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Concrete-Filled Steel Tube Column System : Recent Research and
Construction in Japan
Article · January 2002
Source: OAI
CITATIONS
16
READS
1,785
1 author:
Shosuke Morino
Independent Researcher
132 PUBLICATIONS 1,993 CITATIONS
SEE PROFILE
All content following this page was uploaded by Shosuke Morino on 11 February 2015.
The user has requested enhancement of the downloaded file.
Concrete-Filled Steel Tube Column System : Recent Research and
Construction in Japan
Article · January 2002
Source: OAI
CITATIONS
16
READS
1,785
1 author:
Shosuke Morino
Independent Researcher
132 PUBLICATIONS 1,993 CITATIONS
SEE PROFILE
All content following this page was uploaded by Shosuke Morino on 11 February 2015.
The user has requested enhancement of the downloaded file.
CONCRETE-FILLED STEEL TUBE COLUMN SYSTEM
- RECENT RESEARCH AND CONSTRUCTION IN JAPAN -
Shosuke Morino
Mie University
Japan
Keywords: concrete-filled steel tube, column, beam-column, design formula, construction data
1 INTRODUCTION
It has been verified by extensive investigations since 1970 that a framing system consisting of
concrete-filled steel tube (CFT) columns and H-shaped beams has more benefit compared with ordinary
reinforced concrete and steel systems. Main advantages are as follows. i) Interaction between steel tube
and concrete: Occurrence of local buckling of steel tube is delayed by the restraint of concrete, and the
strength deterioration after the local buckling is moderated. Strength of concrete is increased by the
confining effect provided from the steel tube, and the strength deterioration is not very severe, because
the concrete spalding is prevented by the tube. It is easy to design a beam-to-column connection panel
stronger than adjacent beams and columns. Drying shrinkage and creep of concrete are much smaller
than ordinary reinforced concrete. ii) Cross-sectional properties: Steel ratio in a CFT cross section is
much larger than those in reinforced concrete and concrete-encased steel cross sections, and steel in
the CFT section is well plastified under bending since it is located most outside the section. iii) Construc-
tion efficiency: Works for forms and reinforcing bars are omitted, and concrete casting is done by Tremie
tube or pump-up method, which lead to saving of manpower and constructional cost and time, and clean
construction site. Concrete with low water content is usually cast to avoid void spaces forming under-
neath the diaphragm of the beam-to-column connection due to bleeding, which leads to the concrete of
high-quality. iv) Fire resistance: Concrete improves the fire resistance performance, and the amount of
fireproof material can be reduced or its use can be omitted. v) Cost performance: Because of the advan-
tages mentioned above, a better cost performance is achieved by replacing steel columns by CFT col-
umns. vi) Ecology: Environmental burden can be reduced by omitting form works, and by reusing steel
tubes and high-quality concrete as recycled aggregates.
A weak point of the CFT column system is the compactness of concrete around the beam-to-
column connection; the gap between concrete and steel may be produced by the bleeding of concrete
underneath the diaphragm. There is no way so far to assure the compactness and to repair the defects,
and thus it is tried in the construction practice to cast the concrete with low water-content and high
workability by the use of superplasticizer.
This paper introduces the recent Japanese research, construction data, and some design formu-
las concerning the CFT column system.
2 RESEARCH ON CFT COLUMN SYSTEM
2.1 Research Activities
Figure 1 shows number of abstracts of technical papers on composite concrete and steel struc-
tures and on the CFT column system, which have been presented at annual meetings of Architectural
Institute of Japan (AIJ). It is said that the first technical paper on CFT in Japan was written by Profs.
Naka, Kato, et al. in 1961, which dealt with a circular CFT compression member applied to the power
transmission tower. The number of abstracts presented at AIJ’s annual meeting was about 10 or less
every year until 1986.
In 1985, the Ministry of Construction, Japan, invited the proposal on the structural system for
urban type of apartment houses for the 21st century, and the CFT column system was selected, which
was jointly proposed by 5 general contractors and a steel manufacturer. Since then, a series of experi-
mental investigation, so-called “New Urban Housing Project (NUHP)”, started by these industries and
the Building Research Institute (BRI) of the Ministry of Construction, Japan. This project covered the
tests of centrally-loaded stub columns and beam-columns under combined compression, bending and
1
Composite structuresSession 5
- RECENT RESEARCH AND CONSTRUCTION IN JAPAN -
Shosuke Morino
Mie University
Japan
Keywords: concrete-filled steel tube, column, beam-column, design formula, construction data
1 INTRODUCTION
It has been verified by extensive investigations since 1970 that a framing system consisting of
concrete-filled steel tube (CFT) columns and H-shaped beams has more benefit compared with ordinary
reinforced concrete and steel systems. Main advantages are as follows. i) Interaction between steel tube
and concrete: Occurrence of local buckling of steel tube is delayed by the restraint of concrete, and the
strength deterioration after the local buckling is moderated. Strength of concrete is increased by the
confining effect provided from the steel tube, and the strength deterioration is not very severe, because
the concrete spalding is prevented by the tube. It is easy to design a beam-to-column connection panel
stronger than adjacent beams and columns. Drying shrinkage and creep of concrete are much smaller
than ordinary reinforced concrete. ii) Cross-sectional properties: Steel ratio in a CFT cross section is
much larger than those in reinforced concrete and concrete-encased steel cross sections, and steel in
the CFT section is well plastified under bending since it is located most outside the section. iii) Construc-
tion efficiency: Works for forms and reinforcing bars are omitted, and concrete casting is done by Tremie
tube or pump-up method, which lead to saving of manpower and constructional cost and time, and clean
construction site. Concrete with low water content is usually cast to avoid void spaces forming under-
neath the diaphragm of the beam-to-column connection due to bleeding, which leads to the concrete of
high-quality. iv) Fire resistance: Concrete improves the fire resistance performance, and the amount of
fireproof material can be reduced or its use can be omitted. v) Cost performance: Because of the advan-
tages mentioned above, a better cost performance is achieved by replacing steel columns by CFT col-
umns. vi) Ecology: Environmental burden can be reduced by omitting form works, and by reusing steel
tubes and high-quality concrete as recycled aggregates.
A weak point of the CFT column system is the compactness of concrete around the beam-to-
column connection; the gap between concrete and steel may be produced by the bleeding of concrete
underneath the diaphragm. There is no way so far to assure the compactness and to repair the defects,
and thus it is tried in the construction practice to cast the concrete with low water-content and high
workability by the use of superplasticizer.
This paper introduces the recent Japanese research, construction data, and some design formu-
las concerning the CFT column system.
2 RESEARCH ON CFT COLUMN SYSTEM
2.1 Research Activities
Figure 1 shows number of abstracts of technical papers on composite concrete and steel struc-
tures and on the CFT column system, which have been presented at annual meetings of Architectural
Institute of Japan (AIJ). It is said that the first technical paper on CFT in Japan was written by Profs.
Naka, Kato, et al. in 1961, which dealt with a circular CFT compression member applied to the power
transmission tower. The number of abstracts presented at AIJ’s annual meeting was about 10 or less
every year until 1986.
In 1985, the Ministry of Construction, Japan, invited the proposal on the structural system for
urban type of apartment houses for the 21st century, and the CFT column system was selected, which
was jointly proposed by 5 general contractors and a steel manufacturer. Since then, a series of experi-
mental investigation, so-called “New Urban Housing Project (NUHP)”, started by these industries and
the Building Research Institute (BRI) of the Ministry of Construction, Japan. This project covered the
tests of centrally-loaded stub columns and beam-columns under combined compression, bending and
1
Composite structuresSession 5
shear. Total 86 specimens were tested. A sharp increase in the number of abstracts on the CFT system
observed in 1987 and 1988 in Fig. 1 is due to the research related to NUHP.
5-year research project on composite and hybrid structures started in 1993 as the 5th phase of the
U.S.-Japan Cooperative Earthquake Research Program, and the program was organized into following
4 groups: CFT column system; reinforced concrete column + steel beam system; hybrid wall system; and
research for innovation of new materials, elements and systems. Program of the experimental study for
the CFT system conducted by the Japanese side consisted of centrally-loaded stub columns, eccentri-
cally loaded stub columns, beam-columns, and beam-to-column connections. Total 154 specimens were
tested. Unique feature of this test program was that it covered high-strength materials, such as 800 MPa
steel and 90 MPa concrete, it covered large D/t ratio, and some of the beam-column specimens were
tested under the variable axial load. Another sharp increase starting in 1993 in Fig. 1 is related to the
research in the U.S.-Japan Program. The number of abstracts has been decreasing after the finish of this
program.
2.2 Research Topics and Findings
Research topics treated in the papers presented at the annual meetings of AIJ are summarized as
follows. i) Structural mechanics: stiffness, strength, ductility of columns, beam-columns and beam-to-
column connections, post-local buckling behavior, confining effect, and stress transfer mechanism. ii)
Construction efficiency: compactness of concrete, concrete mixture, concrete casting method and con-
struction time. iii) Fire resistance: strength under fire and amount of fireproof material. iv) Structural
planning: application to high-rise and long-span building, and cost performance.
Results of research works mentioned above have provided following knowledges on the CFT
column system:
Compression Members
Difference between ultimate strength and nominal squash load of a centrally loaded circular short
column is provided by the confining effect, and estimated by a linear function of the steel tube yield
strength
[1].
Increase in the strength due to the confining effect cannot be expected in the case of a square
short column. Moreover, the effect of local buckling must be carefully considered.
Buckling strength of a CFT long column can be evaluated by the sum of the tangent modulus
strengths calculated for a steel tube long column and a concrete long column, separately. There is no
confining effect on the buckling strength regardless of the cross-sectional shape
[2].
Eelastic axial stiffness can be generally evaluated by the sum of the stiffness of steel tube and
concrete. However, careful consideration must be given to the effects of stresses generated in steel tube
at the construction site, mechanism transferring beam loads to a CFT column through steel tube skin,
and creep and drying shrinkage of concrete. They might affect the stiffness.
0
20
40
60
80
100
120
140
160
180
1986 87 88 89 90 91 92 93 94 95 96 98 97 99 00 2002
01
Total numbers on composite structures Numbers on CFT structures
Year
Number of Abstracts Presented at AIJ Annual Meetings
87
109
125
102
85
117
137
153
159
135
138
152
140
118
89
77
73
22
3230 3028
37
4544
50
42 42
50
54
41
32
22
10
Fig. 1 Number of Abstracts Presented at AIJ Annual Meetings
Proceedings of the 1st fib Congress
2
observed in 1987 and 1988 in Fig. 1 is due to the research related to NUHP.
5-year research project on composite and hybrid structures started in 1993 as the 5th phase of the
U.S.-Japan Cooperative Earthquake Research Program, and the program was organized into following
4 groups: CFT column system; reinforced concrete column + steel beam system; hybrid wall system; and
research for innovation of new materials, elements and systems. Program of the experimental study for
the CFT system conducted by the Japanese side consisted of centrally-loaded stub columns, eccentri-
cally loaded stub columns, beam-columns, and beam-to-column connections. Total 154 specimens were
tested. Unique feature of this test program was that it covered high-strength materials, such as 800 MPa
steel and 90 MPa concrete, it covered large D/t ratio, and some of the beam-column specimens were
tested under the variable axial load. Another sharp increase starting in 1993 in Fig. 1 is related to the
research in the U.S.-Japan Program. The number of abstracts has been decreasing after the finish of this
program.
2.2 Research Topics and Findings
Research topics treated in the papers presented at the annual meetings of AIJ are summarized as
follows. i) Structural mechanics: stiffness, strength, ductility of columns, beam-columns and beam-to-
column connections, post-local buckling behavior, confining effect, and stress transfer mechanism. ii)
Construction efficiency: compactness of concrete, concrete mixture, concrete casting method and con-
struction time. iii) Fire resistance: strength under fire and amount of fireproof material. iv) Structural
planning: application to high-rise and long-span building, and cost performance.
Results of research works mentioned above have provided following knowledges on the CFT
column system:
Compression Members
Difference between ultimate strength and nominal squash load of a centrally loaded circular short
column is provided by the confining effect, and estimated by a linear function of the steel tube yield
strength
[1].
Increase in the strength due to the confining effect cannot be expected in the case of a square
short column. Moreover, the effect of local buckling must be carefully considered.
Buckling strength of a CFT long column can be evaluated by the sum of the tangent modulus
strengths calculated for a steel tube long column and a concrete long column, separately. There is no
confining effect on the buckling strength regardless of the cross-sectional shape
[2].
Eelastic axial stiffness can be generally evaluated by the sum of the stiffness of steel tube and
concrete. However, careful consideration must be given to the effects of stresses generated in steel tube
at the construction site, mechanism transferring beam loads to a CFT column through steel tube skin,
and creep and drying shrinkage of concrete. They might affect the stiffness.
0
20
40
60
80
100
120
140
160
180
1986 87 88 89 90 91 92 93 94 95 96 98 97 99 00 2002
01
Total numbers on composite structures Numbers on CFT structures
Year
Number of Abstracts Presented at AIJ Annual Meetings
87
109
125
102
85
117
137
153
159
135
138
152
140
118
89
77
73
22
3230 3028
37
4544
50
42 42
50
54
41
32
22
10
Fig. 1 Number of Abstracts Presented at AIJ Annual Meetings
Proceedings of the 1st fib Congress
2
Constitutive laws for concrete and steel in a CFT column have been established that take into
account increase in concrete strength due to confinement, scale effect on concrete strength, strain soft-
ening in concrete, increase in tensile strength and decrease in compressive strength of steel tube due to
ring tension stress, local buckling of steel tube, effect of concrete restraining the progress of local buck-
ling deformation, and strain hardening of steel
[3].
Beam-Columns
Bending strength of a circular CFT beam-column exceeds superposed strength (sum of the strengths
of concrete and steel tube) due to the confining effect. The confining effect cannot be expected in the
case of a square CFT beam-column, and the effect of local buckling must be considered.
Circular CFT beam-columns show larger ductility than square ones.
Use of high strength concrete generally causes the reduction of ductility. However, non-ductile
behavior can be improved by confining concrete with high strength steel tube, in the case of a circular
CFT beam-column.
Empirical formulas to estimate the limit rotation angle of a CFT beam-column have been pro-
posed
[4].
Fiber analysis based on the constitutive laws mentioned above well traces the flexural behavior
and ultimate strength of an eccentrically loaded CFT column
[5].
Effective mathematical model has been established to trace the cyclic behavior of a CFT beam-
column subjected to combined compression, bending and shear, except for the behavior after the local
buckling of steel tube occurs
[6].
Hysteretic restoring force characteristic model for a CFT beam-column has been proposed, which
accurately predicts the behavior within the rotation angle equal to 1.0%
[7].
Beam-to-Column Connections
Figure 2 shows typical beam-to-column connections used in the CFT column system. Design
formulas have been established for outer and through diaphragms, and ring stiffener.
Strength evaluation formulas have been proposed for inner diaphragms, which are derived by the
yield line theory, although they are rather complicated
[4].
Design formulas for a shear panel in the connection have been established, which give the lower
bound of the ultimate shear strength.
Several new types have been proposed, such as the connections using vertical stiffeners
[8], long
tension bolts
[9, 10], and thicker tube at the shear panel without diaphragm
[11].
Stress transfer mechanism has been proposed to trace the load-deformation behavior of a CFT
column subassemblage, which consists of a diagonal concrete strut and a surrounding steel frame formed
by tube walls and diaphragms
[12].
Frames
Tests of subassemblages whose shear panels were designed weaker than beams and columns
showed very ductile behavior
[13]. However, it is usually difficult in the real practice to make the shear
panel weaker, unless the steel tube thinner than the CFT column is used for the shear panel.
Energy dissipation capacity of a column-failing CFT frame is equivalent to that of a steel frame
[14].
Fig. 2
Beam-to-Column Connections
Outer diaphragm
Through diaphragmInner diaphragm Ring stiffener
Composite structuresSession 5
3
account increase in concrete strength due to confinement, scale effect on concrete strength, strain soft-
ening in concrete, increase in tensile strength and decrease in compressive strength of steel tube due to
ring tension stress, local buckling of steel tube, effect of concrete restraining the progress of local buck-
ling deformation, and strain hardening of steel
[3].
Beam-Columns
Bending strength of a circular CFT beam-column exceeds superposed strength (sum of the strengths
of concrete and steel tube) due to the confining effect. The confining effect cannot be expected in the
case of a square CFT beam-column, and the effect of local buckling must be considered.
Circular CFT beam-columns show larger ductility than square ones.
Use of high strength concrete generally causes the reduction of ductility. However, non-ductile
behavior can be improved by confining concrete with high strength steel tube, in the case of a circular
CFT beam-column.
Empirical formulas to estimate the limit rotation angle of a CFT beam-column have been pro-
posed
[4].
Fiber analysis based on the constitutive laws mentioned above well traces the flexural behavior
and ultimate strength of an eccentrically loaded CFT column
[5].
Effective mathematical model has been established to trace the cyclic behavior of a CFT beam-
column subjected to combined compression, bending and shear, except for the behavior after the local
buckling of steel tube occurs
[6].
Hysteretic restoring force characteristic model for a CFT beam-column has been proposed, which
accurately predicts the behavior within the rotation angle equal to 1.0%
[7].
Beam-to-Column Connections
Figure 2 shows typical beam-to-column connections used in the CFT column system. Design
formulas have been established for outer and through diaphragms, and ring stiffener.
Strength evaluation formulas have been proposed for inner diaphragms, which are derived by the
yield line theory, although they are rather complicated
[4].
Design formulas for a shear panel in the connection have been established, which give the lower
bound of the ultimate shear strength.
Several new types have been proposed, such as the connections using vertical stiffeners
[8], long
tension bolts
[9, 10], and thicker tube at the shear panel without diaphragm
[11].
Stress transfer mechanism has been proposed to trace the load-deformation behavior of a CFT
column subassemblage, which consists of a diagonal concrete strut and a surrounding steel frame formed
by tube walls and diaphragms
[12].
Frames
Tests of subassemblages whose shear panels were designed weaker than beams and columns
showed very ductile behavior
[13]. However, it is usually difficult in the real practice to make the shear
panel weaker, unless the steel tube thinner than the CFT column is used for the shear panel.
Energy dissipation capacity of a column-failing CFT frame is equivalent to that of a steel frame
[14].
Fig. 2
Beam-to-Column Connections
Outer diaphragm
Through diaphragmInner diaphragm Ring stiffener
Composite structuresSession 5
3
Quality of Concrete and Casting
As stated above, the gap between concrete and steel may be produced by the bleeding of con-
crete underneath the diaphragm. It is necessary to mix concrete with small water cement ratio, small
water content, and large cement content to reduce the bleeding. Use of superplastisizer is effective to
keep good workability
[4].
Pumping-up method is recommended to cast compact concrete without void area underneath the
diaphragm. Lateral pressure caused on a steel pipe by pumping usually goes upto 1.3 times the liquid
pressure of concrete (= unit weight of fresh concrete times casting height), which causes ring tension
stress in the steel tube. The pressure and stress may distort the square shape of the tube, if the wall
thickness of the tube is too thin
[4].
Tremie tube method is effective when casting height is not too high, with the use of vibrator to
obtain compact concrete. If the vibrator is not used, it is necessary to cast concrete with high flowability
and resistibility against segregation.
Design Characteristics
Lateral story stiffness of the CFT column system is larger than that of the steel system, but story
weight of the former is larger than that of the latter. This leads to no major differences in the vibration
characteristics of both systems.
No significant difference in elasto-plastic behavior or energy dissipation capacity is observed be-
tween the CFT and steel systems, as long as the overall frame mechanism with plastic hinges mainly
forming in beams is adopted in the design
[15].
Total steel amount of the CFT column system is about 10% less than that of the steel system
[15].
Fire Resistance
CFT columns elongate at early stage of heat loading, and then shorten to the failure.
CFT columns can sustain axial load by filled concrete after the capacity of steel tube is lost by
heating, and thus fireproof material can be reduced or omitted.
Rigidity at the beam-to-column connection reduces by the heat loading, which leads to the reduc-
tion of bending moments transferred from beams to columns. Thus, the column carries only axial load at
final stage of heat loading
[16].
Fire tests of CFT beam-columns forced to sway by the thermal elongation of adjacent beams have
shown that square and circular CFT beam-columns could sustain the axial load for 2 hours and 1 hour
under the axial load ratio of 0.45 and sway angle of 1/100, respectively, but CFT beam-columns could not
resist bending caused by the forced sway after 30 minutes of heating
[17].
3. DESIGN OF CFT COLUMN SYSTEM
3.1 Design Recommendations
First edition of AIJ standard for the composite concrete and circular steel tube structures was
published in 1967, based on the research works carried in early 1960’s. This edition was written for three
types of circular composite sections, so-called concrete-encased tube, CFT and concrete-encased and
filled tube sections, and was revised in 1980 to include sections using square tubes. This standard was
absorbed into AIJ standard for the composite concrete and steel (SRC) structures in 1987, which newly
included the formulas to evaluate ultimate strength of circular and square CFT columns, beam-columns
and beam-to-column connections. English version of this Standard is available at AIJ
[18]. The newest
edition of SRC Standard of AIJ
[19] was published in 2001, which increased the upper limit of design
standard strength of normal concrete to 60 MPa, and revised several parts of design provisions for the
CFT column system, according to the contents of CFT Recommendations
[4] explained below.
CFT Recommendations
[4] was published by AIJ in 1997, based on the recent research develop-
ments, which are characterized by covering following topics. i) Special type of CFT members such as
braces and truss members, in addition to compression members, beam-columns and connections. ii)
Formulas to evaluate deformation capacity of CFT columns and frames. iii) Structural characteristics
under fire. iv) Manufacturing of steel tube and mixture of concrete, v) analysis of the behavior of CFT
columns and frames. vi) Strength formulas used in the world.
Results of investigation carried under NUHP were published in CFT Reports
[20], and it has been
used for the design of the CFT system. This report is characterized as the first document in Japan which
allowed to count the strength increase of confined concrete of circular CFT members, and showed for-
mulas to evaluate the deformation capacity. Evaluation of the deformation capacity of CFT beam-col-
Proceedings of the 1st fib Congress
4
As stated above, the gap between concrete and steel may be produced by the bleeding of con-
crete underneath the diaphragm. It is necessary to mix concrete with small water cement ratio, small
water content, and large cement content to reduce the bleeding. Use of superplastisizer is effective to
keep good workability
[4].
Pumping-up method is recommended to cast compact concrete without void area underneath the
diaphragm. Lateral pressure caused on a steel pipe by pumping usually goes upto 1.3 times the liquid
pressure of concrete (= unit weight of fresh concrete times casting height), which causes ring tension
stress in the steel tube. The pressure and stress may distort the square shape of the tube, if the wall
thickness of the tube is too thin
[4].
Tremie tube method is effective when casting height is not too high, with the use of vibrator to
obtain compact concrete. If the vibrator is not used, it is necessary to cast concrete with high flowability
and resistibility against segregation.
Design Characteristics
Lateral story stiffness of the CFT column system is larger than that of the steel system, but story
weight of the former is larger than that of the latter. This leads to no major differences in the vibration
characteristics of both systems.
No significant difference in elasto-plastic behavior or energy dissipation capacity is observed be-
tween the CFT and steel systems, as long as the overall frame mechanism with plastic hinges mainly
forming in beams is adopted in the design
[15].
Total steel amount of the CFT column system is about 10% less than that of the steel system
[15].
Fire Resistance
CFT columns elongate at early stage of heat loading, and then shorten to the failure.
CFT columns can sustain axial load by filled concrete after the capacity of steel tube is lost by
heating, and thus fireproof material can be reduced or omitted.
Rigidity at the beam-to-column connection reduces by the heat loading, which leads to the reduc-
tion of bending moments transferred from beams to columns. Thus, the column carries only axial load at
final stage of heat loading
[16].
Fire tests of CFT beam-columns forced to sway by the thermal elongation of adjacent beams have
shown that square and circular CFT beam-columns could sustain the axial load for 2 hours and 1 hour
under the axial load ratio of 0.45 and sway angle of 1/100, respectively, but CFT beam-columns could not
resist bending caused by the forced sway after 30 minutes of heating
[17].
3. DESIGN OF CFT COLUMN SYSTEM
3.1 Design Recommendations
First edition of AIJ standard for the composite concrete and circular steel tube structures was
published in 1967, based on the research works carried in early 1960’s. This edition was written for three
types of circular composite sections, so-called concrete-encased tube, CFT and concrete-encased and
filled tube sections, and was revised in 1980 to include sections using square tubes. This standard was
absorbed into AIJ standard for the composite concrete and steel (SRC) structures in 1987, which newly
included the formulas to evaluate ultimate strength of circular and square CFT columns, beam-columns
and beam-to-column connections. English version of this Standard is available at AIJ
[18]. The newest
edition of SRC Standard of AIJ
[19] was published in 2001, which increased the upper limit of design
standard strength of normal concrete to 60 MPa, and revised several parts of design provisions for the
CFT column system, according to the contents of CFT Recommendations
[4] explained below.
CFT Recommendations
[4] was published by AIJ in 1997, based on the recent research develop-
ments, which are characterized by covering following topics. i) Special type of CFT members such as
braces and truss members, in addition to compression members, beam-columns and connections. ii)
Formulas to evaluate deformation capacity of CFT columns and frames. iii) Structural characteristics
under fire. iv) Manufacturing of steel tube and mixture of concrete, v) analysis of the behavior of CFT
columns and frames. vi) Strength formulas used in the world.
Results of investigation carried under NUHP were published in CFT Reports
[20], and it has been
used for the design of the CFT system. This report is characterized as the first document in Japan which
allowed to count the strength increase of confined concrete of circular CFT members, and showed for-
mulas to evaluate the deformation capacity. Evaluation of the deformation capacity of CFT beam-col-
Proceedings of the 1st fib Congress
4
umns is needed to calculate the structural characteristic factor D
s
used in the seismic design. In 1996,
those industries which originally joined NUHP established the Association of New Urban Housing Tech-
nology (ANUHT). The ANUHT consists of more than 100 member companies which are related to the
CFT building construction, and authorizes the structural design of newly-planned CFT buildings, accord-
ing to the ANUHT’s CFT Recommendations
[21].
The ANUHT Recommendations cover the following design and construction items. i) Strength
design of columns and beam-columns. ii) Evaluation of deformation capacity of beam-columns. iii) Fire
resistant design of beam-columns. iv) Production of CFT members including compaction of filled con-
crete by centrifugal method. v) Quality control of materials and construction work.
CFT investigation carried out in the 5th phase of the U.S.-Japan Cooperative Earthquake Re-
search Program produced CFT Guidelines
[22]. The Guidelines are characterized by covering following
topics. i) Flow charts for the seismic design, based on the conventional method using the structural
characteristic factor
D
s
, and performance-based design method which is specified in the recent revision
of the Building Standard Law of Japan. ii) Constitutive laws for concrete and steel tube derived from the
test results of centrally-loaded stub columns, method of analysis for the moment-curvature relation, method
of analysis for the load-deformation relation of a beam-column under combined compression, bending
and shear, and model for the restoring-force characteristics of a beam-column which may be used in the
analysis of an overall CFT frame. iii) Formulas to evaluate stiffness, ultimate strength and deformation
capacity of a CFT beam-column, taking into account the confining and scale effects on concrete, and
triaxial state of stress and local buckling of the steel tube. iv) Stress transfer mechanism around a beam-
to-column connection, and mathematical model for shear force-deformation relation of a connection
panel. v) Material, manufacturing and fabrication of steel tube, concrete mixture and casting. vi) Design
example using 11-story office building, written for beginners of designing the CFT column system. vii)
Investigation of advantages of the CFT column system by the trial design of 10-, 24- and 40-story CFT
frames. Research results that formed the background of this Guidelines are summarized in English in
BRI Research Paper
[23].
3.2 Design Formulas for CFT Members
This section introduces design formulas for CFT members shown in the 2001 edition of SRC
Standard of AIJ
[19].
Ultimate Compressive Strength of a CFT Column
Ultimate compressive strength of a CFT column is calculated by Eqs. (1) through (4).
l
k
D
≤ 4 ; N
u1
= N
u
c
+(1+ η ) N
u
s
(1)
4<
l
k
D
≤ 12 ; N
u2
=N
u1
0.125 ( N
u1
N
u3
)(
l
k
D4) (2)
12 <
l
k
D
; N
u3
= N
cr
c
+ N
cr
s
(3)
where
l
k
: effectivelength of a CFT column
D :
width or diameterof a steel tube section
η =0 for a square CFT column
η = 0.27
for a circular CFT column
(4)
N
u1
, N
u2
, N
u3
: ultimatestrength of a CFT column
N
u
c
: ultimatestrength of a concretecolumn
N
u
s
: ultimatestrength of a steel tube column
N
cr
c
: bucklingstrength of a concretecolumn
N
cr
s
: bucklingstrength of a steel tube column
N
u 1
in Eq. (1) gives cross-sectional strength of a CFT column, in which strength of confined concrete is
considered for a circular CFT column. Derivation of η = 0.27 is shown in Ref.
[1]. N
u 3
in Eq. (3) gives
buckling strength of a long column as the sum of the buckling strengths separately computed for the
filled-concrete and steel tube long columns. Accuracy of Eq. (3) compared with the tangent modulus load
of the CFT column is discussed in Ref.
[2].
Composite structuresSession 5
5
s
used in the seismic design. In 1996,
those industries which originally joined NUHP established the Association of New Urban Housing Tech-
nology (ANUHT). The ANUHT consists of more than 100 member companies which are related to the
CFT building construction, and authorizes the structural design of newly-planned CFT buildings, accord-
ing to the ANUHT’s CFT Recommendations
[21].
The ANUHT Recommendations cover the following design and construction items. i) Strength
design of columns and beam-columns. ii) Evaluation of deformation capacity of beam-columns. iii) Fire
resistant design of beam-columns. iv) Production of CFT members including compaction of filled con-
crete by centrifugal method. v) Quality control of materials and construction work.
CFT investigation carried out in the 5th phase of the U.S.-Japan Cooperative Earthquake Re-
search Program produced CFT Guidelines
[22]. The Guidelines are characterized by covering following
topics. i) Flow charts for the seismic design, based on the conventional method using the structural
characteristic factor
D
s
, and performance-based design method which is specified in the recent revision
of the Building Standard Law of Japan. ii) Constitutive laws for concrete and steel tube derived from the
test results of centrally-loaded stub columns, method of analysis for the moment-curvature relation, method
of analysis for the load-deformation relation of a beam-column under combined compression, bending
and shear, and model for the restoring-force characteristics of a beam-column which may be used in the
analysis of an overall CFT frame. iii) Formulas to evaluate stiffness, ultimate strength and deformation
capacity of a CFT beam-column, taking into account the confining and scale effects on concrete, and
triaxial state of stress and local buckling of the steel tube. iv) Stress transfer mechanism around a beam-
to-column connection, and mathematical model for shear force-deformation relation of a connection
panel. v) Material, manufacturing and fabrication of steel tube, concrete mixture and casting. vi) Design
example using 11-story office building, written for beginners of designing the CFT column system. vii)
Investigation of advantages of the CFT column system by the trial design of 10-, 24- and 40-story CFT
frames. Research results that formed the background of this Guidelines are summarized in English in
BRI Research Paper
[23].
3.2 Design Formulas for CFT Members
This section introduces design formulas for CFT members shown in the 2001 edition of SRC
Standard of AIJ
[19].
Ultimate Compressive Strength of a CFT Column
Ultimate compressive strength of a CFT column is calculated by Eqs. (1) through (4).
l
k
D
≤ 4 ; N
u1
= N
u
c
+(1+ η ) N
u
s
(1)
4<
l
k
D
≤ 12 ; N
u2
=N
u1
0.125 ( N
u1
N
u3
)(
l
k
D4) (2)
12 <
l
k
D
; N
u3
= N
cr
c
+ N
cr
s
(3)
where
l
k
: effectivelength of a CFT column
D :
width or diameterof a steel tube section
η =0 for a square CFT column
η = 0.27
for a circular CFT column
(4)
N
u1
, N
u2
, N
u3
: ultimatestrength of a CFT column
N
u
c
: ultimatestrength of a concretecolumn
N
u
s
: ultimatestrength of a steel tube column
N
cr
c
: bucklingstrength of a concretecolumn
N
cr
s
: bucklingstrength of a steel tube column
N
u 1
in Eq. (1) gives cross-sectional strength of a CFT column, in which strength of confined concrete is
considered for a circular CFT column. Derivation of η = 0.27 is shown in Ref.
[1]. N
u 3
in Eq. (3) gives
buckling strength of a long column as the sum of the buckling strengths separately computed for the
filled-concrete and steel tube long columns. Accuracy of Eq. (3) compared with the tangent modulus load
of the CFT column is discussed in Ref.
[2].
Composite structuresSession 5
5
Ultimate compressive strength
N
c
u
and buckling strength
N ccr of a concrete column are calcu-
lated by Eqs. (5) and (6), respectively.
N
u
c
= A
c
r
u
F
c
(5)
N
cr
c
= A
c
σ
c
r
c
(6)
where
A
c
: cross — sectionalarea of a concretecolumn
F
c
: design standardstrength of filled concrete
σ
cr
c
: critical stress of a concretecolumn
r
u
=0.85: reductionfactor for concretestrength
Critical stress σ
c
cr
is given by Eqs. (7) through (11).
λ
c
≤ 1.0;
σ
cr
c
=
2
1+ λ
c
4
+1
r
u
F
c
(7)
1.0 <λ
c
; σ
cr
c
= 0.83 exp { C
c
(1
λ
c
)}r
u
F
c (8)
where
λ
c
=
λ
c
π
ε
u c
(9)
ε
u
c
=0.93( r
u
F
c
)
1
4
× 10
3
(10)
C
c
=0.568 + 0.00612 F
c (11)
λ
c
: slendernessratio of a concretecolum n
Equations (7) and (8) are obtained by curve fitting numerical results of tangent modulus load of long
concrete columns. Strength increase of confined concrete is not considered. Details are shown in Ref.
[2].
Ultimate compressive strength
N
s
u
of a steel tube column is calculated by Eq. (12).
N
u
s
= A
s
F
s
(12)
where
A
s
: cross — sectionalarea of a steel tube column
F
s
: design standardstrength of steel tube
Buckling strength N
s
cr
of a steel tube column is calculated by Eqs. (13) through (17).
λ
s
≤ 0.3 ; N
cr
s
= A
s
F
s
(13)
0.3≤λ
s
< 1.3 ; N
cr
s
={1
0.545 (λ
s
0.3 ) } A
s
F
s (14)
1.3≤λ
s
; N
cr
s
=
N
E
s
1. 3
(15)
where
λ
s
=
λ
s
π
F
s
E
s
(16)
N
E
s
=
π
2
E
s
I
s
l
k
2
(17)
λ
s
: slendernessratio of a steel tube column
E
s
: Young’s modulusof steel tube
I
s
: cross — sectionalmoment of inertia of a steel tube column
Equations (13) through (15) are the expressions of column curves used in Japan for the plastic design of steel structures
[24].
Ultimate Bending Strength of a CFT Beam-Column
Ultimate bending strength
M
u
of a CFT beam-column subjected to axial load
N is calculated by the
following procedure. First,
M
u
of a beam-column not longer than 12 times width or diameter of the steel
tube section is calculated by Eqs. (18) and (19).
Proceedings of the 1st fib Congress
6
N
c
u
and buckling strength
N ccr of a concrete column are calcu-
lated by Eqs. (5) and (6), respectively.
N
u
c
= A
c
r
u
F
c
(5)
N
cr
c
= A
c
σ
c
r
c
(6)
where
A
c
: cross — sectionalarea of a concretecolumn
F
c
: design standardstrength of filled concrete
σ
cr
c
: critical stress of a concretecolumn
r
u
=0.85: reductionfactor for concretestrength
Critical stress σ
c
cr
is given by Eqs. (7) through (11).
λ
c
≤ 1.0;
σ
cr
c
=
2
1+ λ
c
4
+1
r
u
F
c
(7)
1.0 <λ
c
; σ
cr
c
= 0.83 exp { C
c
(1
λ
c
)}r
u
F
c (8)
where
λ
c
=
λ
c
π
ε
u c
(9)
ε
u
c
=0.93( r
u
F
c
)
1
4
× 10
3
(10)
C
c
=0.568 + 0.00612 F
c (11)
λ
c
: slendernessratio of a concretecolum n
Equations (7) and (8) are obtained by curve fitting numerical results of tangent modulus load of long
concrete columns. Strength increase of confined concrete is not considered. Details are shown in Ref.
[2].
Ultimate compressive strength
N
s
u
of a steel tube column is calculated by Eq. (12).
N
u
s
= A
s
F
s
(12)
where
A
s
: cross — sectionalarea of a steel tube column
F
s
: design standardstrength of steel tube
Buckling strength N
s
cr
of a steel tube column is calculated by Eqs. (13) through (17).
λ
s
≤ 0.3 ; N
cr
s
= A
s
F
s
(13)
0.3≤λ
s
< 1.3 ; N
cr
s
={1
0.545 (λ
s
0.3 ) } A
s
F
s (14)
1.3≤λ
s
; N
cr
s
=
N
E
s
1. 3
(15)
where
λ
s
=
λ
s
π
F
s
E
s
(16)
N
E
s
=
π
2
E
s
I
s
l
k
2
(17)
λ
s
: slendernessratio of a steel tube column
E
s
: Young’s modulusof steel tube
I
s
: cross — sectionalmoment of inertia of a steel tube column
Equations (13) through (15) are the expressions of column curves used in Japan for the plastic design of steel structures
[24].
Ultimate Bending Strength of a CFT Beam-Column
Ultimate bending strength
M
u
of a CFT beam-column subjected to axial load
N is calculated by the
following procedure. First,
M
u
of a beam-column not longer than 12 times width or diameter of the steel
tube section is calculated by Eqs. (18) and (19).
Proceedings of the 1st fib Congress
6
N = N
u
c
+ N
u
s
(18)
M
u
= M
u
c
+ M
u
s
(19)
The strengths appearing on the right hand side of Eqs. (18) and (19) are given as follows:
For a square CFT beam-column:
N
u
c
= x
n
D
c
2
r
u
F
c
(20)
M
u
c
=
1
2(1x
n
) x
n
D
c
3
r
u
F
c (21)
N
u
s
=2(2 x
n
1)D
c
2
tF
s (22)
M
u
s
={(1 t
D
) D
2
+2(1 x
n
) x
n
D
c
2
} tF
s (23)
For a circular CFT beam-column:
N
u
c
=
1 4
(θ
n
sinθ
n
cosθ
n
) D
c
2
σ’
B (24)
M
u
c
=
1
12sin
3
θ
n
D
c
3
σ’
B (25)
N
u
s
={β
1
θ
n
+β
2
(θ
n
π )}(1
t
D
) DtF
s (26)
M
u
s
=
1 2
(β
1
+β
2
) sinθ
n
(1
t
D
)
2
D
2
tF
s (27)
where
x
n
=
x
n
D
c (28)
θ
n
= cos
1
(12 x
n
) (29)
σ’
B
= r
u
F
c
+
1. 5 6 tF
s
D
2 t
(30)
β
1
=0.89, β
2
=1.08 (31)
D
c
: width or diameterof a concretesection
t : thicknessof a steel tube section
x
n
: position parameterof neutral axis
Equilibrium conditions between internal and external forces are given by Eqs. (18) and (19), and
axial and bending strengths carried by concrete and steel tube beam-columns at the ultimate state are
calculated by Eqs. (20) ~ (31), based on the stress distributions shown in Fig. 3 with the neutral axis at
the distance
x
n
from the extreme compression fiber. P δ effect is not considered, and thus they are the
cross-sectional strengths. The strength increase of confined concrete is considered in σ’
B
, and the changes
in axial compressive and tensile yield stresses of steel tube due to ring tension are considered by β
1
and
β
2
, respectively
[1].
M
u
of a CFT beam-column longer than 12 times width or diameter of the steel tube section is
calculated by Eqs. (32) and (33).
N≤N
cr
c
;
M
u
=
1
C
M
{M
u
c
+M
p
(1N
N
k
)}
(32)
N>N
cr
c
;
M
u
=
1
C
M
M
u
s
(1N
cr
c
N
k) (33)
Fig. 3 Stress Blocks for Ultimate Bending Strength
Concrete Steel
σ'
B
β
1
Fs
β
2
Fs
DDc
t
x
n
xx
SteelConcrete
F
s
F
s
r
u
F
c
DDc
t
x
nxx
Composite structuresSession 5
7
u
c
+ N
u
s
(18)
M
u
= M
u
c
+ M
u
s
(19)
The strengths appearing on the right hand side of Eqs. (18) and (19) are given as follows:
For a square CFT beam-column:
N
u
c
= x
n
D
c
2
r
u
F
c
(20)
M
u
c
=
1
2(1x
n
) x
n
D
c
3
r
u
F
c (21)
N
u
s
=2(2 x
n
1)D
c
2
tF
s (22)
M
u
s
={(1 t
D
) D
2
+2(1 x
n
) x
n
D
c
2
} tF
s (23)
For a circular CFT beam-column:
N
u
c
=
1 4
(θ
n
sinθ
n
cosθ
n
) D
c
2
σ’
B (24)
M
u
c
=
1
12sin
3
θ
n
D
c
3
σ’
B (25)
N
u
s
={β
1
θ
n
+β
2
(θ
n
π )}(1
t
D
) DtF
s (26)
M
u
s
=
1 2
(β
1
+β
2
) sinθ
n
(1
t
D
)
2
D
2
tF
s (27)
where
x
n
=
x
n
D
c (28)
θ
n
= cos
1
(12 x
n
) (29)
σ’
B
= r
u
F
c
+
1. 5 6 tF
s
D
2 t
(30)
β
1
=0.89, β
2
=1.08 (31)
D
c
: width or diameterof a concretesection
t : thicknessof a steel tube section
x
n
: position parameterof neutral axis
Equilibrium conditions between internal and external forces are given by Eqs. (18) and (19), and
axial and bending strengths carried by concrete and steel tube beam-columns at the ultimate state are
calculated by Eqs. (20) ~ (31), based on the stress distributions shown in Fig. 3 with the neutral axis at
the distance
x
n
from the extreme compression fiber. P δ effect is not considered, and thus they are the
cross-sectional strengths. The strength increase of confined concrete is considered in σ’
B
, and the changes
in axial compressive and tensile yield stresses of steel tube due to ring tension are considered by β
1
and
β
2
, respectively
[1].
M
u
of a CFT beam-column longer than 12 times width or diameter of the steel tube section is
calculated by Eqs. (32) and (33).
N≤N
cr
c
;
M
u
=
1
C
M
{M
u
c
+M
p
(1N
N
k
)}
(32)
N>N
cr
c
;
M
u
=
1
C
M
M
u
s
(1N
cr
c
N
k) (33)
Fig. 3 Stress Blocks for Ultimate Bending Strength
Concrete Steel
σ'
B
β
1
Fs
β
2
Fs
DDc
t
x
n
xx
SteelConcrete
F
s
F
s
r
u
F
c
DDc
t
x
nxx
Composite structuresSession 5
7
where
M
u
c
=
4 N
0. 9 N
cr
c
(1
N
0. 9 N
cr
c
)
C
b
C
b
+λ
c
2
M
0
c (34)
M
0
c
=
r
u
F
c
D
3
8
for a square CFT beam — column
M
0
c
=
r
u
F
c
D
3
12
for a circular CFT beam — column
(35)
N — N
cr
c
N
cr
s+
M
u
s
(1N — N
cr
c
N
E
s) M
p
=1
(36)
M
p
: full plastic moment of a steel tube section
N
k
=
π
2
(
E’
c
I
c
5
+ E
s
I
s
)
l
k
2
(37)
E’
c
=(3.32 F
c
+6.90) × 10
3
(38)
C
M
=10.5 ( 1
M
1
M
2
)
N
N
k
≥ 0.25 for sideswayprevented
C
M
=1 for sideswaypermitted
(39)
M
1
, M
2
: end moments, M
2
being numerically larger one. M
1
/M
2
is positive when the member is
bent in single curvature, and negative when it is bent in reverse curvature.
C
b
=0.923
0. 0045 F
c (40)
Equations (32) and (33) are derived from the concept that the
M-N interaction curve for a long
composite column is given by superposing two
M-N interaction curves separately computed for a long
concrete portion and a long steel portion, which was proposed by Prof. Wakabayashi
[25, 26]. M-N inter-
action formulas used here for the concrete portion and the steel potion are given by Eqs. (34) and (36),
respectively. Equation (34) is newly proposed in Ref.
[2], and Equation (36) is a well-known and world-
wide design formula for steel beam-columns. A simple superposition of these two interaction curves
contains the conflict that the deformations of the concrete portion and the steel portion do not coincide.
The term
(1 -
N/N
k
) appearing in Eqs. (32) and (33) takes care of additional P δ effect generated by
making two deformations coincide.
Equation (32) corresponds to the case that the axial load
N is small enough to be carried by the
concrete portion only, and the total bending strength of a CFT beam-column is given by the sum of the
remaining bending strength of the concrete portion and the bending strength of the steel portion. On the
other hand, Eq. (33) corresponds to the case that the concrete portion carries the axial load equal to its
full strength, since the axial load
N is too large, and the steel portion carries the remaining axial load and
bending. Details of Eqs. (32) through (40) and their accuracy are discussed in Ref.
[2].
Ultimate Tensile Strength of Diaphragms
Ultimate strength
P
u
of diaphragms subjected to tension from the adjacent beam flange is given by
the following formulas:
For an outer diaphragm of a square CFT connection (Fig. 4(a)):
P
u
=1.42 { 2 ( 4t+t
s
)tF
1
+
4
3
h
s
t
s
F
2
} (41)
For an outer diaphragm of a circular CFT connection (Fig. 4(b)):
P
u
=1.42 [ 1.53 { ( 0.63 + 0.88
B
f
D
)Dt+t
s
}tF
1
+1.77h
s
t
s
F
2
] (42)
For a through diaphragm of a square CFT connection (Fig. 4(c)):
P
u
=1.42(D+2h
s
d
f
)
2
B
f
t
s
d
f
2
F
2 (43)
Proceedings of the 1st fib Congress
8
M
u
c
=
4 N
0. 9 N
cr
c
(1
N
0. 9 N
cr
c
)
C
b
C
b
+λ
c
2
M
0
c (34)
M
0
c
=
r
u
F
c
D
3
8
for a square CFT beam — column
M
0
c
=
r
u
F
c
D
3
12
for a circular CFT beam — column
(35)
N — N
cr
c
N
cr
s+
M
u
s
(1N — N
cr
c
N
E
s) M
p
=1
(36)
M
p
: full plastic moment of a steel tube section
N
k
=
π
2
(
E’
c
I
c
5
+ E
s
I
s
)
l
k
2
(37)
E’
c
=(3.32 F
c
+6.90) × 10
3
(38)
C
M
=10.5 ( 1
M
1
M
2
)
N
N
k
≥ 0.25 for sideswayprevented
C
M
=1 for sideswaypermitted
(39)
M
1
, M
2
: end moments, M
2
being numerically larger one. M
1
/M
2
is positive when the member is
bent in single curvature, and negative when it is bent in reverse curvature.
C
b
=0.923
0. 0045 F
c (40)
Equations (32) and (33) are derived from the concept that the
M-N interaction curve for a long
composite column is given by superposing two
M-N interaction curves separately computed for a long
concrete portion and a long steel portion, which was proposed by Prof. Wakabayashi
[25, 26]. M-N inter-
action formulas used here for the concrete portion and the steel potion are given by Eqs. (34) and (36),
respectively. Equation (34) is newly proposed in Ref.
[2], and Equation (36) is a well-known and world-
wide design formula for steel beam-columns. A simple superposition of these two interaction curves
contains the conflict that the deformations of the concrete portion and the steel portion do not coincide.
The term
(1 -
N/N
k
) appearing in Eqs. (32) and (33) takes care of additional P δ effect generated by
making two deformations coincide.
Equation (32) corresponds to the case that the axial load
N is small enough to be carried by the
concrete portion only, and the total bending strength of a CFT beam-column is given by the sum of the
remaining bending strength of the concrete portion and the bending strength of the steel portion. On the
other hand, Eq. (33) corresponds to the case that the concrete portion carries the axial load equal to its
full strength, since the axial load
N is too large, and the steel portion carries the remaining axial load and
bending. Details of Eqs. (32) through (40) and their accuracy are discussed in Ref.
[2].
Ultimate Tensile Strength of Diaphragms
Ultimate strength
P
u
of diaphragms subjected to tension from the adjacent beam flange is given by
the following formulas:
For an outer diaphragm of a square CFT connection (Fig. 4(a)):
P
u
=1.42 { 2 ( 4t+t
s
)tF
1
+
4
3
h
s
t
s
F
2
} (41)
For an outer diaphragm of a circular CFT connection (Fig. 4(b)):
P
u
=1.42 [ 1.53 { ( 0.63 + 0.88
B
f
D
)Dt+t
s
}tF
1
+1.77h
s
t
s
F
2
] (42)
For a through diaphragm of a square CFT connection (Fig. 4(c)):
P
u
=1.42(D+2h
s
d
f
)
2
B
f
t
s
d
f
2
F
2 (43)
Proceedings of the 1st fib Congress
8
where
h
s
: width of a diaphragmat A — A section
t
s
: thicknessof a diaphragm
B
f
: width of a beam flange
d
f
: diameterof an openingfor concretecasting
F
1
, F
2
: design standardstrengthsof steel tube and diaphragm, respectively
The ultimate strengths, Eqs. (41) through (43), have been empirically obtained as 1.42 times the
yield strength. The yield strengths in Eqs. (41) and (42) have been derived based on the mechanism, in
which the diaphragm plate at section A-A yields in tension and shear, and the tube wall with an effective
width yields in tension. The yield strength in Eq. (43) corresponds to the following mechanism: Yielding
occurs at section A-A of a fixed-end beam with width
t
s
, depth ( D + 2h
s
- d
f
)/2 and length d
f
, which is
subjected to the load
P/B
f
distributed along the distance B
f
at the center of the beam. Detailed derivation
of Eqs. (41) and (43) is given in Ref.
[27], and that of Eq. (42) in Ref.
[28].
Formulas for a through diaphragm of a circular CFT connection and for an inner diaphragm have
been derived by the yield line theory and experiments, although their expressions are complicated
[29, 30].
Ultimate Shear Strength of a CFT Shear Panel
Figure 5(a) shows an internal beam-to-column connection with bending moments and shear forces
acting at member ends, and Fig. 5(b) shows shear forces
Q
pc
and
Q
pb
acting on a square CFT shear
panel as resultants of member end forces. The ultimate strength of a shear panel
Q
p
u
to resist
Q
pc
is
Fig. 4 Design of Diaphragm
s
D
t
t
s
h
s
h
s
P
P
A
A
(a) Outer diaphragm
P
P
h
s
hs
t
A
A
d
f
Bf
D
t
s
(c) Through diaphragm
P
P
h
s
t
D t s
A
A
Bf
45°
(b) Outer diaphragm
Q
cu
M
cu
Q
cl
M
cl
Q
brM
br
Q
blM
bl
d
b
As
Q
pb
Q
pc
Ac
Fig. 5 Shear Panel
(a) Member end forces(b) Shear forces on the panel
Composite structuresSession 5
9
h
s
: width of a diaphragmat A — A section
t
s
: thicknessof a diaphragm
B
f
: width of a beam flange
d
f
: diameterof an openingfor concretecasting
F
1
, F
2
: design standardstrengthsof steel tube and diaphragm, respectively
The ultimate strengths, Eqs. (41) through (43), have been empirically obtained as 1.42 times the
yield strength. The yield strengths in Eqs. (41) and (42) have been derived based on the mechanism, in
which the diaphragm plate at section A-A yields in tension and shear, and the tube wall with an effective
width yields in tension. The yield strength in Eq. (43) corresponds to the following mechanism: Yielding
occurs at section A-A of a fixed-end beam with width
t
s
, depth ( D + 2h
s
- d
f
)/2 and length d
f
, which is
subjected to the load
P/B
f
distributed along the distance B
f
at the center of the beam. Detailed derivation
of Eqs. (41) and (43) is given in Ref.
[27], and that of Eq. (42) in Ref.
[28].
Formulas for a through diaphragm of a circular CFT connection and for an inner diaphragm have
been derived by the yield line theory and experiments, although their expressions are complicated
[29, 30].
Ultimate Shear Strength of a CFT Shear Panel
Figure 5(a) shows an internal beam-to-column connection with bending moments and shear forces
acting at member ends, and Fig. 5(b) shows shear forces
Q
pc
and
Q
pb
acting on a square CFT shear
panel as resultants of member end forces. The ultimate strength of a shear panel
Q
p
u
to resist
Q
pc
is
Fig. 4 Design of Diaphragm
s
D
t
t
s
h
s
h
s
P
P
A
A
(a) Outer diaphragm
P
P
h
s
hs
t
A
A
d
f
Bf
D
t
s
(c) Through diaphragm
P
P
h
s
t
D t s
A
A
Bf
45°
(b) Outer diaphragm
Q
cu
M
cu
Q
cl
M
cl
Q
brM
br
Q
blM
bl
d
b
As
Q
pb
Q
pc
Ac
Fig. 5 Shear Panel
(a) Member end forces(b) Shear forces on the panel
Composite structuresSession 5
9
given by
Q
u
p
=A
c
τ
u
c
+
A
s
2
τ
u s
(44)
where
τ
u c
,τ
u s
: ultimateshear stressesof concreteand steel tube, respectivel y
Equation (44) gives the ultimate shear strength as a sum of the strengths of concrete and two webs of a
steel tube, and it is also applicable to a circular CFT shear panel. The ultimate shear stresses are given
as follows:
τ
u
c
=β×min(0.12F
c
,1.8 + 0.036F
c
) (45)
τ
u s
=
1.2
3
F
s (46)
where
β=2.5
D
d
b
and≤ 4 for a square CFT shear panel
β=2.0
D
d
b
and≤ 4 for a circular CFT shear pane l
(47)
d
b
: center — to — center distanceof beam flanges adjacent to the shear pane l
The shear force acting on a concrete panel may be actually resisted by the horizontal force carried by a
diagonal strut forming in the shear panel, and it becomes larger as the inclination angle of the strut
becomes smaller, in other words, the value of
D/d
b
becomes larger. The parameter
β considers this
effect.
The panel shear force
Q
pc
caused by the member end forces is approximately given by
Q
pc
=
M
bl
+M
br
d
b
h’
h
(48)
where
M
bl
, M
br
: bendingmomentsat beam ends adjacent to the shear panel
h , h’:
center — to — center story height and clear story height , respectively
4. CONSTRUCTION OF CFT COLUMN SYSTEM
The Association of New Urban Housing Technology (ANUHT) established in 1996 has been in-
specting the structural and fire resistance designs of newly planned CFT buildings, and authorizing the construction of those structures. The Association has inspected 216 structural designs and 123 fire resistance designs of CFT buildings lower than 60 m as of September, 2001. In addition to these inspec- tion works, the Association provides design and construction technology of the CFT system, educates the member companies, and promotes the research on the CFT system. Shown below are the construc- tion data provided by the Association.
Structural designs of 156 CFT buildings in total were inspected by the Association from April, 1998
to September 2001, and their statistical details are shown in Figs. 6 and 7. Some of the data are missing for the buildings inspected before this period, and the data after this period are not yet gathered, and thus they are omitted from the analysis. Observations made from the data are as follows: i) Among 156 buildings, 43(27%) are shops and warehouses, and their total floor area shares 42% (Figs. 6(a), (b)). Application of CFT to those buildings indicates building designer's recognition to the spannabil- ity of the CFT system. The CFT system is quite often applied to the buildings of large scale (Figs. 7(a)~(c)). ii) The CFT system is not very often applied to braced frame buildings (Fig. 6(c)). It may not be necessary to use the braces, since the tube section has identical strength and stiffness in both x- and y-directions. It is not very often either to use the structural wall with the CFT system. iii) Floor area carried by one column is much larger than that in the ordinary reinforced concrete or pure steel buildings. The floor area exceeds 90 m
2 in 37% of all buildings, and in 43% of office buildings (Figs.
6(d), (e)). This emphasizes again application of the CFT system to the long-span buildings. iv) Aspect ratio in Figs. 6(f) and (g) is the ratio of the longer distance between two columns to the shorter one in x- and y-directions of a floor plan.Variety of the aspect ratio of span grid indicates the CFT system's possibility for free planning about the span grid. In the case of office buildings, rectangular span grid of 8
m × 18 m is fairly often used, and the aspect ratio exceeds 2.2, while the span grid of shop buildings is
fairly close to square. v) Both square and circular sections are mixedly used in a number of buildings (Fig. 6(h)) . Size of tube
Proceedings of the 1st fib Congress
10
Q
u
p
=A
c
τ
u
c
+
A
s
2
τ
u s
(44)
where
τ
u c
,τ
u s
: ultimateshear stressesof concreteand steel tube, respectivel y
Equation (44) gives the ultimate shear strength as a sum of the strengths of concrete and two webs of a
steel tube, and it is also applicable to a circular CFT shear panel. The ultimate shear stresses are given
as follows:
τ
u
c
=β×min(0.12F
c
,1.8 + 0.036F
c
) (45)
τ
u s
=
1.2
3
F
s (46)
where
β=2.5
D
d
b
and≤ 4 for a square CFT shear panel
β=2.0
D
d
b
and≤ 4 for a circular CFT shear pane l
(47)
d
b
: center — to — center distanceof beam flanges adjacent to the shear pane l
The shear force acting on a concrete panel may be actually resisted by the horizontal force carried by a
diagonal strut forming in the shear panel, and it becomes larger as the inclination angle of the strut
becomes smaller, in other words, the value of
D/d
b
becomes larger. The parameter
β considers this
effect.
The panel shear force
Q
pc
caused by the member end forces is approximately given by
Q
pc
=
M
bl
+M
br
d
b
h’
h
(48)
where
M
bl
, M
br
: bendingmomentsat beam ends adjacent to the shear panel
h , h’:
center — to — center story height and clear story height , respectively
4. CONSTRUCTION OF CFT COLUMN SYSTEM
The Association of New Urban Housing Technology (ANUHT) established in 1996 has been in-
specting the structural and fire resistance designs of newly planned CFT buildings, and authorizing the construction of those structures. The Association has inspected 216 structural designs and 123 fire resistance designs of CFT buildings lower than 60 m as of September, 2001. In addition to these inspec- tion works, the Association provides design and construction technology of the CFT system, educates the member companies, and promotes the research on the CFT system. Shown below are the construc- tion data provided by the Association.
Structural designs of 156 CFT buildings in total were inspected by the Association from April, 1998
to September 2001, and their statistical details are shown in Figs. 6 and 7. Some of the data are missing for the buildings inspected before this period, and the data after this period are not yet gathered, and thus they are omitted from the analysis. Observations made from the data are as follows: i) Among 156 buildings, 43(27%) are shops and warehouses, and their total floor area shares 42% (Figs. 6(a), (b)). Application of CFT to those buildings indicates building designer's recognition to the spannabil- ity of the CFT system. The CFT system is quite often applied to the buildings of large scale (Figs. 7(a)~(c)). ii) The CFT system is not very often applied to braced frame buildings (Fig. 6(c)). It may not be necessary to use the braces, since the tube section has identical strength and stiffness in both x- and y-directions. It is not very often either to use the structural wall with the CFT system. iii) Floor area carried by one column is much larger than that in the ordinary reinforced concrete or pure steel buildings. The floor area exceeds 90 m
2 in 37% of all buildings, and in 43% of office buildings (Figs.
6(d), (e)). This emphasizes again application of the CFT system to the long-span buildings. iv) Aspect ratio in Figs. 6(f) and (g) is the ratio of the longer distance between two columns to the shorter one in x- and y-directions of a floor plan.Variety of the aspect ratio of span grid indicates the CFT system's possibility for free planning about the span grid. In the case of office buildings, rectangular span grid of 8
m × 18 m is fairly often used, and the aspect ratio exceeds 2.2, while the span grid of shop buildings is
fairly close to square. v) Both square and circular sections are mixedly used in a number of buildings (Fig. 6(h)) . Size of tube
Proceedings of the 1st fib Congress
10
section often used is between 500 and 700 mm in the case of square CFT columns, and 600 and 711 mm
in the case of circular CFT columns (Figs. 7(d), (e)). Circular tubes (diameter: 400 to 1117 mm, diameter-
thickness ratio: 16 to 90) are mainly used for the building with irregular plan grid, and square and rectan-
gular tubes (width: 300 to 950 mm, width-thickness ratio: 10 to 54) are used for the case of regular plan.
Most of tubes are cold-formed, which are inexpensive and widely available in the market. Box sections
built-up by welding are used when the plate becomes thick and/or large ductility is required. Cast-steel
tube is used to simplify the beam-to-column connection. Annealing to remove residual stresses are
scarcely done in Japan.
vi) Inner or through diaphragms are used in most cases of beam-to-column connections (Fig. 6(i)). Type
of diaphragm seems to be determined according to plate thicknesses of the column and the beam:
through diaphragm is often employed when the beam plate is thicker than the column plate, and other-
(e) Floor area carried by one column of offices and shops
(i) Offices (ii) Shops
36%
≤
45m
2
105m
2
<
45m
2
~60m
2
60m
2
~75m
2
75m
2
~90m
2
90m
2
~105m
2
3%
12%
27%
9%
12%
(a) Number of constructions and use (b) Total floor area and use
shop
warehouse
hotel
hospital
other
21%
6%
5%
3%
19%
office
45%
shop
warehouse 5%
hotel
hospital
other
37%
3%
5%
22%
office
28%
≤
45m
2
105m
2
<
45m
2
~60m
2
60m
2
~75m
2
75m
2
~90m
2
90m
2
~105m
2
9%
10%
17%
21%
26%
17%
(c) Type of structural frames
22%
(d) Floor area carried by one column
unbraced frame
braced frame
20%
80%
≤
45m
2
105m
2
<
45m
2
~60m
2
60m
2
~75m
2
75m
2
~90m
2
90m
2
~105m
2
9%
13%
22%
19%
15%
Fig. 6 Construction Data in 1998 - 2001: Structural Planning
11
Composite structuresSession 5
in the case of circular CFT columns (Figs. 7(d), (e)). Circular tubes (diameter: 400 to 1117 mm, diameter-
thickness ratio: 16 to 90) are mainly used for the building with irregular plan grid, and square and rectan-
gular tubes (width: 300 to 950 mm, width-thickness ratio: 10 to 54) are used for the case of regular plan.
Most of tubes are cold-formed, which are inexpensive and widely available in the market. Box sections
built-up by welding are used when the plate becomes thick and/or large ductility is required. Cast-steel
tube is used to simplify the beam-to-column connection. Annealing to remove residual stresses are
scarcely done in Japan.
vi) Inner or through diaphragms are used in most cases of beam-to-column connections (Fig. 6(i)). Type
of diaphragm seems to be determined according to plate thicknesses of the column and the beam:
through diaphragm is often employed when the beam plate is thicker than the column plate, and other-
(e) Floor area carried by one column of offices and shops
(i) Offices (ii) Shops
36%
≤
45m
2
105m
2
<
45m
2
~60m
2
60m
2
~75m
2
75m
2
~90m
2
90m
2
~105m
2
3%
12%
27%
9%
12%
(a) Number of constructions and use (b) Total floor area and use
shop
warehouse
hotel
hospital
other
21%
6%
5%
3%
19%
office
45%
shop
warehouse 5%
hotel
hospital
other
37%
3%
5%
22%
office
28%
≤
45m
2
105m
2
<
45m
2
~60m
2
60m
2
~75m
2
75m
2
~90m
2
90m
2
~105m
2
9%
10%
17%
21%
26%
17%
(c) Type of structural frames
22%
(d) Floor area carried by one column
unbraced frame
braced frame
20%
80%
≤
45m
2
105m
2
<
45m
2
~60m
2
60m
2
~75m
2
75m
2
~90m
2
90m
2
~105m
2
9%
13%
22%
19%
15%
Fig. 6 Construction Data in 1998 - 2001: Structural Planning
11
Composite structuresSession 5
wise inner diaphragm is employed. This tendency leads to the use of through diaphragm for cold-formed
tubes, and inner diaphragm for built-up tubes. Inner and through diaphragms have an opening with
diameter of 200 to 300 mm for concrete casting, and several small holes for air passage. Outer dia-
phragm is used as an easy solution to assure compactness of concrete.
vii) Embedded type column bases are most popularly used, which is the best in view of the structural
reliability (Fig. 6(j)). This also indicates that the CFT system is often applied to large scale buildings.
viii) Ratio of the buckling length to the tube size is much larger than that in the ordinary reinforced
concrete or pure steel buildings (Fig. 7(f)). This indicates large axial load carrying capacity of the CFT
column.
iv) Design standard strength of steel often used is 325 MPa, and that of concrete is 36 and 42 MPa (Figs.
7(g), (h)).
(f) Aspect ratio of span grid
2.2
≤
1.0~1.3
1.3~1.61.6~1.9
1.9~2.2
37%
16%
13%
13%
16%
square
circular
mixed
65%
23%
12%
(h) Shape of steel tubes
embedded
bare
encased
unclear
17%
61%
3%
19%
(i) Type of beam-to-column connections (j) Type of column bases
through
outer
mixed
unclear
29%
7%
7%
4%
inner
53%
(g) Aspect ratio of span grid of offices and shops
(ii) Shops
2.2
≤
1.0~1.3
1.3~1.6
1.6~1.9
1.9~2.2
55%
15%
15%
6%
9%
(i) Offices
2.2≤
1.0~1.3
1.3~1.6
1.6~1.9
1.9~2.2
17%
13%
17%
17%
36%
Fig. 6 Construction Data in 1998 - 2001: Structural Planning (continued)
Proceedings of the 1st fib Congress
12
tubes, and inner diaphragm for built-up tubes. Inner and through diaphragms have an opening with
diameter of 200 to 300 mm for concrete casting, and several small holes for air passage. Outer dia-
phragm is used as an easy solution to assure compactness of concrete.
vii) Embedded type column bases are most popularly used, which is the best in view of the structural
reliability (Fig. 6(j)). This also indicates that the CFT system is often applied to large scale buildings.
viii) Ratio of the buckling length to the tube size is much larger than that in the ordinary reinforced
concrete or pure steel buildings (Fig. 7(f)). This indicates large axial load carrying capacity of the CFT
column.
iv) Design standard strength of steel often used is 325 MPa, and that of concrete is 36 and 42 MPa (Figs.
7(g), (h)).
(f) Aspect ratio of span grid
2.2
≤
1.0~1.3
1.3~1.61.6~1.9
1.9~2.2
37%
16%
13%
13%
16%
square
circular
mixed
65%
23%
12%
(h) Shape of steel tubes
embedded
bare
encased
unclear
17%
61%
3%
19%
(i) Type of beam-to-column connections (j) Type of column bases
through
outer
mixed
unclear
29%
7%
7%
4%
inner
53%
(g) Aspect ratio of span grid of offices and shops
(ii) Shops
2.2
≤
1.0~1.3
1.3~1.6
1.6~1.9
1.9~2.2
55%
15%
15%
6%
9%
(i) Offices
2.2≤
1.0~1.3
1.3~1.6
1.6~1.9
1.9~2.2
17%
13%
17%
17%
36%
Fig. 6 Construction Data in 1998 - 2001: Structural Planning (continued)
Proceedings of the 1st fib Congress
12
Number of buildings
020406 080100
5.0< h
4.5< h ≤5.0
4.0< h ≤4.5
3.5< h ≤4.0
3.0< h ≤3.5
h ≤3.0
(c) Height of typical story (h : m)
24< l
k / D
22<
lk / D ≤24
20<
lk / D ≤22
18<
lk / D ≤20
16<
l
k / D ≤18
14<
l
k / D ≤16
12<
l
k / D ≤14
10<
l
k / D ≤12
l
k / D ≤10
010203040
Number of buildings
(f) Ratio of buckling length
(lk) to tube size(D)
010 6020 40 5030
Number of buildings
10 20 30 400
Number of buildings
(a)Total Floor Area
(Af : m
2
) (b)Height of Buildings(H : m)
48000< A
f
24000< A f ≤48000
12000< A
f ≤24000
6000< A
f ≤12000
3000< A
f ≤6000
A
f ≤3000
55< H
H ≤10
50< H ≤55
45< H ≤50
40< H ≤45
35< H ≤40
30< H ≤35
25< H ≤30
20< H ≤25
15< H ≤20
10< H ≤15
490
325
295
235
0204 06080 100 120
Number of buildings
(g) Design standard strength of steel (MPa)
0204 06080
60
48
42
39
33
27
Number of buildings
(h) Desigh standard strength of concrete (MPa)
54
45
40
36
30
24
010203 04050
Number of buildings
(d) Size of square tubes (mm)
950×950
850×850
750×750
700×700
650×750
600×600
550×550
500×450
400×400
350×350
900×900
800×800
750×600
700×600
650×650
550×650
500×500
450×450
350×500
300×300
Number of buildings
024681012
1117
900
800
711
650
558
500
400
(e) Size of circular tubes (mm)
1024
914
850
812
762
750
700
660
609
600
550
508
457
450
Fig. 7 Construction Data in 1998 - 2001: Properties of CFT Systems and Columns
13
Composite structuresSession 5
020406 080100
5.0< h
4.5< h ≤5.0
4.0< h ≤4.5
3.5< h ≤4.0
3.0< h ≤3.5
h ≤3.0
(c) Height of typical story (h : m)
24< l
k / D
22<
lk / D ≤24
20<
lk / D ≤22
18<
lk / D ≤20
16<
l
k / D ≤18
14<
l
k / D ≤16
12<
l
k / D ≤14
10<
l
k / D ≤12
l
k / D ≤10
010203040
Number of buildings
(f) Ratio of buckling length
(lk) to tube size(D)
010 6020 40 5030
Number of buildings
10 20 30 400
Number of buildings
(a)Total Floor Area
(Af : m
2
) (b)Height of Buildings(H : m)
48000< A
f
24000< A f ≤48000
12000< A
f ≤24000
6000< A
f ≤12000
3000< A
f ≤6000
A
f ≤3000
55< H
H ≤10
50< H ≤55
45< H ≤50
40< H ≤45
35< H ≤40
30< H ≤35
25< H ≤30
20< H ≤25
15< H ≤20
10< H ≤15
490
325
295
235
0204 06080 100 120
Number of buildings
(g) Design standard strength of steel (MPa)
0204 06080
60
48
42
39
33
27
Number of buildings
(h) Desigh standard strength of concrete (MPa)
54
45
40
36
30
24
010203 04050
Number of buildings
(d) Size of square tubes (mm)
950×950
850×850
750×750
700×700
650×750
600×600
550×550
500×450
400×400
350×350
900×900
800×800
750×600
700×600
650×650
550×650
500×500
450×450
350×500
300×300
Number of buildings
024681012
1117
900
800
711
650
558
500
400
(e) Size of circular tubes (mm)
1024
914
850
812
762
750
700
660
609
600
550
508
457
450
Fig. 7 Construction Data in 1998 - 2001: Properties of CFT Systems and Columns
13
Composite structuresSession 5
5. CONCLUDING REMARKS
Rational design method for the CFT column system has been established through extensive re-
search done by Architectural Institute of Japan, New Urban Housing Project and U.S.-Japan Coopera-
tive Earthquake Research Program, and several design standards, recommendations and guidelines
are available
[4, 19 ~ 22] . More than 40 buildings have been constructed every year in the last 5 years in
Japan, and it has been known that the CFT structures are mainly used for shop, office and hotel con-
structions, and the merits are applicability to high-rise and long-span structures, and construction effi-
ciency that is the saving of construction cost, time and manpower. Trial designs of unbraced frames have
shown that the structural characteristics of the CFT and steel systems are almost the same, but the total
steel consumption of the CFT system for entire building is about 10 % less than that of the steel system.
It must be noticed that the deformation at which a CFT beam-column reaches the maximum strength
is fairly large: some of the specimens attained the maximum strength after the chord rotation angle
became larger than 1/100. In addition, it becomes known that the dynamic characteristics of the CFT
system are almost the same as those of the steel system. These facts indicate that the CFT system is not
very stiff against lateral load, and thus the investigation on other structural system rather than the mo-
ment frame is now needed to utilize the large axial load carrying capacity of the CFT column more
effectively, such as braced frame or combination of reinforced concrete shear wall and CFT columns in
which CFT columns mainly carry the vertical load.
Weak point of the CFT system is the connections: beam-to-column connections, brace-to-frame
connections and column bases. The outer diaphragm type of beam-to-column connection is sometimes
avoided because the diaphragm sticking outward disturbs the arrangement of curtain walls, and the
through type is most popularly used. Through diaphragm type of connection is fabricated by cutting the
steel tube into three pieces first, then welding them together with two diaphragms, as shown in Fig. 8(a).
Therefore, it requires large amount of welding. Moreover, in the case that levels of beams coming into a
connection are different, or a brace is attached to a CFT column with a gusset plate and diaphragms,
filled concrete in the tube is separated into more layers than an ordinary beam-to-column connection, as
shown in Figs. 8(b) and (c). These cases require more amount of welding and increase the possibility to
make defects in cast concrete. Therefore, it is needed to develop a new type of connection without
cutting the column body and without using welding, such as the connection using long bolts or steel tube
whose wall thickness is partly increased at the connection, as shown in Fig. 9. A few research work has
been done on these new types, but design formulas are not yet well prepared.
CFT column base is usually designed by the same way as for an ordinary steel column base,
without any special consideration. However, there must be more suitable design method to utilize the
CFT characteristics. Investigation on this point has just started.
Most of design engineers have treated the CFT system as an alternative to the steel system, trying
to cut the cost by reducing the steel consumption. However, isn’t it possible to look at the CFT system as
an alternative to the reinforced concrete system? The CFT system has the following ecological advan-
tages against the R/C system in addition to structural advantages such as high strength and high ductil-
ity: neither form works nor works for reinforcing bars are needed, which leads to very clean construction
site; steel tube is easily peeled off from filled concrete and reused when the building is pulled down; filled
concrete is of high quality and easily crashed because it does not contain reinforcing bars, which is also
Fig. 8
Arrangement of Diaphragms
(a) (b)
(c)
Proceedings of the 1st fib Congress
14
Rational design method for the CFT column system has been established through extensive re-
search done by Architectural Institute of Japan, New Urban Housing Project and U.S.-Japan Coopera-
tive Earthquake Research Program, and several design standards, recommendations and guidelines
are available
[4, 19 ~ 22] . More than 40 buildings have been constructed every year in the last 5 years in
Japan, and it has been known that the CFT structures are mainly used for shop, office and hotel con-
structions, and the merits are applicability to high-rise and long-span structures, and construction effi-
ciency that is the saving of construction cost, time and manpower. Trial designs of unbraced frames have
shown that the structural characteristics of the CFT and steel systems are almost the same, but the total
steel consumption of the CFT system for entire building is about 10 % less than that of the steel system.
It must be noticed that the deformation at which a CFT beam-column reaches the maximum strength
is fairly large: some of the specimens attained the maximum strength after the chord rotation angle
became larger than 1/100. In addition, it becomes known that the dynamic characteristics of the CFT
system are almost the same as those of the steel system. These facts indicate that the CFT system is not
very stiff against lateral load, and thus the investigation on other structural system rather than the mo-
ment frame is now needed to utilize the large axial load carrying capacity of the CFT column more
effectively, such as braced frame or combination of reinforced concrete shear wall and CFT columns in
which CFT columns mainly carry the vertical load.
Weak point of the CFT system is the connections: beam-to-column connections, brace-to-frame
connections and column bases. The outer diaphragm type of beam-to-column connection is sometimes
avoided because the diaphragm sticking outward disturbs the arrangement of curtain walls, and the
through type is most popularly used. Through diaphragm type of connection is fabricated by cutting the
steel tube into three pieces first, then welding them together with two diaphragms, as shown in Fig. 8(a).
Therefore, it requires large amount of welding. Moreover, in the case that levels of beams coming into a
connection are different, or a brace is attached to a CFT column with a gusset plate and diaphragms,
filled concrete in the tube is separated into more layers than an ordinary beam-to-column connection, as
shown in Figs. 8(b) and (c). These cases require more amount of welding and increase the possibility to
make defects in cast concrete. Therefore, it is needed to develop a new type of connection without
cutting the column body and without using welding, such as the connection using long bolts or steel tube
whose wall thickness is partly increased at the connection, as shown in Fig. 9. A few research work has
been done on these new types, but design formulas are not yet well prepared.
CFT column base is usually designed by the same way as for an ordinary steel column base,
without any special consideration. However, there must be more suitable design method to utilize the
CFT characteristics. Investigation on this point has just started.
Most of design engineers have treated the CFT system as an alternative to the steel system, trying
to cut the cost by reducing the steel consumption. However, isn’t it possible to look at the CFT system as
an alternative to the reinforced concrete system? The CFT system has the following ecological advan-
tages against the R/C system in addition to structural advantages such as high strength and high ductil-
ity: neither form works nor works for reinforcing bars are needed, which leads to very clean construction
site; steel tube is easily peeled off from filled concrete and reused when the building is pulled down; filled
concrete is of high quality and easily crashed because it does not contain reinforcing bars, which is also
Fig. 8
Arrangement of Diaphragms
(a) (b)
(c)
Proceedings of the 1st fib Congress
14
reusable as aggregates. Only question is the cost performance, and thus the investigation by trial design
is needed to clarify advantages and disadvantages of the CFT system against the R/C system, including
life cycle assessment.
Acknowledgments
Construction data presented in Chapter 4 have been generously provided by the Association of
New Urban Housing Technology. The author wishes to express sincere gratitude to the Association.
References
[1] Sakino, K. Ninakawa, T. Nakahara, H. and Morino, S.: Experimental Studies and Design Recommen-
dations on CFT Columns - U.S.-Japan Cooperative Earthquake Research Program -, Proc. Structural
Engineers World Congress, San Francisco, CD Rom, Paper No. T169-3, 1998.7.
[2] Tsuda, K., Matsui, C. and Fujinaga, T.: Simplified Design Formula of Slender Concrete-Filled Steel
Tubular Beam-Columns, Proc. 6th ASCCS Conference on Composite and Hybrid Structures, Los Ange-
les, Vol. 1, pp. 457-464, 2000.3.
[3] Nakahara, H., Sakino, K. and Inai, E.: Analytical Model for Compressive Behavior of Concrete Filled
Square Steel Tubular Columns, Transactions of Japan Concrete Institute, Vol. 20, pp. 171-178, 1998.6.
[4] Recommendations for Design and Construction of Concrete Filled Steel Tubular Structures, Architec-
tural Institute of Japan (AIJ), 1997.10. (in Japanese)
[5] Nakahara, H. and Sakino, K.: Flexural Behavior of Concrete Filled Square Steel Tubular Beam-Col-
umns, Proc. 12th WCEE, Auckland, CD-Rom, No. 1923, 2000.1.
[6] Sakino, K., Inai, E. and Nakahara, H.: Tests and Analysis on Elasto-Plastic Behavior of CFT Beam-
Columns -U.S.-Japan Cooperative Earthquake Research Program-, Proc. 5th Pacific Structural Steel
Conference, Seoul, Vol. 2, pp. 901-906, 1998.10.
[7] Inai, E., Noguchi, T. Mori, O. and Fujimoto, T.: Deformation Capacity and Hysteretic Model of Con-
crete-Filled Steel Tubular Beam-Columns, Proc. 6th ASCCS Conference on Composite and Hybrid Struc-
tures, Los Angeles, Vol. 1, pp. 605-612, 2000.3.
[8] Kawano, A. and Matsui, C.: New Connections Using Vertical Stiffeners and Hollow or Concrete-Filled
Square Tubular Columns, Proc. Engineering Foundation Conference on Composite Construction in Steel
and Concrete III, Irsee, pp. 172-185, 1996.6.
[9] Kanatani, H., Tabuchi, M., Kamba, T., Ji, H. and Ishikawa, M.: A Study on Concrete Filled RHS Col-
umn to H-Beam Connections Fabricated with HT Bolts in Rigid Frames, Proc. Engineering Foundation
Conference on Composite Construction in Steel and Concrete, Henniker, pp. 614-635, 1987.7
[10] Ji, H., Kanatani, H., Tabuchi, M., Kamba, T. and Ishikawa, M.: Behavior of Concrete Filled RHS
Column to H-beam Connections Fabricated with HT Bolts, Tubular Structures III, Proc. 3rd International
Symposium on Tubular Structures, Lappeenranta, pp. 196-203, 1989.9.
[11] Morita, K., Ebato, K., Furuhara, K., Fujita, K. and Hamano, K.: Experimental Study of Structural
Behavior of Beam-to Column Connections Reinforced by Increasing Plate Thickness of Column without
Diaphragms, Tubular Structures VIII, Proc. 8th International Symposium on Tubular Structures, Sin-
gapore, pp. 585-594, 1998.8.
Fig. 9 Connections without Diaphragms
(a) Long bolts (b) Increased thickness
15
Composite structuresSession 5
is needed to clarify advantages and disadvantages of the CFT system against the R/C system, including
life cycle assessment.
Acknowledgments
Construction data presented in Chapter 4 have been generously provided by the Association of
New Urban Housing Technology. The author wishes to express sincere gratitude to the Association.
References
[1] Sakino, K. Ninakawa, T. Nakahara, H. and Morino, S.: Experimental Studies and Design Recommen-
dations on CFT Columns - U.S.-Japan Cooperative Earthquake Research Program -, Proc. Structural
Engineers World Congress, San Francisco, CD Rom, Paper No. T169-3, 1998.7.
[2] Tsuda, K., Matsui, C. and Fujinaga, T.: Simplified Design Formula of Slender Concrete-Filled Steel
Tubular Beam-Columns, Proc. 6th ASCCS Conference on Composite and Hybrid Structures, Los Ange-
les, Vol. 1, pp. 457-464, 2000.3.
[3] Nakahara, H., Sakino, K. and Inai, E.: Analytical Model for Compressive Behavior of Concrete Filled
Square Steel Tubular Columns, Transactions of Japan Concrete Institute, Vol. 20, pp. 171-178, 1998.6.
[4] Recommendations for Design and Construction of Concrete Filled Steel Tubular Structures, Architec-
tural Institute of Japan (AIJ), 1997.10. (in Japanese)
[5] Nakahara, H. and Sakino, K.: Flexural Behavior of Concrete Filled Square Steel Tubular Beam-Col-
umns, Proc. 12th WCEE, Auckland, CD-Rom, No. 1923, 2000.1.
[6] Sakino, K., Inai, E. and Nakahara, H.: Tests and Analysis on Elasto-Plastic Behavior of CFT Beam-
Columns -U.S.-Japan Cooperative Earthquake Research Program-, Proc. 5th Pacific Structural Steel
Conference, Seoul, Vol. 2, pp. 901-906, 1998.10.
[7] Inai, E., Noguchi, T. Mori, O. and Fujimoto, T.: Deformation Capacity and Hysteretic Model of Con-
crete-Filled Steel Tubular Beam-Columns, Proc. 6th ASCCS Conference on Composite and Hybrid Struc-
tures, Los Angeles, Vol. 1, pp. 605-612, 2000.3.
[8] Kawano, A. and Matsui, C.: New Connections Using Vertical Stiffeners and Hollow or Concrete-Filled
Square Tubular Columns, Proc. Engineering Foundation Conference on Composite Construction in Steel
and Concrete III, Irsee, pp. 172-185, 1996.6.
[9] Kanatani, H., Tabuchi, M., Kamba, T., Ji, H. and Ishikawa, M.: A Study on Concrete Filled RHS Col-
umn to H-Beam Connections Fabricated with HT Bolts in Rigid Frames, Proc. Engineering Foundation
Conference on Composite Construction in Steel and Concrete, Henniker, pp. 614-635, 1987.7
[10] Ji, H., Kanatani, H., Tabuchi, M., Kamba, T. and Ishikawa, M.: Behavior of Concrete Filled RHS
Column to H-beam Connections Fabricated with HT Bolts, Tubular Structures III, Proc. 3rd International
Symposium on Tubular Structures, Lappeenranta, pp. 196-203, 1989.9.
[11] Morita, K., Ebato, K., Furuhara, K., Fujita, K. and Hamano, K.: Experimental Study of Structural
Behavior of Beam-to Column Connections Reinforced by Increasing Plate Thickness of Column without
Diaphragms, Tubular Structures VIII, Proc. 8th International Symposium on Tubular Structures, Sin-
gapore, pp. 585-594, 1998.8.
Fig. 9 Connections without Diaphragms
(a) Long bolts (b) Increased thickness
15
Composite structuresSession 5
[12] Fukumoto, T. and Morita, K.: Elasto Plastic Behavior of Steel Beam to Square Concrete Filled Steel
Tube (CFT) Column Connections, Proc. 6th ASCCS Conference on Composite and Hybrid Structures,
Los Angeles, Vol. 1, pp. 565-572, 2000.3.
[13] Fujimoto, T., Inai, E., Kai, M., Mori, K., Mori, O. and Nishiyama, I.: Behavior of Beam-to Column
Connection of CFT Column System, Proc. 12th WCEE, Auckland, CD-Rom, No. 2197, 2000.1.
[14] Kawaguchi, J. Morino, S. and Sugimoto, T.: Elasto-Plastic Behavior of Concrete-Filled Steel Tubular
Frames, Proc. Engineering Foundation Conference on Composite Construction in Steel and Concrete
III, Irsee, pp. 272-281, 1996.6.
[15] Uchikoshi, M. Hayashi, Y. and Morino, S.: Merits of CFT Column System -Results of Trial Design of
Theme Structures-, Composite and Hybrid Structures, Proc. of 6th ASCCS Conference, Los Angeles,
pp.135-142, 2000.3.
[16] Kimura, M., Ohta, H., Kaneko, H. and Kodaira, A.: Fire Resistance of Concrete-Filled Square Steel
Tubular Columns Subjected to Combined Load, Journal of Structural and Construction Engineering,
Transaction of AIJ, No. 417, pp. 63-70, 1994.4. (in Japanese)
[17] Saito, H. and Saito, H.: Fire Resistance of Concrete-Filled Square Steel Tubular Columns under
Deformation to Simulate the Elongation of Steel Beams, Journal of Structural and Construction Engi-
neering, Transaction of AIJ, No. 458, pp. 163-169, 1990.11. (in Japanese)
[18] Standard for Structural Calculation of Steel Reinforced Concrete Structures, English Ed., AIJ, 1991.9.
[19] Standard for Structural Calculation of Steel Reinforced Concrete Structures, 5th Ed., AIJ, 2001.1. (in
Japanese)
[20] Reports of Committee for Evaluation of Structural Performance of Concrete-Filled Steel Tubular
Columns, Building Center of Japan (BCJ), 1989.3. (in Japanese)
[21] Design Recommendations for Concrete Filled Steel Tube Structures (2000), Association of New
Urban Housing Technology (ANUHT), 4th Ed., 2000.8. (in Japanese)
[22] Guidelines for the Structural Design of CFT Column System (draft), BCJ, 1998.10. (in Japanese)
[23] Nishiyama, I. Morino, S., Sakino, K., et al.: Summary of Research on Concrete-Filled Structural
Steel Tube Column System Carried out under the US-Japan Cooperative Research Program on Com-
posite and Hybrid Structures, BRI Research Paper No. 147, 2002.1
[24] Recommendations for the Design of Plastic Design of Steel Structures, AIJ, 1975.9.
[25] Wakabayashi, M.: A Proposal For Design Formulas of Composite Columns and Beam-Column,
Preliminary Report, 2nd International Colloquium on Stability, Tokyo, pp. 65-87, 1976.9.
[26] Wakabayashi, M.: A New Design Method of Long Composite Beam-Columns, 2nd International
Colloquium on Stability, Washington, pp. 65-87, 1977.3.
[27] Matsui, C.: Strength and Behaviour of Frames with Concrete Filled Square Steel Tubular Columns
under Earthquake Loading, Proc. International Specialty Conference on Concrete Filled Steel Tubular
Structures, Harbin, pp. 104-111, 1985.8.
[28] Kurobane, Y., Togawa, T. and Matsuo, O.: Beam-to-Concrete Filled Tubular Column Connection with
Stiffener Rings, Part 1 ~ 3, Abstracts, Structure II, Annual Meeting of Architectural Institute of Japan, pp.
1275-1280, 1987.10.
[29] Ito, H., Fu, G., Nagata, M., Nakamura, H. and Morita, K.: Structural Behaviours of Connections
between Concrete Filled Steel Tubular Column and Steel Beam, Part 1 and 2, Abstracts, Structure III,
Annual Meeting of Architectural Institute of Japan, pp. 831-834, 1995.8. (in Japanese)
[30] Morita, K., Yokoyama, Y., Kawamata, Y. and Matsumura, H.: Effect of Inner Ring Stiffener on the
Strength of Connection between Steel Beam and Concrete-Filled Square Tube Column, Journal of Struc-
tural and Construction Engineering, Transactions of AIJ, No. 422, pp. 85-96, 1991.4. (in Japanese)
Proceedings of the 1st fib Congress
16
View publication stats
Tube (CFT) Column Connections, Proc. 6th ASCCS Conference on Composite and Hybrid Structures,
Los Angeles, Vol. 1, pp. 565-572, 2000.3.
[13] Fujimoto, T., Inai, E., Kai, M., Mori, K., Mori, O. and Nishiyama, I.: Behavior of Beam-to Column
Connection of CFT Column System, Proc. 12th WCEE, Auckland, CD-Rom, No. 2197, 2000.1.
[14] Kawaguchi, J. Morino, S. and Sugimoto, T.: Elasto-Plastic Behavior of Concrete-Filled Steel Tubular
Frames, Proc. Engineering Foundation Conference on Composite Construction in Steel and Concrete
III, Irsee, pp. 272-281, 1996.6.
[15] Uchikoshi, M. Hayashi, Y. and Morino, S.: Merits of CFT Column System -Results of Trial Design of
Theme Structures-, Composite and Hybrid Structures, Proc. of 6th ASCCS Conference, Los Angeles,
pp.135-142, 2000.3.
[16] Kimura, M., Ohta, H., Kaneko, H. and Kodaira, A.: Fire Resistance of Concrete-Filled Square Steel
Tubular Columns Subjected to Combined Load, Journal of Structural and Construction Engineering,
Transaction of AIJ, No. 417, pp. 63-70, 1994.4. (in Japanese)
[17] Saito, H. and Saito, H.: Fire Resistance of Concrete-Filled Square Steel Tubular Columns under
Deformation to Simulate the Elongation of Steel Beams, Journal of Structural and Construction Engi-
neering, Transaction of AIJ, No. 458, pp. 163-169, 1990.11. (in Japanese)
[18] Standard for Structural Calculation of Steel Reinforced Concrete Structures, English Ed., AIJ, 1991.9.
[19] Standard for Structural Calculation of Steel Reinforced Concrete Structures, 5th Ed., AIJ, 2001.1. (in
Japanese)
[20] Reports of Committee for Evaluation of Structural Performance of Concrete-Filled Steel Tubular
Columns, Building Center of Japan (BCJ), 1989.3. (in Japanese)
[21] Design Recommendations for Concrete Filled Steel Tube Structures (2000), Association of New
Urban Housing Technology (ANUHT), 4th Ed., 2000.8. (in Japanese)
[22] Guidelines for the Structural Design of CFT Column System (draft), BCJ, 1998.10. (in Japanese)
[23] Nishiyama, I. Morino, S., Sakino, K., et al.: Summary of Research on Concrete-Filled Structural
Steel Tube Column System Carried out under the US-Japan Cooperative Research Program on Com-
posite and Hybrid Structures, BRI Research Paper No. 147, 2002.1
[24] Recommendations for the Design of Plastic Design of Steel Structures, AIJ, 1975.9.
[25] Wakabayashi, M.: A Proposal For Design Formulas of Composite Columns and Beam-Column,
Preliminary Report, 2nd International Colloquium on Stability, Tokyo, pp. 65-87, 1976.9.
[26] Wakabayashi, M.: A New Design Method of Long Composite Beam-Columns, 2nd International
Colloquium on Stability, Washington, pp. 65-87, 1977.3.
[27] Matsui, C.: Strength and Behaviour of Frames with Concrete Filled Square Steel Tubular Columns
under Earthquake Loading, Proc. International Specialty Conference on Concrete Filled Steel Tubular
Structures, Harbin, pp. 104-111, 1985.8.
[28] Kurobane, Y., Togawa, T. and Matsuo, O.: Beam-to-Concrete Filled Tubular Column Connection with
Stiffener Rings, Part 1 ~ 3, Abstracts, Structure II, Annual Meeting of Architectural Institute of Japan, pp.
1275-1280, 1987.10.
[29] Ito, H., Fu, G., Nagata, M., Nakamura, H. and Morita, K.: Structural Behaviours of Connections
between Concrete Filled Steel Tubular Column and Steel Beam, Part 1 and 2, Abstracts, Structure III,
Annual Meeting of Architectural Institute of Japan, pp. 831-834, 1995.8. (in Japanese)
[30] Morita, K., Yokoyama, Y., Kawamata, Y. and Matsumura, H.: Effect of Inner Ring Stiffener on the
Strength of Connection between Steel Beam and Concrete-Filled Square Tube Column, Journal of Struc-
tural and Construction Engineering, Transactions of AIJ, No. 422, pp. 85-96, 1991.4. (in Japanese)
Proceedings of the 1st fib Congress
16
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