gchddtuorf8o6tiytiyf

1. Journal of Constructional Steel Research 62 (2006) 1333–1340
www.elsevier.com/locate/jcsr
Improving seismic performance of concrete-filled tube to base connections
H.-L. Hsu∗, H.-W. Lin
Department of Civil Engineering, National Central University, Chung-Li 32054, Taiwan
Received 19 September 2005; accepted 8 February 2006
Abstract
This study is focused on experimentally investigating the relationship between design details and column base performance, such as strength
and rigidity. Results from specimens tested under combined axial and lateral loads were used to define the effectiveness of the base connections.
It was observed from test comparisons that the rigidities of the base connections increased when the embedded depths were increased. It was also
found that the base connections possessed higher stiffness and energy dissipating capability when stiffeners were added to the base connections.
Finally, an empirical expression for base rigidity estimation is proposed for design references.
c 2006 Elsevier Ltd. All rights reserved.
Keywords: Concrete-filled tube; Base connections; Embedded depth; Seismic performance
1. Introduction
Concrete-filled tubes (CFT) possess high compressive
strength and significant ductility and thus are effective
structural forms for earthquake-resistant purposes [1–3].
The significant seismic performance of such designs can
be guaranteed only when adequate design details are
employed. Current studies on the behavior of CFT designs
under earthquake excitation are focused primarily on the
response of CFT members under axial load, bending or a
combination [4–7]. Information on the CFT column to base
design details and the influence of the connection effectiveness
upon the structural performance is still limited.
To fully develop the strength and ductility of CFT
members, the column base must possess sufficient strength
and rigidity to prevent premature failure. It has been
indicated in the investigation report from the 1995 Kobe
earthquake that much structural damage was directly related
to the failure of foundations [8]. Therefore, adequate
recommendations for base connection designs that promote
sustained structural performance are essential to CFT structural
system applications.
In general, column to base connections can be designed
in exposed or embedded forms. Exposed base connection
fabrication is easier than embedded connections. However,
∗ Corresponding author.
exposed connections are less reliable because the connection
rigidity relies solely on the effectiveness of the preset anchor
bolts. In such cases, anchor bolt yielding, as shown in Fig. 1(a),
which is disadvantageous to foundation rigidity, becomes
inevitable. Unless large numbers of anchor bolts and extremely
thick end plates are employed, which should be avoided
when construction costs are a concern, the base connection
effectiveness during earthquakes is susceptible [9,10].
For embedded base connections, connection rigidities can be
improved by the additional resistance of the concrete within the
embedded depth. Although the high stress at the anchor bolts
can be alleviated in such designs, concentrated stresses are still
likely to occur on the concrete at the interface between the
CFT column and surrounding base concrete if the connection
is not adequately detailed. Once the concrete at the contact
surface becomes crushed due to heavy stress, as shown in
Fig. 1(b), the rigidity of the base connections will be reduced,
and the structural performance will degrade accordingly. In
such situations, high level stress at the anchor bolts will
return and the effectiveness of the base connections becomes
questionable. Interaction among the embedded CFT column,
anchor bolts and surrounding concrete in the foundation
incurs a complicated load transmission mechanism. To prevent
undesirable stress distribution within the base concrete, a
simple and efficient remedy to improve the base connection
effectiveness is essential.
0143-974X/$ - see front matter c 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jcsr.2006.02.002

2. 1334 H.-L. Hsu, H.-W. Lin / Journal of Constructional Steel Research 62 (2006) 1333–1340
Fig. 1. Fabrication of column base connections: (a) exposed base connection; (b) embedded base connection; (c) proposed CFT to base connections.
A strengthening scheme involving adding stiffeners at the
embedded segments of the CFT columns, as shown in Fig. 1(c),
is proposed in this study. This strengthening scheme is intended
to minimize the tendency of the concrete to crush, shift the
critical section from the column bottom to the top of the base
concrete, improve the connection rigidity, and enhance the
design efficiency of CFT structural systems. To accomplish this
goal, a series of combined load tests on the CFT column base
connections with various stiffeners were conducted. Results
from specimens tested under combined axial and lateral loads
were used to define the relationship between the design
details and base connection performance, such as rigidity and
energy dissipation capability, and establish references for CFT
structural system design.
2. Experimental program
2.1. Specimens
Seven square CFT members at various lengths (0.5D, 1.0D
and 1.5D; D = section depth) embedded in corresponding
foundations were fabricated for testing. The dimensions of the
steel tubes for all CFT members were 350 × 350 × 7 mm
(width×depth×thickness),which resulted in a width/thickness
ratio of 50. The yield strength of the steel tube was 324 N/mm2.
Each steel tube was fillet-welded to a 32 mm thick end plate
before concrete casting. Foundation concrete was cast in two
pours: one for the anchor bolt mounting and a second to
complete the foundation and fill the core of the CFT members.
Concrete strengths were determined from cylinder tests for the

3. H.-L. Hsu, H.-W. Lin / Journal of Constructional Steel Research 62 (2006) 1333–1340 1335
Fig. 2. Specimen details: (a) top view; (b) side view.
first and second pours at 24.3 and 35.4 N/mm2, respectively.
Four ASTM A325 M27 anchor bolts were used to erect the CFT
member in each foundation. To investigate the feasibility of
this strengthening scheme, three specimens were stiffened with
different stiffener lengths and embedded into the foundations.
The stiffeners were made from 75×16 mm (width×thickness)
ASTM A36 steel plates and were attached to the CFT members
by fillet welding. This was conducted to investigate whether
surface failure could be moved from the highly-stressed CFT
embedded area to the surface of the foundation so that the
foundation integrity could be sustained and CFT member
performance fully developed. The stiffened and unstiffened
specimens were labeled S-series and U-series, respectively.
Specimen details are shown in Fig. 2. Table 1 also lists the
compositions of the specimens.
2.2. Test setup
Each specimen was mounted onto the strong floor with
four high strength rods. To obtain the flexural rigidity of
the base connection for future comparisons and establish
the relationship between the base rigidity and corresponding
fabrication details, specimens were first subjected to only
lateral loads within the elastic range. After the elastic load
tests, the specimens were unloaded and then tested under
combined constant axial and cyclic lateral loads. Two sets of
Table 1
Specimen compositions and the corresponding rigidities
Specimen Embedded depth (D) Stiffener Anchor bolt α
U-05 0.5D No Yes 2.13
U-10 1.0D No Yes 2.58
U-15 1.5D No Yes 2.80
S-00 0 Yes Yes 1.08
S-05 0.5D Yes Yes 2.41
S-10 1.0D Yes Yes 2.79
S-15 1.5D Yes Yes 2.92
D= depth of CFT = 350 mm.
Fig. 3. Displacement history.
load apparatus were used to generate the required loads. The
lateral load was generated using a servo-controlled hydraulic
actuator through a series of prescribed cyclic displacements,
as shown in Fig. 3. The constant axial load was generated
using two hydraulic jacks pushing against a stiffened loading
beam. The test setup is shown in Fig. 4. The magnitude of
the axial load for all specimens was set to 15% of the CFT
member’s compressive strength (Po), which could be calculated
as follows:
Po = As Fys + Ac fc (1)
in which As and Ac are the cross-sectional areas of the
steel tube and the in-filled concrete, and Fys and fc are the
yield strength of the tube and the compressive strength of the
concrete, respectively. Strain gages mounted on the anchor
bolts and the CFT tubes were used to measure the specimen
responses.
3. Observations
Fig. 5 shows the hysteretic relationships and the correspond-
ing failure patterns for specimens with various base com-
positions. It can be observed from the figure that inelastic
elongation of the anchor bolts was exhibited in the exposed
base connection, i.e. S-00. Only poor hysteretic performance
can be achieved with this specimen. Anchor bolt yielding was
significantly alleviated when the CFT members were embed-
ded in the foundations. From Fig. 5(b)–(g), whenever the base
connections possessed sufficient strength, the CFT members
were capable of developing full bearing capacity, such as U-10,
U-15, S-05, S-10, and S-15 specimens.

4. 1336 H.-L. Hsu, H.-W. Lin / Journal of Constructional Steel Research 62 (2006) 1333–1340
Fig. 4. Test setup: (a) front view; (b) side view.
Note that when further comparing the responses of the
S-05 and U-05 specimens, although the CFT columns in both
specimens were embedded in the foundations the same depth,
the stiffened S-05 successfully developed full CFT column
strength, while the unstiffened U-05 exhibited premature
cracking in the base concrete before the CFT member reached
its moment capacity. These phenomena revealed that the critical
embedded lengths, i.e. lengths to prevent base concrete failures,
can be significantly reduced when the strengthening scheme
is adopted. The critical embedded length for the unstiffened
specimen in this study was approximately 1D. This value
was significantly reduced to 0.5D in the stiffened specimen.
This reduction indicated an improved efficiency and justified
the effectiveness of the proposed strengthening scheme in
the base connection design. Further benefits of adopting the
stiffeners in the base connection designs can be validated by
the enhancements in the rigidity of the base connections and the
energy dissipation of the system, as discussed in the subsequent
sections.
4. Comparisons of test results
4.1. Rigidity of base connections
To effectively predict the structural responses of CFT
systems under earthquakes, the member stiffness and the
corresponding boundary conditions of the foundations must
first be adequately defined. For a column member rigidly
attached to the foundation and subjected to lateral load at
the member tip, i.e. a cantilever member as shown in Fig. 6,
the system’s flexural stiffness (K) can be evaluated using the
following expression:
K = 3E I/L3
. (2)
However, the theoretical rigid boundary does rarely exist in
a real structure. Therefore, a reasonable estimation on the
boundary rigidity with specified fabrication details is more
feasible and essential for structural designs. This concern is
particularly critical for the design of CFT structures, because
the structures usually possess higher strength and stiffness than
steel or reinforced concrete structures with similar member
dimensions. In such cases, influence due to discrepancy in base
rigidity estimation will become significant because analytical
results based on inadequate information will be biased.
To account for the effectiveness of the base connections, the
flexural stiffness of specimens with various fabrication details
can be expressed in the following form:
K = αE I/L3
(3)
in which α is a coefficient related to the various foundation
placement details. The α values for the specimens are evaluated
using the information obtained from the elastic load tests, listed
in Table 1 and compared in Fig. 7.
As indicated in the previous section the failures of base
connections are usually governed by the length of the embedded
segment. Once the embedded member depth is larger than
the critical value, the CFT member reaches its capacity prior
to foundation failure. Under this circumstance, it can still be
observed from Fig. 7 that the base rigidities varied even though
the specimens possessed sufficient strength and similar failure
patterns, such as U-10 and S-10. In general, the base rigidities
increased with increasing embedded length; however, the
increments were at different rates for stiffened and unstiffened
bases. They were:
(For U-series) α = −0.4763D2
+ 1.627D + 1.4313 (≤3) (4)
(For S-series) α = −0.4878D2
+ 1.4888D + 1.7882 (≤3). (5)
It can be observed from Fig. 7 that significant improvement
was achieved whenever the strengthening stiffeners were
presented. It is also interesting to find that when the embedded
length reached an extreme value, 1.5D, which is too costly
in construction projects, the achievable member stiffness is
still less than the theoretical 3E I/L3, therefore, the above

5. H.-L. Hsu, H.-W. Lin / Journal of Constructional Steel Research 62 (2006) 1333–1340 1337
Fig. 5. Hysteretic relationships and the corresponding failure patterns for specimens with various placement details: (a) S-00; (b) U-05; (c) S-05; (d) U-10; (e) S-10;
(f) U-15; (g) S-15.

6. 1338 H.-L. Hsu, H.-W. Lin / Journal of Constructional Steel Research 62 (2006) 1333–1340
Fig. 5. (continued)
expressions can serve as more feasible design references for
design purposes.
4.2. Load transmission mechanism
For base connections with embedded CFT segments and
anchor bolts, the connection strength can be evaluated by
summing the component strengths of the anchor bolts and
confining concrete. As stated in previous sections that reduction
in stress concentration to avoid anchor bolt yielding is a major
concern in the effective CFT base connection design. This
concern can be investigated by comparing the stress levels
of anchor bolts in base connections with various placement
details. Fig. 8 shows the comparisons of the ratios between the
moments resisted by the anchor bolts, (Mbolt), and the whole
base connection, (Mbase). The load induced on the anchor bolts
was calculated using the strain measurements installed on the
anchor bolts.
It can be found from the figure that for CFT base connection
with less embedded segment, strength requirement for the
anchor bolts became extremely critical. For example, the anchor
bolts of the U-05 specimen were required to provide 33%
of the base connection’s moment resisting capacity when the
column top reached 3% drift ratio. This induced heavy stress

7. H.-L. Hsu, H.-W. Lin / Journal of Constructional Steel Research 62 (2006) 1333–1340 1339
Fig. 6. Cantilever member subjected to lateral load at member tip.
concentration on the anchor bolts and increased the possibility
of concrete fracturing at the bolt locations. However, when
pairs of stiffeners were added to the embedded CFT segment
at the base connection with details the same as U-05, i.e. S-
05, the induced stress, at the same drift, on the anchor bolts
was significantly reduced, approximately to 2/3 of that of
U-05. These phenomena, in conjunction with the achievable
Fig. 7. Comparisons of base rigidities.
base rigidities described in previous sections, demonstrated the
effectiveness of the stiffening scheme to the improvement of
base connection performance.
4.3. Energy dissipation
To further investigate the effect of the proposed strength-
ening scheme on the seismic performance of CFT base con-
nections, the systems’ energy dissipation capacities were com-
Fig. 8. Comparisons of moment resisting ratios: (a) specimens with different embedded depths; (b) specimens with various stiffening schemes.

8. 1340 H.-L. Hsu, H.-W. Lin / Journal of Constructional Steel Research 62 (2006) 1333–1340
Fig. 9. Comparisons of energy dissipations.
pared. Energy dissipation was calculated by the total area
bounded by the hysteresis loops. Fig. 9 shows energy dissipa-
tion comparisons for the tested specimens. It can be found from
the figure that the exposed base connection, i.e. S-00, mounted
with anchor bolts only, exhibited poor performance due to in-
adequate energy dissipation mechanism. An improved energy
dissipating performance was achieved in specimen S-05 when
the CFT member was embedded in the foundation. The energy
dissipation capability continued to increase when the embedded
depth increased. This phenomenon stabilized when the embed-
ded depth, for the stiffened specimens, reached a critical value,
approximately 1.0D. Further comparisons of energy dissipa-
tion capabilities between the stiffened and unstiffened spec-
imens also showed that the former designs possessed higher
seismic resistance than the latter when they were embedded to
the same depth, which justified the applicability of the strength-
ening schemes.
5. Conclusions
This study focused on the experimental investigation of
the relationship between design details and column base
performance. Results from specimens tested under combined
axial and lateral loads were used to define the effectiveness
of the base connections. Test results showed that the rigidities
of the base connections increased when the embedded depths
were increased. It was also found that the base connections
possessed higher stiffness and energy dissipation capability
when stiffeners were added to the embedded segment. An
empirical expression for base rigidity estimation is proposed
for design references.
Acknowledgments
This study was partially supported by the National Science
Council of the Republic of China under Grant No. NSC
90-2625-Z-008-011 and NSC 91-2625-Z-008-011, which is
gratefully acknowledged.
References
[1] Usami T, Ge H. Ductility of concrete-filled steel box columns under cyclic
loading. J Struct Engng, ASCE 1994;120(7):2021–40.
[2] Boyd PF, Cofer WF, McLean DI. Seismic performance of steel-encased
concrete columns under flexural loading. ACI Struct J 1995;92(3):355–64.
[3] Ninakawa T, Sakino K. Inelastic behavior of concrete filled circular steel
tubular columns subjected to uniform cyclic bending moment In: Proc.
11th world confer. earthquake eng. vol. 1385; 1996.
[4] Kitada T. Ultimate strength and ductility of state-of-the-art concrete-filled
steel bridge piers in Japan. Eng Struct 1998;20(4):347–54.
[5] Hsu HL, Lin JL. Experimental evaluation on seismic performance
of sandwich box columns. Earthquake Engng Struct Dyn 1999;28(8):
823–40.
[6] Uy B. Strength of concrete filled steel box columns incorporating local
buckling. J Struct Engng, ASCE 2000;126(3):341–52.
[7] Hsu HL, Yu HL. Seismic performance of concrete-filled tubes with
restrained plastic hinge zones. J Constr Steel Res 2003;59(5):587–608.
[8] Atorod A, Ghosh SK. Steel reinforced concrete structures in 1995
Hygoken-nanbu earthquake. J Struct Engng, ASCE 1997;123(8):986–92.
[9] Kallolil JJ, Chakrabarti SK. Experimental investigation of embedded
steel plates in reinforced concrete structures. Engng Struct 1998;20(1–2):
105–12.
[10] Pertold J, Xiao RY, Wald F. Embedded steel column bases I experiments
and numerical simulation. J Constr Steel Res 2000;56:253–7.