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SHEAR BEHAVIOR OF REINFORCED CONCRETE BEAMS WITH VERTICAL AND
TRANSVERSE OPENINGS
Article in TEST ENGINEERING AND MANAGEMENT · April 2020
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March-April 2020
ISSN: 0193-4120 Page No. 22120- 22133
SHEAR BEHAVIOR OF REINFORCED
CONCRETE BEAMS WITH VERTICAL AND
TRANSVERSE OPENINGS
Ahmed S. Hamzah1, Ammar Y. Ali2
M.Sc. Student, College of Engineering, University of Babylon, Hilla, Babil, Iraq.
2
Professor, Ph.D., College of Engineering, University of Babylon, Hilla, Babil, Iraq.
sengahmed@yahoo.com1 , dr_ammaraljanabi2005@yahoo.com2
1
Article Info
Volume 83
Page Number: 22120- 22133
Publication Issue:
March - April 2020
Article History
Article Received:19 October 2019
Revised: 27 December 2019
Accepted: 29 March 2020
Publication: 30 April 2020
Abstract:
Providing vertical opening for reinforced concrete beams becomes very common in
building construction. It is used to pass various services, especially for buildings of
limited size and height. In this paper, an experimental study is conducted on the shear
behavior of RC beams with opening. The main aim of this study is to investigate the
effect of the vertical opening on the shear strength and behavior of RC simply
supported beams. The study consisted of tested five RC beams, one of them without
opening represents the reference beam, while all the other beams having an opening
at their mid-shear span. The variables adopted here included openings direction,
openings shape, and the details of replacing the reinforcement that will obstruct the
openings penetration. The experimental results showed that the presence of a vertical
opening slightly reduced the ultimate load capacity. In addition, it has an insignificant
effect on increasing the maximum deflection at the service loads. On the other hand,
it is found that the circular shape of openings exhibited a lower effect on reducing
the ultimate load capacity than square openings. With regard to methods that used to
replace the obstruction rebars, the results showed that using stirrup on each side of
the opening in the longitudinal direction is adequate to enhance the ductility index
and recover the lost strength. In addition to the above, it is found that the transverse
openings have a significant effect on the ultimate load compared with the vertical
opening’s effect.
Keywords: Reinforced concrete beams; Vertical opening; Transverse opening; Shear
behavior
I. INTRODUCTION
Floor system for all multistory buildings requires
large slab openings for stairwells and elevator
shafts, in addition, it requires multiple small beam
and slab penetrations for fire protection pips,
routing of plumbing, and ductwork between floors
or in the same floor.
There are two types of openings in RC beams
according to their direction; horizontal (transverse)
opening which crosses the beams in the direction
of their width, and vertical opening which crosses
the beams in the direction of their height. In Iraq
and other countries, the use of reinforced concrete
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beams with vertical openings become very
common in building construction to pass various
services. However, special consideration must be
given to the analysis and design of such beams
depending on the effect of the openings. In the last
five decades, intensive research has been
conducted on the effect of transverse openings on
RC beams. In fact, vertical openings no less
important, there is almost no multi-story building
without beams having small or even large vertical
opening especially for buildings of limited size and
height; see Figure 1.
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Fig. 1. Service pipes passing through RC beams with vertical openings in a limited height building.
In general, the influence of openings on the
beam's strength and deformation relies upon the
openings' location and size. If the place is selected
wisely, quite large openings on RC beam structures
can be provided without affecting the ultimate load.
However, even small openings positioned in an
unfavorable place can cause a dramatic decrease in
the beam's strength. For that reason, it is necessary
to plan the size and place of the openings
accurately. However, the existence of the openings
results in discontinuities or disturbances in normal
stress flow, resulting in a concentration of stress
and early cracking at the opening vicinity.
Therefore, special reinforcement at opening region
should be provided in adequate amounts to control
the cracks’ width and avoid any possible premature
failure of the beam [1].
Vertical openings in RC beams are used instead
of small slab penetrations, especially for buildings
of limited size and height (low – rise buildings). In
fact, the effect of small openings in the structural
behavior of RC slabs is often not considered due to
the structure ability to redistribute stresses [2].
However, it will take up useful space and make the
services visible, thus may not be aesthetically
convenient and need a suspended ceiling or special
decoration to be acceptable from the aesthetic
viewpoint. On the other hand, most ducts and pipes
passaging through vertical openings in beams will
be hidden in the partitions until its reach the
specified location, for this reasons, vertical
Published by: The Mattingley Publishing Co., Inc.
openings in RC beams become commonly used.
However, unlike the horizontal opening, which
can be placed properly without cutting the
compression zone of the beam and does not affect
the ultimate moment capacity [3], the vertical
opening will always damage apart from it and thus
decreases the area of concrete needed to ultimately
develop the complete compressive stress block.
The reduced area of concrete in compression
should be taken into account in design because it
will cause a reduction in both flexural and shear
ultimate strength of the beam. In addition, the
provision of a vertical opening has a possibility of
cutting or obstructs the flexural and shears
reinforcement bars.
Several studies had investigated the effects of
transverse openings with various shape, size, and
location on the flexural, shear, and torsion behavior
of RC beams. In addition, there are a large number
of researches about the design and strengthening of
such beams [4]–[10]. However, very little
researches were found in the literature about the
effect of vertical opening on the behavior of RC
beams. Aziz and Ajeel [11] studied the shear
behavior of RC T-beams with vertical opening or
cold joints in a different location of the flanges.
They found that the presence of flange openings
reduce the shear capacity and cracking load of the
beams. Al-Jazaeri and Dawood [12] reported a
similar study of Aziz and Ajeel but CFRP
laminates have been used to strength the flange of
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the T-beam specimens having openings. They
found that the use of CFRP sheet to upgrade the
reinforced concrete T- beams with flange openings
has a significant effect on enhancing overall
behavior such as the ultimate load, crack width, and
deflection.
With regard to vertical openings crossing the
entire depth of the beam, Silva et al. [13],
conducted an experimental and numerical
investigation on RC beams with multiple vertical
openings. They found that how more was opening
close to support, more the main tensile grew, thus
may result in a possible diagonal tensile failure.
However, they discovered that when the openings
were located adjacent to the support, it does not
affect the ultimate load of the beam.
On the other hand, the main practice codes for
designing RC structures do not provide provisions
for the design of RC beams with vertical openings.
Currently, the American Concrete Institute (ACI),
in its guide of simplified design for reinforced
concrete buildings ACI 314R-16 [14], recommends
passing the opening vertically through a beam,
girder, or joist by using the guidelines illustrated in
Figure 2.
Fig. 2. Location of conduits and pipes passes vertically through girders, beams, and joists according to ACI
314-16
In addition to the American code, the Brazilian
Association of Technical Standards in its Brazilian
code for design of concrete structures - Procedure
NBR 6118 (2014) [15], recommends passing the
opening vertically through a beam by using the
following guidelines:
 The vertical openings should not have
diameters greater than one-third of the
beam's width.
 The minimum distance from the opening to
the closest face of the beam shall be at least
5 cm and twice the cover provided on that
face.
 If a set of openings is required, they must be
aligned and the distance between their faces
must be at least 5 cm or the opening
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diameter and each gap must contain at least
one stirrup.
It is well known from the literature mentioned
above that there is lack of knowledge about the
effect of vertical openings on the strength and
behavior of reinforced concrete beam with vertical
openings, especially when the opening penetrates
the entire depth of the beam and obstruct the
reinforcement bars. In addition, no information is
available about the difference between the effect of
the vertical and the transverse openings on the
strength and behavior of RC beams. On the other
hand, all the building design codes and guidelines
introduce very limited recommendations for beams
with vertical openings. This study program is
dedicated to fill in this gap by evaluating and
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enhance the information about the behavior of RC
beams with a vertical opening when it crossing the
entire depth of the beams and obstruct the
reinforcement bars. The absence of researches and
publications in this issue makes this paper of
significant relevance.
II. Experimental Program
For the experimental investigation, five square
RC beams were fabricated and tested, one of them
without opening and four beams with an opening.
The variables adopted here included openings
direction, openings shape, and the details of
replacing the reinforcement that will obstruct the
openings penetration. The following sections
below provide details of the experimental program.
1.1. Materials Properties
The properties of the materials used in this study
were obtained experimentally. All tested beams
were prepared at the same time and cured under the
same conditions to ensure that the compressive
strength was nearly the same. Ordinary Portland
cement and a maximum size of 4.75 and 14 mm of
sand and gravel respectively were used throughout
this investigation for casting all the specimens.
Table 1 shows the mixture proportions (by weight)
of the selected concrete mix. The cube compressive
strength was obtained by testing six cubs with
dimensions of 150 × 150 × 150 mm using
Compressive Testing Machine (CTM). The
average tested the compressive strength of concrete
at 28 days was 37 MPa. The splitting tensile
strength was carried out on cylindrical concrete
specimens of 150 mm diameter and 300 mm height
using Compressive Testing Machine (CTM). The
average tested the splitting tensile strength of
concrete at 28 days was 3 MPa.
Direct Tensile test was carried out on Ø12, Ø10
and Ø6 mm diameter deformed steel bars. Three
specimens for each bar size were tested. The yield
strength was 550 MPa for Ø12, and Ø10 mm
diameter steel bars and 450 MPa for Ø6 mm
diameter steel bar.
Table 1
Mixture proportions for the selected concrete mix
Materials
Water/cement
ratio
Water
(kg/m3)
The selected
0.49
207
concrete mix
1.2. Specimen Details
All beams have a square cross-section of
200×200 mm, with a total length of 2000 mm. They
were designed to fail in shear (shear-critical beams)
by reinforcing them for flexure with 6Ø12mm
deformed steel bares distributed in two layers, four
bars for the first layer with a clear cover of 20 mm
and two bars for the second layer spaced apart
vertically with a clear spacing of 20 mm. The
effective depth (d) was 162 mm. The shear-span
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Cement
(kg/m3)
Fine aggregate
(kg/m3)
Coarse
aggregate
(kg/m3)
423
732
934
was
reinforced with 6-mm-diameter deformed stirrups
spaced every 175 mm. For all beams there are no
stirrups were provided in the pure bending zone,
2Ø10 mm steel deformed bars were used as top
reinforcement to hold stirrups.
The opening was located at the mid-shear span
of the beams. Its size was kept the same for all
specimens; their dimension was 30% of the beam
width or height (60 × 60) mm. The reference beam
tests layout is detailed in Figure 3.
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.
Fig. 3. Geometric and reinforcement details for the reference beam (all dimensions in mm)
In order to make it easy to recognize the
description of each beam, all beams were given a
designation representing their variable, and
whether they were solid or with an opening. Figure
4 shows the arrangement and definition of
specimen naming convention.
Fig. 4. The arrangement and definition of specimen naming convention
Two bars from the longitudinal main
reinforcement as well as the stirrup that located at
the mid-shear span for the reference beam will
obstruct the openings’ penetration for the rest
beams. Details of each specimen are described in
Table 2. VB2-SVo-D1 and VB4-CVo-D1 specimens
were provided with square and circular vertical
openings respectively by redistribute the main
rebars and replace the obstructed stirrup by singleleg stirrup at each transverse sides of the opening
(Detail No.1). The VB3-SHo-D2 specimen was
provided with a square horizontal opening by
replacing the obstructed stirrup by rectangular
stirrup at each cord of the opening (Detail No.2).
The VB5-SVo-D3 specimen was provided with
square vertical opening by replacing the obstructed
stirrup by using stirrup at each longitudinal side of
the opening (Detail No.3). The details of
replacement the obstruction reinforcement for all
specimens are shown in Figure 5.
Table 2
Details of the tested specimens
Specimen
designation
Opening
shape
Opening
direction
Dimensions
of opening
(mm)
RVB1
--------
---------
----------
VB2-SVOD1
Square
Vertical
60×60
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Details of
replacement the
obstruction
rebars
--------D1 (single-leg
stirrup at each
transverse sides
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of the opening)
VB3-SHoD2
Square
Horizontal
60×60
D2 (using stirrup
at both cords
above and below
the opening)
VB4-CVOD1
Circular
Vertical
60
D1
60×60
D3 (using stirrup
at each
longitudinal side
of the opening)
VB5-SVOD3
Square
Vertical
(a) Detail No.1 (D1) used for VB2 and VB4 specimens
(b)Detail No.2 (D2) used for VB3 specimen
(c) Detail No.3 (D3) used for VB5 specimen
Fig. 5. Details of replacement the obstruction rebars for beams (dimensions in mm).
1.3. Test Setup
All beams were tested under two-point loading
until failure by using 600 kN capacity hydraulic
testing machine. Specimens were placed from one
end on a roller and the other on a hinge. The
supports located 100 mm from each end of the
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beam allowing a simply supported span of 1800
mm; the shear span was 550 mm, the distance
between loads being 700 mm. The load was applied
to the tested beam through a spreader steel beam.
Bearing plates with a dimension of (200*75*12)
mm were used at supports and loading points to
avoid local crushing of concrete. In order to
measure the deflection of the tested beams, two
vertical digital dial gauges were used, one at the
mid-span section and the other vertical dial gauges
were added under one of the concentrated point
load ( 550 mm from the supports). Both dial gauges
were attached to vertical metallic elements and
fixed in vertical position during the test. These dial
gauges have a maximum measurement of 25 mm
and an accuracy of 0.01 mm. The load was applied
in stages by a load control mode at a load rate of
0.1 kN/sec. Observations of crack development on
the concrete beams were marked with a heavy felt
pen to make easy the positioning and identification
of cracks during and after the test. The beam test
layout is shown in Figure 6.
Fig. 6. Beam test layout (all dimensions in mm)
III. RESULTS AND DISCUSSION
Table 3 gives a summary of the load capacity and
failure modes of the tested beams. The observation
Specimen
designation
RVB1
VB2-SVoD1
VB3-SHoD2
of specimens during the tests and detailed
discussions are presented in the following.
Table 3
Summary of experimental results with failure mode
Failure
Deflection at
Max.
Reduction in
Failure mode
load
service load *
Deflection ultimate load
(KN)
(mm)
(mm)
(%)
145
5.79
11.5
----Flexural-Shear
133
57
5.81
-----
10.61
3.93
8.27
Flexural-Shear
60.69
Web-Shear
(beam-type
failure)
VB4-CVo138
5.8
10.95
5.07
Flexural-Shear
D1
VB5-SVo142
5.8
11
2.06
Flexural-Shear
D3
* Assumed service load = Ultimate load of reference beam / 1.7 [16]
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1.4. Ultimate Load and Modes of Failure
As previously explained, all RC beams were
designed to fail by shear. During the tests, this
mode of failure was clearly obtained. The shear
failure took place in the shear-span at one side of
the beam after the formation of an inclined crack
between the support and the nearest concentrated
load. At about 70% of the ultimate load, this crack
began to widen and opens more rapidly. ultimately,
all the specimens were failed due to yielding and
rupture of the web reinforcement, except the beam
with the horizontal opening (VB3 specimen), which
collapse immediately following the formation of
the first shear crack. That is due to absence of the
web reinforcement through the diagonal crack at
the opening location.
Figure 7 shows the failure modes obtained for
the tested beams. For reference beam, the failure
was occurred at one side of the beam, while for
VB2, VB3, and VB4 specimens, the failure occurred
at the side of the beam containing the opening,
passing through it. For the beam with the transverse
opening (VB3 specimen), the observed failure
mode is web-shear failure, since the inclined failure
plane of angle about 45° traversed through the
center of the opening, this failure is referred to as a
beam-type failure as described by Mansur [16]; see
Figure 7b. For VB5 specimens, the failure occurred
on the other side of the concrete beam opposite to
the opening.
The experimental load capacity (Pu) for each
tested beams and the comparison with the reference
beam were presented in Table 3. It is observed that
the presence of the transverse opening reduced the
ultimate load of about 60.69%. While the reduction
in the ultimate load for beams with vertical opening
is 8.27 and 5.07 % for VB2 and VB4 specimens,
respectively. In fact, the significant reduction in the
ultimate load caused by providing the transverse
opening is because that, the stirrups above and
below the opening does not work appropriately,
thus it does not compensate the stirrup which was
lifted due to the opening penetration. With regard
to the details of replacing the obstruction stirrup for
beams with vertical opening, it is found that using
two stirrups on each side of the opening (VB5
specimen) restored the shear capacity of the beam.
(a) Typical failure of beams with and without
(b) Failure of the beam with transverse
vertical opening (RVB1 specimen)
opening (VB3 specimen)
Fig. 7. Failure modes of tested beams
1.5. Cracking Behavior
At the early stages of loading, reinforced
concrete beams are free from any cracks, with
increasing the load and when the stresses exceed
the tensile strength of the concrete, cracks will
progress from the tensile portion of the crosssection. The experimental first cracking load (Pcr)
Published by: The Mattingley Publishing Co., Inc.
was about 25 KN for all tested beams. The first
crack was a vertical flexural crack formed at the
bottom of the pure bending zone at mid-span
section, except for beam with transverse opening
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(VB3 specimen) which its first crack appeared
under the point load close to the opening.
Figure 8 shows the crack pattern at failure for
all the tested beams. Initially fine vertical flexural
cracks develop at the mid span of the beams. After
that, the bending moment in the shear-bending
segment increases with the load, the flexural
(tensile) crack appears there near the bottom and is
perpendicular to the beam axis. As the load
increases, additional inclined cracks appear due to
the effect of shear forces on the principal tensile
stresses at mid-depth of the beams near the
supports, it is referred to as web-shear cracks. With
increasing the load, two or three diagonal cracks
develop and extend from the flexural crack at the
shear span. This kind of crack is called a flexuralshear crack. With further increase of the load, the
flexural crack in the pure bending segment is
stagnated, while the flexural-shear crack in the
shear-bending segment extends continuously. As
the cracks stabilise, one of the diagonal cracks
widens into a principal tension crack and propagate
in to the top compression fibres of the beam. When
the load is increased further, the widths of these
cracks are increased continuously but their number
and shape do not change again. As shown in Figure
8, because of the lower load capacity for VB3
specimen, it has the least amount of cracks.
(a) RVB1 specimen
(b) VB2-SVo-D1 specimen
(c) VB3-SHo-D2 specimen
(d) VB4-CVo-D1 specimen
(e) VB5-SVo-D3 specimen
Fig. 8. Cracks patterns at failure for tested beams
1.6. Load – Deflection Response
The mid-span deflection is registered with a
vertical dial gauge placed at the mid-span position.
Figure 9 shows the load-midspan deflection
relationship for all the tested beams. Generally,
there was insignificant deflection at the first stage
of loading representing the uncrack stage, however,
the deflection increased after the formation of the
first flexural crack in the beam. At about 70% of
the ultimate load, the inclined crack, which formed
between the support and the nearest concentrated
load began to widen and opens more rapidly until
it separates and causes loss of the interface shear
transfer by the aggregates interlock. At this stage of
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loading, a sudden slightly increase in the deflection
is observed.
As is shown in Figure 9, all tested beams have
almost similar load-deflection curve until the
separation of the inclined cracks for each beam,
except the beam with the transverse opening
(VB3 specimen), which failed immediately
following the formation of the first shear crack as
it mentioned previously. For beam with the vertical
opening (VB2 specimen), the load-deflection curve
show a sudden little increase in the deflection at a
load before that of the reference beam as shown in
Figure 9a. Thus, before the ultimate load is
achieved, the curves have separated, and shown
slightly increase in the deflection for VB2 specimen
compared with the reference beam for the same
load level. For shape of the vertical opening, it is
observed that both VB2 and VB4 specimens have
almost similar load-deflection curve until the
failure as shown in Figure 9b.
With regard to the details of replacing the
obstruction stirrup for beams with vertical opening,
it is found that using two stirrups on each
longitudinal side of the opening (VB5 specimen)
enhance the load-deflection curve related to the
VB2 specimen. Where, this beam has almost
similar deflection curve to the reference beam as
shown in Figure 9c.
As shown in Table 3, providing vertical opening
at mid-shear span for all specimens does not affect
the maximum deflection at the service loads. For
comparison, the limitation of the allowable
deflection imposed by the ACI 318-14 Code [17]
in such circumstances is ℓ/240 = 1800/240 = 7.5
mm., indicating that the stiffness of the proposed
beams are sufficient.
(a) Effect of the opening direction
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(b) Effect of the opening shape
(c) Effect of the details of reinforcement
Fig. 9. Load - Midspan deflection curves for tested beams.
1.7. Ductility
was found more suitable to use the second measure
Ductility is the ability of the RC member to
of ductility. Figure 10 shows displacement energy
sustain large inelastic deformations without
measured for all the tested beams.
excessive strength deterioration. It can be
from Figure 10, it can be noticed that the
represented either in terms of ratio of the maximum
presence of opening caused decrease in the
displacement to the yield displacement, both
ductility by 13.5% for beam with vertical opening
measured at mid span (Deflection ductility index),
(VB2 specimen) and 86.62% for beam with
or in terms of the displacement energy consumed
transverse opening (VB3 specimen) when
by the specimen during the test. The displacement
compared with the solid beam. The significant
energy measured as the area under the load
reduction in the ductility for VB3 specimen is due
displacement curve up to the failure load (Energy
to the considerable reduction in the ultimate load
dissipation index)[18], [19]. Since the flexure
capacity. For opening shape, VB4 specimen has
mode of failure was prevented for all beam
ductility more than VB2 specimen does. With
specimens to allow for the shear mode of failure, it
regard to the details of replacement the obstruction
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stirrup for beams with vertical opening, it is found
that using two stirrups on each side of the opening
(VB5 specimen) enhance the ductility related to the
VB2 specimen. Where, it has almost similar
ductility to the reference beam.
926
1000
850
Displacement energy
(Kn.mm)
801
905
500
124
0
RFB1
FB2-SVO-D1
FB3-SHO1
FB4-CVO-D1
FB5-SVO-D2
Fig. 9. Displacement energy for the tested beams
IV. Conclusions
During the experimental work, it is found that
when a vertical opening having a size of 30% from
the beam's width is provided at mid-shear span for
shear critical beams, the following conclusions can
be derived:
 In general, within the bounds of opening size and
location considered in this study, the presence of
vertical opening caused slightly decreased in the
ultimate load capacity with an average of about
6.6 %.
 As the opening represents a source of weakness,
thus if no additional reinforcement was added,
the failure plane will always pass through it.
 The presence of vertical openings influences the
crack pattern, such that more cracks form with
closer average spacing at openings vicinity.
 Providing vertical openings for shear-critical
beams does not affect the maximum deflection
at the service loads, however, it caused a slight
decrease in the ductility index.
 Circular openings exhibited lower effect than
square openings in term of ductility index, and
ultimate load. Thus, it considers a good choice
for passing the serves vertically through the
buildings.
 Using single-leg stirrups at the opening region
gave a good response in compensation the lifted
one, while for transverse opening, the stirrups in
each cord above and below the opening did not
work
properly.
Therefore,
additional
reinforcement enclosing the opening should be
provided in order to prevent possible premature
failure.
 Using stirrup on each longitudinal side of the
opening is adequate to enhance the ductility
index and recover the lost strength.
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22131
V. Acknowledgements
The completing of the current work was in the
Department of Civil Engineering at Engineering
College of Babylon University. Therefore, the
moral support that was provided is gratefully
acknowledged.
VI. Conflicts of Interest
The authors declare no conflict of interest.
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[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
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slabs–An experimental and numerical
study,”
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Building
Materials, vol. 21, no. 4, pp. 810–826, 2007.
M. A. Mansur and K.-H. Tan, Concrete
beams with openings: Analysis and design,
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M. A. Mansur, S. K. Ting, and S.-L. Lee,
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openings,”
Journal
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bending,”
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Engineering
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