the width and strength of an infilled frame. This approach of modeling the struts is based on the initial stiffness of the infill wall. Fig 2.1 and 2.2 shows how the infill panels behave when it is designed as equivalent diagonal strut when subjected to lateral load. Smith and Carter (1969) expressed the parameter, λh, as follows Where, Es = elastic modulus of the equivalent strut Ec = elastic modulus of the column in the bounding frame Ic = moment of inertia of the column h'= clear height of infill wall (Fig. 2) h = height of column between centre lines of beams t = thickness of infill wall θ = slope of the infill wall diagonal to the horizontal A relationship between the ratio of axial load in the equivalent strut (Ri ) to the capacity of the strut under corner crushing (Rc ), and width (w) was derived by Ramesh (2003) from the plot given by Smith and Carter (1969), as given by The parameter w’ accounts for the panel aspect ratio. An expression for w’/d is as given: The strength of the equivalent strut is taken as the minimum of the two failure modes, i.e. (i) Local crushing (Rc) of infills in the corners (ii) Shear cracking (Rs) along the bed joint of the brickwork. The failure load corresponding to corner crushing mode was expressed in terms of λh as: Where fm’ is the compressive strength of the masonry infill wall. The following relationship was proposed for the diagonal load causing shear cracking failure (Rs) by Govindan et. al. (1987), using the curves given by Smith and Carter, 1969. Where fbs’ is the bond shear strength between the masonry and mortar Another equation by Mainstone for the determination of the equivalent strut width is Where d’ = is the clear diagonal length of the infill walls. This expression yields a constant strut width, independent of parameters such as axial load on the diagonal strut and infill wall panel aspect ratio. Paulay and Priestley (1992) suggested that the width of the strut can be taken as 1/4th of the diagonal length of the infill panel. Al-Chaar (2002) proposed an eccentric equivalent strut (Fig.2.3) which was pin connected to the column at a distance le from the face of the beam to model the masonry infill wall. Where le = w/cosθ and w is calculated using above equation.
- 5. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME 212 3.2 ULTIMATE LOAD APPROACH Saneinejad and Hobbs (1995) proposed a new model that accounts for the interface stresses and the nonlinear inelastic behavior of the infill wall. The area of the equivalent strut is calculated from the diagonal load at failure. This approach is based on the ultimate strength of the equivalent strut and the strength of the strut is calculated from the three modes of failure: 1) Corner crushing failure at the compressive corners 2) Shear cracking failure along the bedding joints of the brickwork 3) Diagonal compression failure of the slender infill wall The applicability of the two approaches stated above for different types of building analysis was investigated. The calculation of the strut properties by both the approaches was presented through a case study by Asokan (2006) and the justification of using either of the methods was presented. He selected a two bay frame of an existing five storey building which was infilled in the entire four stories except for the ground floor. The beams and column frames were of the same size. The infill wall thickness was 120 mm and he from his study concluded that the EA approach is simple in the calculation. A higher strut width gives higher stiffness and hence, higher base shear in a building. Since the EA approach gives the higher strut width, it is conservative in estimating the base shear. For estimating the lateral drift of a building, since the UL approach gives lower stiffness of a strut, it is more conservative. To carry out a linear analysis of the building by the equivalent static method (static analysis) or the response spectrum method (dynamic analysis), modeling of the infill walls by the simpler EA approach would prove to be adequate. But in a pushover analysis (nonlinear static analysis) of a building, the UL approach would be preferred. 3.3 APPROACH BASED ON PLASTIC ANALYSIS Experimental results (Smith 1962) show that there is a considerable nonlinearity in the infilled frames before they collapse. The nonlinearity arises mainly from cracking and crushing of the infill wall material, confinement of the infill walls in the frames, and formation of plastic hinges in the frame members. In the elastic stage, stress concentration occurs at all four corners. As cracks develop and propagate, the stresses at the tensile corners are relieved while those near the compressive corners are significantly increased. The frame moments increase significantly when the infill wall degrades leading to the formation of plastic hinges and collapse of the structure. A plastic theory was developed for integral and non-integral (without shear connectors) infilled frames by Liauw and Kwan (1983). The stress redistribution in the frames towards collapse was taken into account and the friction was neglected for strength reserve for the non-integral infilled frames. The theory was based on the findings from nonlinear finite element analysis and experimental investigation. The local crushing of the infill wall corner is associated with a plastic hinge formation either in the beam or in the column. The following modes of failure were identified. • Corner crushing mode with failure in columns: This mode of failure is associated with weak columns and strong infill wall. Failure occurs in the columns with subsequent crushing of the infill wall at the compressive corners. • Corner crushing mode with failure in beams: This mode of failure predominates when beam is relatively weak and the infill wall is strong. Failure occurs in beam after the failure of the infill wall at the compressive corners. • Diagonal crushing mode: With relatively strong frame and weak infill wall, failure occurs in the infill wall by crushing at the loaded corners with subsequent failure in the joints of the frame. Based on plastic theory, following are the mathematical expressions were developed for the above modes of failure.
- 6. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME 213 1. For failure mode 1 2. For failure mode 2 3. For failure mode 3 Where Hu = lateral load causing the failure Mpc = the plastic moment of resistance of the column Mpb = the plastic moment of resistance of the beam σc= contact stresses in the column 3.4 APPROACH BASED ON FINITE ELEMENT ANALYSIS Finite element analysis was done by many researchers to study the behavior of the infill wall under lateral load. The different parameters influencing the infill walls under lateral loads were investigated. A finite element model was developed by Mallick and Severn (1967) to incorporate the effect of slip and interface friction between the frame and infill wall. Riddington and Smith (1977) studied the effect of different parameters such as aspect ratio, relative stiffness parameter, number of bays and beam stiffness. It was found that the bending moments in the frame members were reduced in the presence of the infill wall. Hence, the infilled frame can be modeled as truss elements. Dhanasekar and Page (1986) developed a finite element program and concluded that the behavior of a frame not only depends on the relative stiffness of the frame and infill wall but also on the properties of masonry, such as shear and tensile bond strengths. 4. PROBLEM STATEMENT[6] For the analysis purpose two models have been considered namely as: Model A: Fully infilled frame (S.M.R.F infill frame with masonry effect considered) Model B: Bare frame (S.M.R.F infill frame with masonry effect not considered) 4.1 STRUCTURAL DETAILS The plan layout of the special reinforced concrete moment resisting frame (SMRF) building with one open storey and Un-reinforced brick infill walls in the other stories, chosen for this study is shown in Fig. 3. The building is deliberately kept symmetric in both orthogonal directions in the plan to avoid torsional response under pure lateral forces. The building is considered to be located in the seismic zone V and intended for commercial use. The building is founded on hard soil through isolated footings (of size 2m×2m) under the columns. Elastic moduli of concrete and masonry are 28,500 MPa and 3,500 MPa, respectively, and their Poison’s ratio is 0.25. Performance factor (K) has been taken as 1.0 (assuming ductile detailing). The unit weights of concrete and masonry are taken as 25 kN/m3 and 20 kN/m3 is considered. The other building parameters are as follows.
- 7. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME 214 Table No. 1 1 Type of Structure Multistory rigid jointed plane frame (SMRF) 2 Seismic Zone V 3 Number of stories Four, G+3 4 Floors Height 3.2 m 5 Infill wall 250mm thick brick masonry wall along X direction & 150 mm thick brick masonry wall along Y direction 6 Type of soil Hard 7 Size of column 250 mm X450 mm 8 Size of Beam 250 mm X 400 mm 9 Depth of Slab (RCC) 100 mm 10 Live load a) On roof = 1.5 KN/sqm b) On floor = 4 Kn/sqm 11 Material M 20 Grade concrete & Fe 415 Reinforcement 12 Unit weights a) Concrete = 25 KN/Cum b) Masonry = 20 KN/Cum 13 Damping in structure 5% 14 Importance factor 1.5 Fig:4 Plan of the model Calculation of Lumped Mass The seismic weight W is the full dead load & the appropriate imposed load or live load at the corresponding floor level as mentioned below for the computation of seismic forces. W= DL +0.5 LL (for LL > 3.00 KN/ sqm)
- 8. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME 215 Table No. 2 FLOORS DEAD LOAD (KN) LIVE LOAD (KN) SEISMIC WEIGHT OF FLOORS (KN) Ground Floor 2364 900 2814 First Floor 2364 900 2814 Second Floor 2364 900 2814 Third Floor 1613.25 Not considered 1613.25 Seismic weight of the structure (W) = M1 + M2 + M3 + M4 = 10055.25 KN Fig:5 Plane frame structure and its lumped mass model Model A: The natural period (Tn) of the structure with infilled wall is estimated as follows Tn = 0.09 h/ (d)0.5 Where, h= 12.8m & d = 15m So, Tn= 0.297 Response Acceleration Coefficient for 5% damping and hard soil (Sa/g) = 2.5 Z (Zone factor for zone V) = 0.36 Importance factor (I) = 1.5 Response reduction factor (for SMRF), R = 5 So horizontal seismic coefficient is Ah = (Z/2)(I/R)(Sa/g) So, Ah = 0.135 The base shear is (VB)’ = Ah X W So, (VB)’ =1357.45 KN
- 9. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME 216 Table No. 3 Storey Weight in KN, Wi hi (m) from the base Wi hi 2 Wi hi 2 /∑ Wi hi 2 Qi=(VB)’X (Wi hi 2 /∑ Wi hi 2 ) 4 1613.25 12.8 264314.9 0.395841001 537.3343669 3 2814 9.6 259338.2 0.388387928 527.2171927 2 2814 6.4 115261.4 0.172616857 234.3187523 1 2814 3.2 28815.36 0.043154214 58.57968808 ∑ 10055.2 667729.9 1 1357.45 Therefore the base shear is 1357.45KN and the lateral forces at the storey levels are Q1 = 537.33, Q2 = 527.22 KN , Q3 = 234.32 KN and Q4 = 58.58 KN. Fig:6 Lateral load distribution at various floors Model B: The natural period (Tn) of the structure without infilled wall is estimated as follows Tn = 0.075h0.75 Where, h= 12.8m So, Tn= 0.508 Response Acceleration Coefficent for 5% damping and hard soil (Sa/g) = (1/Tn) =1.969 Z (Zone factor for zone V) = 0.36 Importance factor (I) = 1.5 Response reduction factor (for SMRF), R = 5 So horizontal seismic coefficient is Ah = (Z/2)(I/R)(Sa/g) So, Ah = 0.106 The base shear is (VB)’ = Ah X W So, (VB)’ = 1065.86 KN
- 10. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME 217 Table No. 4 Storey Weight in KN, Wi hi (m) from the base Wi hi 2 Wi hi 2 /∑ Wi hi 2 Qi=(VB)’X (Wi hi 2 /∑ Wi hi 2 ) 4 1613.25 12.8 264314.9 0.395841001 421.9110894 3 2814 9.6 259338.2 0.388387928 413.9671568 2 2814 6.4 115261.4 0.172616857 183.985403 1 2814 3.2 28815.36 0.043154214 45.99635075 ∑ 10055.2 667729.9 1 1065.86 Therefore, the base shear is 1065.86 KN and the lateral forces at the storey levels are Q1 = 421.91KN, Q2 =413.97 KN, Q3 = 183.99 KN and Q4 = 45.99 KN. Fig:6 Lateral load distribution at various floors 5. COMPARISON OF RESULTS Table No. 5 Considering the stiffness of the wall (Model A) Neglecting the stiffness of infill (Model B) Natural period , Tn 0.297 0.508 Base shear 1357.45 1065.86 6. CONCLUSION The Indian standard provides different expressions for the estimation of the natural period of the building structure considering or neglecting the stiffness of the infill wall. The consideration of stiffness of masonry infill increases the stiffness of the structure and hence reduce the natural period and consequently increase the response acceleration and hence the seismic forces (i.e. base shear and correspondingly the lateral forces at each storey.
- 11. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME 218 7. REFERENCES [1] A.S. Kasnale & Sanjay Jamkar “Analysis of lateral stiffness for infilled frame with opening”, International Journal of advanced technology in civil engineering, ISSN:2231-5721 Vol I Issue 3,2012. [2] Hyun Ko , Yong-Koo Park and Dong- Guen Lee “Evaluation of Seismic Behavior for low rise RC Moment Resisting with masonry infill wall”, the 14th world conference on earthquake engineering October 12-17, 2008, Beijing, China. [3] Wakchaure M.R. , Ped S.P. “Earthquake analysis of high rise building with and without infilled walls”, IJEIT ISSN:2277-3754 Vol II Issue 2 August 2012. [4] A. Asokan, (2006) Modeling of Masonry Infill Walls for Nonlinear Static Analysis of Buildings under Seismic Loads. M. S. Thesis, Indian Institute of Technology Madras, Chennai. [5] Shenash patel “Earthquake resistant design of low-rise open ground storey framed building “ M.Tech Thesis, NIT Rourkela. [6] Dr. Vinod Hosur “ Earthquake – Resistant design of building structures”, ISBN No. 978-81- 265-3859-1 Publisher Wiley Precise Textbook. [7] Machhindra S.Purkar and Sunil Y. Kute, “Numerical Modeling of Reinforced Soil Segmental Wall Under Surcharge Loading”, International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 1, 2013, pp. 1 - 15, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316. [8] Mohammed S. Al-Ansari, “Flexural Safety Cost of Optimized Reinforced Concrete Beams”, International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 2, 2013, pp. 15 - 35, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316. [9] Misam.A and Mangulkar Madhuri.N., “Structural Response of Soft Story-High Rise Buildings Under Different Shear Wall Location”, International Journal of Civil Engineering & Technology (IJCIET), Volume 3, Issue 2, 2012, pp. 169 - 180, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.

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