The document investigates the effect of confining pressure on the compressive strength and ductility of square concrete-filled steel tube (CFT) columns. It finds that: 1) Increasing confining pressure from 11.4-34.3% of concrete strength slightly improved compressive strength by 3.0-19.3%, with more improvement at pressures over 22.9%. 2) Preconfining the steel jacket did not enhance the ductility or energy absorption of the square CFT columns. 3) The non-uniform hoop stress in square sections under preconfining pressure resulted in poorer strength and ductility gains compared to circular CFT columns.

1. EFFECT OF CONFINING PRESSURE ON
COMPRESSIVE STRENGTH AND DUCTILITY
OF SQUARE SECTION CONCRETE FILLED
TUBE COLUMN
Department of Civil and Environmental Engineering, Faculty of
Science and Engineering, Kasetsart University Chalermprakait
Sakon Nakhon Province Campus, Sakon Nakhon, Thailand
Asst.Prof. Dr.Vatwong Greepala

2. #1
CONTENT
INTRODUCTION OBJECTIVE
EXPERIMENTAL
INVESTIGATION
RESULTS AND
DISCUSSION
CONCLUSIONS

3. INTRODUCTION
Structural
Properties
Construction
Cost
Construction
Time
Fire
Protection
WHAT IS CFT? WHY USE CFT?
Concrete-filled steel tubes (CFT) is a type of structural
element where structural concrete is placed inside a
structural steel shell.

4. INTRODUCTION
Infill concrete in steel tube delays local
buckling of the tube and so increases the
stability and strength of the column
Steel tube performs as primary longitudinal
main reinforcement to concrete core
Confinement of steel tube improves
concrete compressive strength and ductility
Ductility of CFT is significantly enhanced
when compared with steel tube and
concrete alone
Advantages of CFT column

5. #4
INTRODUCTION
Statement of problems
The square or rectangular
CFT columns present less
gain of the advantages
than the circular CFT due
to some loss of the
confinement effect
How to improve it?
Pre-confining?

6. #5
OBJECTIVE
 The purpose of this study is to investigate the influence of
preconfining pressure on the strength and ductility performance of
the square sections of CFT columns.

7. #6
EXPERIMENTAL INVESTIGATION
SPECIMEN
 The steel tubes with external dimensions of
98x98x2.3 mm (width x depth x thickness) were
used to confine the infill concrete with
compressive strength of 20.98 MPa (BS1881-116
1983).
 The width-to-thickness ratio (B/t) is 42.6.

8. #7
EXPERIMENTAL INVESTIGATION

9. #8
Start
Specimen
Preparation
Compressive
Strength Testing
Determination of
Compressive Strength
Determination of
Ductility Factor (µcu)
Conclusion
End
Max. Compression Forced Axial Stress-Strain Relation
Step 1
Step 2
Step 3
Step 4
EXPERIMENTAL INVESTIGATION
Concrete compressive strength of
20.98 MPa (BS1881-116 1983)

10. #9
Specimen Preparation
Casting CFT specimen Jacketing Preconfining CFT Specimen

11. Test Setup
Wielding line
LVTD
Dial Gauge
LVTD
Dial Gauge
Strain Gauge No.1
Strain Gauge No.1
Strain Gauge No.3
Strain Gauge No.4
Axial Load
Axial Load
Bearing Plate
Bearing Plate
LVTD
Dial Gauge
LVTD
Dial Gauge
98mm
CFT
Specimen
98mm
Core
Concrete
350mm
Steel Tube
Thickness 6mm
Welding
Welding
A A
Strain Gauge No.2
Section A-A
Strain Gauge
No.1 & No.2
Strain Gauge
No.3 & No.4
Strain Gauge No.2

12. #11
Determination of ductility factor
1
cu
cu
ε
µ
ε
=
( )
'
max 1
cu
cu
c
e
W
f ε
=
×
Ductility factor
Work index
Energy absorption capacity factor (ecu) is
defined as the area under the stress-strain
curve up to the rupture of the CFT jacket
Suggested by Sheikh et al. (1994)

13. #12
RESULTS AND DISCUSSION

14. #13
RESULTS AND DISCUSSION

15. #14
RESULTS AND DISCUSSION
The squared
section did not
provide a large
confining effect
especially when
the width-to-
thickness ratio
was large
(B/t>30. In this
study B/t is 42.6
therefore the
crushing plane
of concrete
show less
efficiency
confinement.

16. #15
RESULTS AND DISCUSSION

17. #16
Compressive Strength

18. Ductility
Ductility Factor (µcu) Work Index (Wcu)

19. l
f
a
σ
l
σ
l
f
l
f
l
f
l
σ
l
σ
l
σ
a
σ
High Tensile
Stress
Uniform
Tensile
Stress
RESULTS AND DISCUSSION
In preconfining process of most of
the hoop stress and hoop strain of
the square section occurred at the
pre-stressing side unlike the hoop
stress and hoop strain in a circular
which are constantly distributed
along the hoop ring. At the loading
state, the ultimate hoop stress and
hoop strain at the pre-stressing side
may reach the yield limit early results
in poor improvement of compressive
strength and ductility of the square
section.

20. Conclusion
The influence of the preconfining pressure of CFT column on its compressive strength
and ductility was experimentally investigated. The following conclusions can be drawn:
▫ 1) The increasing of confining pressure of 11.4%-34.3% of concrete strength slightly
improved the compressive strength of the square section of CFT column by 3.0-
19.3% and the more improvement occurs at the preconfining pressure beyond
22.9%.
▫ 2) The use of preconfining steel jacket is not enhancing its ductility and energy
absorption.

21. THANK YOU