Earthquake-Resistant Design of Coupling Beam Elements
Incorporating High-Performance Fiber Reinforced Concrete
James K. Wight 1 and Rémy D. Lequesne 2
Abstract
This paper highlights an ongoing study aimed at investigating the use of strainhardening, high performance fiber reinforced concrete (HPFRC) materials in the critical
regions of coupled walls that laterally brace many structures. During a large earthquake it
is anticipated that the coupling beams will undergo significant inelastic deformations and it
is important for these beams to have a high energy dissipation capability and good stiffness
retention. However, placing the steel reinforcement required in reinforced concrete (RC)
coupling beams to resist earthquake-induced deformations is labor intensive and costly,
which often leads practicing engineers to discard the use of such coupling beams. The
concept behind this study was conceived from the idea that the next generation of RC
structures should utilize ductile concrete in critical regions, rather than extensive
reinforcement detailing to provide shear resistance and concrete confinement and thus,
achieve an increase in deformation capacity and stiffness retention of structural members
and systems.
This paper will discuss the observed behavior of precast HPFRC coupling beams,
based on results from large-scale reversed cyclic displacement tests of individual coupling
beam specimens at the University of Michigan. These test results indicate that HPFRC can
be used as a replacement for normal confinement reinforcement, to provide additional
shear resistance, and to increase coupling beam damage tolerance.
The next phase of the project will be the testing of four-story coupled wall systems
under displacement reversals at the University of Michigan. The design and construction
of those specimens, which includes the use of precast coupling beams, is now in progress.
1. Introduction
Coupled concrete structural wall systems are an efficient lateral bracing system for
both medium-rise steel frame and concrete structures. These wall systems can develop
significant lateral strength with good stiffness retention through large displacement
reversals. The efficiency of the system is improved through proper coupling of two or
more consecutive structural walls through the use of short coupling beams. The demand
for flexural rigidity of the individual walls is reduced by taking advantage of the axial
stiffness of the structural walls through the coupling action provided by the short coupling
beam elements.
1
2
Professor, University of Michigan, Ann Arbor, USA
Graduate Student Research Assistant, University of Michigan, Ann Arbor, USA
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These coupling beams must ideally retain a significant shear force capacity through
large displacement reversals, while suffering a modest degradation of stiffness for
satisfactory performance of this structural system during a seismic event. For span-todepth ratios of less than two, it has been shown[1] that special diagonal reinforcement
detailing is required to eliminate the development of brittle diagonal tension or sliding
shear failures and to provide adequate ductility and stiffness retention to achieve the design
goals of the system. However, the detailing required to provide stable behavior of the
diagonally reinforced coupling beam is difficult to construct, and often fails to sustain the
integrity of the full concrete element through large displacement reversals. Other
alternatives have been proposed and investigated[2], including steel and hybrid steelconcrete beams. Despite simplified detailing and improved hysteretic behavior, providing
proper anchorage of the steel element without disrupting reinforcement in the structural
wall is a significant challenge.
High performance fiber reinforced concrete (HPFRC) provides significantly improved
ductility over more traditional cement based materials, and is therefore ideal for
consideration as an improvement to the seismic design of coupling beams. The improved
tensile properties of HPFRC have been shown[3] to not only enhance bond with the
reinforcement, but also to provide adequate confinement of the diagonal reinforcement,
eliminating the need for steel confinement for each group of diagonal bars. This
significantly simplifies detailing and construction of the elements.
Furthermore, HPFRCs offer the potential, through their enhanced tensile strength at
high strains (beyond 5% in some cases)[4], to support a significant portion of the applied
shear stress after cracking, thereby reducing the need for heavy diagonal reinforcement
while improving the ductility of the entire element. Essentially, coupling beam behavior
would no longer be reduced to relying solely on the two sets of diagonal bars to sustain the
applied shear at large drifts. Rather, the HPFRC would maintain its integrity through large
displacement reversals, and thus, continue to share the diagonal tension with the diagonal
reinforcement.
2. Research Significance
It has been shown previously[3] in tests of short coupling beams with a span-to-depth
ratio of 1.0 that incorporating high performance fiber reinforced cementitious composites
(HPFRCC) can: 1) eliminate the need for steel confinement of the diagonals, 2) delay the
development of crack localization by dispersing damage over multiple finer cracks, and 3)
provide improved shear capacity, drift capacity, and stiffness retention over conventional
diagonally reinforced coupling beams, thereby significantly improving energy dissipation.
It is the goal of this current research to investigate the effects of incorporating HPFRC
in the design of coupling beams with larger aspect ratios (l/d = 1.75), where flexural
deformations play a larger role, interacting with shear to contribute to the drift of the
element. To further simplify the construction process and reduce costs, this research will
continue to investigate the possibility of precasting the coupling beam, as in previous
research by Canbolat et al.[3], but with a focus on evaluating possible embedment strategies
that provide sufficient development of the precast section into the structural wall without
interfering with wall reinforcement.
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3. Materials
The tests performed by Canbolat et al.[3], which are discussed below, allow a
comparison of the behavior of a conventionally reinforced concrete coupling beam to one
incorporating an HPFRCC composite. The concrete in Specimen 1 was conventional, with
a 28-day compressive strength of 43.0 MPa (6.2 ksi). Specimen 4 consisted of a mortar
mixture reinforced with a 1.5% volume fraction of twisted steel (Torex) fibers. The 28-day
compressive strength was 59.6 MPa (8.6 ksi); the peak tensile strength was 5.9 MPa (860
psi).
The current project selected a concrete mixture developed recently at the University of
Michigan[5], which reinforces a concrete matrix with a 1.5% volume fraction of high
strength hooked steel fibers, while remaining highly flowable to ease placement. Unlike
many previous fiber reinforced mixtures, this mixture includes crushed aggregate to
improve the economy of the mixture and bring it more inline with current practice.
Minimal vibration was required for placement, despite narrow openings between steel
reinforcement and forms. Cylinder tests, performed for each specimen, averaged 38 MPa
(5.5 ksi). All tensile tests showed strain-hardening behavior with an average peak tensile
strength of 2.4 MPa (350 psi) occurring at about 0.2% strain. Five beam tests were also
conducted according to ASTM C1609[6], which consistently showed deflection-hardening.
The mean maximum equivalent bending stress was 45% higher than the mean bending
stress at first cracking.
4. Previous Research
Because conventional detailing of short coupling beams with an effective span-todepth ratio less than two has been shown by Paulay[7], and others since, to exhibit
unacceptably brittle behavior due to either diagonal tension failure or sliding shear failure
at the ends of the beam, several solutions have been investigated by researchers to improve
the hysteretic behavior of these elements.
In 1996, Tassios et al.[8] presented the results from a series of tests which
systematically compared the most commonly proposed reinforcement schemes applied to
beams with l/d ratios of 1.0 and 1.66. Their results illustrated several key points,
summarized as follows:
• while dowel bars in the end regions of the beam may help prevent sliding shear
failure, stiffness degradation and severe pinching in the hysteretic response are not
improved,
• although a rhombic layout of diagonal reinforcement requires less complicated
detailing than diagonally reinforced coupling beams, and exhibits less stiffness
degradation than conventionally reinforced coupling beams, severe pinching of the
hysteretic loops is still present,
• diagonal reinforcement does appear to provide the most stable behavior and
highest energy dissipation, but providing sufficient confinement to ensure stability of
the diagonal bars is “indispensable”, and presents significant field placement
difficulties.
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With these difficulties in mind, researchers have explored other options, as discussed
by Harries, Gong, and Shahrooz[2], including steel and hybrid coupling beam designs.
These alternatives respond very favorably in shear, showing stable behavior with wide
hysteretic loops. However, the steel element requires a long embedment into the structural
wall to ensure full development of its capacity, which inevitably interferes with critical
transverse reinforcement providing confinement to the boundary region of the structural
walls.
4.1 HPFRCC Coupling Beam Tests
To address these issues, Canbolat et al.[3] proposed using HPFRCC to simplify the
design of reinforced concrete coupling beams by providing sufficient confinement to the
diagonal reinforcement, while improving the cyclic behavior in terms of energy
dissipation, stiffness retention, and damage mitigation. Reproduced in Fig. 1 are the
reinforcement details for the series of tests which were included in the study. Specimen 1
is conventional concrete reinforced with diagonal steel, and cast monolithically with the
structural walls as is done in practice today. Specimen 2 explores eliminating the diagonal
reinforcement entirely and relying on the HPFRCC for ductility, while Specimen 3
considers removing only the confining reinforcement around the diagonal steel.
Specimens 2 and 3 also explored the possibility of precasting the coupling beam to further
simplify jobsite construction and to reduce material costs by incorporating fiber
reinforcement only in the most critical regions. Finally, Specimen 4 ties these efforts
Figure 1: Reinforcement details for specimens included in Canbolat [9]
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together into a precast, diagonally reinforced HPFRCC section with bent diagonals to ease
placement of the precast section on the jobsite.
Of most interest to the current discussion is a comparison of the hysteretic behavior of
Specimens 1 and 4, as presented in Fig. 2. While the cross-sectional area and orientation
of diagonal reinforcement are nearly equivalent, the shear stress and drift capacities are
both markedly improved in the HPFRCC member. This shows clearly that stability of the
diagonal reinforcement can be adequately maintained by the surrounding HPFRCC matrix.
Further, it is evidence that the enhanced properties of HPFRCC contribute appreciably to
increasing the shear strength of the member up to drifts nearing 6%, even in members that
commonly exhibit significant deterioration of shear strength and stiffness under reversed
cyclic loading. The more gradual stiffness degradation in the HPFRCC member is
evidence that the HPFRCC suffers less localization of damage than the reinforced concrete
member at comparable drifts.
Figure 2: Shear stress vs. drift response comparison of Specimen 1 (reinforced concrete)
and Specimen 4 (HPFRCC) from Canbolat[9]
5. Current project
5.1 Embedment
The idea that HPFRC may be advantageously utilized in critical regions to improve the
ductility of structural elements and alleviate congestion is a major thrust behind the current
research project. The cost, however, of incorporating fiber reinforcement throughout a
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Figure 4: Reinforcement details
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structure is prohibitive. The solution proposed previously[3], to precast the HPFRC section
and embed it into the reinforced concrete structural walls, has proven promising. This
approach has the added benefit of providing a more controlled environment for
manufacturing, thereby ensuring high quality of construction of the coupling beams.
However, similar to the steel and hybrid coupling beams discussed earlier, interference
with critical transverse reinforcement in the boundary region of the wall is still an issue.
The current research project proposes to precast the HPFRC section, but to terminate
the precast section only 25 mm (1 inch) into the wall. Reinforcement extending into the
adjacent walls is relied upon to transfer shear and moment between the coupling beam and
the structural wall. This deep embedment of only the reinforcing bars allows for easier
placement of the precast beam through the wall reinforcement. To enhance the sliding
shear resistance at the interface between the coupling beam and the structural wall, shear
keys were provided in the first two coupling beam specimens. To demonstrate that there is
no need to rely on shear keys for sliding shear resistance, the end regions of the third
specimen were designed to transfer the full shear force through dowel action and shear
friction provided by the steel protruding from the end of the precast section. No
localization or sliding was observed at the interface, so shear keys may be considered to be
a supplement to, rather than replacement for, dowel action and shear friction.
It is desirable to force plastic rotations to occur within the precast element where the
fiber reinforcement is active, rather than at the interface between the HPFRC coupling
beam and the structural wall. Two reinforcement details were evaluated for moving the
location of the plastic hinge away from the interface. The first two specimens had two Ushaped reinforcing bars extended 150 mm (6 in.) into the coupling beam before being bent
for anchorage. This results in a situation where the ratio of moment capacity to moment
demand at the termination of the U-shaped bar in the coupling beam is similar to the
equivalent ratio at the cold joint. Test results, shown in Fig. 5, verified that this detail
prevented localization of rotations at the interface. The third specimen takes advantage of
a second, simpler, detail consisting of straight dowel bars provided across the interface,
which was intended to better distribute plastic rotations throughout the hinge. It was
assumed that the stress in the dowel bar varies linearly from zero at the point where it is
terminated to yield at the interface. This development of the dowel bar results in a
gradually diminishing contribution of the dowel bar to the moment capacity of the
coupling beam at cross sections away from the interface. As a result, the moment capacity
and demand follow a similarly sloped decline throughout the plastic hinge, which
encourages a more desirable distribution of flexural yielding. A satisfactory distribution of
plastic rotations was observed in the test of the third specimen.
Note that only 200 mm (8 in.) of development was provided for a 12 mm bar (#4).
This reduced development length is possible due to the enhanced bond stress capacity
inherent in fiber reinforced concrete, as discussed by Chao[10].
5.2 Interaction of HPFRC and reinforcement
The hysteretic shear vs. drift response of the HPFRC coupling beam tests in the current
study are shown in Fig. 5.
At first glance, it is clear that the response of the first specimen degrades prematurely,
around 2.5% drift. This is a direct result of a detailing issue which was addressed in
subsequent tests. To understand the detailing issue, first note in Fig. 4 that the diagonal
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Specimen CB-1
Shear Stress (MPa)
8
4
0
-4
-8
-8%
-6%
-4%
-2%
0%
2%
4%
6%
8%
4%
6%
8%
4%
6%
8%
Drift
Specimen CB-2
Shear Stress (MPa)
8
4
0
-4
-8
-8%
-6%
-4%
-2%
0%
2%
Drift
Specimen CB-3
Shear Stress (MPa)
8
4
0
-4
-8
-8%
-6%
-4%
-2%
0%
2%
Drift
Figure 5: Shear stress vs. drift response
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reinforcement is bent before entering the wall. This is done to ease placement of the
precast section and to increase the shear contribution of the diagonal bars by increasing the
angle of elevation relative to the horizontal. This results in a force component that will
tend to burst through the top and bottom of the coupling beam if not sufficiently restrained
by stirrups. Insufficient transverse reinforcement was provided to both adequately restrain
this force component and confine the plastic hinge region. In subsequent tests this problem
was addressed by providing tighter spacing of stirrups in the plastic hinge region, and by
providing additional stirrup legs through the center of the hinge to restrain lateral
expansion.
It is worth noting that the behavior of the first specimen, despite this detailing issue,
showed no evidence of instability of the diagonal bars up to 2.5% drift. The responses of
the second and third specimens, which showed no instability of the diagonal bars past 5%
drift, were consistent with this observation. These results support the assertion that
HPFRC can be relied on to provide adequate confinement of the diagonal reinforcement.
The third specimen was unintentionally subjected to an unsymmetrical displacement
history resulting from an instrumentation issue on the day of the test. Despite this issue,
the responses of the second and third specimens show that the use of HPFRC and proper
reinforcement detailing results in a stable response. Even under very high shear stresses of
over 6 MPa (870 psi), the hysteretic loops are full and stable at member drifts of 5%.
The ductility and energy absorption of the HPFRC coupling beams is partially a result
of the multiple cracking behavior which is regularly ascribed to strain hardening fiber
reinforced composites. This effect manifests as an increased number of narrow and tightly
spaced cracks when compared to conventional reinforced concrete. The first two
specimens, which had transverse reinforcement ratios of 0.6% over the middle portion of
the span, exhibited no severe diagonal crack growth throughout the test. Likewise, no
severe opening of diagonal cracks was observed in the third specimen despite a lower
transverse reinforcement ratio of 0.45%. This result attests to the significant role fiber
reinforcement plays in maintaining the integrity of the element and distributing damage.
Another interesting observation from the testing of these specimens relates to the
improved bond developed between the HPFRC and the steel reinforcement, compared to
conventional reinforced concrete. Due to the double curvature condition imposed on
coupling beams, a single longitudinal steel bar, placed along either the top or bottom of the
beam, should theoretically have one-half of the bar experience tension and the other half
experience compression. Due to a severe degradation of bond, coupled with elongation of
the beam, conventional concrete coupling beams have been observed to exhibit tension of
the longitudinal bars over the entire length of the beam[1]. These tests presented here of
HPFRC coupling beams show that even at large drifts, a longitudinal bar will show
compressive strains at one end of the beam while yielding in tension at the other end.
6. Conclusions
The tests of HPFRC coupling beams described herein show evidence that the enhanced
material properties of HPFRC can both simplify and enhance the design of short coupling
beams by preventing brittle failure mechanisms from developing. The following
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observations are indicative of the potential benefits that HPFRC coupling beams may
offer:
• HPFRC can be relied on to provide adequate confinement for the diagonal
reinforcement of coupling beams, which significantly simplifies detailing and construction
of coupling beams.
• HPFRC will disperse damage to a structural element through more numerous,
finer cracks than conventional concrete, thus delaying the localization of damage and the
subsequent development of an undesirable failure mechanism. The structural implications
of this are evidenced by the more gradual degradation of stiffness through multiple
displacement reversals. This has important implications for a coupled wall system, where
dissipating energy and maintaining a consistent degree of coupling are critical for adequate
behavior of the structure during a seismic event.
• It is possible to adequately develop both moment and shear capacities of the
coupling beam without interfering with structural wall reinforcement. From a construction
viewpoint, this makes precast HPFRC a more attractive option for coupling beams than
structural steel members.
• Short coupling beams that incorporate HPFRC show a higher shear stress capacity
than traditional diagonally reinforced coupling beams, as well as an improved drift
capacity. This results in more stable hysteretic behavior and greater energy dissipation.
7. Continuing research
The current research project will continue to study the effects of incorporating HPFRC
in the design of coupling beams with larger aspect ratios (l/d = 1.75). Further, the design
concepts proven by the series of component tests will be applied to a large-scale 4-story
coupled wall system to study how the behavior of precast HPFRC coupling beams impacts
the seismic response of the coupled wall system.
Acknowledgements
The current research project is funded by NSF grant #CMS 0530383, and is a part of
the NEES research program. The efforts of Dr. Canbolat and Professor Parra-Montesinos
are also acknowledged for working at length with Professor Wight to complete the series
of short HPFRCC coupling beam tests discussed herein. Professor Parra-Montesinos is
also acknowledged as an integral part of the research team conducting the current research
project.
References
[1] Paulay, T. and Binney, J.R., ‘Diagonally Reinforced Coupling Beams of Shear Walls’,
Shear in Reinforced Concrete, ACI Publication SP-42 by ACI-ASCE Shear
Committee, 2 (1974) 579-598.
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[2] Harries, K., Gong, B., and Shahrooz, B., ‘Behavior and Design of Reinforced
Concrete, Steel, and Steel-Concrete Coupling Beams’, Earthquake Spectra 16 (4)
(2000) 775-799.
[3] Canbolat, B.A., Parra-Montesinos, G., and Wight, J., ‘Experimental Study on Seismic
Behavior of High-Performance Fiber-Reinforced Cement Composite Coupling Beams’,
ACI Structural Journal 102 (1) (2005) 159-166.
[4] Li, V.C., Wang, S., and Wu, C., ‘Tensile Strain Hardening Behavior of PVA-ECC’,
ACI Materials Journal 98 (6) (2001) 64-97.
[5] Liao, W.C., Chao, S.H., Park, S.Y., and Naaman, A.E., ‘Self-Consolidating High
Performance Fiber Reinforced Concrete (SCHPFRC) – Preliminary Investigation’
Technical Report, University of Michigan, Ann Arbor, Report No. UMCEE 06-02
(2006).
[6] ASTM C1609/C1609M–06, Standard Test Method for Flexural Performance of
Fiber-Reinforced Concrete (Using Beam With Third-Point Loading), ASTM
International.
[7] Paulay, T., ‘Coupling Beams of Reinforced Concrete Walls’, Journal of the Structural
Division, ASCE 97 (ST3) (1971) 843-862.
[8] Tassios, T., Moretti, M., and Bezas, A., ‘On the Behavior and Ductility of Reinforced
Concrete Coupling Beams of Shear Walls’, ACI Structural Journal 93 (6) (1996) 711720.
[9] Canbolat, B.A., Seismic Behavior of High-Performance Fiber Reinforced
Cementitious Composite Coupling Beams, Ph.D. Dissertation, University of Michigan,
Ann Arbor, Report No. UMCEE 04-11 (2004).
[10] Chao, S.H., Bond Characterization of Reinforcing Bars and Prestressing Strands in
High Performance Fiber Reinforced Cementitious Composites Under Monotonic and
Cyclic Loading, Ph.D. Dissertation, University of Michigan, Ann Arbor, (2005).
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