seismic performance of masonry in-fills reinforced ...

_____________________________________________________________________________________________ UKIERI Concrete Congress - Innovations in Concrete Construction

SEISMIC PERFORMANCE OF MASONRY IN-FILLS REINFORCED CONCRETE BUILDINGS M Danish1, M Shoeb2, A Masood1, M Shariq1 1. Aligarh Muslim University, India 2. National Institute of Technology Hamirpur, India

ABSTRACT. Masonry in-fills are the integral part of R.C.C frame structure and steel structures. Masonry in-fills are frequently used to fill the voids between the vertical and horizontal resisting elements of the building frames with the assumption that these in-fills will not take part in resisting any kind of load either axial or lateral. Hence, its significance in the analysis of frame generally neglected. In fact, an in-fill wall considerably enhances the rigidity and strength of the frame structure. Researches pointed that the frame considering no in-fill has comparatively lesser stiffness and strength than the in-fill frame and therefore their ignorance cause failure of many multi-storey buildings when subjected to seismic loads. Hence, the common practice of ignoring in-fills and designing the buildings as bare frame is not always conservative.In the present study, a finite element analysis of R.C.C frame with and without in-fills (bare frame), frame considering in-fills at all storeys except the first and further also with shear walls has been carried out. The seismic performance of the above RC structures with masonry in-fills carried out by response spectrum method conforming IS1893: 2002. Number of stories has also been varied from G+3 to G+9 and the behaviour of these buildings under Gravity and Seismic loads has been observed. Keywords: FEM, Seismic performance, RSM, Masonry in-fills, RCC frame Mr Mohd Danish is a M. Tech. student in the Department of Civil Engineering, Z.H. College of Engineering & Technology, Aligarh Muslim University, India. His research interest includes Seismic performance of masonry and RC frame structures. Mr Mohd Shoeb is a M. Tech. student in the Department of Civil Engineering, National Institute of Technology Hamirpur, India. His research interest includes Seismic analysis of masonry and reinforced concrete buildings. Dr Amjad Masood is a Professor of Civil Engineering at Department of Civil Engineering, Z.H. College of Engineering & Technology, Aligarh Muslim University, India. His research interests are in the area of earthquake- design, analysis restoration and retrofitting. Dr Mohd Shariq is an Assistant Professor of Civil Engineering at CES, University Polytechnic, Faculty of Engineering and Technology, Aligarh Muslim University, India. His research interests are Seismic performance of masonry and RC buildings, Behaviour of confined concrete at elevated temperature.

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UKIERI _____________________________________________________________________________________________ Concrete Congress - Innovations in Concrete Construction

INTRODUCTION An earthquake force is a very peculiar force and behaves quite differently than other types of loads, such as, gravity and wind loads. It strikes the weakest spot in the whole three dimensional building. This should be an eye opener for designers and builders. Due to ignorance in design and poor quality of construction, results many weaknesses in the structure to cause serious damage to life and property. One of the examples, which shook the country on 26th January 2001, is Bhuj Earthquake, which caused thousands of casualties with over 300,000 buildings collapsed. Masonry in-fill is the integral part of R.C.C frame structure and some steel structures. Masonry in-fill are frequently used to fill the void between the vertical and horizontal resisting elements of the building frames with the assumption that these in-fill will not take part in resisting any kind of load either axial or lateral. Hence, its significance in the analysis of frame generally neglected. As recent studies have shown a properly designed in-filled frames can be superior to a bare frame in terms of stiffness, strength and energy dissipation. From structural point of view, the composite action between in-fill panels and frames give more lateral resistance and in-plane stiffness, as a result, total and inter storey drift is reduced. In non-linear range, in-fill acts as a good damper by dissipating energy through cracking. Subsequent to cracking of in-fill, the centre of stiffness gets shifted towards the stiffer portion of the building and the eccentricity between the centre of stiffness and the centre of mass get increased, thus, torsion dominates the structural behaviour of the building and extra shear stress get induced in frame elements. It is also been observed that for many buildings, ground storey is kept open by removing all in-fill for parking. The removal of in-fill leads to more ductility demand in the open ground storey. All the inelasticity gets concentrated in the open ground storey and it can damage severely. Past studies also carried out on the behaviour of R.C frame with in-fills and the modelling and analysis of the R.C frame with and without in-fills and shear walls. Smith [1, 2, 3] used an elastic theory to proposed the effective width of the equivalent strut and concluded that this width should be a function of the stiffness of the in-fill with respect to that of bounding frame. By analogy to a beam on elastic foundation, he defined the dimensionless relative parameters to determine the degree of frame in-fill interaction and thereby, the effective width of the strut. Singh [4] found in his research that in the dynamic analysis of a complete building system, the inclusion of the effect of in-fill is essential for a realistic prediction of the behaviour; he further concluded that there is very limited literature available on dynamic response of 3-D in-filled reinforcement concrete frames. Bell and Davidson [5] found that a review of international research and guidelines indicate that in-fill panels, where present in a regular arrangement, have a significant beneficial influence on the behaviour of RC buildings. These contrasts with New Zealand guidelines, which can give an impression that in-fill masonry panels, have a detrimental influence on the behaviour of buildings due to soft storey effects. The reviewed sources indicate that due to stiffness, strength, and damping effects of in-fill panels, deformations are below that required for a soft storey mechanism. A review of international research and guidelines indicate that in-fill panels, where present in a regular arrangement, have a significant beneficial influence on the behaviour of RC buildings. These contrasts with New Zealand guidelines, which can give an impression that in-fill masonry panels have a detrimental influence on the behaviour of buildings due to soft storey effects. The reviewed sources indicate that due to stiffness, strength, and damping effects of in-fill panels, deformations are below that required for a soft storey mechanism.

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Das and Murty [6] carried out non-linear pushover analysis on 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 equivalent braced frame method given in literature. In-fills are found to increase the strength and stiffness of the structure, and reduce the drift capacity and structural damage. In-fills reduce the overall structure ductility, but increase the overall strength. Building designed by the equivalent braced frame method showed better overall performance. Amato et al. [7] discussed the mechanical behaviour of single storey-single bay in-filled frames and generalised analytical procedures available in the literature for the identification of a pin-jointed strut equivalent to the in-fill to take the influence of vertical loads into account. Detailed numerical investigation on in-filled meshes has proved that in the presence of vertical loads it is possible that a strong correlation between the dimension of the equivalent diagonal strut model and a single parameter, which depends on the characteristics of the system. A family of curves has obtained for different values of vertical load. Baran and Sevi [8] have found through various analytical and experimental studies that hollow brick infills could not only increased both strength and stiffness of RC frames but also adequately be modelled by diagonal compression struts. Asteris et al. [9] conducted quasi-static experiments on frames with masonry in-fill panels with openings that reveal important insights regarding the global as well as the local response of the tested in-fill frames. In particular, the experimental results indicate that the failure modes of the in-filled frames classified into distinct modes. Such a classification of the failure modes (crack patterns) enhances considerably the understanding of the earthquake resistant behaviour of in-filled frames and leads to improved comprehension of their modelling, analysis and design. Mohan and Prabha [10] concluded that Equivalent Static Method can be used effectively for symmetric buildings up to 25m height. For higher and unsymmetrical buildings, response spectrum method shall used. For important structures, time history analysis shall performed as it predicts the structural response more accurately in comparison with other two methods since it incorporates P-Δ effects and material non-linearity, which is true in real structures. Therefore, the presence of in-fill influence the behaviour of moment resisting frame and the characteristic configuration of the in-fill panels can alter the predominant mode of structural action particularly when the frames subjected to lateral loads. .

OBJECTIVES OF THE PRESENT STUDY The objectives of the present study are as follows: i) To study the effect of in-fill on G+3, G+5, G+7 and G+9 storeyed RC frame buildings. ii) To study the effect of shear wall on G+3, G+5, G+7 and G+9 storeyed RC frame buildings. iii) To compare the seismic response of the building in terms of base shear, storey drift, mode participation factor, time-period of vibration. Linear analysis and design of all RC frame structures has been performed as per IS: 1893 [11] and IS: 456 [12]. The thickness of the in-fill walls have been taken as 150mm and 250mm and only in-plane stiffness of masonry is considered. In-fill panels have been modelled as strut elements. The behaviour of buildings studied with the help of Response spectrum analysis (SRSS method) using Finite element method (FEM) based software. __________________________________________________________________________________________ 2171


SEISMIC EVALUATION OF RC FRAMES Methodology In present study, effect of in-fill is considered in the analysis of RC frame to take the maximum advantage of in-fill and to relate the results with the practical field. IS: 1893 [11] recommends two types of dynamic analysis. Response spectrum method has been used to carry out the analysis of RC frame for the present study. Parametric Study To study the stiffness change in RCC frame building four types of frames have been considered: first a bare frame, second a frame with in-fill at all storeys, third a frame with no in-fill at first storey, fourth frame with shear wall at all storeys (considering in-fill). The infill in the frame that is modelled as diagonal struts and the FEM based software carries the analysis.The analytical models for the G+3 storeys frames i.e. bare frame, frame with no infill at first storey, frame with in-fill, are shown in Figs. 1 to 3 respectively. Details of frame are given in Table 1. The time-period of frames obtained after the plane frame analysis as shown in Table 2. The time-period of bare frame has found to be 2.9 times the time-period of frame with in-fill at all storeys for G+3, which shows that stiffness is considerably increased by effect of in-fill. The mass participation factor (%) and base shear for G+3, G+5, G+7, G+9 building frames has been given in Table 3 and 4 respectively. The variation of storey drift with respect to height is shown in Figures 4 to 7, which show considerable amount of decrease in storey drift when the effect of in-fill is considered in analysis. Verification of Modelling Firstly, a G+3 plane bare frame is considered subjected to only gravity loads (as shown in Fig. 8), so that the validity of model can be verified. The results obtained from computational analysis are compared with manual calculation carried by using Kani’s Method [13]. Table 5 shows the bending moments calculated manually when compared with results of computational analysis are found to be approximately same, which shows that the present modelling of the frame, is correct and hence, further study has been carried out using this model. Discussion Results obtained from plane frame analysis are discussed here as under. Time Period Time period of a frame (in 1st mode) is observed to be more in case of bare frame for G+3 fame configuration. The Time period is observed to be decreased by 37% with the inclusion of in-fills at all the storeys above first storey. Further, inclusion of in-fills at the first storey makes it more rigid by the reduction in the time period by 65%. Similarly, in case of G+5, G+7 and G+9, the time period of bare frame becomes twice to that of the frame with no infills at first storey and the time period gets reduced by 60% with the in-fills at all storeys. Thus, it can be said that the in-fills play an important role in modifying time period of vibration. The time period of frame also observed to be increasing by considerable amount by

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increasing number of floors from G+3 to G+9. The time period of G+9 building frame is observed to be approximately 3 times than that of G+3 building frame. Similar trend followed with the inclusion of in-fills in frame. Hence, the inclusion of in-fills makes a building frame more rigid but the increment in height makes it more vulnerable to vibration. Storey Drift The storey drifts of frame with in-fills when compared with bare frame (see plot of storey drift v/s height for G+3 building frame fig. 4) shows considerable reduction in drifts in the storey where the in-fills are included. Moreover, when the in-fill not considered from the first storey, the storey drift abruptly increased at this level. The similar trend is followed for other frame configurations i.e. from G+5 to G+9. S.NO. 1 2 3 4

Table 1 Details of RC frames* PARTICULARS Size of Columns (mm) Size of beams (mm) Each storey height (m) Thickness of external in-fill panels (mm)

DETAIL 600 × 500 500 × 300 3.35 250

*plane frame analysis

TYPE OF BUILDING G+3 G+5 G+7 G+9

Table 2 Time-period (sec) of frames FRAME WITH NO INBARE FRAME FILL AT FIRST STOREY

FRAME WITH IN-FILL AT ALL STOREYS

MODE1

MODE2

MODE3

MODE1

MODE2

MODE3

MODE1

MODE2

MODE3

0.273 0.440 0.613 0.793

0.082 0.134 0.191 0.249

0.042 0.071 0.102 0.135

0.172 0.237 0.308 0.389

0.041 0.068 0.097 0.128

0.022 0.032 0.044 0.057

0.094 0.155 0.229 0.316

0.031 0.049 0.069 0.092

0.020 0.028 0.038 0.048

Table 3 Mass participation factor for RC buildings MASS PARTICIPATION FACTOR NUMBER OF FRAME (%) STOREYS CONFIGURATION MODE 1 MODE 2 MODE 3 Bare frame 81.09 12.50 4.81 G+3 With in-fill 82.33 12.10 3.49 With no in-fill at 1st storey 98.69 1.18 0.09 Bare frame 79.60 11.72 4.52 G+5 With in-fill 78.93 13.38 3.99 With no in-fill at 1st storey 96.81 2.92 0.19 Bare frame 78.86 11.47 4.25 G+7 With in-fill 76.18 14.98 4.04 93.80 5.72 0.35 With no in-fill at 1st storey Bare frame 78.38 11.44 4.08 G+9 With in-fill 73.84 16.52 4.16 With no in-fill at 1st storey 89.93 9.31 0.56 __________________________________________________________________________________________ 2173


NUMBER OF STOREY G+3

G+5

G+7

G+9

Table 4 Base shear FRAME BASE SHEAR CONFIGURATION (KN) Bare frame 42.80 with in-fill 50.57 No in-fill at 1st 73.26 storey Bare frame 56.29 with in-fill 93.40 No in-fill at 1st 136.95 storey Bare frame 63.56 with in-fill 150.48 No in-fill at 1st 179.40 storey Bare frame 64.66 with in-fill 184.69 No in-fill at 1st 204.25 storey

Vb/Vb 1.035 1.64 1.075 -

Table 5 Comparison of computational analysis and manual calculation BENDING MOMENT (kNm) BEAM NODE COMPUTATIONAL MANUAL ANALYSIS CALCULATION 2 64.82 65.13 5 7 -65.63 -66.92 3 65.94 65.13 6 8 -65.29 -66.92 4 67.13 65.13 7 9 -65.45 -66.92 5 67.00 69.57 8 10 -71.25 -73.38 6 9.73 10.69 9 7 -20.71 -17.73 13 -20.97 -19.43 19 14 -17.91 -19.43 12 61.36 62.71 21 17 -60.68 -60.85 13 61.07 62.71 22 18 -61.74 -60.85 14 61.24 62.30 23 19 -62.89 -61.67

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Figure 1 Analytical model of bare frame

Figure 2 Analytical model of frame with no in-fill at first storey

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HEIGHT (M)

Figure 3 Analytical model of frame with in-fill at all storeys

DRIFT (cm)

Figure 4 Relationship between drift and storey height for G+3 building

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HEIGHT (M)


DRIFT (cm)

HEIGHT (M)


DRIFT (cm)


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HEIGHT (M)


DRIFT (cm)


Figure 8 G+3 Plane bare frame

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SEISMIC EVALUATION OF RC BUILDINGS To carry out the parametric study, symmetric G+3, G+5, G+7, G+9 buildings have been considered with the plan as shown in Fig. 9. The comparative study between the bare frame buildings and the buildings with in-fill has been carried out with respect to dynamic characteristics that are fundamental time periods & mass participation factors and storey drifts with the help of Square root of summation of squares (SRSS) method.

Figure 9 Plan of the building

Modelling of Structures The overall plan dimensions of the RC frame structures of 14.4 × 24.4m, measured along the central line of the columns. The same plan (Fig 9) is adopted for all the four buildings i.e. G+3, G+5, G+7, and G+ 9. All the buildings are assumed to be fixed at ground level. All storey heights are taken to be 3.35m each. A solid RCC slab of 110mm thickness has been considered. Dead Load and Live Load intensities for roof and floor are given in Table 6 and 7 respectively. The section details for beams, columns and in-fill walls are given in Table 8. The thickness of exterior and interior brick masonry in-fill has been considered as 250mm and 150mm respectively. No reduction in weight of in-fill due to openings is considered. Design consideration for staircase is not taken into account, however its dead load and location are considered in modelling. A 3D view of building skeleton is shown in Fig 10.

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S. No. 1 2 3

S. No. 1 2 3 4 5 6 7 8

Table 6 Dead loads LOAD TYPE Terrace Water Proofing Floor Finish Sanitary Blocks including filling Table 7 Live loads LOAD TYPE Roof Library Assembly Hall Sanitary Blocks Office Floors Officer’s Chamber Stairs Corridor

INTENSITY (kN/m2) 2.5 1.0 2.5 INTENSITY (kN/m2) 1.5 10 5 3 4 3 5 5

Table 8 Section details S. No. MEMBER 1 Beams (Transverse*) 2 Roof Beams (Longitudinal**) 3 Corridor Beams(Longitudinal) 4 External Beams (Longitudinal) 5 Columns 6 External Walls 7 Internal Walls 8 Slab * along z-direction; **along x-direction

SIZE (mm) 500 × 300 300 × 300 300 × 300 350 × 300 600 × 500 250 150 110

Figure 10 Three-dimensional view of the building __________________________________________________________________________________________ 2180


Equivalent Strut Model The in-fill walls considered without opening in the present study are modelled as equivalent diagonal strut as proposed by Smith [1, 2, 3]. The use of Equivalent Strut Model is attractive from practical point of view. The properties required for defining the strut model depend on type of analysis. For linear type of analysis (as in present study), only the area, length of the strut and modulus of elasticity are required to calculate the elastic stiffness of in-fill strut. The following expressions have been used to determine the parameters (as shown in Fig. 11) required for modelling the diagonal strut.

αh =

αL =

w=

π

4

2

π 2

4

4E f I c h

[1]

E m t sin 2θ

4E f I b L

[2]

E m t sin 2θ

1 α h2 + α L2 2

[3]

Where, Em is Elastic Modulus of masonry wall, Ef is Elastic Modulus of masonry of frame material, t is Thickness of the in-fill wall, h is Height of the in-fill wall, L is Length of the infill wall, Ic is Moment of Inertia of the column of the frame, Ib is Moment of Inertia of the beam of the frame, θ is tan-1 (h/L) and W is Width of the Equivalent Strut. The calculations for width of equivalent diagonal struts using above expressions are given in Table 9. Table 9 Calculations for width of equivalent diagonal strut for both external and internal masonry in-fill* H (m)

L (m)

X

Sin2x

Ef

Em

Ic

Ib

t

ah

al

W

w'

3.35

6.2

0.495

0.836

22360

15000

0.009

0.00313

0.25

1.511

2.707

1.550

1.55

3.35

6.2

0.495

0.836

22360

15000

0.009

0.00313

0.15

1.717

3.075

1.761

1.76

3.35

6

0.509

0.851

22360

15000

0.009

0.00313

0.25

1.505

2.673

1.533

1.54

3.35

6

0.509

0.851

22360

15000

0.009

0.00313

0.15

1.710

3.037

1.742

1.74

3.35

2.2

0.989

0.917

22360

15000

0.009

0.00313

0.25

1.477

2.041

1.259

1.28

3.35

2.2

0.989

0.917

22360

15000

0.009

0.00313

0.15

1.678

2.319

1.431

-

3.35

3.05

0.832

0.995

22360

15000

0.009

0.00107

0.25

1.447

1.660

1.101

1.1

3.05

0.832

0.995

22360

15000

0.009

0.00068

0.15

1.644

1.681

1.175

1.2

3.35

*

Remarks outer in-fill along z- direction inner in-fill along z- direction outer in-fill along z- direction inner in-fill along z- direction outer in-fill along z- direction inner in-fill along z- direction outer in-fill along x- direction inner in-fills along x-direction

The equivalent strut shall have the same thickness and modulus of elasticity as the in-fill panel it represent.

Where, H is storey height in m, L is length of member in m, x is Tan-1(H/L) in radian, Ef is modulus of elasticity of concrete in MPa, Em is modulus of elasticity of masonry in MPa, Ic is moment of inertia of column in m4, Ib is moment of inertia of beam in m4, t is thickness of

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masonry walls in m, w is width of equivalent diagonal strut in m and w' is width of equivalent diagonal strut adopted for analysis in m.

Figure 11 Equivalent diagonal strut Method of Analysis

Modal analysis based on response spectrum has been adopted to dynamically analyse the structure with the help of FEM based software. The following Response Spectrum 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 is shown in Fig 12. ⎧ ⎫ ⎪1 + 15T , 0.00 ≤ T ≤ 0.10 ⎪ Sa ⎪ ⎪ = ⎨2.50, 0.10 ≤ T ≤ 0.40⎬ g ⎪ ⎪ 1 ⎪ 0.4 ≤ T ≤ 4.0 ⎪ ⎩T ⎭

PERIOD (sec)

Figure 12 Spectra curve for finding base shear from fundamental time period __________________________________________________________________________________________ 2182


A comparison of the dynamic characteristics of the bare frame buildings; frame with in-fill at all storeys and frame with no in-fill at first storey is observed. wherein the time period, mass participation factor (%), design base shear and storey drift obtained from the analysis results corresponding to mode 1, mode 2 and mode 3 as given by FE software are observed. Time Period and Fundamental Time Period

When a building is subjected to dynamic action it develops a vibratory motion in the building due to its elastic properties and mass. The vibration is similar to the vibration of a violin string, which consists of a fundamental tone and the additional contribution of various harmonics. Similarly, the vibration of a building consists of a fundamental mode of vibration and the additional contribution of various modes, which vibrates at higher frequencies. On the basis of time period the building may be classified as Rigid (T < 0.3 sec), Semi-Rigid (0.3 sec < T < 1 sec), and Flexible Structure (T > 1). Fundamental period of vibration can be determined by code base empirical formula. The time period obtained from dynamic analysis of G+3, G+5, G+7 and G+9 buildings in Zdirection of seismic force for first three modes are given in Table 10. The fundamental time periods for G+3, G+5, G+7 and G+9 buildings, estimated by using the empirical expression given in IS 1893 (Part 1): 2002 are given in Table 11, which shows decrease in time periods with the inclusion of in-fill and similar trend is followed by increasing the number of storeys. The results obtained in space frame analysis matches approximately with that of plane frame analysis. Table 10 Time periods obtained from dynamic analysis of RC buildings in Z-direction NUMBER OF STOREYS G+3

G+5

G+7

G+9

FRAME CONFIGURATION Bare frame With in-fill With no in-fill at 1st storey Bare frame With in-fill With no in-fill at 1st storey Bare frame With in-fill With no in-fill at 1st storey Bare frame With in-fill With no in-fill at 1st storey

TIME PERIODS (Sec) MODE 1

MODE 2

MODE 3

0.404 0.172

0.323 0.100

0.123 0.058

0.250

0.187

0.064

0.632 0.257

0.505 0.147

0.198 0.085

0.341

0.246

0.100

0.868 0.353

0.691 0.193

0.275 0.115

0.438

0.300

0.139

1.111 0.464

0.879 0.240

0.354 0.147

0.546

0.353

0.180

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Table 11 Fundamental Time Period (Sec) for RC building NUMBER OF STOREYS G+3 G+5 G+7 G+9

FRAME CONFIGURATION Bare frame With in-fill Bare frame With in-fill Bare frame With in-fill Bare frame With in-fill

FUNDAMENTAL TIME PERIOD (Sec) Z 0.525 0.318 0.712 0.477 0.883 0.635 1.044 0.795

Mass Participation Factor

The effective modal mass provides a means for judging the significance of a particular mode of vibration in the dynamic analysis. It has been observed during this study that with the different frame configurations (i.e. bare frames, frames with in-fill at all storeys and frames with no in-fill at first storey), the mass participation factors (in Z-direction) for the first mode gets increased when effect of in-fill is considered as given in Table 12. Similar trend is followed by increasing the number of storeys (i.e. G+3, G+5, G+7 and G+9). Table 12 Mass participation factor for RC buildings NUMBER OF STOREYS G+3

G+5

G+7

G+9

FRAME CONFIGURATION Bare frame With in-fill With no in-fill at 1st storey Bare frame With in-fill With no in-fill at 1st storey Bare frame With in-fill With no in-fill at 1st storey Bare frame With in-fill With no in-fill at 1st storey

MASS PARTICIPATION FACTOR (%) MODE 1 MODE 2 MODE 3 82.38 0.00 11.81 92.55 0.10 5.72 98.28

0.01

1.51

80.72 87.84

0.00 0.55

11.27 9.38

96.29

0.08

3.24

79.84 83.64

0.00 1.10

11.15 12.04

93.46

0.25

5.64

79.27 80.12

0.00 1.57

11.17 14.02

90.08

0.50

8.46

Design Base Shear

The design base shear ‘VB’ given in Table 13 determined as per code IS 1893 (Part 1): 2002 with the following conditions: __________________________________________________________________________________________ 2184


Table 13 Design base shear

NUMBER OF STOREY G+3

G+5

G+7

G+9

FRAME CONFIGURATION Bare frame with in-fill No in-fill at 1st storey Bare frame with in-fill No in-fill at 1st storey Bare frame with in-fill No in-fill at 1st storey Bare frame with in-fill No in-fill at 1st storey

BASE SHEAR (kN) 665.45 961.49 1221.15 794.08 1728.25 1734.52 784.15 2062.48 2100.06 737.31 2267.08 2308.15

Vb / V B 1.094 1.067 1.112 1.176 1.234 -

Response spectra curve is shown in Fig 12, Zone factor, Z = 0.16 (Zone III), Importance factor, I = 1.5, Soil site type = hard soil, Response reduction factor, R = 3 and Damping is assumed to be 5 %. The design base shear Vb as per IS: 1893 [11] shall be calculated by the following formula: Vb = AhW

[4]

Where,

Ah =

ZIS a 2 Rg

[5]

W is Seismic Weight of the building If ‘VB’ is less than ‘Vb’, all the response quantities shall be multiplied by the ratio Vb/VB. Storey Drift

The inter storey drift is restricted so that the minimum damage would take place during earthquake and posing less psychological effect in the mind of people. The Indian Seismic Code IS 1893 (Part 1): 2002 recommends that “The storey drift in any storey due to the minimum specified designed lateral force, with partial load factor of 1.0, shall not exceed 0.004 times the storey height.” The variation of storey drift with height as observed during plane frame analysis is shown in Figs. 4 to 7 and as observed during space frame analysis is shown in Figs. 13 to 16 (for different frame configurations as mentioned above). It has been __________________________________________________________________________________________ 2185


observed that the storey drifts are considerably reduced when the effect of in-fill are considered. All the drifts are found to be within permissible limit i.e. 1.34 cm.

HEIGHT (M)

Storey Drifts in case of shear wall (considering in-fills) is observed to be minimum when compared to all other frame configurations, which shows that the shear wall are best suited for increasing the lateral stiffness of building.

DRIFT (cm)

HEIGHT (M)


DRIFT (cm)


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HEIGHT (M)


DRIFT (cm)

HEIGHT (M)


DRIFT (cm)


EFFECT OF SHEAR WALL ON SEISMIC EVALUATION OF RC BUILDINGS Modelling of Shear Wall

Analysis of shear wall is carried out by finite element method based software. The shear walls start at ground level and are continuous throughout the building height in present study. These walls are considered to be of uniform thickness of 250mm. The analysis is done by considering the effect of in-fill along with the shear wall. The in-fill is provided as equivalent __________________________________________________________________________________________ 2187


diagonal struts (having the same properties as described above). The position of shear wall is provided symmetrically at the corners of the building as shown in Fig. 17.

Figure 17 Position of shear walls

Discussion of Results

A comparison of the dynamic characteristics of the bare frame buildings and frame with shear wall is observed, wherein the time period, mass participation factor (%), design base shear and storey drift obtained from the analysis results corresponding to mode 1, mode 2 and mode 3 as given by the software are observed. Time Period

The time period obtained from dynamic analysis of G+3, G+5, G+7 and G+9 buildings in Zdirection of seismic force for first three modes are given in Table 14. Table 14 Time periods obtained from dynamic analysis of shear wall buildings NUMBER OF STOREYS 4 6 8 10

TIME PERIODS (Sec)

FRAME CONFIGURATION

MODE 1

MODE 2

MODE 3

Bare frame With Shear Wall Bare frame With Shear Wall Bare frame With Shear Wall Bare frame With Shear Wall

0.404 0.115 0.632 0.170 0.868 0.236 1.111 0.316

0.323 0.067 0.505 0.097 0.691 0.130 0.879 0.164

0.123 0.047 0.198 0.065 0.275 0.083 0.354 0.101

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Mass Participation Factor

The Mass Participation Factor (in z-direction) for first three modes for the frame with shear wall by varying number of storeys i.e. G+3, G+5, G+7 and G+9 are given in Table 15 while the same has been already discussed above for other frame configurations. Table 15 Mass participation factor NUMBER OF STOREYS G+3 G+5 G+7 G+9

MASS PARTICIPATION FACTOR (%) MODE 1 MODE 2 MODE 3 88.09 0.01 4.70 84.69 0.00 10.06 80.73 0.09 14.02 77.36 0.26 16.62

Design Base Shear

The design base shear for the frame with provision of shear wall given in Table 16: Table 16 Design base shear NUMBER OF STOREYS G+3 G+5 G+7 G+9

BASE SHEAR (kN) 864.89 1531.37 2369.63 2813.38

Vb/VB 1.30 -

Storey Drift

Storey drift variation with height for the frame with provision of shear wall with different number of storeys is also shown in Figs 13 to 16. The considerable amount of reduction in drift is observed by an amount of 93.4% in maximum storey drift for the frame with shear wall when compared with bare frame is observed. It is also found that the storey drift for the frame with shear wall is minimum with respect to all other configuration under study.

CONCLUDING REMARKS The following conclusions are drawn from the present study: i)

The natural period of vibration of the building frame depends upon its mass and lateral stiffness. Masonry in-fill panels increases both the mass and stiffness of the building, though the contribution of the latter is more significant.

ii)

Time periods of frames obtained after plane frame analysis gets substantially reduced by the inclusion of in-fills. The time period of bare frame has been found to be 2.9 times the time period of frame with in-fill at all storeys for G+3, which shows that stiffness is

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considerably increased by effect of in-fill, and the similar pattern is followed with the increase in number of storeys and modes. iii)

Fundamental time periods as estimated by using empirical expression given in IS: 1893 (Part 1): 2002 has been found to be decreasing with the inclusion of in-fill.

iv)

The mass participation factor decreases considerably with the increase in number of modes.

v)

Time periods of frames obtained from space frame analysis also follows the same pattern as in case of plane frame analysis.

vi)

It has been observed that the storey drifts are considerably reduced when the effect of in-fill are considered. All the drifts are found to be within permissible limit, i.e. 1.34 cm.

vii)

When there are no in-fills at the ground storey, the storey drifts is found to be considerably greater than that observed when the effect of in-fill at ground storey is considered. Hence, stilt buildings are more vulnerable to collapse due to soft storey formation.

viii)

The maximum storey drift reduced by 93.4% for the frame with shear wall when compared with bare frame. It is also found that the storey drift for the frame with shear wall is found to be minimum with respect to all other configuration under study.

REFERENCES 1.

SMITH B S, The Composite Behaviour of Infilled Frames. In Proceedings of a Symposium on Tall Buildings with Particular Reference to Shear Wall Structures, University of Southampton, Department of Civil Engineering. (Oxford: Pergamon Press), 1966.

2.

SMITH B S, Lateral stiffness of infilled frames, Journal of Structural division, ASCE, 88 (ST6), 1962, pp. 183-199.

3.

SMITH B S, Behaviour of square infilled frames, Journal of Structural division, ASCE, 92 (ST1), 1966, pp. 381-403.

4.

SINGH H, Response of Reinforced Concrete Frames With Infilled Panels Under Earthquake Excitation, PhD Thesis, Department of Civil Engineering, Thapar Institute of Engineering & Technology, March 1995.

5.

BELL D K AND DAVIDSON B J, Evaluation of Earthquake Risk Buildings with Masonry Infill Panels, NZSEE Conference, Paper No.4.02.01, 2001.

6.

DAS D AND MURTY C V R, Brick Masonry Infills in Seismic Design of RC Frame Buildings: Part 2- Behaviour, The Indian Concrete Journal, 2004.

7.

AMATO G, CAVALERI L, FOSSETTI M, AND PAPIA M, Infilled Frames: Influence of Vertical Load on The Equivalent Diagonal Strut Model, The 14th World Conference on Earthquake Engineering, Beijing, China, 2008.

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8.

BARAN M AND SEVI T, Analytical and experimental studies on infilled RC frames, International Journal of the Physical Sciences, 2010, Vol. 5, No.13, pp. 1981-1998.

9.

ASTERIS P G, KAKALETSIS D J, CHRYSOSTOMOU C.Z., SMYROU E.E., Failure Modes of In-filled Frames, Electronic Journal of Structural Engineering, 2011,Vol. 11, No. 1.

10. MOHAN R AND PRABHA C, Dynamic Analysis of RCC Buildings with Shear Wall, International Journal of Earth Sciences and Engineering ,, ISSN 0974-5904, 2011, Vol. 04, No 06, pp 659-662. 11. IS 1893 (Part 1), Indian Standard: Criteria for Earthquake Resistance Design of Structures, New Delhi, 2002. 12. IS 456, Indian Standard: Plain and Reinforced Concrete- Code of Practice, New Delhi, 2000. 13. KARVE S R AND SHAH V L, Illustrated Design of Reinforced Concrete Buildings, Structures Publishers, Pune, 1994.

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