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10NCEE
Tenth U.S. National Conference on Earthquake Engineering
Frontiers of Earthquake Engineering
July 21-25, 2014
Anchorage, Alaska
STRENGTHENING WITH WING WALLS
FOR SEISMICALLY SUBSTANDARD R/C
BEAM-COLUMN JOINTS
Yuebing Li1 and Yasushi Sanada2
ABSTRACT
A large number of reinforced concrete (RC) buildings designed for gravity loads according to
older seismic design codes exist even in regions of moderate seismicity, and some of them
contain no transverse reinforcement in the beam-column joints. In consideration of economic and
technical conditions in developing countries, a seismic strengthening method using RC wing
walls is proposed for this kind of seismically substandard beam-column joint. This paper
presents a design procedure for applying the proposed method to exterior beam-column joints. In
this study, three 3/4-scale substandard exterior beam-column joint specimens were constructed
with the same structural details and tested with a new experimental technique. Two of the
specimens were strengthened by wing walls with different strengthening details. The test results
showed that the strengthened specimens exhibited ductile failure modes with beam yielding,
whereas the unstrengthened control specimen failed at the joint in a brittle manner. We consider
the effectiveness of seismic strengthening, focusing on strength and ductility, and discuss the test
results in terms of the design calculations presented. Our seismic strengthening approach is valid
for improving the seismic performance of substandard RC beam-column joints.
1
PhD Candidate, Dept. of Global Architecture, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka,
Suita, Osaka 565-0871, Japan
2
Associate Professor, Dept. of Global Architecture, Graduate School of Engineering, Osaka University, 2-1
Yamadaoka, Suita, Osaka 565-0871, Japan
Yuebing Li, Yasushi Sanada. Strengthening with wing walls for seismically substandard R/C beam-column joints.
Proceedings of the 10th National Conference on Earthquake Engineering, Earthquake Engineering Research
Institute, Anchorage, AK, 2014.
10NCEE
Tenth U.S. National Conference on Earthquake Engineering
Frontiers of Earthquake Engineering
July 21-25, 2014
Anchorage, Alaska
Strengthening With Wing Walls For Seismically Substandard R/C
Beam-Column Joints
Yuebing Li1 and Yasushi Sanada2
ABSTRACT
A large number of reinforced concrete (RC) buildings designed for gravity loads according to
older seismic design codes exist even in regions of moderate seismicity, and some of them contain
no transverse reinforcement in the beam-column joints. In consideration of economic and
technical conditions in developing countries, a seismic strengthening method using RC wing walls
is proposed for this kind of seismically substandard beam-column joint. This paper presents a
design procedure for applying the proposed method to exterior beam-column joints. In this study,
three 3/4-scale substandard exterior beam-column joint specimens were constructed with the same
structural details and tested with a new experimental technique. Two of the specimens were
strengthened by wing walls with different strengthening details. The test results showed that the
strengthened specimens exhibited ductile failure modes with beam yielding, whereas the
unstrengthened control specimen failed at the joint in a brittle manner. We consider the
effectiveness of seismic strengthening, focusing on strength and ductility, and discuss the test
results in terms of the design calculations presented. Our seismic strengthening approach is valid
for improving the seismic performance of substandard RC beam-column joints.
Introduction
Reinforced concrete (RC) buildings that were designed according to older seismic design codes,
or without complying with current seismic codes, containing no transverse reinforcement in the
beam-column joint regions (called “unreinforced joint”), still widely exist, particularly in
buildings designed before the 1970s in the western U.S. and in other seismically active regions
worldwide [1]. Model tests have shown that unreinforced joints possess lower strength, ductility,
and energy dissipation [2], and shear failure in joints may cause such buildings to collapse, as
observed in recent severe earthquake disasters. Effective and economic strengthening methods
for such buildings are urgently needed.
Several studies have examined seismic strengthening of unreinforced joints using steel
props [3], GFRP [4], etc. and have achieved significant strengthening effects, but particularly for
developing countries, these methods are not easily implemented due to technical level, available
1
PhD Candidate, Dept. of Global Architecture, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka,
Suita, Osaka 565-0871, Japan
2
Associate Professor, Dept. of Global Architecture, Graduate School of Engineering, Osaka University, 2-1
Yamadaoka, Suita, Osaka 565-0871, Japan
Yuebing Li, Yasushi Sanada. Strengthening with wing walls for seismically substandard R/C beam-column joints.
Proceedings of the 10th National Conference on Earthquake Engineering, Earthquake Engineering Research
Institute, Anchorage, AK, 2014.
materials, and construction cost. Installing wing walls beside columns seems to be a realistic way
of upgrading these seismically substandard buildings.
This paper describes a series of tests conducted to clarify the strengthening effects of
installing wing walls, and discusses the results.
Investigated Building and Joint
The study focused on an exterior beam-column joint of a three-story RC moment-resisting frame
structure which collapsed in the 2009 Sumatra Earthquake, as shown in Fig. 1. It was built in
2005 with a floor height of 3,000 mm and span length of 7,000 mm. In this building, damage
such as buckling of longitudinal bars and concrete spalling was concentrated in the joints.
According to a field survey [5], there was no transverse reinforcement in the joints, as shown in
Fig. 2.
Figure 1. Earthquake-damaged building and
beam-column joint [2].
Figure 2. Dimensions and reinforcing details
of the structure [5].
Proposed Strengthening Design
Seismic Behavior of Exterior Beam-Column Joint
In a previous study [2], a small-scale (1/3) model test of the same structure was conducted and
the deformation behavior was found to be as follows. One crack along the diagonal line of the
joint panel (called “diagonal crack”) and the other extending from the tensile corner of the beam
and column to the center of the diagonal crack (called “corner crack”) appeared under seismic
loading in the positive/negative directions, as shown in Fig. 3. Each diagonal crack and corner
crack widened with rotation at A, B, C/D, E, F under positive/negative loading, respectively.
Concept of Seismic Strengthening Method
Shiohara [6] proposed a resisting model for beam-column joints called a nine-parameter model
and defined that the joint strength depends on its moment capacity (Mju), which is the total
moment acting on the joint center (denoted by O in Fig. 4), mainly produced by tension of the
reinforcements crossing the cracks of the joint and compression of concrete around the rotation
points in Fig. 3. Kusuhara and Shiohara [7] developed this model and formulas for calculating
the moment resistance of exterior joints considering their details, and found good agreement with
experimental results.
Figure 3. Deformation behavior of exterior joint.
Figure 4. Strengthening concept.
Seismic bending moments around the joint panel (Mb, Mc), as shown in Fig. 4, induce a
diagonal crack and a corner crack at the joint, and rotations of the divided three parts (upper
column, lower column and beam). It is conceivable that, when the seismic moment at the joint
center (Mb or 2Mc, called “joint moment”) exceeds the moment capacity, the joint will fail. It is
expected that, by installing wing walls to the columns, the tension (bTay, cTay) of the anchor bolts
that are used to connect the pulled wing wall to the existing structure, and the compression (N) of
the strut in the pushed wall, will increase the moment capacity of the existing joint, thus
preventing brittle failure of the joint and shifting it to a beam flexural hinging mechanism.
For designing the wing walls, the following assumptions are made:
1) The moment capacity of the existing joint is known. This study refers to the previous test
conducted by the authors [8]. The capacity of the strengthened joint is set to be higher than
the joint moment when the beam yields at the end of the wing wall (byMj). The difference
between both is the required strengthening.
2) Firstly, the wing walls are individually designed for the pulled/pushed side because of the
unclear contribution ratio, even though they seem to act together. The final strengthening
scheme is adopted as the safer one for which more strengthening is needed, and is
symmetrically applied to both sides.
3) The width of the strut along the edge of the beam (Lc) is half the width of the wing wall (bw).
The angle (α) between the strut and column is 45o.
4) The amount of anchor bolt groups installed in the beam and column are equivalent, and their
centroids are at the middle of the wing wall.
Based on the above assumptions, the design formulas are as follows:
Strengthening design for the pulled wing wall:
bTay ⋅b La +cTay ⋅c La ≥by M j −exM j
(1)
Strengthening design for the pushed wing wall:
N ⋅ cosα ⋅ LP ≥by M j −exM j
(2)
Symbols in the formulas are stated above or shown in Fig. 4.
Experimental Program
Specimens
Three 3/4-scale specimens of the exterior beam-column joint shown in Figs. 1 and 2 were
prepared. They were modeled up to the inflection points of the upper/lower column and beam,
however, pin supports attached to the column ends and a loading method described later were
considered. Fig. 5 shows the dimensions and reinforcement details. One of them was the control
specimen, J2, representing the existing frame. The other two were strengthened by installing
wing walls; J2-W2 was strengthened at both sides, and J2-W1 only at one side.
Figure 5. Dimensions and reinforcement details of the existing part of the specimens.
The wing walls were designed as stated above. In this study, however, when determining
the width of the wing wall, the effective width of the pulled wall for the partial specimen was
assumed to be ebw in Fig. 4. This is equivalent to the length from the column interior surface to
the intersection of the line extending from the corner crack and the upper edge of the specimen,
and was 328mm. Using Eq. 1 for the pulled wall, the area of beam and column anchors should
each be more than 330mm2, when their yield strength is 295N/mm2. Using Eq. 2 for the pushed
wall, the thickness of the wing wall should be not less than 114mm when concrete compressive
strength is 22N/mm2. According to the Japanese guideline for retrofitting RC buildings [9],
however, the minimum thickness limit of the wing wall should be not less than 150mm,
considering the scale of 3/4. Moreover, the width of wing walls was eventually increased to
340mm, because of the Japanese code restrictions on the minimum spacing of anchors and the
minimum thickness of concrete cover. Fig. 6 shows the details of wing walls.
Figure 6. Strengthened specimens.
Figure 7. Validation of strengths
of anchors.
J2-W2 was strengthened at both sides of the beam, where wing walls were symmetrically
installed to the upper and lower columns. To confirm the contribution of the pulled/pushed wing
wall, only one wall was installed to the lower column for J2-W1. Two weeks after the existing
part was constructed, anchor bolts were embedded and then concrete was cast for the wing walls.
The tested cylindrical compressive strength of the concrete and the properties of the
reinforcement and anchor bolts are shown in Table 1.
Table 1. Properties of concrete, reinforcement, and anchor bolts (N/mm2).
J2
J2-W2
J2-W1
Region
－
Existing
wall
Existing
wall
Ec
2.55×104
2.57×104
2.62×104
2.80×104
2.62×104
Fc
20.2
22.7
26.9
22.6
27.7
ft
1.9
2.0
2.5
2.1
2.4
D16
Φ9
D10
D13
Es
1.75×105
1.78×105
1.68×105
1.65×105
Fy
373
344
380
361
Tu
529
455
554
523
where, Ec: Young’s modulus of concrete, Fc: compressive strength of concrete, ft: tensile strength
of concrete, Es: Young’s modulus of steel, Fy: yield strength of reinforcement, and Tu: ultimate
tensile strength of reinforcement. The properties of the concrete and reinforcement are similar to
those of the studied building [2].
Strength Evaluation for Existing Frame, J2
The strengths of the members of J2 were calculated according to the Japanese standard: flexural
strengths of the beam and column by Eqs. 3 and 4, respectively [10], and shear strength of the
joint by Eq. 5 [11]. The shear strengths of the beam and column are considerably larger than
their flexural strengths, and are not given here.
b Mu
= 0.9atσ y d
cMu
⎛
N
= 0 .8 a tσ y D + 0 .5 ND ⎜⎜ 1 −
bDF c
⎝
(3)
⎞
⎟⎟
⎠
(4)
V ju = κ ⋅ φ ⋅ F j ⋅ b j ⋅ D j
(5)
where, at: area of tensile bars, σy: yield strength of tensile bar, d: effective depth of beam,
D: depth of column, N: axial force, b: width of the member, Fc: compressive strength of
concrete, κ: joint shape factor (0.7 for exterior joint), φ: factor depending on existence of
orthogonal beams (0.85 for joint without it), Fj: nominal value of shear strength for joint (Fj =
0.8Fc0.7), bj: effective width (= b, when beam and columns have the same width), and Dj:
effective depth of joint.
By calculations, the ultimate strengths are 134kN.m (142kN.m), 71kN.m (177kN.m), and
278kN (124kN.m) for the beam, column, and joint, respectively. Values in parentheses are the
conversions to joint moment. The strength of joint is the minimum, meaning that joint failure
will occur first for the existing frame.
Validation of Strength of Anchor Bolt Groups
Post-installed adhesive anchor bolts were used in this study. When an adhesive anchor bolt is
pulled, it may fail in three patterns: fracture of anchor, bond failure or concrete cone failure. The
minimum is taken as its tensile strength. However, when anchor bolts are close to one another,
the existing concrete may fail as shown in Fig. 7. According to the Japanese guideline [9], the
tensile strengths of anchor bolt groups, together with their conversions to joint moment and the
required strengthening (referring to Eq. 1), are shown in Table 2. The shear strengths and their
corresponding requirements are also given in the table. As shown in Fig. 7, the shear
strengthening requirements are considered as the shear forces on the beam and column (Vb, Vc)
when the beam yields, and are resisted by the column and beam anchor groups, respectively. As
shown in Table 2, strengths of the anchor groups were confirmed.
Table 2. Strengths of anchor bolt groups and the corresponding design loads.
Tensile
Shear
Anchor
Strength Total moment on joint Requirement Strength Requirement
Place
bolts
kN
kN.m
kN.m
kN
kN
Beam
Column
7-D10
5-D13
135.9
190.3
135.4
76.0
102.9
131.0
54.0
63.1
Test Set-up, Instrumentation, and Loading Program
Fig. 8 shows the test set-up, and Fig. 9 shows the strain gage arrangement. The specimens were
installed on the loading facilities, rotated by 90o. The left column (lower floor column) was
supported by a pin hinge, and the right was supported by a roller. In order to measure the shear
force of the column, a load cell was incorporated into the roller support. Horizontal reversed
cyclic loading was applied to the beam tip controlled by the displacement of the beam tip.
Meanwhile, an additional moment in proportion to the horizontal load was applied by two
vertical hydraulic jacks, to represent a realistic moment distribution of the full-scale beam. The
loads of the vertical jacks were controlled by Eq. 6. Axial load was not applied to the column.
N = ± ( 2 .625 − 1 .7 ) × Q / 3 .5 = ± 0 .264 Q
Figure 8. Test set-up and loading method.
(6)
Figure 9. Strain gage arrangement.
Beam drift ratio is defined as R = δ/L, where δ is the horizontal displacement at the beam
tip measured by a displacement transducer, and L is the distance between the transducer and joint
center, as shown in Fig. 8. The loading program was one cycle for R = 1/800, 1/400, 1/200,
1/133, 1/100, 1/67, 1/50, 1/33, and 1/25 rad., followed by a pushover to R = +1/17.
Experimental Results
Fig. 10 shows damage to the specimens after the cycles to R = 1/200, and 1/67 rad., during which
the joint diagonal crack appeared and the ultimate strength was recorded, respectively, for the
control specimen, together with the final damage. The relationships between joint moment and
beam drift ratio together with the maximum strength are also given in the same figure. The joint
moments are the product of the distance between pin centers and the shear force of the column.
Control specimen, J2
Diagonal cracks appeared at the joint panel during the cycle to R = ±1/200, as shown in Fig. 10,
then extended along the external longitudinal column bars with increasing plastic deformation.
The maximum strength was recorded at R = ±1/67. During the cycle to R = ±1/25, column
longitudinal bars became exposed and their buckling was observed. After that, the diagonal
cracks pierced the outside of the column, and a substantial amount of concrete peeled off.
Figure 10. Damage to specimens, joint moment–beam deflection relationships.
Specimen strengthened at two sides, J2-W2
During the cycle to R = ±1/200, the first beam anchor bolt (ABU/L4, referring to Figs. 6 and 9)
yielded, and diagonal cracks appeared at the beam end where the walls were attached. During the
cycle to R = ±1/133, the beam longitudinal reinforcement yielded at the location where the first
anchor bolt was inserted (BU/L4), and diagonal cracks appeared at the existing joint panel.
During the cycle to R = +1/17, the longitudinal reinforcement of the beam became exposed and
buckling was observed. The damage concentrated at the beam where the wing walls ended.
Obvious damage to the wing walls was not observed.
Specimen only strengthened at one side, J2-W1
The main characteristic for J2-W1 was the asymmetry between positive and negative loadings.
During the cycle to R = +1/200, a diagonal crack appeared from the existing joint to the part of
the beam where the wing wall was attached. During the cycle to R = ±1/133, diagonal cracks
appeared at the existing joint panel, and in the positive loading, the first beam anchor (ABL4)
yielded. During the cycle to R = +1/100, the third beam anchor (ABL3) yielded, then the beam
reinforcement (BL4) yielded at the location where the first anchor was placed, and a crack
appeared along the beam depth where beam anchors were buried, as shown in Fig. 11. It seemed
to be the start of cone failure of concrete, as shown in Fig. 7. During the cycle to R = -1/50, when
the wing wall was pushed, shear cracks appeared at the wing wall, at an angle of about 45o to the
column, as shown in Fig. 11. During the cycle to R = +1/17, the column reinforcement became
exposed and buckling was observed. The maximum strengths between positive and negative
loading, when the wall was pulled or pushed, varied widely. The damage was also asymmetric.
Discussion
The skeleton curves of the hysteresis loops, the strengths considering beam yielding and joint
failure based on Eq. 3 and 5, and the ratios of the maximum strength of strengthened specimens
to the control specimen are illustrated in Fig. 12.
J2 behaved in brittle failure mode, showing low strength and deformability. Its ultimate
strength was obviously below the value calculated according to the Japanese standard [10]
(±124, depending on the joint strength).
The failure mode of F2-W2 successfully shifted from brittle joint failure to ductile beam
yielding, showing high strength, exceeding the beam yield strength (±164, assuming yield at wall
ends), and good deformability.
For J2-W1, when the wing wall was pushed, the strength was improved by 81%, about
the same as J2-W2. When it was pulled, the strength was improved by 39% compared with J2,
reaching the beam yield strength (±142, assuming yielding at column face) of the existing frame,
but less than the yield strength at the wing wall end (±164). The reason seemed to be cone failure
of concrete where beam anchors were buried, as shown in Fig. 11, although the strengthening
capacity was validated as described above. Because the anchors further from the joint center
carried a larger tension, the anchor bolts did not share the tension equally as had been assumed.
250
1.73MJ2(+)
200
164
142
124
Joint Moment (kN.m)
150
100
MJ2(+)
50
0
-50
MJ2(-)
-100
-150
-200 1.85MJ2(-) 1.81MJ2(-)
-250
-4
-3
-2
-1
0
o
Figure 11. Cone-style failure, and 45
cracks at wing wall for J2-W1.
1.39MJ2(+)
J2
J2-W2
J2-W1
Ultimate strength
Calculations
Joint strength
Beam yielding (at column face)
Beam yielding (at wall end)
1
2
3
Beam Drift Angle (%rad.)
4
5
6
Figure 12. Skeletons, calculated strengths,
and strengthening effect.
Conclusions
This paper proposed and verified a strengthening method using RC wing walls for exterior beamcolumn joints where shear reinforcement was not provided. Three partial frame specimens were
tested. The major findings were as follows:
1. In the case of the control specimen, damage concentrated at the joint, showing a brittle
failure mode. The maximum strength was less than the calculated design value.
2. For the specimen strengthened by installing wing walls to both upper and lower columns, the
failure mode shifted from brittle joint failure to ductile beam yielding. The strength and
deformability were significantly improved. It was verified that the strengthening method
effectively inhibited premature failure of the joint.
3. For the specimen strengthened on one side, the strengthening effect when the wing wall was
pushed was greater than when the wall was pulled, although the strengthening capacity by
the tension of the anchor bolt groups had been validated when designing.
4. The contribution of strengthening by pulling and pushing for the joint strengthened on two
sides needs further examination.
References
1.
Park S, Mosalam KM, Shear strength models of exterior beam-column joints without transverse reinforcement.
Pacific Earthquake Engineering Research Center Report 2009; 2009/106.
2.
Sashima Y, Nitta Y, Tomonaga T, Sanada Y, Seismic Loading Test on an R/C Exterior Beam-Column Joint
without Shear Reinforcements in Indonesia. Proceedings of the Thirteenth Taiwan-Japan-Korea Joint Seminar
on Earthquake Engineering for Building Structures 2011; pp. 68-77.
3.
Sharbatdar MK, Kheyroddin A, Emami E, Cyclic performance of retrofitted reinforced concrete beam–column
joints using steel prop. Construction and Building Materials 2012; Vol. 36, pp. 287-294.
4.
Ghobarah A, Said A, Shear strengthening of beam-column joints. Engineering Structures 2002; Vol. 24, pp.
881-888.
5. Sanada Y, Kishimoto I, Kuroki M, Sakashita M, Choi H, Tani M, Hosono Y, Fauzan, Musalamah S, Farida F,
Preliminary Report on Damage to Buildings due to the September 2 and 30, 2009 Earthquakes in Indonesia.
Proceedings of the Eleventh Taiwan-Korea-Japan Joint Seminar on Earthquake Engineering for Building
Structures 2009; pp. 297-306.
6. Shiohara H, New Model for Shear Failure of RC Interior Beam-column Connections. Journal of Structural
Engineering 2001; Vol. 127, Issue 2, pp. 152-160.
7. Kusuhara F, Shiohara H, Ultimate moment of reinforced concrete exterior beam-column joint. Journal of
Structural and Construction Engineering (Transactions of AIJ) 2013; Vol. 78(693), pp. 1949-1958. (in Japanese)
8. Sanada Y, Tomonaga T, Li Y, Watanabe Y, Behavior of an R/C Exterior Beam-Column Joint without Concrete
Confinement under Seismic Loading. Proceedings of the Seventh International Conference on Concrete under
Severe Conditions – Environment and Loading 2013; Vol. 2, pp. 1598-1606.
9.
The Japan Building Disaster Prevention Association (JBDPA), Guidelines for seismic retrofit of existing
reinforced concrete buildings, (2001).
10. The Japan Building Disaster Prevention Association (JBDPA), Standard for seismic evaluation of existing
reinforce concrete buildings, (2001).
11. Architectural Institute of Japan (AIJ), Design Guidelines for Earthquake Resistant Reinforce Concrete
Buildings Based on Inelastic Displacement Concept, (1999). (in Japanese)

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