Advances in Steel Structures - part 42 pptx

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CONCRETE-FILLED STEEL TUBES AS
COUPLING BEAMS FOR RC SHEAR WALLS
J.G. Teng 1, J.F. Chen 2 and Y.C. Lee 1
1 Department of Civil and Structural Engineering
Hong Kong Polytechnic University, Hong Kong, China
2 Built Environment Research Unit, School of Engineering and the Built
Environment, Wolverhampton University, Wulfruna Street,
Wolverhampton WV 1 1SB UK
ABSTRACT
Coupling beams in a reinforced concrete coupled shear wall structure are generally designed to provide
a ductile energy dissipating mechanism during seismic attacks. This paper explores the use of
concrete-filled rectangular tubes (CFRTs) as coupling beams and describes an experimental
investigation into this form of construction to study their load carrying capacity, ductility and energy
absorption characteristics. Results from six tests on simplified CFRT coupling beam models subject to
static and cyclic loads are presented. These results demonstrate that CFRT beams have good ductility
and a good energy absorption capacity. They are therefore suitable as coupling beams for shear walls
particularly if the effect of local buckling is minimised by the use of steel plates of an appropriate
thickness.
KEYWORDS
Coupling beams, concrete-filled steel tubes, shear walls, tall buildings, seismic design, ductility.
INTRODUCTION
Reinforced concrete (RC) coupled shear walls are commonly found in high-rise buildings. For
buildings subject to seismic attacks, properly designed coupled walls offer excellent ductility through
inelastic deformations in the coupling beams, which can dissipate a great amount of seismic energy. It
is thus essential that the coupling beams be designed to possess sufficient ductility.
The traditional way of constructing a ductile RC coupling beam is to use a large amount of steel
reinforcement, particularly diagonal reinforcement (e.g. Pauley and Binney, 1974, Park and Paulay,
1975). However, diagonal reinforcement is only effective for coupling beams with span-to-depth ratios
less than two. For larger span-to-depth ratios, the inclination angle of diagonal bars becomes too small
for them to contribute effectively to shear resistance (Shiu et al, 1978). However, deep coupling beams

391
392 J.G. Teng et al.
are often not desirable because their depths may interfere with clear floor height. Furthermore, with the
increased use of high strength concrete, it is more difficult to achieve ductility in RC beams as the
section size reduces and the brittleness of the concrete increases. Therefore, the exploration of
alternative coupling beam forms offering good ductility is worthwhile.
As an alternative to RC coupling beams, Harries et al. (1993) and Shahrooz et al. (1993) studied the
use of steel I-beams as coupling beams. As a structural material, steel is much stronger and much more
ductile than concrete. However, steel beams may suffer from inelastic lateral buckling and local
buckling which limit their ductility. Although local buckling may be prevented by the proper use of
lateral stiffeners (Harris et al., 1993), such stiffening is labour intensive and may lead to uneconomic
designs.
More recently, steel coupling beams encased in normally reinforced concrete have been studied (Liang
and Han, 1995; Wang and Sang, 1995; Gong et al., 1997). These studies show that the encasement of
concrete leads to increases in stiffness and strength which should be properly considered in design and
that the concrete is likely to spall during cyclic deformations.
This paper explores the use of concrete-filled rectangular tubes (CFRTs) as coupling beams and
describes an experimental investigation into this form of construction to study their load carrying
capacity, ductility and energy absorption characteristics. Extensive recent research has been carried out
on the behaviour of concrete filled steel tubes, particularly as columns (e.g. Ge and Usami, 1992;
Shams and Saadeghvaziri, 1997; Uy, 1998). In such tubes, the concrete infill prevents the inward
buckling of the tube wall while the steel tube confines the concrete and constrains it from spalling. The
combination of steel and concrete in such a manner makes the best use of the properties of both
materials and leads to excellent ductility. To the authors' best knowledge, CFRTs have not previously
been used as coupling beams, although their use in buildings and other structures, particularly as
columns, has been extensive. Apart from ductility considerations, CFRT beams are much simpler to
construct than RC beams because both the placement of complicated reinforcement and temporary
formwork are eliminated. Compared with steel coupling beams, CFRT beams are more economic due
to significant savings in steel.
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SPECIMEN DESIGN AND PREPARATION
Modelling of Coupling Beams
During an earthquake, the coupling beams provide an important energy dissipation mechanism in a
coupled wall structure through inelastic deformations. These beams are subject to large shear forces
Concrete-Filled Steel Tubes as Coupling Beams for RC Shear Walls
393
and bending moments, with the effect of axial forces being small. The shear force and bending
moment distributions in a coupling beam with the point of contraflexure at the mid-span are shown in
Figure 1 a. These force distributions can be modelled by a cantilever beam under a point load at its free
end (Figure l b). This cantilever beam system was thus used in the present study to simulate the
behaviour of a coupling beam under seismic loading. The effect of embedment was not considered and
the wall was assumed to provide a rigid support to the beam. In practical applications, a sufficient
embedment length should be used to prevent premature failures in the embedment zones. An existing
approach for designing the concrete embedment for steel coupling beams (Marcakis and Mitchell,
1980; Harries et al., 1993) can be used for designing the concrete embedment for CFRT coupling
beams.
Design of Specimens
Eight cantilever beams were tested in this study, consisting of two control rectangular hollow section
(RHS) tubes and six CFRTs. All tubes had a wall thickness of 2 mm, with a cross-sectional height of

200 mm and width of 150 mm. The variable for the CFRTs was the concrete strength, designed to cube
strengths of 40, 60 or 90 MPa (referred to as Grade 40, Grade 60 and Grade 90 concrete respectively in
the paper). The eight specimens were divided into two series, each consisting of one RHS tube and
three CFRTs filled with concrete of different grades. The two series of specimens were tested under
static loads and cyclic loads respectively.
Preparation of Specimens
The fabrication of the RHS tubes was by cold-bending and welding. Two channels were first made
from steel sheets using a bending machine. Subsequently, the two channels, with their edges facing
each other, were welded together to form a RHS tube with a welding seam at the mid-height of each
web. Two types of steels with slightly different properties were used (Table 1). These properties were
determined by tensile tests using samples from the same plates used for fabricating the RHS tubes.
TABLE 1
SPECIMEN DETATILS
Specimen Steel properties, MPa Concrete properties, MPa
Yield stress
Ultimate
stress
Young' s
modulus
Compressive
Strength,
28th day
Compressive
strength,
day of beam test
Splitting tensile
strength,
28 th days
RHSs 290 441 194,000 N/A N/A N/A
TG40s 290 441 194,000 45.3 45.2 3.03

TG60s 290 441 194,000 87.6 84.3 4.57
TG90s 290 441 194,000 92.5 94.1 4.68
RHSc 290 365 216,000 N/A N/A N/A
TG40c 290 441 194,000 41.3 45.1 3.36
TG60c 290 441 194,000 87.6 90.62 4.57
TG90c 290 365 216,000 112.0 109.5 6.51
Test
type
Static
Static
Static
Static
Cyclic
Cyclic
Cyclic
Cyclic
The fabricated RHS tubes were then filled with fresh concrete. For each of the specimens, six
100x100x100 mm 3 concrete cubes and three concrete cylinders with a diameter of 100mm and a height
of 200mm were cast to test their compressive and splitting tensile strengths. Measured concrete
properties are shown in Table 1. The actual concrete strength for Grade 60 (Specimens TG60s and
TG60c) was as high as that for Grade 90 (Specimens TG90s and TG90c) probably due to mixing
problems. While this was undesirable, the specimens were still suitable for the present study and are
still referred to using their intended concrete grades (ie TG60 and TG90) in this paper.
394
J.G. Teng et al.
In order to prevent the concrete core from being pushed out when a CFRT specimen was loaded, two 6
mm thick steel plates were welded to the ends of each CFRT beam when the concrete age was 28 days.
This simulated the antisymmetric condition at the point of contraflexure in a full coupling beam.
EXPERIMENTAL SET-UP
The experimental set-up for static loading tests is shown in Figure 2. Beam specimens were clamped

between two large angle plates, which were in turn fixed on the floor by four high strength bolts. The
embedment length of the beams was 440mm. Loads were applied at 460 mm from the fixed end. The
span to depth ratio of the beams was 460/200=2.3 which was the smallest value possible because of
restrictions of the pre-installed anchor plates on the strong floor. A hydraulic jack was fixed onto the
floor to load the beam horizontally for convenience. Displacements at the loading position, the mid-
span and near the fixed end were measured by electronic displacement transducers. Furthermore, a
number of strain gauges were installed near the fixed end (Figure 2). Two displacement transducers
were also used to measure the translation and rotation of the fixed end support. The effect of small
support movements has been removed in the values of displacements presented in this paper.
For cyclic loading tests, two hydraulic jacks were used. Because of this arrangement, the displacement
transducer at the loading point was moved to the tip of the beam. The positions of other transducers
were the same as in the static tests. The deflection at the loading position was inferred from the
measured values at the tip in an approximate manner assuming either the beam deformed elastically or
rigid-plastically with a plastic hinge at the fixed support. Details are given in Lee (1998). No strain
measurement was undertaken in the cyclic tests.
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Figure 2: Experimental set-up for static loading test
TEST PROCEDURE
Static Loading Test
In static loading tests, the specimens were monotonically loaded until failure. The strains and
displacements were recorded at different load levels, from which load-deflection curves were plotted.
These curves were used to determine the values of the 'yield load' Py and the corresponding deflection
at the loading position dy (Lee, 1998). Based on observations during the experiments, the 'yield load'
was defined as the load when local buckling of the compression flange occurred and corresponds to a
strong change in slope of the load deflection curve. This yield load Py and the deflection dy were later
used to control the load/displacement levels in the cyclic loading tests.
Concrete-Filled Steel Tubes as Coupling Beams for RC Shear Walls
395
Cyclic Loading Test
The loading sequence used in the cyclic loading tests is shown in Figure 3. Load control was used
before the yield load was reached. Two cycles of reversed cyclic loading were carried out at a load
level of P=0.8Py. Three additional cycles were then carried out at P = Py. Thereafter, deflection control
at multiples of dy was used. Three complete cycles were carried out at each selected value of deflection
until the specimen failed. The loads or displacements were carefully controlled during cyclic tests,
nevertheless, some small deviations from the intended values still existed. Displacements were
monitored and recorded throughout the test.
Figure 3: Loading history for cyclic tests
Figure 4: Load-deflection curves
under static loading
STATIC TEST RESULTS
Figure 4 shows the load-deflection curves of the loading point for all four static test specimens. The
rapidly descending load-deflection curve after buckling of the RHS tube indicates that its load carrying
capacity was reduced quickly, exhibiting very limited ductility. The ultimate strengths of the concrete
filled tubes are almost triple that of the corresponding RHS tube. The extended plateaux in the load-
deflection curves alter yielding show that CFRT beams are very ductile. These effects of the concrete
infill are well known. The ductile behaviour of the CFRT beams was terminated by tensile rupture of

the tension flange which occurred significantly earlier in Specimen TG90s than in the other two CFRT
beams. Specimens TG40s and TG60s showed similar ductility, though they were filled with concrete
of rather different strengths. The effect of the concrete strength on ductility is thus believed to be
small.
Although local buckling of the steel tube was observed in all tests, the final failure modes were
different for RHS and CFRT specimens (Figure 5). The local buckling of the compression flange near
the fixed end occurred at a load of approximately 40 kN, leading to immediate collapse of Specimen
RHSs. Shear buckling occurred on both webs at the same load. No crack was found on the tensile
flange of Specimen RHSs.
For the three CFRT beams, outward local buckling was observed on the compression flanges at a load
of approximately 80kN. Shear buckling occurred later on the webs at about 100kN. Clearly, the
concrete infill constrained the plate to buckle only away from it, which led to a higher buckling
strength, as has been shown by many authors (eg Wright, 1993; Smith et al., 1999). Strain readings
showed that the tensile flange had yielded and the compression flange was close to yielding when local
buckling occurred. Fracture cracks were found on the tension flanges of CFRT specimens at final
failure, indicating the full use of the steel strength. The final failure of CFRT members was by rupture
of steel of the tension flange and is referred to as a flexural failure.
396
J.G. Teng et al.
Figure 5: Static loading test: buckling of the compression flange
Table 2 shows the experimental ultimate loads for all the static test specimens. The calculated ultimate
flexural failure loads according to the approach in BS 8110 (1985) for reinforced concrete beams and
using the ultimate stress of steel are also listed for comparison. Clearly, experimental observations are
in good agreement with theoretical predictions for CFRT specimens, with discrepancies within 3%.
These calculations did not consider local buckling effects, so the calculated ultimate flexural failure
load of Specimen RHSs of 93.78kN is more than double the value actually achieved during the test
(42.51kN). The chief contribution of the concrete infill is thus to provide constraint to the steel tube.
The ultimate strength of CFRT beams increases with the concrete strength. However, this increase is
small. Table 2 shows that the concrete strength for TG60s and TG90s is almost twice that for TG40s,
but the increase in the experimental ultimate load is only less than 3% while the theoretical increase is

less than 6%.
TABLE 2
STATIC ULTIMATE LOADS
Specimen fcu, MPa Test ultimate load, Predicted ultimate Test / Prediction
kN load, kN
RHSs N/A 42.51 93.78 0.453
TG40s 45.2 117.14 114.2 1.026
TG60s 84.3 120.24 119.55 1.006
TG90s 94.1 119.57 120.38 0.993
CYCLIC TEST RESULTS
Test Observations and Failure Modes
Local buckling was observed on both flanges of all the cyclic test specimens. During load reversal, a
buckled flange was straightened again under tension. The compression-tension cyclic stresses caused
degradation in both steel and concrete, so that the maximum load reached in a cyclic test is
considerably lower than that in the corresponding static test.
For Specimen RHSc, local buckling was observed in both flanges. No crack developed in the flanges,
indicating that the steel tensile strength was not fully utilised. By contrast, cracks developed in both
flanges of TG40c and TG60c, and in one of the flanges of TG90c at final failure. Figure 6 shows one
Concrete-Filled Steel Tubes as Coupling Beams for RC Shear Walls
397
of the flanges for each of the three cyclic test specimens after final failure. All the CFRT specimens
failed after 14 to 15 loading cycles.
Figure 6: Failure mode under cyclic loading
Hysteretic Responses
The hysteretic load-deflection responses of the loading position from all four cyclic tests are shown in
Figure 7. The load carrying capacity of Specimen RHSc (Figure 7a) was quickly reduced from about
40kN in the first few cycles, to less than 20kN at the 8 th cycle and to less than 10kN at the 14 th cycle,
confirming the lack of ductility as observed in the static loading test. Because the areas surrounded by
the hysteresis loops represent the amount of energy absorbed by the test specimen, the energy
absorption capacity of RHS tubes is thus very limited and reduces quickly under large cyclic

deformations.
As observed in the static tests, the ultimate strength of CFRT beams is significantly higher than their
hollow counterparts. While the differences in the load carrying capacity in the plastic range are not
large between the three CFRT cyclic test specimens, it is worth noting that TG90c, which had the
highest concrete strength (Table 1), showed the lowest load carrying capacity.
Compared with the results from the static loading tests, the maximum load carrying capacities of
CFRT beams under cyclic loading are about 20-30% lower, with the difference between the two TG40
specimens being the smallest and that between the two TG90 specimens the largest. This indicates that
a CFRT beam with a lower strength concrete behaves better than one filled with concrete of a higher
strength.
The CFRT beams exhibited strength and stiffness degradations under reversed cyclic loading and
pinching is seen for all of them (Figure 7). The main reason is believed to be the degradation of
concrete in strength and stiffness when subject to reversed cyclic loading which leads to shear cracks
in both directions. Slipping between the steel tube and the concrete may also have been a significant
factor. The slipping behaviour may be improved by using shear connectors such as those used by
Shakir-Khalil et al. (1993).
Overall, the hysteretic responses of these beams are good and are better than normal reinforced
concrete beams, but are not as good as deep RC beams with proper diagonal reinforcement (Park and
398
J.G. Teng et al.
Paulay, 1975). Significant improvements to the cyclic behaviour of these beams should be achievable
by using thicker steel plates so that the effect of local buckling is minimised.
Figure 7: Hysteretic load-deflection responses at loading position
CONCLUSIONS
This paper has explored the use of concrete filled steel tubes as coupling beams for reinforced concrete
coupled shear wall structures. Six concrete filled rectangular steel tubes and two rectangular hollow
steel tubes have been tested under static and cyclic loadings. The mutual constraints of the steel tube
and the concrete infill lead to higher strength and good ductility. The strength and ductility of these
beams are insensitive to concrete strength, but cyclic degradation seems to increase with concrete
strength. The use of high strength concrete thus seems to be undesirable. The hysteretic responses of

these beams under cyclic loads show that they have a good energy absorption capacity. Therefore,
these beams are suitable as coupling beams, particularly if local buckling is minimised by using
relatively thick steel plates and slipping between the steel and concrete is reduced using some form of
shear connectors. Further research is required to better understand this form of coupling beams.
Concrete-Filled Steel Tubes as Coupling Beams for RC Shear Walls
ACKNOLWEDGEMENTS
The authors are grateful to Dr. Y.L. Wong for helpful discussions on the subject.
399
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