Design of Concrete Filled
Steel Tubes and Application
Prof. Mahendrakumar Madhavan Ph.D., P.E., F.SEI
Department of Civil Engineering
IIT Hyderabad
Guangzhou Tower
CFST Structure
Design of Concrete filled steel tubes
1
Introduction
2
5
6
Materials and Component Behavior
3
Application and Design Procedure for CFST
4
Connections
Economical Benefit of CFST structure
Fire Performance and Limitation
Design of Concrete filled steel tubes
1
Introduction
2
5
6
Materials and Component Behavior
3
Application and Design Procedure for CFST
4
Connections
Economical Benefit of CFST structure
Fire Performance and Limitation
Introduction
Composite Construction
 Composite Member refers to two load carrying structural member that are integrally connected
and deflect as a single unit.
Concrete
Concrete
beam
II beam
Slip
Composite beam
Composite beam
 Concrete encasement restrain steel against buckling.
 Steel brings ductility into the structure.
 Concrete provides protection against corrosion and fire.
=
Non Composite beam
Non Composite beam
Concrete
Concrete
I beam
Introduction
Concrete Filled Steel Tubes (CFST)
• Concrete filled steel tubular section comprising a hollow steel tube infilled with concrete with or
without additional reinforcement or steel section.
Reducing labor
and material cost
Conventional RCC member
Composite member
Increasing the
efficiency of
construction
Outer tube as
form work
Composite member can be adapted to replace RC member so that structure can benefit over Composite member.
Introduction
Concrete Filled Steel Tubes (CFST)
• The concrete produces stronger compressive strength and deformation ability.
• Due to the restrictive function of the steel tube, the concrete is capable of
bearing the stress from three directions.
a
• The local deformation of the steel tube can be prevented due to the presence of
concrete in the tube.
c
b
The combination of both materials can help conquer their respective weakness
and bring their strong points into full play.
A composite structure system has a bright future largely depending on the
economic benefit of the system.
Concrete in a steel tube
bearing the stress from
three directions
Reduces site works, reduces material waste
and improves quality
Buildability
Why CFST ?
Great
Bearing
capacity
70% greater bearing capacity than that of
the sum of individual steel tube and
concrete
safety
Quality and
Easy to
construction
Great
Economic
benefit
Structures can be designed to meet the
highest seismic and wind load
specifications in any part of the country
Steel tube is factory made and
Rebar construction activity can be
left out.
CFST can save concrete by 70%
and steel by 50% therefore
construction cost can be reduced
by 40%.
7
Types and Cross Section of CFST
Typical cross section of CFST
CFST with additional reinforcement
Concrete encased CFST
Concrete filled double skin tubes CFDST
Design of Concrete filled steel tubes
1
Introduction
2
5
6
Materials and Component Behavior
3
Application and Design Procedure for CFST
4
Connections
Economical Benefit of CFST structure
Fire Performance and Limitation
Materials for concrete-filled steel tubes
Steel
• Various kinds of steel can be used in concrete filled steel tubular members, such as normal carbon (mild)
steel, high strength steel, high-performance fire-resistant steel, weathering steel, etc.
Hot rolled steel structural
members and members
built up of plates.
Two main families of
construction in steel
structures
Cold formed steel structural
members.
Materials for concrete-filled steel tubes
Concrete
Compressive strength
20 to 40 MPa
Workability
More workability
Load bearing
Comparatively less
Compressive strength
Above 40 MPa
Workability
Less workability
Load bearing
Higher load carrying
capacity
Schematic view of concrete placement
Pump filling
Gravity filling
• Pumping concrete from the
bottom of the column.
• Placing concrete from the top of the
column.
• The self-consolidating concrete
(SCC) and the thoroughly mixed
concrete are favored
• The depth of the fresh concrete is
usually made in steps of 300 mm to
500 mm, and should be vibrated
after being placed.
• The end of the pipe is recommended to
be placed below the concrete surface
to ensure the compactness of concrete.
Component Behaviour
• The circular cross section provides the strongest confinement to the core concrete, and the local buckling is
more likely to occur in square or rectangular cross-sections.
• The CFST with square hollow section (SHS) and rectangular hollow section (RHS) are used in construction,
for the reasons of being easier in beam-to-column connection design, high cross-sectional bending stiffness
and for aesthetic reasons
Circular and Square Hollow Section
• In CFST members, the confinement of concrete is provided by the steel tube, and the local buckling of the
steel tube is improved due to the support of the concrete core.
Schematic failure modes of hollow steel tube, concrete and CFST stub
columns
• The both inward and outward buckling is found in the steel tube, and shear failure is exhibited for the plain
concrete stub column.
• For the concrete-filled steel tube, only outward buckling is found in the tube, and the inner concrete fails in
a more ductile fashion.
Axial Compressive behaviour of CFST stub Column
The ductility of the concrete-filled steel tube is significantly enhanced, when compared to those of the steel
tube and the concrete alone.
Ref : L.-H. Han et al
Schematic failure modes of steel tube, concrete and CFST under
tension
• The CFST member in tension behaved in a
ductile manner.
• Cracks of the inner concrete are evenly
distributed along the member.
Schematic failure modes of steel tube, concrete and CFST under
bending
• The inner concrete can change the failure
modes of the outer tubes under bending, with
the wave like buckling exhibited in the
compressive area of the member.
Schematic failure modes of steel tube, concrete and CFST under
torsion
• The hollow steel tube exhibit
torsional buckling when the
torsion is applied.
• CFST in torsion, the compressive
force is developed in the inner
concrete while the tensile force in
the diagonal direction is
developed in the steel tube.
Earthquake Resistant Behavior of CFST Column
• Under the action of the earthquake, the building with a concrete-filled steel structure will not result in
brittle failure or collapse.
• It can meet the earthquake resistance demand of “being interstitial but collapse”.
Improved deformation capacity
Seismic force Q
Under the action of a moving load or earthquake, the
concrete-filled steel tube structure possesses good
CFST
RC
ductility and energy absorbing power, which is much
stronger than that of the reinforced concrete structure.
Deformation
Design of Concrete filled steel tubes
1
Introduction
2
5
6
Materials and Component Behavior
3
Application and Design Procedure for CFST
4
Connections
Economical Benefit of CFST structure
Fire Performance and Limitation
Application of CFST
 Main columns in subway stations.
 Workshops and power plant buildings.
 In high-rise and multistorey buildings as columns and beam-columns, and as beams in low-rise industrial
buildings.
 CFT members can serve as piers, bridge towers, arches and they can also used in the bridge deck system.
Applications
Electrical Transmission Line in Zhoushan China
CST Column Height – 262 m
Diameter of tube – 2300 mm
Wall Thickness – 28 mm
Compressive Strength of Concrete -50Mpa
Applications
Fleet Place house London, UK (2000)
 8 Storey high
concrete filled
external CHS
columns office block
building.
 External diameter of
CHS vary from 323.9
x 30mm to 323.9 x
16mm
China Zun (2011 – 2018)
 Height – 528m
with 108 floor
 8 concrete filled
tube mega
columns are
located at the
corners
Applications
Montevetro Apartment Block, London
Peckham Library, London
Queensberry House, London
Applications
Wushan Yangtze River Bridge, Chongqing,
China (2001 – 2005 )
Ganhaizi Bridge, Yaan, Sichuan Province, China
CFST
 The world's longest
concrete filled steel
tubular truss bridge.
 The world's highest
bridge piers of
concrete filled steel
tubular lattice.
 Height – 103m
 Arc Span – 460 m
 Length –1811m
 Height of Lattice piers – 107 m
Development of Codal Provision
 The first edition of the standard for composite concrete and circular steel tubular structures was
published by AIJ (Architectural Institute of Japan) in 1967, based on the research conducted in the
early 1960 .
 Structural Stability Research Council proposed a specification for the design of steel-concrete
composite columns in 1979, which was subsequently adopted in the 1986 AISC-LRFD Code.
Available national codes for CFST
AISC 2010
British bridge
code BS5400
Australian
bridge design
standard AS5100
Japanese code
AIJ 2001
IS 11384 -1985
Chinese code
DBJ/T13-51
Eurocode 4
(2004)
Design equations from codes for compressive load carrying capacity of CFST columns
As per AISC 360 -16
Start
Circular Section
Circular cross section
provides the strongest
confinement to the
core concrete than
rectangular section
Rectangular
Load carrying capacity
Load Carrying Capacity
Compact Section
Pp = N u= As fy+ 0.9 Ac fc
Load carrying capacity
Load Carrying Capacity
Slender Section
N u= As fcr+ 0.7 Ac fc
Elastic critical load buckling Stress
− .
01
λ 𝑟= 2.26
2
𝑓𝑐𝑟 = 0.72 𝑓𝑦
𝑡
𝑑
𝑓𝑦
𝑓𝑐𝑘
𝑁 𝑢 = 𝑃𝑝 –
𝐸 0.5
𝑓𝑦
λ 𝑟=
Load Carrying Capacity
(𝑃𝑝−𝑃𝑦)
(𝜆𝑟 −𝜆𝑦 )^2
λ𝑟 − λ𝑦
and λ 𝑦 = 3
0.15 𝐸𝑠
𝑓𝑦
and λ 𝑦=
N u= As fy+ 0.7 Ac fc
𝐸 0.5
𝑓𝑦
t = Thickness of steel tube
d = Outer diameter of the circular cross section
B = Breadth of the cross section
Pp = Plastic strength
Py = Yield strength
Elastic critical load buckling Stress
𝐵
fcr = 0.9 Es ( )-2
𝑡
0.19 𝐸𝑠
𝑓𝑦
Semi Compact
2
Pp = N u= As fy+ 0.85 Ac fc
Summation of
Load carrying
Capacity of
concrete and
steel
Py = As fy+ 0.7 Ac fc
As = Area of Steel
fy = Design yield strength of steel
Ac = Area of in-filled concrete
fc = Design compressive strength of concrete
Load Carrying Capacity
N u= Pp –
(𝑃𝑝−𝑃𝑦 )
(𝜆𝑟 −𝜆𝑦 )^2
λ𝑟 − λ𝑦
2
Es = Modulus of elasticity of steel
Is and Ia = Moments of inertia of the steel and the concrete
λ 𝑟 = Slenderness ratio of compact / semi compact
λ 𝑦 = Slenderness ratio of Semi compact / Slender
Design equations from codes for compressive load carrying capacity of CFST columns
As per BS EN 1994-1-1
Start
Circular Section
Other than Circular Section
No
Yes
Plastic resistance to compression
e = 0
N pl,Rd = Aa fyd+ 0.85 Ac fcd + As fsd
η𝑎 = η𝑎o = 0.25 (3 + 2 𝜆 )
η𝑐 = η𝑐o = 4.9 – 18.5 𝜆 + 17 𝜆 2
Yes
No
e > 0.1d
or 𝜆 > 0.5
𝜆=
η𝑎 = 1 ; η𝑐 = 0
η𝑎 = η𝑎o + (1- η𝑎o )
Plastic resistance to compression
N pl,Rd = η𝑎 Aa fyd+ [ 1 + η𝑐
𝑡 𝑓𝑦
𝑑 𝑓𝑐𝑘
] Ac fcd + As fsd
e = Eccentricity of loading
d = Outer diameter of the circular cross section
As = Area of reinforcement bar
fsd = Design yield strength of reinforcement
Aa =
fyd =
Ac =
fcd =
Relative Slenderness
10 𝑒
𝑑
Summation of Load carrying Capacity
of steel tube, concrete and
reinforcement
Npl, 𝑅𝑘
Ncr
η𝑐 = ηco + (1-
10 𝑒
𝑑
)
η𝑎 and η𝑐 are the factors to increase
the compressive strength of the
concrete due to the confinement
provided by the hollow circular
section (only significant in stocky
column)
Npl,Rk = Plastic resistance of the compression
Area of tubular section
Design yield strength of steel
(EI)eff = Effective flexural stiffness
Area of in-filled concrete
Design compressive strength of concrete Is and Ia = Moments of inertia of the steel and the concrete,
Flow chart for design of compressive resistance of the column by using simplified method
Doubly -symmetrical and uniform cross-sections over the column length
As per BS EN 1994-1-1
Start
Compressive strength
Checks for applicability
2
0.2 ≤ δ (steel contribution ratio) ≤ 0.9.
𝜙 = 0.5 1 + 𝛼 𝜆 − 0.2 + 𝜆
𝜆=
Npl, 𝑅𝑘
Ncr
Slenderness ratio 𝜆 ≤ 2.0.
Aspect ratio, 0.2 ≤
Design Compressive Resistance
𝑁𝑏, 𝑅𝑑 = 𝜒 × N pl,Rd
Ncr =
N Ed
<1
𝜒 N pl,Rd
Ley = ky L
Aa =
fyd =
Ac =
fcd =
Aa fyd
N pl,Rd
N pl,Rd = Aa fyd+ 0.85 Ac fcd + As fsd
𝑆𝑡𝑟𝑒𝑠𝑠 𝑅𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝐹𝑎𝑐𝑡𝑜𝑟
1
𝜒=
[𝜙 + 𝜙 2 − λ2 0.5 ]
Check:
δ=
Area of tubular section
Design yield strength of steel
Area of in-filled concrete
Design compressive strength of concrete
𝜋 2 𝐸𝐼
𝑒𝑓𝑓, 𝑦
2
Ley
N pl,Rk = Aa fyk+ 0.85 Ac fck+ As fsk
(EI)eff,y = Ea Iay + Ke Ecm Icy + Es Isy
As = Area of reinforcement bar
fsd = Design yield strength of reinforcement
N pl,Rd = Plastic resistance to compression
Ncr = Critical elastic buckling load
N Ed = Factored axial compression force
h
𝑏
≤5
cross-sectional area of bar reinforcement does
not exceed 0.06 Ac
For fully encased steel section, maximum thickness
of concrete cover, Cz = 0.3h, Cy = 0.4 b
Ecm = Secant modulus of elasticity of concrete
𝐸𝐼 𝑒𝑓𝑓, 𝑦 = Effective flexural stiffness
Ley = Effective length of the column
Ky = Effective length coefficient
Design Procedure for composite columns using Concrete-Filled Steel
Tubes (CFST) As per BS EN 1994-1-1
N
1
Mz,1
My,1
Mz, 2
My,2
N2
Intermediate rectangular column
section subject to compression and
bending
Inputs
Column details
= 4.8 m
ky = 1.0 (major axis direction)
Effective length coefficients
kz = 0.85 (minor axis direction)
z
Column Length, L
ta =20
h = 500
Cross-section dimensions = 500 x 300 x 20 mm thick RHS
y
Material properties
Structural steel (Hot-Rolled) = Grade S355 (fy = 355 N/mm2)
Concrete
= C35 (fck = 35 N/mm2)
Note: Reinforcement is not provided for the in-filled concrete.
Factored force demands
Factored axial compression force N Ed = 11000 kN
Modulus of elasticity
Ea = 210 kN/mm2
Secant modulus of elasticity Ecm= 33.5 kN/mm2
Design of CFST column subject to axial compression
b=300
Cross-section of CFST column
Cross-section geometry & properties of composite section
Properties of Tubular section
=
300 mm
Area of tubular section, Aa
=
30.4 x 103 mm2
Depth, h
=
500 mm
Major axis moment of inertia, Iay
=
1016 x 106 mm6
20 mm
Minor axis moment of inertia, Iaz
=
415 x 106 mm6
Thickness, ta =
z
ta =20
Properties of in-filled concrete
Ac = 260 x 460 = 119.6 x 103 mm2
Major axis moment of inertia, Icy =
260 x 4603
12
460 x 260
= 2109 x 106 mm4
460
Area of in-filled concrete,
y
h = 500
Breadth, b
260
3
Minor axis moment of inertia, Icz =
12
= 674 x 106 mm4
Design strength of material
b=300
fy
355
1.0
Cross-section of CFST column
fy = yield strength of steel
a
𝛾a = partial safety factor of steel = 1.0 (as per BS EN 1994 -1-1)
fck = Characteristic Strength of Concrete
fck 35
2
Design compressive strength of concrete, fcd = 𝛾 =
= 23.3 N/mm 𝛾 = partial safety factor of concrete (BS EN 1994 -1-1)
1.5
c
c
Design yield strength of steel, fyd = 𝛾 =
= 355 N/mm2
Design of CFST column subject to axial compression
Inputs
Column details
Effective length coefficients
ky = 1.0 (major axis direction)
kz = 0.85 (minor axis direction)
For a braced column ,0.5 ≤ K ≤ 1
IN LONGITUDINAL DIRECTION
𝛽1 = 𝛽2 =
𝐾𝑐
𝐾𝑐 + 𝐾𝑏
2 x1016 x106
Ʃ𝐾𝑐 =
= 423333.33 𝑚𝑚3
4800
𝐾𝑏,𝑖 =
𝛾 𝑖 𝐼𝑖
𝑙𝑖
γi = 1(if ends are built-in)
2x1 x 77991 x 103
Ʃ𝐾𝑏 =
= 22283.32 𝑚𝑚3
7000
β1= 0.95 β2= 1 (column is pinned at base)
Design of CFST column subject to axial compression
Wood’s Curve
Inputs
Column details
Effective length coefficients
ky = 1.0 (major axis direction)
kz = 0.85 (minor axis direction)
For a braced column ,0.5 ≤ K ≤ 1
IN TRANSVERSE DIRECTION
𝛽1 = 𝛽2 =
𝐾𝑐
𝐾𝑐 + 𝐾𝑏
2 x415 x106
Ʃ𝐾𝑐 =
= 172916.67 𝑚𝑚3
4800
𝛾𝑖 𝐼𝑖
𝐾𝑏,𝑖 =
𝑙𝑖
γi = 1(if ends are built-in)
2x1 x 1008 x 106
Ʃ𝐾𝑏 =
= 28571.42 𝑚𝑚3
7000
β1= 0.6 β2= 1 (column is pinned at base)
Design of CFST column subject to axial compression
Wood’s Curve
Relative slenderness ratio of the composite column
Relative slenderness 𝜆 =
Npl, 𝑅𝑘
Ncr
N pl,Rk = Characteristics value of plastic resistance to compression
N pl,Rk = Aa fyk+ 1.0 Ac fck + As fsk
= [(30.4 (355)) + (1.0 (119.6) (35)) + (0)] x 103 x 10-3 = 10792 + 4186 = 14978 kN
About the major axis
About the minor axis
(EI)eff,z = Ea Iaz + Ke Ecm Icz + Es Isz
(EI)eff,y = Ea Iay + Ke Ecm Icy + Es Isy
Ea Iay = 210 x 1016 x 106 x 10-6 = 213360 kN-m2
Ke Ecm Icy = 0.6 x 33.5 x 2109 x 106 x 10-6 = 42390 kN-m2
(EI)eff,y = 213360 + 42390 = 255750 kN-m2
Ncr y =
𝜆𝑦 =
𝑒𝑓𝑓 , 𝑦
Ley 2
14978
109560
π2 x 255750
=
4.8²
Ke Ecm Iz = 0.6 x 33.5 x 674 x 106 x 10-6 = 13547 kN-m2
(EI)eff,z = 94710 + 13547 = 108257 kN-m2
Lez = kz L = 0.85 x 4.8 = 4.08 m
Ley = ky L = 1.0 x 4.8 = 4.8 m
𝜋 2 𝐸𝐼
Ea Iaz = 210 x 451 x 106 x 10-6 = 94710 kN-m2
= 109560 kN
= 0.37 (Major axis slenderness ratio)
Ncr z =
𝜆𝑧 =
Design of CFST column subject to axial compression
𝜋 2 𝐸𝐼
𝑒𝑓𝑓 , 𝑧
Lez2
14978
64165
=
π2 x 108257
4.08²
= 64165 kN
= 0.48 (minor axis slenderness ratio)
Check for applicability of simplified method
z
The scope of the simplified method in BS EN 1994-1-1 is limited, as follows:
2) 0.2 ≤ δ (steel contribution ratio) ≤ 0.9. O.K.
ta =20
460
3) Slenderness ratio 𝜆 ≤ 2.0. O.K.
y
4) The cross-sectional area of bar reinforcement does not exceed 0.06 Ac. O.K.
5) The aspect ratio, 0.2 ≤
h
≤ 5, where h =column depth and b = column width. O.K.
𝑏
h 500
= 300 = 1.67
N pl,Rd = Aba fyd+
1.0 Ac fcd + As fsd
= [(30.4 (355)) + (1.0 (119.6) (23.3)) + (0)] x 103 x 10-3 = 10792 + 2786 = 13578 kN
δ=
Aa fyd 30.4 x 355 x 10−3 x 103
= 0.795
N pl,Rd=
13578
𝜆𝑦 = 0.37 < 2.0 ; 𝜆𝑧 = 0.48 < 2.0
Design of CFST column subject to axial compression
h = 500
1) The column is doubly symmetrical and uniform cross-section over its length. O.K.
260
b=300
Cross-section of CFST column
Aa = Area of tubular section
fyd = Design yield strength of steel
Ac = Area of in-filled concrete
fcd = Design compressive strength of concrete
As = Area of reinforcement bar
fsd= Design yield strength of reinforcement
N pl,Rd = Plastic resistance to compression
Compressive resistance of the column
𝜙 = 0.5 1 + 𝛼 𝜆 − 0.2 + 𝜆2 = 0.5 [ 1+ 0.21 (0.48 -0.2 ) + 0.482] = 0.646
𝜒=
1
[𝜙+ 𝜙2 − λ2 0.5 ]
=
1
0.646 + 0.6462 −0.482 0.5
= 0.93 ≤ 1
χ N pl,Rd = 0.93 x 13578 = 12628 kN > NEd = 11000 kN
χ = stress reduction factor for column buckling
using buckling curve a for concrete filled RHS
OK
Resistance of member in axial compression
N Ed
11000
=
= 0.87 < 1
χ N pl,Rd 12628
Unity factor for buckling in pure compression is 0.87
Design is satisfactory for pure axial compression.
Design of CFST column subject to axial compression
COMBINED AXIAL LOAD AND BENDING
• Structural members are subject to combined compressive axial load and bending is referred to as beamcolumn.
• The bending may result from eccentric loading, transverse loads or applied moments.
• In structures, beams are usually supported by columns through framing angles or brackets on the sides of
the columns. Reaction from the beams are referred as eccentric loading which produce bending moment.
e p
e P
Subject to
eccentric load
p
p
M1
P
Subject to
transverse load
M2
P
Subject to end
moment
Simplified Interaction Curve Cl 6.7.3.2
• Point A defines the plastic compression resistance.
• Point B corresponds to the plastic bending resistance.
• Point C, the plastic resistances of the cross-section in
compression and in bending.
• Point D, the plastic neutral axis coincides with the
centroidal axis of the cross section.
fcdffcd
fcdcd
hhn
n
hn
As per BS EN 1994-1-1
-
--
fydffyd
fydyd
-++ +
fsdffsdfsdsd
M
M
M
===
pl,Rd
M
M
0M
BCD
pl,Rd
max,Rd
A
==
0=NNpm,Rd
N
NAN=
N
BNC
D pl,Rd
C/ 2
Npm, Rd = plastic resistance of concrete in compression
Design of CFST column subject to combined axial compression and bending
Simplified Interaction Curve Cl 6.7.3.2
The neutral axes (hn) for points B and C can be determined
from the difference in stresses at points B and C.
-
NA = Npl,Rd
fyd
fcd
As per BS EN 1994-1-1
MB = Mpl,Rd
hn
+
hn
hn
NB = 0
fyd
fsd
MC = Mpl,Rd
-
-
NC = Npm,Rd
+
fyd
fcd
Point D occurs the
neutral axis lies at
mid-depth.
fsd
-
-
fcd
Point C occurs when the
neutral axis hn below the
plastic centroid
fsd
MA = 0
hn the neutral axis outside the section, for
this pure axial compression condition
(Point A)
hn is the distance of the neutral axis above
the centroid, for this pure flexure
condition (Point B)
fyd
fcd
-
+
Design of CFST column subject to combined axial compression and bending
fsd
MD = Mmax,Rd
ND = NC / 2
Inputs
NEd
Column details
z
My,max, Ed
= 4.8 m
ky = 1.0 (major axis direction)
Effective length coefficients
t =20
kz = 0.85 (minor axis direction) a
Cross-section dimensions = 500 x 300 x 20 mm thick RHS
h = 500
Column Length, L
y
Material properties
Structural steel (Hot-Rolled) = Grade S355 (fy = 355 N/mm2)
Concrete
= C35 (fck = 35 N/mm2)
Note: Reinforcement is not provided for the in-filled concrete.
Factored force demands
b=300
Cross-section of CFST column
r x My,max, Ed
Factored axial compression force N Ed = 11000 kN
M y,max, Ed =75 kN-m [about y-y (major) axis]
NEd
Factored bending moment
Uniaxial bending
Ea = 210 kN/mm2
M z,max, Ed = 0 kN-m [about z-z (minor) axis] Modulu of elasticity
End moment ratio, r
= - 0.5
Secant modulus of elasticity Ecm = 33.5 kN/mm2
Design of CFST column subject to combined axial compression and bending
Inputs
NEd
Considering, End moment ratio r = - 0.5
• r is either called as end moment ratio or bending coefficient
My,max, Ed
• To take into account for non-uniformity, a bending coefficient was introduced by Kirby and
Nethercot (1979).
• The ratio of end moments 𝑀1 ∕ 𝑀2 is positive when 𝑀1 and 𝑀2 have the same sign (single
curvature bending), and negative when they are of opposite signs (reverse curvature bending).
r x My,max, Ed
NEd
Design of CFST column subject to combined axial compression and bending
Bending resistance of the column
Calculation of plastic section modulus y-y axis
260 x 4602
Plastic Section Modulus of Concrete , Zpc =
= 13.7 x 106 mm3
4
Plastic Section Modulus of steel, Zpa =
b x h2
4
- Zpc =
300 x 5002
4
- 13.7 x 106 = 4950 x 103 mm3
Depth of plastic neutral axis
2 bhnfcd + 4thn 2fyd − fcd = Ac fcd − Asn (2 fsd − fcd)
hn =
𝐴𝑐 fcd − Asn (2 fs𝑑− fcd)
2 𝑏 fcd + 4𝑡 2fyd − fcd
119.6 × 103 × 23.3 − 0
= 2 ×300 ×23.3+4 ×20 ×(2 ×355 −23.3 ) = 40.4 mm
Zpcn = (b - 2t) x hn2 - Zpsn = (300 -2 x 20) x 40.42 – 0 = 424 x 103 mm3
Zpan = b x hn2 - Zpcn - Zpsn = 300 x 40.42 - 424 x 103 = 65.6 x 103 mm3
fyd
fcd
hn
hn
-
+
fsd
MC = Mpl,Rd
NC = Npm,Rd
Zpcn = Plastic Section Modulus of Concrete within region of hn,
Zpan = Plastic Section Modulus of steel within region of hn
Design of CFST column subject to combined axial compression and bending
Establish the key points on the axial force – moment interaction diagram
Point A - Plastic compression resistance.
N pl,Rd = Aa fyd+ 1.0 Ac fcd + As fsd = [(30.4 (355)) + (1.0 (119.6) (23.3)) + (0)] x 103 x 10-3 = 10792 + 2786 = 13578 kN
Point B - Plastic bending resistance.
Mpl Rd = Mmax Rd – Mpn Rd
Mpl Rd = fyd (Zpa – Zpan) + 0.5 fcd (Zpc – Zpcn) + fsd (Zps – Zpsn)
= [ 355 x (4950 x 103 - 65.6 x 103)+ 0.5 x 23.3 x (13.7 x 106 – 424 x 103)] x 10-6 = 1734 + 155 = 1889 kN-m
Point D
Mmax,Rd = fyd Zpa + 0.5 fcd Zpc = [ 355 x 4950 x 103 + 0.5 x 23.3 x 13.7 x 106 ] x 10-6 = 1917 kN-m
Npm, Rd / 2 = fcd Ac = (23.3 x 119600 x 10-3 ) / 2 = 1393 kN
Npm, Rd = plastic resistance of concrete in compression
Point C - Plastic resistance to compression and bending
Npm, Rd = fcd Ac = 23.3 x 119600 x 10-3 = 2786 kN
MC = Mpl Rd
Design of CFST column subject to combined axial compression and bending
Establish the key points on the axial force – moment interaction diagram
Point E
Δ ME = (b h2E – ZcE) fyd + 0.5 ZcE fcd
hE = 0.25h + 0.5hn = 0.25 (500) + 0.5 (40.4) = 145.2 mm
ZcE = (b – 2t )
fyd
fcd
hn
h2
E
-
-
hn
hE
+
3 mm3
= (300 – 2 x 20) x 145.22 = 5481Plastic
x 10resistance
to compression
ZaE = bhE2 - ZcE
3
3
3
= 300 x 145.22 – 5481
NE x 10 = 844 x 10 mm
Plastic resistances of the cross-section in compression and in bending
ΔME = [ (300 x
145.22 –
5481 x
103)
x 355 + 0.5 x 5481 x 103 x 23.3 ] x 10-6
Plastic neutral axis coincides with the centroidal axis of the cross section.
= 299.6 + 63.9 = 363.5 kN-m
ME = Mmax ,Rd – ΔME = 1917 – 363.5
ME = 1553.5 kN-m
NE = 4 hE t fyd + [0.5 Ac + (b – 2t) hE ] fcd
Plastic bending resistance
= { 4 x 145.2 x 20 x 355 + [0.5 x 119600 + (300 – 2 x 20) x 145.2] x 23.3 } x 10-3
= 4124 + 2273 = 6397 kN
Design of CFST column subject to combined axial compression and bending
fsd
MD = Mmax,Rd
ND = NC / 2
Simplified interaction curve of the composite cross section subject to uniaxial bending
𝑥 −𝑥
x = (𝑦2 −𝑦1 ) (y – y1) + x1
2
Plastic resistance to compression
MEd =
1
𝑀𝐸 − 0
(𝑁𝐸 −𝑁𝑝𝑙 𝑅𝑑 )
( NEd – Npl Rd) + 0
1553.5− 0
MEd = (6397 −13578) (11000 – 13578) + 0
MEd = 558 kN-m
Plastic resistances of the cross-section in
MEd = μd Mpl,Rd
compression and in bending
MEd
Plastic neutral axis coincides with
μdthe
= centroidal
Mpl,Rd =
axis of the cross section.
558
1889 = 0.29 < 1
Plastic bending resistance
Design of CFST column subject to combined axial compression and bending
Amplification of compressive force of the column using second order elastic analysis
About the major axis
Ea Ia = 210 x 1016 x 106 x 10-6 = 213360 kN-m2
KeII Ecm Ic = 0.5 x 33.5 x 2109 x 106 x 10-6 = 35325 kN-m2
(EI)eff,II = Ko (Ea Ia + Es Is + KeII Ecm Ic) = 0.9 x (213360 + 0 + 35325) = 223816 kN-m2
Ncr,eff =
𝜋2 𝐸𝐼
𝑒𝑓𝑓,𝐼𝐼
Le2
L
ea = 300 =
4800
300
=
π2 x 223816
=
4.8²
95876 kN
= 16 mm
Mi = NEd ea = 11000 x 16 x 10-3 = 176 kN-m
K=
β
NEd
1−
Ncr,eff
NEd 11000
Second order effects should be considered
Ncr,eff = 95876 = 0.11 > 0.1
=
1
11000
1−95876
Cl 6.7.3.4
= 1.14
MEd,II,I = 1.14 x 176 = 198.9 kNm
Design of CFST column subject to combined axial compression and bending
Check for compressive resistance for second order effects
According to the simplified interaction curve, 0.9 x MEd,II,I = 179.0 kNm
N − 13578 11000 − 13578
N − Npl,Rd NEd− Npl,Rd
=
= −4.622  𝑁 = 13578 − 4.622 M
=

M
558
M
MEd
𝑁𝑅𝑑 = 13578 − 4.622 179 = 12750𝑘𝑁 > 11000𝑘𝑁 (= 𝑁𝐸𝑑 )
Design is satisfactory for axial compression due to second order effects.
Amplification of bending moment in the column using second order elastic analysis
Assume bending ratio, r = -0.5
β = 0.66 + 0.44 r = 0.66 + 0.44 (-0.5) = 0.44
𝑘𝑚 =
β
1 − NEd
Ncr,y,eff
=
0.44
1 −11000
95876
= 0.497
MEd,II,m = km My,Ed = 0.497 x 75 = 37.275 kN-m
Design of CFST column subject to combined axial compression and bending
Check for combined compression and bending resistance for second order effects
Total moment at the mid-height, MEd,max = 198.9 + 37.27
= 236.2 kN-m > My,Ed = 75 kN-m
• Moment due to second-order effects is higher than first-order moment.
• Hence, it is essential to check for bending resistance against second-order moments.
μ = moment resistance ratio obtained from the N-M interaction curve
MEd
Mpl N Rd
=μ
MEd
α Mpl Rd
d M
MEd
Mpl N Rd
=
≤1
236.2
=
0.29 x 0.9 x 1889
0.4 < 1
Cl:6.7.3.6
Design is satisfactory for combined compression and bending.
Summary
CFST of 500 x 300 RHS is acceptable for column of 4.8m length
Design of CFST column subject to combined axial compression and bending
Inputs
NEd
Column details
z
My,max, Ed
= 4.8 m
ky = 1.0 (major axis direction)
Effective length coefficients
t =20
kz = 1.0 (minor axis direction) a
Cross-section dimensions = 500 x 300 x 20 mm thick RHS
Mz,max, Ed
h = 500
Column Length, L
y
Material properties
Structural steel (Hot-Rolled) = Grade S355 (fy = 355 N/mm2)
Concrete
= C35 (fck = 35 N/mm2)
Note: Reinforcement is not provided for the in-filled concrete.
Factored force demands
b=300
Cross-section of CFST column
Factored axial compression force N Ed = 11000 kN
M y,max, Ed = 75 kN-m [about y-y (major) axis]
NEd
Factored bending moment
Ea = 210 kN/mm2
M z,max, Ed = 10 kN-m [about z-z (minor) axis] Modulu of elasticity
End moment ratio, r
= - 0.5
Secant modulus of elasticity Ecm = 33.5 kN/mm2
Design of CFST column subject to combined axial compression and bending
Bending resistance of the column
Calculation of plastic section modulus Z-Z axis
2602 x 460
Plastic Section Modulus of Concrete , Zpc =
= 7.77 x 106 mm3
4
Plastic Section Modulus of steel, Zpa =
b x h2
4
- Zpc =
3002 x 500
4
- 7.77 x 106 = 3480 x 103 mm3
Depth of plastic neutral axis
2 hhnfcd + 4thn 2fyd − fcd = Ac fcd − Asn (2 fsd − fcd)
hn =
𝐴𝑐 fcd − Asn (2 fs𝑑− fcd)
2 ℎ fcd + 4𝑡 2fyd − fcd
=
119.6 × 103 × 23.3 − 0
2 × 500 ×23.3+4 ×20 ×(2 ×355 −23.3 )
= 35.6 mm
Zpcn = (h - 2t) x hn2 - Zpsn = (500 -2 x 20) x 35.62 – 0 = 583 x 103 mm3
Zpan = h x hn2 - Zpcn - Zpsn = 500 x 35.62 - 583 x 103 = 50.6 x 103 mm3
fyd
fcd
hn
hn
-
+
fsd
MC = Mpl,Rd
NC = Npm,Rd
Zpcn = Plastic Section Modulus of Concrete within region of hn,
Zpan = Plastic Section Modulus of steel within region of hn
Design of CFST column subject to combined axial compression and bending
Establish the key points on the axial force – moment interaction diagram
Point A - Plastic compression resistance.
N pl,Rd = Aa fyd+ 1.0 Ac fcd + As fsd = [(30.4 (355)) + (1.0 (119.6) (23.3)) + (0)] x 103 x 10-3 = 10792 + 2786 = 13578 kN
Point B - Plastic bending resistance.
Mpl Rd = Mmax Rd – Mpn Rd
Mpl Rd = fyd (Zpa – Zpan) + 0.5 fcd (Zpc – Zpcn) + fsd (Zps – Zpsn)
= [ 355 x (3480 x 103 - 50.6 x 103)+ 0.5 x 23.3 x (7.77 x 106 – 583 x 103)] x 10-6 = 1217 + 84= 1301 kN-m
Point D
Mmax,Rd = fyd Zpa + 0.5 fcd Zpc = [ 355 x 3480 x 103 + 0.5 x 23.3 x 7.77 x 106 ] x 10-6 = 1325 kN-m
Npm, Rd / 2 = fcd Ac = (23.3 x 119600 x 10-3 ) / 2 = 1393 kN
Npm, Rd = plastic resistance of concrete in compression
Point C - Plastic resistance to compression and bending
Npm, Rd = fcd Ac = 23.3 x 119600 x 10-3 = 2786 kN
MC = Mpl Rd
Design of CFST column subject to combined axial compression and bending
Establish the key points on the axial force – moment interaction diagram
Point E
Δ ME = (b h2E – ZcE) fyd + 0.5 ZcE fcd
hE = 0.25b + 0.5hn = 0.25 (300) + 0.5 (35.6) = 92.8 mm
ZcE = (h – 2t )
-
-
hn
hn
hE2
fyd
fcd
hE
Plastic resistance to compression
+
= (500 – 2 x 20) x 92.82 = 3961 x 103 mm3
ZaE = hhE2 - ZcE
= 500 NxE 92.82 – 3961 x 103 = 345 x 103 mm3
ΔME = [ (500 x
92.82 –
Plastic resistances of the cross-section in compression and in bending
3961 x
103)
x 355 + 0.5 x 3961 x 103 x 23.3 ] x 10-6
= 122.5 + 46.1 = 168.6kN-m
Plastic neutral axis coincides with the centroidal axis of the cross section.
ME = Mmax ,Rd – ΔME = 1325 – 168.6 = 1156.4 kN-m
NE = 4 hE t fyd + [0.5 Ac M
+ (h – 2t) hE ] fcd
E
= { 4 x 92.8 x 20 x 355 + [0.5 x 119600Plastic
+ (500
2 x 20) x 92.8] x 23.3 } x 10-3
bending –
resistance
= 2635 + 1054 = 3659 kN
Design of CFST column subject to combined axial compression and bending
fsd
MD = Mmax,Rd
ND = NC / 2
Simplified interaction curve of the composite cross section subject to uniaxial bending
A
Npl,Rd = 13578
x=
Plastic resistance to compression
N (kN)
NEd
𝑥2 −𝑥1
(𝑦2 −𝑦1)
(y – y1) + x1
𝑀 −0
μd Mpl Rd
E
NE
𝐸
MEd = (𝑁𝐸 −𝑁𝑝𝑙
(1156 , 3659 )
MEd =
Plastic resistances of the crosssection in compression and in
bending
(1301
Plastic neutral axis coincides
the, 2786 )
C with
centroidal axis of the cross section.
D (1325 , 1393 )
Npm,Rd
Npm,Rd
2
MEd
ME
Plastic bending resistance
M (kNm)
B
Mpl,Rd = 1301
)
𝑅𝑑
( NEd – Npl Rd) + 0
1156.4− 0
(3659 −13578)
(11000 – 13578) + 0
MEd = 300 kN-m
MEd = μd Mpl,Rd
M
300
μd = M Ed = 1301 = 0.23 < 1
pl,Rd
Design of CFST column subject to combined axial compression and bending
Amplification of compressive force of the column using second order elastic analysis
About the major axis
Ea Ia = 210 x 415 x 106 x 10-6 = 87150 kN-m2
KeII Ecm Ic = 0.5 x 33.5 x 674 x 106 x 10-6 = 11290 kN-m2
(EI)eff,II = Ko (Ea Ia + Es Is + KeII Ecm Ic) = 0.9 x (87150+ 0 + 11290) = 88596 kN-m2
Ncr,eff =
𝜋2 𝐸𝐼
𝑒𝑓𝑓 ,𝐼𝐼
Le2
L
ea = 300 =
4800
300
=
π2 𝑥 88596
=
4.8²
3795 kN
= 16 mm
Mi = NEd ea = 11000 x 16 x 10-3 = 176 kN-m
K=
β
NEd
1−
Ncr,eff
NEd 11000
Second order effects should be considered
Ncr,eff = 3795 = 2.89 > 0.1
=
1
11000
1− 3795
Cl 6.7.3.4
= 0.52 ≤ 1
MEd,II,I = 1 x 176 = 176 kNm
Design of CFST column subject to combined axial compression and bending
Check for compressive resistance for second order effects
According to the simplified interaction curve, 0.9 x MEd,II,I = 158.4 kNm
N − 13578 11000 − 13578
N − Npl,Rd NEd− Npl,Rd
=
= −8.6 
=

M
300
M
MEd
𝑁 = 13578 − 8. 6 M
𝑁𝑅𝑑 = 13578 − 8.6 158.4 = 12215𝑘𝑁 > 11000𝑘𝑁 (= 𝑁𝐸𝑑 )
Design is satisfactory for axial compression due to second order effects.
Amplification of bending moment in the column using second order elastic analysis
Assume bending ratio, r = -0.5
β = 0.66 + 0.44 r = 0.66 + 0.44 (-0.5) = 0.44
𝑘𝑚 =
β
1 − NEd
Ncr,y,eff
=
0.44
1 −11000
95876
= 0.497
MEd,II,m = km M,Ed = 0.497 x 10= 4.97 kN-m
Design of CFST column subject to combined axial compression and bending
Check for combined compression and bending resistance for second order effects
Total moment at the mid-height, MEd,max = 176 + 4.97
= 180.97 kN-m > Mz,Ed = 10 kN-m
• Moment due to second-order effects is higher than first-order moment.
• Hence, it is essential to check for bending resistance against second-order moments.
μ = moment resistance ratio obtained from the N-M interaction curve
MEd
Mpl N Rd
=
MEd
μd αM Mpl Rd
MEd,max
=
μ M pl,Rd
≤1
180.97
=
0.23 x 0.9 x 1301
0.5 < 0.9
Cl:6.7.3.6
Design is satisfactory for combined compression and bending.
Design of CFST column subject to combined axial compression and bending
Check for combined compression and biaxial bending
For the combined compression and biaxial bending following condition should be satisfied for the stability check.
My Ed
μd Mpl y Rd
M ZEd
Mpl z Rd
d
+μ
≤1
MyEd = Total moment including second order effect in y -y plane
Mz Ed = Total moment including second order effect in z -z plane
Mpl y Rd = Plastic bending resistance in y -y plane
Mpl z Rd = Plastic bending resistance in z –z plane
236.2
0.29 x 1889
+
180.97
0.24 x 1301
= 0.9 ≤ 1
Design is satisfactory for combined compression and biaxial bending.
Summary
CFST of 500 x 300 RHS is acceptable for column of 4.8m length
Design of CFST column subject to combined axial compression and bending
Design of Concrete filled steel tubes
1
Introduction
2
5
6
Materials and Component Behavior
3
Application and Design Procedure for CFST
4
Connections
Economical Benefit of CFST structure
Fire Performance and Limitation
Connections
The connections can be classified according to the behavior in terms of moment versus rotation relationship
Simple connection:
• It able to transmit internal forces but unable to transmit bending
moments.
Semi-rigid connection:
• The connection can transfer internal forces and moments.
• Rotation between connected members cannot be neglected.
Rigid connection:
• It can withstand moment by maintaining the perfect angle between
the two members.
Connections
Simple Connection
Simple connections between steel beams to concrete
filled steel tubular column with fin plates
Simple beam-column connection with steel corbel
Connections
Simple Connection
CFST column to hollow section beam
connection
Stiffened seat connection
Ref: Kurobane et al. 2004
Connections
Simple Connection
Shear plate connection
T-connection
“Through-plate” connection
Ref: Kurobane et al. 2004
Connections
Moment Connection
Steel beam to column moment connection with
external diaphragm plates
Detailing for edge and corner columns with external
diaphragm plates
Connections
Rigid connection
Connection with RC ring
Connection with variable width RC beam
Connection with anchor stiffeners
Ref: Kurobane et al. 2004
Design of Concrete filled steel tubes
1
Introduction
2
5
6
Materials and Component Behavior
3
Application and Design Procedure for CFST
4
Connections
Economical Benefit of CFST structure
Fire Performance and Limitation
Economical Benefits of CFST Structure
Analysis on building use Function
Building type
: Hospital building in Tianjin - China as a reference building.
Number of stories :
15
Floor area
:
17,385 m²
Structure type
:
Reinforced concrete structure
Column Type
Steel Consumed
Cement Consumed
Concrete Consumed
Reinforced concrete
column
100 %
100 %
100 %
Concrete filled steel
tube column
99.4 %
57 %
57 %
Ref: Jinming Liu Et.al
Cement consumption in CFST Structure is 57% of RC structure
Economical Benefits of CFST Structure
Analysis on building use Function
Column Section
Average RC column Section
Average CFST column Section
Centre and Side Column Section
0.49 m2
0.28 m2
Column Weight of CFST is 59% of RC
Ref: Jinming Liu Et.al
Weight of CFST structure is 40 % lighter than RCC building
(construction cost would be saved)
The cost of formwork and manpower of erecting and stripping of formwork can be saved
If steel tubes are installed instead of erecting the formwork, the savings in cost is 85%
Economical Benefits of CFST Structure
Case Study
Ref : Ketan Patel and Sonal Thakkar
Plan of building :
38.4m X 32m
Typical storey height :
3m
20 Storey building
Type of building
Column Size
Design Capacity (kN)
Steel Building
ISWB 600
10250
Less than 19.1 % than
CFST building
RCC building
GF to 10th floor D=900
11th to 20th floor D = 700
11402
Less than 27.3 % than
CFST building
CFST building
D = 800 and t = 9mm
14094
Economical Benefits of CFST Structure
Case Study
Ref : Ketan Patel and Sonal Thakkar
Plan of building :
38.4m X 32m
Typical storey height :
3m
30 storey building
Type of building
Column Size
Design Capacity (kN)
Steel Building
D = 1000 and t = 20mm
19032
RCC building
GF to 10th floor D=1100
11th to 20th floor D = 900
20th to 30th floor = 700
Less than 11.8 % than
CFST building
16675
Less than 22.8 % than
CFST building
CFST building
D = 1000 and t = 11mm
21558
Instead of increase the size of column in RC structure, will reduce the size of column in high rise building by
adopting Composite columns which provides good strength and stability.
CFST Structure for Sustainable Construction
There is need to build all living and working spaces in minimum use of scarce resources like energy and water.
By CFST construction,
 Environmental burden can be reduced by omitting the formwork.
 Using concrete made from recycled aggregates
 Steel which is a major constituent material of CFST, itself being
almost totally recycled material is a green material
Design of Concrete filled steel tubes
1
Introduction
2
5
6
Materials and Component Behavior
3
Application and Design Procedure for CFST
4
Connections
Economical Benefit of CFST structure
Fire Performance and Limitation
Fire performance
• The fire resistance of unprotected hollow steel tubular columns in high-rise buildings is normally found to be
less than 30 minutes.
• For CFST columns, the filled concrete can significantly increase the fire resistance. Because the heat is
absorbed by the core concrete, the temperature in the steel tube increases much slower than that of the bare
hollow steel tubes.
• The outer tube provides a confinement to the core concrete during the fire exposure, the spalling of the core
concrete can be prevented.
Behaviour of CFST in Fire
• The steel tube expands faster than the concrete core, in such a way that the steel section carries higher load
than the concrete core.
• The heat gets transferred from the steel to concrete core which has lower thermal conductivity.
• After 20 minutes the strength of steel tube
starts to decrease due to its elevated
temperature.
• As the temperature advances through the
concrete core its strength decreases until
it eventually fails either due to buckling
or compression
Ref : Espinos(2012)
• Higher ratio of concrete load capacity
to overall column strength.
Material
strength
Column
Size
• The concrete core of a larger sized
column will support a greater
proportion of the total load than a
smaller one.
• Short column fails by material degradation
and long column fails by instability.
Effective
Length
Parameters affecting
fire performance
Applied
load
• The lower the level of the applied
load, the lower the stresses produced,
and the longer the period of fire
stability of the column.
75
Fire performance
• Fire proof coating are used to protect steel and steel-concrete composite structures and they can
form an effective fire insulation layer.
Non Reactive type – Fire Coating
Non-reactive type
Sprays
Cement
mortar
Concrete
encasement
Reactive type
Thin film
Intumescent
fire coating
(IFC)
• It have relatively constant physical and thermal properties and
no chemical reaction would occur when heated.
• It have several defects such as high thickness, heavy weight,
poor durability and difficult maintenance.
Reactiveencasement
type – Fire Coating Sprays and Cement Mortar
Concrete
• IFC is composed of constituents including resin, dehydrating
catalyzer, carbonizing agent, vesicant, fillers and additives.
• Light weight, high performance,
construction,
aesthetic
Thin filmsimple
Intumescent
fire coating
appearance, easy to maintenance.
(IFC)
Changing process of intumescent fire coating in fire
Before testing
After testing
Ref : Qian-Yi Song et al.(2018)
 Chemical reaction would occur when IFC is heated, and large amount of heat is absorbed by smoke and fume
Inproduced
practices,inthe
fire proofMeanwhile,
coatings canthe
be intumescent
used to protect
steel and
composite
structural
members
thenon-reactive
chemical reaction.
material
would
be charred,
significantly
satisfying
fire fluffy
resistance
rating
up which
to 180 has
minrelatively
in standard
fire.heat conductivity.
swell and gradually change
to a thick
foamed
layer
small
Limitations
• Installation difficulties and corrosion of steel plate causes de-bonding of members.
• If the steel tube dilates more from its actual position than that of concrete. It results in lack of confining
pressure causes reduction in the strength and stiffness of column.
• Design guidelines for CFST (IS 11384-1985) in India should be improved, as far as now no proper design
guidelines are available in India
Thank you