report on the comparison between CFSTC and HSTC and Comparison of CFSTC strength based on their strengths

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CHAPTER 1
INTRODUCTION
1.1 CONCRETE FILLED STEEL TUBES (CFST)
Over the years, the construction industry has witnessed various composite
structures for high performance column design to fare better in terms of high strength,
stiffness, ductility and seismic resistance. Concrete-filled steel tube (CFST) columns
are one such composite structure that outperforms the conventional column system in
various different aspects.
Concrete filled steel tubular (CFST) members utilize the advantages of both
steel and concrete. They comprise of a steel hollow section of circular or rectangular
shape filled with plain or reinforced concrete. They are widely used in high-rise and
multistorey buildings as columns and beam-columns, and as beams in low-rise
industrial buildings where a robust and efficient structural system is required.
There are a number of distinct advantages related to such structural systems in
both terms of structural performance and construction sequence. The inherent
buckling problem related to thin-walled steel tubes is either prevented or delayed due
to the presence of the concrete core. Furthermore, the performance of the concrete in-
fill is improved due to confinement effect exerted by the steel shell. The distribution
of materials in the cross section also makes the system very efficient in term of its
structural performance. The steel lies at the outer perimeter where it performs most
effectively in tension and bending. It also provides the greatest stiffness as the
material lies furthest from the centroid. This, combined with the steel's much greater
modulus of elasticity, provides the greatest contribution to the moment of inertia. The
concrete core gives the greater contribution to resisting axial compression.
The use of concrete filled steel tubes in building construction has seen
resurgence in recent years due mainly to its simple construction sequence, apart from
its superior structural performance. Typically, it was used in composite frame
structures. The hollow steel tubes that are either fabricated or rolled were erected first
to support the construction load of the upper floors. The floor structures consist of

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steel beams supporting steel sheeting decks on which a reinforced concrete slab is
poured.
Such structural system has the advantage of both steel and reinforced concrete
frame. It has the structural stiffness and integrity of a cast-on-site reinforced concrete
building, and the ease of handling and erection of a structural steelwork.
The hollow tubes alone were designed in such a way that they are capable of
supporting the floor load up to three or four storey height. Once the upper floors were
completed, the concrete was pumped into the tubes from the bottom. To facilitate easy
pumping the tubes were continuous at the floor level. Modern pumping facility and
high performance concrete make pumping three or four storey readily achievable. Due
to the simplicity of the construction sequence, the project can be completed in great
pace.
Fig.1 Plan and section of CFST columns
1.2 VARIOUS TYPES OF CFST COLUMNS
There are two types of composite columns generally used in buildings, steel
section encased in concrete and steel section in-filled with concrete. A concrete filled
steel tubular (CFST) structure consists of steel tube of square, rectangular or circular

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cross-section filled with plain or reinforced concrete. Following are the various types
of CFST columns:
1. Composite column systems
2. Reinforced composite column systems
3. Concrete-filled double skin tubes (CFDST)
4. Reinforced Concrete-filled double skin tubes (CFDST)
5. Concrete-encased CFST columns
6. Stiffened CFST columns
Fig.2 Various types of CFST columns

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1.3 OBJECTIVES
 To study the behaviour of CFST and HST columns in axial compression.
 To determine the axial load carrying capacity of CFST and HST columns.
 To study the behaviour of square and circular CFST columns in axial
compression and to determine the axial load capacity of the respective
columns.
 To compare the experimental results of CFST with AISC-LRFD 2005 and
Eurocode-4.

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CHAPTER 2
EXPERIMENTAL WORK
2.1 COMPARATIVE STUDY BETWEEN HOLLOW STEEL TUBES (HST)
AND CONCRETE FILLED STEEL TUBE (CFST) COLUMNS
For experimental investigation axial load is applied on HST & CFST
specimen. While testing care taken that the end surfaces on which concrete filled steel
tubes keeping for testing should be the plane. All specimens were tested in
Compression Testing Machine and are simply supported at both ends.
2.1.1 DETAILS OF COLUMN SPECIMEN
 All the steel tubes have same c/s as 145mm X 82mm X 4.8mm.
 A total of 18 columns were tested (6 HST and 12 CFST).
 2 HST and 4 CFST columns of height 0.5m were tested for axial compression
loading.
 2 HST and 4 CFST columns of height 1m were tested for axial compression
loading.
 2 HST and 4 CFST columns of height 1.5m were tested for axial compression
loading
2.1.2 MATERIALS USED
1. Hollow steel tubular section
2. Nitowrap 410 (epoxy)
3. Cement
4. Fine aggregate
5. Coarse aggregate
6. Steel bars
7. Water
8. Curing compound

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Table 1 Details of column specimen
SL
NO.
SPECIMEN
DIMENSIONS
(MM) LENGTH(MM) L/D
CONCRETE
GRADE
B D t
1 HSTC-01 82 145 4.8 500 3.45 …..…
2 HSTC-02 82 145 4.8 500 3.45 …….
3 HSTC-03 82 145 4.8 1000 6.90 …….
4 HSTC-04 82 145 4.8 1000 6.90 …….
5 HSTC-05 82 145 4.8 1500 10.34 …….
6 HSTC-06 82 145 4.8 1500 10.34 …….
7 CFSTC-01 82 145 4.8 500 3.45 M20
8 CFSTC-02 82 145 4.8 500 3.45 M20
9 CFSTC-03 82 145 4.8 500 3.45 M40
10 CFSTC-04 82 145 4.8 500 3.45 M40
11 CFSTC-05 82 145 4.8 1000 6.90 M20
12 CFSTC-06 82 145 4.8 1000 6.90 M20
13 CFSTC-07 82 145 4.8 1000 6.90 M40
14 CFSTC-08 82 145 4.8 1000 6.90 M40
15 CFSTC-09 82 145 4.8 1500 10.34 M20
16 CFSTC-10 82 145 4.8 1500 10.34 M20
17 CFSTC-11 82 145 4.8 1500 10.34 M40
18 CFSTC-12 82 145 4.8 1500 10.34 M40
2.1.3 PROPERTIES OF MATERIAL USED
2.1.3.1 Hollow Steel Tubes
It confirms to IS-4923:1997
Table 2 Dimensional and geometric properties of hollow steel

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Table 3 Mechanical properties of cold formed steel section
2.1.3.2 Cement
OPC 53 grade cement confirming to IS 12269:1987 is used in the current
investigation.
Table 3 Properties of cement
2.1.3.3 Fine Aggregate
Manufactured sand confirming to IS-383:1970 belonging to zone II is used in
the current investigation.

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Table 4 Properties of sand
SL NO. PROPERTIS VALUES
2.1.3.4 Coarse Aggregate
Crushed stone aggregates confirming to IS 383:1970 were used as coarse
aggregates. The maximum size of crushed stone dust was 12.5mm. The specific
gravity of crushed stone aggregate used was found to be 2.63 and the water absorption
was found to be 0.72%.
2.1.3.5 Chemical Admixture
The chemical admixture basically used in the concrete for current
experimental investigation is a high performance super plasticizer which is derived
from carboxylic ether.
Table 5 Characteristics of admixture

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2.1.3.6 Concrete
The concrete used in the current experimental investigation was produced in
the Ready Mix Concrete (RMC) plant. Two grades of concrete M20 and M40 were
used. Both the concrete had collapsible slump so that concrete can easily flow into the
steel tube by its own.
2.1.3.7 Curing compound
The curing compound used in the current experimental investigation was
basically based the membrane curing theory. The curing compound used is Master
Kure 181 which is a non degrading, membrane forming liquid basically derived from
the acrylic resin.
Table 6 Characteristics of Master kure 181
2.1.4 EXPERIMENTAL TEST SETUP
The concrete filled steel tube specimens of different cross sections are tested
for their load carrying capacity under axial compression on the compression testing
machine. The actual test setup is as shown in following figure. The specimen of CFST
is placed centrally on plates of compression testing machine and load is applied
gradually. The readings were taken on dial guage and tabulated.

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Fig.3 Mixing of hardener and base of epoxy
Fig.4 Hollow steel tubes sections of 6m long pieces

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Fig.5 Gas cutting of 6m long pieces
Fig.6 Finishing to the concrete exposed surfaces of column

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Fig.7 Test set up of HST and CFST Column for axial loading
Fig.8 Test set up of CFST Column of 1 m an 1.5 m for axial loading

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2.1.4 TEST RESULTS
Table 7 Test result of HSTC
SL NO.
Specimen
Designation
Height of Column
(m)
Ultimate
Compressive Load
(KN)
1 HSTC-01 0.5 749.48
2 HSTC-02 0.5 757.33
3 HSTC-03 1.0 688.66
4 HSTC-04 1.0 680.81
5 HSTC-05 1.5 608.22
6 HSTC-06 1.5 622.94
Table 8 Test result of CFST
SL NO.
Specimen
Designation
Height of
column (m)
Grade of
concrete infilled
Ultimate
Compressive
Load (KN)
1 CFST-01 0.5 M20 884.86
2 CFST-02 0.5 M20 912.33
3 CFST-03 0.5 M40 1020.24
4 CFST-04 0.5 M40 1059.48
5 CFST-05 1.0 M20 797.55
6 CFST-06 1.0 M20 808.34
7 CFST-07 1.0 M40 819.14
8 CFST-08 1.0 M40 830.91
9 CFST-09 1.5 M20 725.94
10 CFST-10 1.5 M20 741.64
11 CFST-11 1.5 M40 755.37
12 CFST-12 1.5 M40 769.10

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2.1.6 COMPARISON OF TEST RESULTS
2.1.6.1 EUROCODE 4
In this research, similar to for end –loaded braced members, the axial force
Nsd and the maximum end moment Msd are determined from a first order structural
analysis. For each of the bending axis of the column it has to be verified that
Where χk is a reduction factor due to buckling. The buckling curves can also be
described in the form of an equation:
Where,
Where α depends on the buckling effects, a value of 0.21 was adopted for CFST
column . The relative slenderness of λ is given by:
In which Ncr is the critical buckling stress resultant given by:

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Where Le is the effective length and (EI)e is the actual elastic stiffness.
In this research it is proposed:
Where βc is the load effect;
Ic,, Is are the concrete , steel moments of inertia;
Es is the Young’s modulus of steel;
Ec is the secant modulus for the concrete determined for the appropriate concrete
grades, equal to 9500(fc’+8)1/3
In MPa:
Fc
,
is the characteristic compressive cylinder strength of concrete at 28 days.
The value of βc is adopted as:
For n≤0.5
Where n is the ratio of design load to the capacity:
and for n>0.5
βc =0.735

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Above Eqn. is approximately safety factor of 1.35.
The secondary moment effect due to lateral deflection is accounted for by the use of a
moment magnifier δb.
M*
=δb Msd
Where Msd is the maximum first order bending moment and
Where Cm is the moment factor, equal to
r is the ratio of the smaller to larger end moment and is positive when the member is
bent in single curvature.
2.1.6.2 AISC-LRFD 2005
Axial load capacity of column is found out by
Where Pn is the axial compressive load, As is the gross area of steel and Fcr is the
critical stress.

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Fmy - modified yield stress
is the column slenderness parameter
K is the effective length factor for prismatic member, L is the unbraced length of
member measured between the center of gravity of the bracing members, rm is the
radius of gyration of steel shape in composite column and Em is the modified
modulus of elasticity.
fy is the specified minimum yield stress of the type of steel being used, fc’ is the
concrete compressive stress, Ac is the net concrete area and As is the gross area of
steel.
Ec is the modulus of elasticity of concrete
W is the unit weight of concrete

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Table 9 Comparison of experimental and design ultimate loads
Sl
No.
Specimen
Designation
Experimen
tal
Ultimate
Load(PEX
P)
(KN)
Predicted Load Design by
AISC-LRFD-2005 Eurocode-4
PLRFD
(KN)
PEXP/
PLRFD
PEC4
(KN)
PEXP/
PEC4
1 CFSTC-01 884.86 752.08 1.18 787.03 1.12
2 CFSTC-02 912.33 752.08 1.21 787.30 1.16
3 CFSTC-03 1020.24 883.33 1.15 943.21 1.08
4 CFSTC-04 1059.48 883.33 1.20 943.21 1.12
5 CFSTC-05 797.55 719.22 1.11 752.40 1.06
6 CFSTC-06 808.33 719.22 1.12 752.40 1.07
7 CFSTC-07 819.14 840.81 0.97 895.41 0.91
8 CFSTC-08 830.91 840.81 0.99 895.41 0.93
9 CFSTC-09 725.94 667.62 1.09 705.26 1.03
10 CFSTC-10 741.64 667.62 1.11 705.26 1.05
11 CFSTC-11 755.64 774.45 0.98 826.65 0.91
12 CFSTC-12 769.10 774.45 0.99 826.65 0.93
2.1.7 FAILURE OF COLUMN
Fig.9 Failure of Hollow Steel Tubular Column

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Fig.10 Failure of CFSTC-03 due to local buckling near mid height
Fig.11 Failure of the HST column Fig.12 Failure of the CFS column
of due to overall buckling 1.5m length due to overall
buckling

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2.1.8 TEST RESULT ANALYSIS
 The axial load carrying capacity of CFST columns was increased by
1. 19.3% and 38% for M20 and M40.
2. 17.3% and 22.2% for M20 and M40.
3. 19.7% and 24.3% for M20 and M40.
 The theoretical axial load carrying capacity of Concrete Filled Steel Tubular
columns evaluated in accordance with AISC-LRFD 2005 and Eurocode 4
were found to be in best agreement.
 The maximum percentage variation for experimental results and theoretical
results of axial load carrying capacity of CFST columns evaluated in
accordance with AISC-LRFD 2005 was around 21%. Eurocode 4 was around
16%.
 Although there was some variation in the results between the experimental and
theoretical results, but the experimental results were on the conservative side.
 The failure of the CFST columns of height 0.5m was basically due to the local
buckling near the mid height compare to the failure of Hollow Steel Tubular
columns which failed due to inward local buckling near the ends.
 The failures of the CFST columns of height 1.0m and 1.5m were basically due
to the overall buckling which was very much similar in case of Hollow Steel
Tubular columns.

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2.2 STUDY ON CIRCULAR AND SQUARE CONCRETE FILLED STEEL
TUBE COLUMNS SUBJECTED TO AXIAL COMPRESSION LOADS
Here we would like to compare the difference of the axial load capacity of the
circular and square CFST columns of high grade of steel for different grades (M20,
M30 & M40) of concrete. Further, the performance indices named Ductility Index
(DI), Strength Index (SI) and the Concrete Contribution Ratio (CCR) were evaluated
and compared for the circular and square CFST columns. Finally, the design steps are
presented along with calculated axial loading capacity of the CFST columns as per the
EC4[6]design codes. Which is best suitable code for design of CFST columns
amongst all other international codes for composite.
2.2.1 PEOPERTIES OF MATERIALS USED
2.2.1.1 STEEL
Table 10 Properties of steel

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2.2.1.2 CONCRETE
Table 11 Concrete strengths
2.2.1.3 SPECIMEN PROPERTIES
Table 12 Specimen properties
2.2.2 EXPERIMENTAL PROGRAM
The column was tested for static axial load with the following arrangements using
Hydraulic Universal Testing Machine (200T). Three different grades of concrete i.e.
M20, M30 & M40 has been used of two different shapes (circular & square) of CFST
columns. The columns were fixed at both ends and axial compressive load was
applied. A pre-load of about 5kN was applied to hold the specimen upright. Dial
gauge was used to measure longitudinal deformations of the columns. The load was
applied in small increments of 50 kN. At each load increment, the deformations were
recorded. All specimens were loaded up to ultimate load.

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Fig.13 Experimental setup of square and circular cfst
2.2.3 EXPERIMENTAL RESULT
The circular and square CFST columns have not definite axial shortening
during the initial linear loading process, which shows the composite action between
steel tube and the concrete core. The axial load much higher for the square CFST
columns than the circular ones with same area of steel & same resisting area under
compression, as expected. However, the axial load capacity is gradually increased for
both of the square & circular CFST columns when the grade of concrete is increased
from M20 to M30 & M40. Axial shortening values at ultimate load for hollow as well
as CFSTs circular significantly more than square ones for all three grades of concrete.
While these values decreases for both circular & square CFST columns when grade of
concrete is increased from M20 to M40 very easily. To compare the ductility, strength
enhancement concrete part in the CFST columns, some important parameters are
defined as below and evaluated for the different filling concrete grade i.e. M20,
M30and M40. Which are Ductility Index (DI), Strength Index (SI) and Concrete
Contribution Factor (CCR) respectively.

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Here, pu,filled is the ultimate load capacity of infilled steel tubes while
pu,hollow is the ultimate load capacity of the steel tubes without filling, δu is the axial
shortening CFST specimens at the ultimate stage; δ85% is the axial shortening of
CFST specimens the ultimate load.
The strength enhancement index (SI) can be defined as the ratio the axial load
capacity of the CFST section to the sum of the strengths of the steel tube and the
concrete combined Role of the concrete part in axial load capacity of CFST columns
are shown by CCR i.e. Ratio of the ultimate load capacity of in-filled to the un-filled
steel tubes. The relationships between the constraining factor and the ductility index,
strength enhancement index and the concrete contribution ratio are shown in Charts
The increase in the DI is 20-25% higher for the circular CFST columns than the
square ones. Contrary to the DI, SI and CCR are much higher for Square CFST
columns.
Chart 1 Axial shortening curve for hollow steel tubes

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Chart 2 Axial shortening curve for square CFST
Chart 3 Axial shortening curve for circular CFST

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Chart 4 Axial shortening curve for M20
Chart 5 Axial shortening curve for M30
Chart 6 Axial shortening curve for M40

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Fig.14 Graph showing axial load vs grade of concrete
Table 13 Measured circular specimen test results

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Table 14 Measured square specimen test results
Table 15 Average values of Performance Indices

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Table 16 Performance Indices

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2.2.4 COMPARISON WITH EUROCODE 4
2.2.4.1 SQUARE SECTION
The plastic resistance of a concrete filled rectangular hollow sectionis given by
Npl,Rd = Aafy / ra +Asfsk / rs + Acfck / rc
Where,
Aa is the area of the steel section
As is the area of the reinforcement
For ease of expression, fy/ ra, fsk / rs and fck / rc presented asdesign strengths of the
respective materials as fyd, fsd and fcd respectively.
2.2.4.2 CIRCULAR SECTION
For composite columns with concrete filled circular hollowsections, the
increased resistance of concrete due to the confining effect of the circular included
hollow section may be included. The resistance of a concrete filled circular hollow
sectionto compression may increase by upto 15 % under simple axial loads when the
effect of triaxial confinement is considered, which is restricted to short column only.
EC4 considers confinement effects for circular sections when relative slenderness (λ)
has value less than 0.5. It is the only code that treats the effects of long-term loading
separately.
Npl,Rd = Aafyd η2 +Asfsd + Acfcd(1+ η1(tfy+dfck))
Where,
t is the wall thickness of the steel hollow section in mm
η 1= η10 (1-10e/d)
for 0 <e ≤ d/10
η2 = η20 + (1- η20) 10e/d
η1 = 0

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for e >d/10
η 2= 1.0
The basic values η10 and η20 depend on the non-dimensionalslenderness ratio λ.
Table 17 Axial load carrying capacity of CFST columns
2.2.5 FAILURE OF COLUMN
Fig.15 M20 grade square and circular CFST columns

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Fig.16 M30 grade square and circular CFST columns
Fig.17 M40 grade square and circular CFST columns

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2.2.6 TEST RESULT ANALYSIS
 Ultimate load (Pu) for square shape CFST sections are quite higher compared
to circular ones. Although, they were kept of same thickness & same resisting
area under compressive load.
 It can be observed that the strength to weight ratio of 80 mm hollow square
column is about 35% more than that of 88.9 mm hollow circular column.
Therefore, about 30% of steel can be saved when square columns are used to
obtain the same load capacity instead of circular columns.
 By increasing grade of concrete, decrement in ultimate axial shortening value
for CFSTs. i.e. stiffness increases with increase in concrete strength but
columns fail due to crushing of concrete for all the specimens.
 Ultimate axial shortening values for circular CFSTs are quite large than square
ones for all grades of concrete. i.e. ductility index of circular CFSTs are better.
 By increasing grade of concrete, Concrete Contribution Ratio(CCR)
increases while Strength Index(SI) decreases. The increase in the Ductility
Index(DI) is20-25% higher for the circular CFST columns than the square
ones.
 By comparing experimental ultimate axial load capacity of CFST columns
with analytical (i.e. by Eurocode4) it is concluded that results are vary by 15-
20% error. Which is not depicts perfect results but acceptable due to validation
by other researchers too.

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CHAPTER 3
3.1 ADVANTAGES OF CFST
CFST column system has many advantages compared with ordinary steel or
reinforced concrete system. The main advantages are listed below.
Interaction between steel tube and concrete:
 The occurrence of the local buckling of the steel tube is delayed, and the
strength deterioration after the local buckling is moderated, both due to the
restraining effect of concrete.
 The strength of concrete is increased due to the confining effect provided from
the steel tube, and the strength deterioration is not very severe, since the
concrete Spalding is prevented by the tube.
 Drying shrinkage and creep of concrete are much smaller than ordinary
reinforced concrete.
Cross-sectional properties:
 The steel ratio in the CFT cross section is much larger than those in the
reinforced concrete and concrete-encased steel cress section.
 Steel of the CFT section is well plasticized under bending since it is located on
the outside the section.
Construction efficiency:
 Forms and reinforcing bars are omitted and concrete easting is done by
tramline tube or pump-up method, which lead to savings of manpower and
constructional cost and time.
 Constructional site remains clean.
Fire resistance:
 Concrete improves the fire resistance performance, and the amount of
fireproof material can be reduced or its use can be omitted.

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Cost performance:
 Because of the merits listed above, a better cost performance is obtained by
replacing a steel structure by CFT structure.
Ecology:
 Environmental burden can be reduced by moiling the form work, and high
quality concrete as recycled aggregates.
The cost advantages of CFT column system against other structural systems
will be discussed later in more detail. One weak point of the CFT system is the
compactness of concrete around the beam-to-column connection, especially in the
case of inner and through type diaphragms, in which the gap between concrete and
steel may be produced by the bleeding of the concrete underneath the diaphragm.
There is no way so far to assure the compactness and to repair the deficiency, and thus
it is common construction practice to cast a high quality concrete with low water
content and good workability by the use of a super plasticizers.
The other advantages of CFST column is listed below:-
1. The size of column is smaller, increases the usable floor area by 3.3%
(5500m2).
2. CFST columns used concrete 62% less and steel 5%~10% less than that
of RC columns.
3. Compared with steel column, CFST ones used steel is 50% less and
decreases cost 45%.
4. It is about 55% lighter than that of RC. Hence, the foundation cost can
be reduced. The force resulting from earthquake is smaller.
5. The cost on transportation and assembly of columns can be reduced
because they are built by hoisting the empty steel tube first, then pour
concrete into it.
6. CFST columns are safer and more reliable in seismic region, The high-
strength concrete can be used and the brittle failure can be prevented.
7. Steel tube of CFST columns are generally less than 40mm thick. It is
easily available, cheap and can be conveniently fabricated and
assembled.

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CHAPTER 4
APPLICATION OF CFST COLUMNS
The first engineering adopted CFST is the No.1 subway of Beijing. The size of
CFST column is smaller than that of RC column, which increases the usable area.
Good economic effect was obtained. Then, all of the platform columns for Beijing
No.2 subway adopted CFST columns.
According to incomplete statistics, in this stage, there are over 200 constructed
engineering adopted CFST structures in China. Some typical engineering are
introduced as follows.
1. The steel ingot work- shop of Benxi steel company, the span is 24m, interval
of column is 6m, which the heavy cranes Q=20t/200t and 10t/50t are
equipped. The length of column is 15.8m. Four limbs column was used, steel
is Q235 and concrete is C40. It was the first industry building adopted CFST
columns. It completed in 1972.
2. The application of CFST in tall buildings, only partial columns of building
adopted in early days, then greater part of columns adopted, then all of the
columns adopted. This process was very short, only a little more than 10 years.
The highest tall building adopted CFST is Shenzhen SEG Plaza building
completed in 1999. It is the highest one in China and abroad. There is no
staying area for construction. It made the construction rather difficult. There
are a lot of new technology and experiences in design, fabrication and
construction of this building. It offers a good example of the adoption of CFST
columns in super tall buildings. It also promotes the development of CFST
structures in our country to a higher level.

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Fig.18 SEG Plaza under construction
3. The concrete filled steel tube (CFST) is a composite material combined by the
thin-walled steel tube and the concrete filled into the steel tube. On one hand,
the concrete in the tube improves the stability of the thin-walled steel tube in
compression; on the other hand, the steel tube confines the filled concrete and
the filled concrete in turn is in compression in three directions. Therefore, the
CFST has higher compression capacity and ductility. It is good for the
application of arch bridge
Fig.19 First CFST Arch Bridge in China: Wangchang East River Bridge
(Span 115 m)

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CHAPTER 5
CONCLUSION
This study mainly presents an experimental investigation of concrete-filled
steel tube columns under an axial compressive load. Here mainly two types of study
are done one is the experimental study between hollow and concrete filled steel tubes
and other is the experimental study between circular and square concrete filled steel
tubes. Then the obtained result is compared with the standard codes AISC LRFD
2005 and EUROCODE-4. The following results were obtained on the basis of
experiment.
1. The axial load carrying capacity of CFST columns compared to hollow steel
tubes was increased by
 19.3% and 38% for M20 and M40.
 17.3% and 22.2% for M20 and M40.
 19.7% and 24.3% for M20 and M40
2. The failure of the CFST columns of height 0.5m was basically due to the local
buckling near the mid height compare to the failure of Hollow Steel Tubular
columns which failed due to inward local buckling near the ends.
3. The failures of the CFST columns of height 1.0m and 1.5m were basically due
to the overall buckling which was very much similar in case of Hollow Steel
Tubular columns.
4. Ultimate load (Pu) for square shape CFST sections are quite higher compared
to circular ones. Although, they were kept of same thickness & same resisting
area under compressive load.
5. It can be observed that the strength to weight ratio of 80 mm hollow square
column is about 35% more than that of 88.9 mm hollow circular column.
Therefore, about 30% of steel can be saved when square columns are used to
obtain the same load capacity instead of circular columns.
6. Ultimate axial shortening values for circular CFSTs are quite large than square
ones for all grades of concrete. i.e. ductility index of circular CFSTs are better.

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