EFFECT OF MODELING OF INFILL WALLS ON PERFORMANCE OF MULTI STORY RC BUILDING by iaemedu

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									International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
   INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND
(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. 243-250
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    EFFECT OF MODELING OF INFILL WALLS ON PERFORMANCE OF
                  MULTI STORY RC BUILDING

                             Dr. Suchita Hirde*, Ms. Dhanshri Bhoite**
      * Professor, Dept. of App. Mechanics Govt. College of Engineering Karad 415124 India
       ** PG student, (Civil- Structure), Govt. College of Engineering, Karad 415 124, India


ABSTRACT

        Buildings with masonry infill wall RC frames are the most common type of structures used
for multistory constructions in the developing countries. In several moderate earthquakes, such
buildings have shown excellent performance during earthquake. While analysing such multi storey
frames using structural engineering software, normally the multistory frames are modeled as bare
frame. The infill walls are not modeled in software and its resistance is not considered to lateral
forces. Hence in this paper attempt has been made to study the effect of modeling of such infill walls
on the performance of multi storey reinforced concrete framed building. Nonlinear static pushover
analysis of multi storey frame is carried out considering it as bare frame. Then the pushover analysis
of same frame is carried out by modeling the infill walls for throughout the height and for modeling
the infill walls excluding ground storey so as to make it as soft storey, since the soft storey feature is
very common in multi storey building to provide the parking place. The results of bare frame
analysis and frame with infill effects are compared in the form of capacity spectrum curve,
performance point and hinge formation at performance point and conclusion are made. It is seen that
the masonry infill contribute significant lateral stiffness, strength, overall ductility and energy
dissipation capacity.

Keywords: Nonlinear static pushover analysis, performance point, performance level, plastic
hinges, bare frame, masonry infill, soft storey

INTRODUCTION

        In a country like India, use of reinforced concrete framed structure is very common in multi
storey building construction. The masonry infill walls which are constructed after completion of
reinforced concrete frames are considered as non-structural elements. Although they are designed to
perform architectural functions, masonry infill walls do resist lateral forces with substantial structural
action. In addition to this infill walls have a considerable strength and stiffness and they have
significant effect on the seismic response of the structural system. From the literature review it has

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

been seen that many research papers are available to understand the behavior of soft storey. However
there is little work carried out by researcher related to finding vulnerability of existing RCC building
with soft storey using pushover analysis. The concept of pushover analysis is rapidly growing in the
area of seismic evaluation now a day. It gives the capacity curve of the building from which lateral
load carrying capacity of the buildings can be calculated. Hence for the safety of the structure during
earthquake, it is important to assess performance level of the building and to suggest retrofitting
schemes for deficient buildings.
        There is a general agreement among the researchers that infilled frames have greater strength
as compared to frames without infill walls. The presence of the infill walls increases the lateral
stiffness considerably. Due to the change in stiffness and mass of the structural system, the dynamic
characteristics change as well. Structural engineering software are used to analyze multi storey
buildings. But while analyzing such structures using software the multistory building are modeled as
bare frames without infill walls. The dead load of wall is simply applied as an external dead load.
Hence in this study three different models of an eight storey building symmetrical in the plan are
considered to find the effect of modeling of such infill walls on performance of the building.
Following three different models are investigated in the study.

       Model I: Multi storey frame modeled as bare frame without infill
       Model II: Multi storey frame with infill excluding ground story so as to make it as soft story.
       Model III: Multi storey frame with masonry infill throughout the height of the building.

        These models of 8 storey building are modeled and pushover analysis (Nonlinear static
analysis) is carried out using SAP 2000 software. The purpose of pushover analysis is to evaluate the
expected performance of structural systems by estimating performance of a structural system by
estimating its strength and deformation demands in design earthquakes by means of static inelastic
analysis, and comparing these demands to available capacities at the performance levels of interest.
A plot of total base shear versus top displacement in a structure is obtained by this analysis that
would indicate any premature failure or weakness. The analysis is carried out upto failure, thus it
enables determination of collapse load and ductility capacity. This type of analysis enables weakness
in the structure to be identified. This type of analysis is useful to take decision for retrofitting of
structure. Based on these results this paper explores the beneficial effects of masonry infill walls on
seismic behavior of RC frame buildings.

MODELING OF MULTI STOREY BUILDING

        The study is carried out on 8 storey reinforced concrete moment resisting frame building
having symmetrical plan layout as shown in figure 1. Building is kept symmetric in both orthogonal
directions in plan to avoid torsional response under pure lateral forces. Plinth height is 1.2m and
story height is 3.1m for each floor level. The building is considered to be located in seismic zone V
and intended for residential use. The building is founded on medium strength soil. Elastic moduli of
concrete and masonry are taken as 28500 Mpa and 3500Mpa respectively and their poisons ratio is
0.2. The unit weights of concrete and masonry are taken as 25.0 KN/ m3 and 20.0 KN/ m3. The floor
finish on the floor is 1 KN/ m2. The roof treatment is taken as 1.5 KN/ m2. The live load on the floor
is taken as 3 KN/ m2 and that of the Roof is taken as 1.5 KN/ m2. In the seismic weight calculations
only 25% of the floor live load is considered. Size of beam for peripheral beams is considered as
300x500mm and internal beams as 300x600mm. Size of exterior columns is considered as 450x450
mm while internal column size is 550x550mm.             Figure 2 shows the 3 D model of bare frame,
Fig. 3 shows the 3 D model of multistory frame with masonry infill excluding ground floor (i.e. soft
storey) and Fig.4 shows 3 D model of multistory frame with masonry infill throughout the height.

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




                            Figure 1: Plan of Building




       Figure 2: Model I – Bare frame          Figure 3: Model II – Multi storey frames
               (without infill)                           with soft storey




               Figure 4: Model III – Multi storey frame with full masonry infill


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

PUSHOVER ANALYSIS

        The pushover analysis provides an insight into the structural aspects, which control the
performance during earthquakes. It also provides data on the strength and ductility of a building. It is
widely accepted that, when pushover analysis is used carefully, it provides useful information that
cannot be obtained by linear static or dynamic analysis procedures. Due to its simplicity, the
structural engineering profession has been using the nonlinear static procedure (NSP) or pushover
analysis. Modeling for such analysis requires the determination of the nonlinear properties of each
component in the structure, quantified by strength and deformation capacities, which depend on the
modeling assumptions. Pushover analysis is carried out from either user-defined nonlinear hinge
properties or default-hinge properties, available in some programs based on the FEMA-356 and
ATC-40 guidelines. While such documents provide the hinge properties for several ranges of
detailing, programs may implement averaged values. In this study, for pushover analysis, beams and
columns are modeled with concentrated plastic hinges for flexure and shear at the column and beam
faces, respectively. Beams have both moment (M3) and shear (V2) hinges, whereas columns have
axial load and biaxial moment (PMM) hinges and shear hinges in two directions (V2 and V3).

COMPARISON OF PERFORMANCE OF VARIOUS FRAMES

        Pushover analysis of frames shown in figure 2, 3 and 4 is carried out. Comparison between
the performance point in terms of base shear and roof displacement obtained from the nonlinear
static analysis and hinge formation pattern of the three models.

EFFECT OF MODELING OF INFILL OF WALLS ON BASE SHEAR AND ROOF
DISPLACEMENTS

Effect of modeling of infill of walls on base shear and roof displacements is presented in table 1.

     Table 1: Comparison of performance of three models in terms of base shear and roof
                                      displacement
                                             Performance Point
                                                                                            Seismic
       Building                 X direction                       Y direction
                                                                                           performa
      (8 storied)               (kN, mm)                          (kN, mm)
                                                                                           nce level
                                          Roof                              Roof
                        Base shear                        Base shear
                                      Displacement                     Displacement
     Bare frame
                         1518.249             194         1507.6362           187            LS-CP
  Open ground soft
                         8072.274             79            7815.3             83            LS-CP
        story
   Full masonry
                         12003.252            69          10458.648            71              B
       infills

         It is observed that the performance level of the bare frame and open ground soft story
structure are in LS-CP range (i.e. life safety to collapse prevention range) whereas full infill masonry
structure is in elastic range. Roof displacement for bare frame is greater than frame with masonary
infill and open ground soft story.



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

       Figure.5 and figure 6 show the comparison of pushover curve of three models in X direction
and in Y direction. These curves clearly represent that the performance of bare frame is modified
after modeling infill walls.




              Figure 5: Comparison of pushover curve for three models in X direction




             Figure 6: Comparison of pushover curve for three models in Y direction



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

EFFECT OF MODELING OF INFILL WALLS ON HINGE FORMATION PATTERN

        Figure 7, figure.8 and figure 9 shows the formation of hinges in structure at performance
level. Figure 7 shows the formation of hinges in model I: bare frame structure by considering push
load in X and Y directions. Figure 8 shows the formation of hinges in model II i.e. model with
masonry infill with soft bottom storey in X and Y directions and figure 9 shows the formation of
hinges in model III with full masonry infill in X and Y directions
        From the pushover curve shown in figure 5 and figure 6, it has been observed that the
performance of structure with masonry infill wall is improved after modeling the masonry infill walls
as compared to bare frame. Performance of bare frame is modified from LS-CP range i.e. life safety
to collapse prevention range to B range i.e. operational range after modeling the infill walls in
structure. In case bare frame, as observed in figure 7 more number of beams are in life safety range and
plastic hinges are concentrated at middle storeys.




        Figure 7: Formation of hinges in model I: Bare Frame in X direction and Y direction




    Figure 8: Formation of hinges in model II: Soft story structure in X direction and Y direction


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




Figure 9: Formation of hinges in model II: Full infill frame structure in X direction and Y direction

        The response of open ground soft story building in terms of plastic hinge properties is also in
collapse level due to formation of hinges some in life safety range, some in collapse prevention range
and some are at C (effective cracking) range which is not acceptable criteria for hinges in column. From
figure 8, it is very clear that since the columns of the soft storey are in LS range, this building will get
sever damages during earthquake. In case of full masonry structure shown in figure 9, all the hinges are
in B range (i.e. operational range) which is acceptable criteria for hinges. The plastic deformation in
case of masonry infill in columns and beams is within limit i.e. linear range but it is crosses collapse
(C) level in case of masonry strut.

CONCLUSION

        The Effect of modeling of masonry infill on the response of multi-storied building under
seismic loading is illustrated using three different models. The presence of full masonry infill panels
modifies the stiffness, performance of structure and formation hinges in elastic range i.e. all hinges
are formed operational. The total storey shear force increases considerably as the stiffness of the
building increases in the presence of masonry infill. In case of soft story at open ground floor the
mode of failure is by soft story mechanism (formation of hinges in LS, CP and C range in ground
floor columns). The lateral load resisting mechanism of the masonry infill frame is essentially
different from the bare frame. The bare frame acts primarily as a moment resisting frame with the
formation of plastic hinges at the joints under lateral loads. In contrast, the infill frame behaves like a
braced frame resisted by a truss mechanism formed by the compression in the masonry infill panel
and tension in the column. The plastic hinges are confined with the joint in contact with the infill
panel. It is seen that the existing building with open ground story is deficient due to formation of
plastic hinges at ground floor column at collapse level and in need to have retrofitting measures to
prevent collapse of such buildings during earthquake..
        The present study will be very useful to Civil Engineers to understand the effect of the
modeling of infill walls on the performance of multi storey framed reinforce concrete building. It is
also useful to understand the contribution of infill walls in formation of plastic hinges in beams and
columns in multistory frame.



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

REFERENCES

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