Docstoc

A COMPARATIVE STUDY OF THE EFFECT OF INFILL WALLS ON SEISMIC PERFORMANCE OF REI

Document Sample
A COMPARATIVE STUDY OF THE EFFECT OF INFILL WALLS ON SEISMIC PERFORMANCE OF REI Powered By Docstoc
					   INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME
                                TECHNOLOGY (IJCIET)
ISSN 0976 – 6308 (Print)
ISSN 0976 – 6316(Online)                                                        IJCIET
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)                     © IAEME
www.jifactor.com




    A COMPARATIVE STUDY OF THE EFFECT OF INFILL WALLS ON
   SEISMIC PERFORMANCE OF REINFORCED CONCRETE BUILDINGS

                    1
                        Prerna Nautiyal*, 2Saurabh Singh* and 3Geeta 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

                                                  208
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME

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

                                                   209
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME

(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

                                                    210
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME

        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.

                                                  211
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME

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.


                                                   212
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME

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.
                                                  213
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME

                                            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)

                                                 214
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME

                                               Table No. 2
                                                                               SEISMIC WEIGHT
          FLOORS              DEAD LOAD (KN)              LIVE LOAD (KN)
                                                                               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

                                                    215
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME

                                              Table No. 3
                    Weight        hi (m)
                                                              Wi hi2/∑ Wi     Qi=(VB)’X (Wi
          Storey    in KN,      from the      Wi hi2
                                                                  hi2          hi2/∑ Wi hi2)
                      Wi          base
             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


                                                  216
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME

                                             Table No. 4
                    Weight       hi (m)
                                                            Wi hi2/∑ Wi      Qi=(VB)’X (Wi
         Storey     in KN,     from the       Wi hi2
                                                                hi2           hi2/∑ Wi hi2)
                      Wi         base
            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     Neglecting the stiffness of
                                        the wall (Model A)              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.

                                                  217
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME

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.




                                               218

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:1
posted:9/5/2013
language:
pages:11