EXPERIMENTAL MODELING OF IN FILLED RC FRAMES WITH OPENING

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International Journal of Civil Engineering and Technology (IJCIET)
Volume 7, Issue 2, March-April 2016, pp. 95–106, Article ID: IJCIET_07_02_007
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ISSN Print: 0976-6308 and ISSN Online: 0976-6316
© IAEME Publication
EXPERIMENTAL MODELING OF IN
FILLED RC FRAMES WITH OPENING
M.E. Ephraim
Department of Civil Engineering, Rivers State University of Science and Technology,
P.M.B 5080 Port Harcourt, Rivers State, Nigeria
T.C. Nwofor
Department of Civil Engineering, University of Port Harcourt,
P.M.B 5323 Port Harcourt, Rivers State, Nigeria
ABSTRACT
Reinforced concrete frames are usually infilled with masonry walls but, in
most designs, both the shear strength capacity of these walls and the
contribution of the infill panel openings on the shear strength of the infilled
frame, especially in critical cases of seismic loading are generally ignored.
This paper reports the results of an experimental study of the influence of
central openings in the infill on the sway stiffness of reinforced concrete plane
frames. A series of 1:4 scaled structural models with opening ratios from 0 to
50 percent in steps of 10 percent were designed, constructed and tested in the
study to obtain the load - displacement profiles. The test results were validated
with output of FE models of the prototype walls using SAP 2000 analysis
software. The results confirm that 1:4 model adequately reproduces the
behavior of infilled frame with openings including lateral stiffness and
anisotropy. The six percent accuracy of predicted shear strength of infilled
frames under lateral loadings as a function of opening ratio is considered
sufficient for engineering design purposes.
Key words: Modeling, Similitude Requirement, Sway Deflection and
Stiffness.
Cite this Article: M.E. Ephraim and T.C. Nwofor, Experimental Modeling of
In Filled RC Frames with Opening, International Journal of Civil Engineering
and Technology, 7(2), 2016, pp. 95–106.
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=7&IType=2

M.E. Ephraim and T.C. Nwofor
1. INTRODUCTION
It has been established that the consideration of the infill panel in the design of RC
frame structures results in a complex modeling problem because of the large number
of interacting parameters and the many possible modes of failure that need to be
evaluated with a high degree of uncertainty[1 - 9]. The need to obtain a deeper
understanding of the influence of openings on the composite behavior of infilled
frames has further led to the development of more and more complex models with
ever increasing number of parameters [10-16]. An experimental study in which all
these factors could be taken into account is difficult to implement for obvious reasons
[17-19]. Thus, in most cases, the use of finite element approach has been considered a
most viable option in spite of its computational complexities and resource
requirements. For these reasons, the need for more simplified models of the composite
behavior of infilled frame has been recognized by researchers. In this regard, perhaps
the most popular of the simplified models remains the one-strut model (OSM),
proposed by Polyakov [10]. However, the major challenges in the development of this
model is in deciding the value of the width of the equivalent strut on the one side and
how to account for the effect of openings on the other. In this study, the shear
strength of infilled frames with openings was investigated using the structural
modeling theory and appropriate experimental techniques. The main aim of the study
was to obtain experimental data to assess the magnitude and trend of variation of the
shear strength of reinforced concrete infilled plane frames as a function of the opening
ratio. The brick masonry infill panel incorporated various sizes of square openings,
centrally located in the infill. The frame thus varied from the fully infilled frame to
the bare frame configurations. The effects of the opening ratio on the strength,
stiffness and drift of the infilled sway frames under lateral racky load were
investigated and the outputs compared with values obtained on the basis of numerical
analysis by the finite element method.
2. GEOMETRICAL CHARACTERISTICS OF MODEL FRAMES
The structural design of the prototype frame was carried out in accordance with
Eurocode 6, BS EN 1996 (2006) the lateral load capacity Q calculated. A series of 1:4
scaled reinforced concrete frame models with centrally located openings of varying
opening ratios was constructed and tested in the Structural Engineering Laboratories
of the Rivers State University of Science and Technology, Port Harcourt, Nigeria. The
details of the models and their construction are presented in 3.2
2.1. Similitude Requirements for Modeling
To obtain the appropriate loading for the models, the theory of dimensional analysis
and similitude mechanics was employed to determine the prediction and operating
dimensionless parameters for modeling the real prototype behavior [20-21]. The
theoretical framework was based on assumptions that the diagonal tensile stress σt of
an infill wall was dependent on the following variables: the magnitude of the racky
load Q, span L, thickness t, the modulus of elasticity E and Poisson’s ratio ν. The
relationship can be implicitly expressed as follows
 Q, , , , , 0F L t E  
(1)
Considering the elastic modulus E and span L as dimensionally independent
variables for static structural modeling, equation (1) can now be expressed in
dimensionless products in the form

Experimental Modeling of In Filled RC Frames with Opening
2 , , , 0
tQG
EL E L


 
 
  (2)
The functional G must be the same for various scales of measurement and hence it
must be same in the model and prototype. Therefore, similitude requirements for
modeling will result from forcing the non-dimensional terms to be equal in model and
prototype. Thus,
Pr2 2ototype Model
Q Q
E L E L
   
   
    (3)
From where
Qp = QM . SE . SL
2
(4)
Here, Qm and QP represent the values of racky load in model and prototype,
respectively;
SE and SL – scale factors for material and geometry
Assuming the same material in prototype and model, and neglecting Poisson’s ratio
distortion, SE = S ν = 1.
Hence, the model load equals
2
L/SM PQ Q
(5)
The linear scale factor SL equals to 4 for 1:4 scaled model adopted in this
investigation.
The design and structural detailing of a typical specimen are given in Table 1 and
Figure 1.
Table 1 Design Details of Prototype and Model RC Frame
Design Characteristics Prototype 1:4 Model
Total height (mm) 2800 725
Total length (mm) 3600 900
Cross section of columns (mm) 300 x 300 75 x 75
Cross section of beam 400 x 300 100 x 75
Longitudinal reinforcement of columns 4 Ø 16 4 Ø 4
Tensile and Compression rein. of the beam 2Ø 16 Top, 3Ø 16 Btm 2 Ø 4, 3 Ø 4
Stirrups Ø 10 @ 150mm c.c Ø 2.5 @ 50mm c.c

Figure 1 Structural Details of the Prototype Model
3. EXPERIMENTAL PROCEDURE
The experimental procedure consisted of instrumentation and testing single-bay,
single-storey reinforced concrete plane frames, infilled with one-quarter scale brick
masonry with centrally located opening of various sizes. The following nomenclature
was adopted for the frames: Model Frame (MF) followed by two digit suffixes
representing the percentage opening. Thus, for example, MF10, MF20, MF30 etc
represent model frames with 10, 20 and 30 percent opening ratios respectively. A total
of seven frame models were constructed and tested as detailed in section 3.2.
Appropriate tests were also conducted to determine the mechanical characteristics
which were required as input for the finite element validation of the experimental
results.
3.1. Modulus of Elasticity and Poisson’s Ratio for Model Materials
The basic mechanical properties of masonry were obtained from tests carried out on
the masonry units used. These mechanical properties are basic input parameters for
the finite element micro modeling of masonry infilled frame structure. The modulus
of elasticity and Poisson’s ratio of the masonry were determined through loading a
four-block wallet vertically and measuring the strains in the longitudinal (X) and
transverse (Y) directions. Mechanical strain gages of sensitivity 0.01mm were used in
the strain measurements. Prototype burnt bricks of dimensions 224 x 106 x 72mm
were set on 13mm mortar. Three mortar mixes, namely 1:3, 1:4.5 and 1:6, were
considered. The load was applied normal and parallel to the bedding and average
values taken as representative of the mechanical properties of the masonry. The tests
were conducted in accordance with BS EN 1996 (2006). Plates 1A, B, C demonstrate
the test set up and failed specimen.

A B
C
Plate 1 Test Setup for Determination of Mechanical Properties of Brickwork and Failed
Specimen
By measuring the compression load and the strains  x and y, the values of
modulus of elasticity (E) and Poisson’s ratio (v) were obtained through the following
basic relationships
y
y
E



; y
x
v



(6)
3.2. Sway Frame Model Construction, Test Set-up and Procedure
The main aim of the experimental program on the single bay, single storey reinforced
concrete infilled frames with openings was to obtain a load-displacement profile for
each specimen in order to capture the degree of reduction in the shear strength or
sway stiffness of the infilled frames as a function of the opening ratio. To maintain
good workmanship, the frame and the infill brickwork were constructed in horizontal
beds. The ground beam was constructed in-situ and allowance made in the column
pits to accommodate the erection of the precast frame and infill. Dial gauges Model
EL83-546 of 0.01mm sensitivity were installed to measure the horizontal
displacement as a result of the lateral in-plane loading. The general pre-test set-up is
shown in Plate 2. The lateral load was applied by the aid of a hydraulic jack at the
level of the horizontal axis of the beam. A 70 kN proofing ring, duly calibrated, was

used to measure the applied lateral load during the tests. The analysis and discussion
of results obtained are presented in section 5.
Plate 2 Test Set-up and Instrumentation for Determination of Sway Stiffness of Infilled
Frames with Various Opening Ratios
4. THE FINITE ELEMENT MODEL
To advance the comparison with another reliable model, the FE micro model was
executed using SAP 2000 version 14, a sophisticated software package for finite
element modeling with capacity to model infill openings. Minor details that do not
significantly affect the analysis were deliberately left out from the models for ease of
analysis. The comparative analysis of the experimental and finite element results is
presented in Table 4 under results.
5. RESULTS
The results obtained from tests on infill wall specimens and infilled frame structures
are presented in the subheadings that follow.
5.1. Mechanical Properties of Model Materials
The summary of mechanical properties for brick infill obtained to aid the finite
element analysis of the models is given in Table 2.
Table 2 Summary of Test Results on Brick-Mortar Wall Specimens
Description
of Loading
Mortar
Mix
No. of
Specimen
Compressive
Strength (Fm)
(N/m2
)
Strains
10-3
Modulus
of
Elasticity
(Em)
(kN/m2
)
Poisson’s
Ratio 
x y
Perpendicular
to bedding
plane
1:3 2 13.46 1.90 0.55 8.41 0.29
1:4 2 11.54 2.00 0.66 7.21 0.33
1:6 2 10.58 4.70 1.69 6.61 0.36
Parallel to
bedding plane
1.3 2 8.50 5.70 1.03 5.32 0.18
1.4 2 7.20 9.20 1.93 4.67 0.21
1.6 2 5.10 8.60 2.41 3.21 0.28

The variation of modulus of elasticity mE with compressive strength m measured
on specimens is shown in Table 2. An average relationship was obtained for modulus
Em and Fm in the form
Em1 = 634.66Fm1 (7)
Em2 = 640.00Fm2 (8)
Where the suffixes 1 and 2 denote values corresponding to compressive load
normal and parallel to mortar bedding respectively.
In order to be consistent as regards suitable mechanical property for masonry
infill, the following average values of modulus of elasticity and Poisson’s ratio were
adopted.
xE = 4.4 x 106
kN/m2
; yE = 7.41x 106
kN/m2
; Poisson’s ratio, xy = 0.22; yx =
0.33
From the above results, the anisotropy of the masonry wall is obvious.
The mechanical properties of the concrete in the frame are considered to be fairly
stable and thoroughly documented. Hence, reference values were obtained from a
previous works for example [1], [14], [18], [21] among others.
Modulus of elasticity, xE = yE = 2.9 x 107
kN/m2
Poisson’s ratio, xy
= yx
= 0.20
5.2. Model and Prototype Deflections and Computed Sway Stiffnesses
The results of experimental tests and numerical analysis are summarized in Tables 3
and 4.
Table 3 Experimental Values of Deflection of Test Models
Model
Loads
Model lateral Displacements (mm)
MF 0 MF 10 MF 20 MF MF 40 MF 50 MF100
(KN) 30
3.125 0.38 0.40 0.42 0.42 0.45 0.77 0.82
6.25 0.72 0.76 0.93 0.95 1.20 1.25 1.31
9.375 0.93 0.96 1.27 1.35 2.20 2.07 2.19
12.5 2.25 2.34 1.97 2.12 2.75 2.87 3.01
15.624 3.64 3.75 4.30 4.00 4.82 5.75 6.15
18.75 5.38 5.75 5.55 6.62 7.00 9.00 9.72

Table 4 Comparative Analysis of Experimental Sway Deflections and Analytical Results
Specimen
Opening
Ratio %
Model
Deflection at different load application (mm)
50kN 100kN 150kN 200kN 250kN 300kN
MF 0 0% FE model 1.47 2.69 3.69 9.10 15.20 20.69
Exp. Model 1.52 2.88 3.72 9.00 14.56 21.52
Diff. % 0.33 6.59 0.81 1.11 4.40 3.86
MF10 10% FE model 1.53 3.10 4.01 9.21 14.59 21.09
Exp. Model 1.60 3.01 3.86 9.34 15.01 23.00
Diff. % 4.40 2.99 3.87 1.39 2.80 8.30
MF20 20% FE model 1.62 3.58 5.12 8.01 16.15 23.22
Exp. Model 1.70 3.70 5.10 7.90 17.20 22.20
Diff. % 4.71 3.24 0.39 1.39 6.50 4.60
MF30 30% FE model 1.67 3.70 5.51 8.20 17.10 25.95
Exp. Model 1.70 3.80 5.40 8.50 16.00 26.50
Diff. % 1.76 2.63 2.04 3.53 6.87 2.07
MF40 40% FE model 1.99 4.51 7.72 11.02 19.11 27.12
Exp. Model 1.80 4.80 8.80 11.00 19.30 28.00
Diff. % 1.06 6.04 12.27 0.18 0.98 3.14
MF50 50% FE model 2.79 5.79 8.05 12.44 22.45 34.12
Exp. Model 3.10 5.00 8.30 11.50 23.00 36.00
Diff. % 0.10 0.16 3.01 8.17 2.39 0.05
MF100 100% FE Model 2.89 5.40 8.50 13.00 23.10 36.02
Exp. Model 3.28 5.24 8.76 12.04 24.60 38.88
Diff. % 11.89 3.05 2.97 7.97 6.10 7.35
The value of lateral load applied to test models and the corresponding lateral
displacements were read from the proofing ring and dial gages. The experimental
results are presented in Table 3. The predicted prototype loads and the corresponding
lateral displacements, based on the similitude requirement obtained in section 2.1, are
presented in Table 4 under the appropriate rows for each model tested. The prototype
loads and deflections were extrapolated from the experimental values obtained from
model tests using the similitude expressions as follows:
2
P M LQ Q S and P M LS  , where
4LS  for 1:4 model.
5.3. Sway Deflection of Infilled Frames and Validation of Results
The dependence of corner deflection with load for the various opening ratios is
presented in Figure 2, from where it can be seen that there is approximately linear
relationship up to a load value of about 150kN for all values of opening ratios
investigated. This portion of the graph is followed by a more rapid increase of
deflection underscoring the non linear character of the force deflection curve.
The comparative analysis of the experimental results with those from numerical
analyses is presented in Table 4. It can be seen that lateral displacements obtained

from the test models are within 4 percent of the values based on the finite element
model. The close agreement between the results from experimental tests and
numerical analysis confirms the adequacy of the model to reproduce the strength and
deformation of the infilled frame including its anisotropy.
Figure 2 Force-Deflection Curves for Test Models with various Opening Ratios
5.4. Variation of Lateral Stiffness of the Infilled Frame with Openings
The values of lateral stiffness, computed as the ratio of the prototype load to the
corresponding sway deflection, are plotted in Figure 3 for different values of opening
ratio  . From the graphs of Figure 3, it can be seen that the observed increase in
lateral deflection due to increase in opening ratio of the infill as depicted in Table 4,
generally leads to a reduction in the computed sway stiffness of the infilled frames. It
was also observed from the plots that the stiffness increases with the solidity ratio
with the curve exhibiting a peak somewhere around 150kN load, followed by a falling
branch of slope gradually reducing with increasing opening ratio. It is important to
note that the peakness or kurtosis of the sway stiffness curves decreased with the
opening ratio, thus reflecting the reduction in the stress concentration effect of the
openings as the ratio increased from 0 to 100 percent.
A highly reduced rate of increase in sway stiffness in the linear zone was observed
for MF50 structural frame corresponding to 0.5  . This in line with the trend
observed in previous investigations [23], namely, that the influence of the opening
ratio beyond 50% is relatively insignificant up to a complete bare frame ( 1.0  ).
0
5
10
15
20
25
30
35
40
45
0 50 100 150 200 250 300 350
LateralDeflection(mm)
Applied Sway load (kN)
MF0
MF10
MF20
MF30
MF40
MF50
MF100

Figure 3 Graphical Plot of Infill Frame Stiffness against Sway Load.
6. CONCLUSIONS
The following specific conclusions can be drawn from this study.
1. The 1:4 experimental model is able to reproduce the shear resistance of the infilled
frame with reasonable accuracy. The experimental values are within 4 percent of the
corresponding results based on finite element model.
2. The experimental values of the elastic modulus in the directions normal and parallel
to the mortar bedding are in the ratio of about 1.68:1. This corresponds to the range of
documented values for burnt clay brick masonry. The close agreement of the results
from the experimental test with those from finite element model confirms that the 1:4
model adequately reproduces the anisotropy of the masonry infill.
3. There is approximately linear force-displacement relationship up to a lateral racky
load of about 150kN for all values of opening ratios investigated. This portion of the
graph is followed by a more rapid increase of deflection, indicating the non linear
character of the force deflection curve beyond this load. At about 150kN, the stiffness
curves exhibit a sharp peak, followed by a falling branch of slope gradually reducing
with increasing opening ratio.
4. The peakness or kurtosis of the sway stiffness curve sharply decreased with the
opening ratio, reflecting the reduction in the stress concentration effect of the opening
ratio as it is increased from 0 to 100 percent.
5. A highly reduced rate of increase in sway stiffness in the linear zone was observed for
the test frame with opening ratio 0.5  . This in line with the observations in
previous investigations namely, that the influence of the opening ratio beyond 50% is
relatively insignificant up to a complete bare frame configuration for which 1  .
0
5
10
15
20
25
30
35
40
45
0 50 100 150 200 250 300 350
Stiffness(kN/mm)
Sway Load (kN)
B=0.1
B=0.2
B=0.3
B=0.4
B=0.5

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