Available online at www.sciencedirect.com
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Procedia Engineering 95 (2014) 132 – 138
2nd International Conference on Sustainable Civil Engineering Structures and Construction
Materials 2014 (SCESCM 2014)
Behavior of ductile beam with addition confinement in compression
zone
Yulita Arni Priastiwia, Iswandi Imranb, Nurojia, Arif Hidayata*
a
Department of Civil Engineering,Diponegoro University, Semarang-50275, Central of Java, Indonesia
Structural Engineering Research Group,Civil Engineering Department, Bandung Institute of Technology , Bandung-40132, West of Java,
Indonesia
b
Abstract
This paper explains the behavior of both reinforced concrete beams with and without confinement in compression zone to
evaluate the influence of confinement in compression zone to the increasing of beam ductility. Four beams with reinforcement
ratio ɏ = 1.9 % and ɏ = 2.5 % have been tested with monotonic loading. The results showed that an increase up to 300 % in
concrete strain for beams with confinement on the compression zone than beams without confinement. The increase also
occurred in the load capacity and the curvature of beams with confinement, while the moment capacity did not show significant
improvement.
©
2014The
TheAuthors.
Authors.
Published
by Elsevier
Ltd.
© 2014
Published
by Elsevier
Ltd. This
is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/3.0/).
Peer-review
under responsibility of organizing committee of the 2nd International Conference on Sustainable Civil Engineering
Peer-review and
underConstruction
responsibility of
organizing
committee of the 2nd International Conference on Sustainable Civil Engineering
Structures
Materials
2014.
Structures and Construction Materials 2014
Keywords:Confinement; compression zone; ductility; strain; monotonic
1. Introduction
In Indonesian Standard SNI 1726-2012 [1] stated that the earthquake resistant structures must be designed to
resist earthquake forces with return period of 2500 years. Basically the procedure of seismic design in SNI 17262012 [1] allows to reduce the earthquake forces with modification factor of structure response (R) that represent of
* Corresponding author. Tel.: 08122858572; fax: +622476480727.
E-mail address:yulita_tiwi@ymail.com
1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/3.0/).
Peer-review under responsibility of organizing committee of the 2nd International Conference on Sustainable Civil Engineering
Structures and Construction Materials 2014
doi:10.1016/j.proeng.2014.12.172
Yulita Arni Priastiwi et al. / Procedia Engineering 95 (2014) 132 – 138
133
structure ductility level. This concept enable the structure undergoes plastification in certain location form plastic
hinges as a mean energy dissipation when the high seismic occur. However, the structure must not collapse [2].
Ductile structures must be supported by structural elements that are ductile anyway, because the structural
elements with high ductility will be able to maintain its strength after a sizeable inelastic deformation without
collapse. Ductile structural elements that will be able to rotate more and able to maintain the energy dissipation
capability by maintaining the hysteresis curve stable and not pinched [3]. With ductile structural elements, they will
assure a plastic hinge mechanism to spread in places that have been determined. In order for the mechanism of
energy dissipation that occurs is very ductile, then the location chosen as the energy dissipation reinforcement
detailing should be good and sufficient, such as confinement in the concrete reinforcement.
One of the mechanism of ductile structural design is Beam Sidesway Mechanism and among other mechanisms
there is Column Sidesway Mechanism with consequences that the structure of the column should be planned
stronger than the beam structure, so that the plastic hinges will be formed first on the beam. To obtain the Beam
Sidesway Mechanism, beam should be able to be have ductile. Ductile behaviour at the beam can actually be
obtained if planning is based on the conditions of under-reinforced beam.
Nomenclature
‫ܣ‬௦
‫ܣ‬ᇱ௦
b
ܿ
d
‫ܧ‬௦
݂௖ᇱ
݂௦
area of tensile reinforcement
area of compressive reinforcement
width of beam
distance from extreme compressive
fiber to neutral axis
effective depth of beam
modulus of elasticity
compressive strength of concrete
tensile strength of reinforcement
݂௬
‫ݏ‬
‫ݏ‬௖
ߝ௖
yield strength of reinforcement
spacing of shear reinforcement
spacing of confinement
strain of concrete
ߝ௦
ߩ
ߩᇱ
ߩ௕௔௟
strain of reinforcement
tensile reinforcement ratio
compressive reinforcement ratio
reinforcement ratio of balance
However the effectiveness in design of structural elements, especially under-reinforced beams, is very low. This
is because the concrete compression section becomes very small compared to the whole concrete section. This is
because the strain on the balance conditions will make the location of the neutral axis increasingly shifted towards
the compression fibre, so that the smaller the area of stressed concrete (Fig. 1).
Fig. 1. Shifting the location of the neutral axis in the balance of strain
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Yulita Arni Priastiwi et al. / Procedia Engineering 95 (2014) 132 – 138
Increasing the ratio of tensile reinforcement of a concrete cross-section, will shift the location of the neutral axis
in the direction of the tension area, so that the extent of concrete used becomes bigger. This condition is
advantageous because the concrete can be utilized optimally in the presence of a bigger compression area. However,
the increase in tensile reinforcement ratio will affect the ductility of the cross-section as shown by the results of
cross-sectional analysis of the two beams of the same section with different tensile reinforcement ratio in Fig. 2.
Section with
ߩ much
Section with ߩ low
Fig. 2. Relationship moment-curvature beam with the same section but different at reinforcement ratio
Although analytically moment capacity increases, the beam with larger reinforcement ratio of concrete will be
first crushed before reinforcement yields with possibly a sudden failure. This is what should be avoided in terms of
structural design.
Starting from these ideas, this paper presents the results of research on improving the ductility of beams by
providing additional confinement stirrup-shaped in cross-section compression zone, in order to obtain more
favorable conditions in terms of moment capacity and ductility when compared with the regular beam without
additional confinement.
Confinement is the chosen way among other ways to get enough ductility in reinforced concrete, as well as
lowering the tensile reinforcement ratio ሺߩሻ, increasing the compressive reinforcement ratio ሺߩԢሻ, lowers the quality
of the steel (݂௬ ), or by improving the quality of concrete (݂௖ᇱ ) [4] due to the mechanical behaviour of concrete
materials showing better performance if given confinement. The provision of confinement in concrete can improve
the strength and strain capacity of the condition in uniaxial compressive. Stress-strain behaviour of concrete as a
brittle material can be converted into a ductile material by providing adequate confinement to the concrete [5].
Confinement effect can increase the deformability of concrete materials so as to produce a significantly more
ductile reinforced concrete structural element, particularly in the post-elastic range.
Research on the confinement of compression zone of concrete has been done by some previous researchers with
this purpose and varies methods of testing. Confinement given to the compression zone of the beam is in the form of
a spiral [6-7,8-9,10-11,12] or stirrups [13,14]. Nearly all research on the confinement of compression zone has been
performed on the pure bending beam. Load delivery system is in the form of concentrated loads at four load points,
in order to obtain regions with pure bending moment without shear forces. Other e.g. Leung and Burgoyne [11]
provided confinement of compression zone on presstressed beam.
A beam under pure bending providing results of such research indicates some improvement in ductility,
rotational capacity, as well as a good influence in terms of energy dissipation [7,9,11], and increased it will be even
greater on over-reinforced concrete beams [12,15] compared to the under-reinforced beam [14]. The results also
show that increasing the concrete compressive stress due to confinement is proportional to the increase in the
volumetric ratio of confinement reinforcement provided [13].
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Yulita Arni Priastiwi et al. / Procedia Engineering 95 (2014) 132 – 138
This paper presents the results of research on the provision of confinement in the compression zone of beam,
designed for the bending moment and shear force representing the region at the end of the beam having a negative
moment due to the earthquake as the beginning of the study to review the effect of confinement on the compression
zone of beam to increased ductility .
2. Method
To determine the improvement of ductility in the beams due to the addition of a stirrup as confinement on the
compression zone four beams are designed with section and properties as shown in Table 1.
Table 1.Properties of beam.
Type
b
h
݂௖
݂௬
‫ܣ‬ᇱ௦
‫ܣ‬௦
2
ߩ
ߩᇱ
‫ݏ‬
ܿ
‫ݏ‬௖
mm
mm
mm
0.0034
‫׎‬8-100
125
0
0.19
0.0034
‫׎‬8-100
125
‫׎‬8-50
0.25
0.0040
‫׎‬8-100
174
0
0.25
0.0040
‫׎‬8-100
174
‫׎‬8-100
2
mm
mm
MPa
MPa
mm
mm
A1
175
300
17.5
400
850.16
157
0.19
A2
175
300
17.5
400
850.16
157
B1
150
300
17.5
400
981.25
157
B2
150
300
17.5
400
981.25
157
A1and B1beams are designed as beams without confinement on compression zone, while A2 and B2 beams are
designed as beams with confinement in compression zone. All beams are designed as a double reinforcement beam
with reinforcement ratio of tensile (ߩ) such that will produce a neutral axis (ܿ) in order to effect significant influence
of confinement. Beam type A is installed with deformed tensile reinforcement 3D19 and deformed compressive
reinforcement 2D10. While beam type B is installed with deform tensile reinforcement 2D25 and 2D10 for
compressive reinforcement. The result of the value, ܿis used as an area to be given confinement. With a 20 mm
cover concrete, for beam type A the confinement height is 105 mm and type B confinement height is 150 mm. As
fastener for the confinement reinforcement longitudinally two plain rounder bars of 10 mm were installed.
Confinement reinforcement itself in the form of plain rebars with a diameter of 8 mm were installed at spacing of 50
mm for A2 and spacing of 100 mm for beam B2. While to resist shear, stirrups ߶8 mounted 100 mm spacing on
each beam in accordance with the calculated shear forces. Section of the beams are shown in Fig.3.
Fig.3. Two types of beams
Quality of normal concrete used is of a maximum aggregate size of 10 mm and slump of 100 mm used to obtain
the compressive strength of concrete as planned, with the proportion of aggregate mixer as shown in Table2.
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Yulita Arni Priastiwi et al. / Procedia Engineering 95 (2014) 132 – 138
Table 2. Proportion of mix design.
Proportion/m3
Material
Cement
300.00
Kg
1 043.56
Kg
Fine aggregate
799.11
Kg
Water
202.33
Kg
Coarse aggregate
For the purpose of testing each specimen is installed with straingauges to determine the strain changes. The
straingauges are mounted on the main reinforcement for both the tensile reinforcement and compressive
reinforcement, at the confinement reinforcement and also of the concrete. Linear Variable Displacement Transducer
(LVDT) is also installed at a predetermined position. Both straingauges and LVDT are connected to the data logger
that serves as a data recorder that is connected to the monitor screen. Set up test can be seen in Fig. 4
Fig.4. Set up experiment of beams
For the test, the monotonic load was carried out in two stages of loading using load control and displacement
control. Loading begins with load control stage and then proceeds with displacement control stage until ultimate
conditions. This is necessary because the purpose of this study is to find the increase inductility due to confinement
in the beam for which data up to ultimate conditions are required
3.
Result and discussion
The test results of the four beams are given a monotonic loading concentrated load is shown in Fig. 5(a) and
Fig. 5(b) below.
From the graph of the relationship between ܲ െ ߝ௖ it can be seen that a significant increase in concrete strain in
beams with confinement on compression zone compared to beams without confinement on compressive zone. It
looks good on the beam withߩ=1.9 % and on the beam withߩ= 2.5 %. This increase reached more than 300% both
in beam with ߩ= 1.9% and beam with ߩ= 2.5%. As for the increase in load capacity, beam with confinement in
compression zone was able to increase nearly 15% for the beam withߩ = 1.9% and reached almost 59% for a beam
with ߩ = 2.5%. Thus the existence of the compression zone confinement in the beam can improve the ductility of the
beam and also increase load capacity significantly for beams with greater tensile reinforcement ratio.
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Yulita Arni Priastiwi et al. / Procedia Engineering 95 (2014) 132 – 138
180
(a)
250
P (KN)
(b)
160
140
Confinement. ρ=1.9%
120
100
Confinement. ρ=2.5%
Non confinement.ρ=1.9%
100
Non confinement.ρ=2.5%
60
50
40
20
-0,002
200
150
80
0
-201E-17
P (KN)
εc
0,002
0,004
0,006
-0,002
0
1E-17
-50
εc
0,002
0,004
0,006
Fig.5. Relationship ܲ െ ߝ௖ at beam (a) with ߩ=1.9% (b) with ߩ=2.5%
The moment curvature graph on beam with ߩ=1.9% is as shown in the Fig. 6 below :
Fig.6. Moment-curvature at beam with ߩ=1.9%
It can be seen that the confinement in the compression zone of a beam can increase curvature up to 26% with
ߩ= 1.9%, compared to the curvature of a beam without confinement in compression zone, yet the increase was not
significant for the maximum moment as increase is about 4% only.
4. Conclusion
Based on the experimental results of the four beams it can be concluded that the presence of confinement in the
compression zone of beam is able to improve the ductility of the beam as characterized by an increased strain and
curvature of the concrete beam significantly. Brittle failure is in the beam with a large tensile reinforcement ratio
can be avoided by administering confinement in compression zone the section of beams.
Acknowledgements
Gratitude to the Ministry of Education and Culture for funding given to this research through grant competition
research programs BOPTN2014, Ditlitabmas Kemendikbud. In addition to structure laboratory Gadjah Mada
University, Yogyakarta over a place of execution and testing tools.
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Yulita Arni Priastiwi et al. / Procedia Engineering 95 (2014) 132 – 138
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