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METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania
RESEARCH IN THE FIELD OF
CONCRETE-FILLED STEEL TUBULAR
STRUCTURAL MEMBERS IN VGTU
Audronis Kazimieras Kvedaras, Antanas Šapalas
Vilnius Gediminas Technical University, Dept. of Steel and Timber
Structures, Lithuania
INTRODUCTION
The concrete-filled steel tubes now obtain wider application in construction practice.
a) b) c) d)
Fig. 1: Composite sections with annular (a, c) and solid (b, d) concrete cores
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
The composite steel and concrete members of annular cross-section are quite efficient tubular structures. Structurally and economically rational for columns, beams and beam-columns are especially such members with hollow concrete cores.
However, the structural performance and reliability of this new type of tubular members are investigated not well enough in spite of some recommendations existing in approved EN 1994-1-1 (Euro code 4).
The main task of this report is presenting the methodical formats for behaviour verifications of tubular composite steel and concrete members of buildings assisted by
testing and based on semi-probabilistic approaches.
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
1. Performance of composite cross-section
It is necessary simply to pay your attention that on the
behaviour of stubby compressed members the resistance
checking of columns, beams and beam-columns is based
by simplified methods.
The interaction between the concrete filled tubes of
circular cross-section components’ interfaces occurring
under compression is calling an increase in strength of
both components and the robustness of the whole
composite member against the member resistance
defined by superposition criterion. That is proved by
numerous test data and results of plastic analysis.
But according to EN1994-1-1 account may be taken of
increase in strength of concrete caused by confinement
provided that the relative slenderness λ does not exceed
0.5 and e/d < 0,1. By the General method of design even
so full composite action up to failure may be assumed
between the steel and concrete components of a member.
For definition the resistance of members in combined
compression and uniaxial bending the value of plastic
bending resistance Mpl,Rd it is necessary to know. There
are no recommendations in EN1994-1-1 how to define
the value of this moment in spite of some peculiarity of
such procedure because of circular shape of cross-section.
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
Therefore, appealing to the above description of the General
method of design about possibility to assume full
composite action up to failure between the steel and
concrete components of the concrete filled tubular
member we carried out the experimental and theoretical
investigation of behaviour of concrete filled tubular
columns, beams and beam-columns.
An approach of above mentioned interaction description
should be based on the postulates the theory of plasticity
of the small elasto-plastic strains and shall to take into
account different values of Poisson’s ratio of components
and an appropriate definition of the strain criteria.
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
x x
y
y
zz
x
x xx
N N N
N N N
d
d d
tt
= +
( )a ( )c ( )d( )b
a c
e
ci ce
ca c
z
z
z
z
Fig.2: Diagrammatic sketch of concentric actions on the
hollow composite member (a), steel tube (b) and hollow
concrete core (c); shape and dimensions of hollow cross-
section (d)
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
zyxim
Ex
εεσ v,
34
From the generalized Hooke’s law, the normal ultimate
stresses of both media have to be expressed so:
where: is the secant modulus of elasticity;
and
are the values of longitudinal and tangential strains,
respectively. Because of the assumed biaxial stress
state for both components the ultimate normal
stresses from Eq. represent the modified values of
components’ strengths.
imE
22, 35.0 ziyzyx axzz E 5.0
and,
where and (1.640 – for solid core) are the
mean value of constraining factors which characterize the
interaction effect of components;
, values of steel yield and concrete specified
compressive strength (Figs 3 and 4)
The rationality of tubular composite members under
compression demonstrates the efficiency factor
the mean value of which in our
tests was equal to 1.17.
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
yf
aaxaf ησ
cf
ccxcf ησ
0741= .a 320.1cη
'
cfyf
ccaytref AfAfRK
Fig. 3. Stress-strain relationship of steel under the unblocked (1) and the blocked (2) strains (in scale of generalized stresses and strains)
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
1f
f2
ai
y
a y
ai
0
Fig. 4. Stress-strain relationship of concrete under unblocked (1) and
blocked by typical (2) and bi-linear (3) strain laws (in scale of
generalized stresses and strains)
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
1
f
f
0 0.5
c c
c
ciciuciu
2
3
ci
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
Because of the assumed equality of biaxial stress state in
steel shell and in concrete core, the ultimate value of the
generalized strain will be the same for both
materials. This generalized strain for concrete-filled steel
tube expresses the deformation criterion corresponding
to the yield strength of shell steel. When the thickness
of a steel tube is less than its ultimate minimum critical
steel strength has to be used instead of .
The second deformation criterion may correspond to the
shear yield strength of shell steel (that might agree with
the ultimate tensile strength ) but at initial design stages
this criterion is unusfull because of not standardised
value of limit of proportionality of steel.
ayiy Ef5.1
crf yf
yf
uf
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
COMPARISON OFCALCULATIONS AND EXPERIMENTAL DATA
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
2. RESISTANCE OF COLUMNS
It was taken recommendation of euro code EC4 that for
simplification for members in axial compression, the
design value of the normal force should satisfy:
Where
is the reduction factor for the relevant buckling mode
in terms of the relevant relative slenderness .
EdN
0,1Rdpl,
Ed N
N
sdscdccydaaRdpl, fAfAfAN
3. RESISTANCE OF BEAM-COLUMNS
If to take recommendation of euro code EC4 the following
expression based on the interaction curve should be
satisfied:
That means necessity to find the value of bending moment
for quite specific circular cross-section. Earlier in BS
5400 and in drafts of EC4 the expressions were
recommended, but in approved version of EC4 no more
similar expressions exist.
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
M
Rdpl,d
Ed
RdN,pl,
Ed
M
M
M
M
Therefore, we investigated this problem and discovered that
bending moment of circular concrete-filled steel tube may
be expressed so:
Mpl,Rd=Npl,a,Rd eu , where Npl,a,Rd=ηa Aafyd
The corresponding relationship is used to find a magnitude
of this design eccentricity eu:
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
Fig. 5. Dimensions of cross-section of hollow concrete filled circular
steel tubular beam and position of plastic neutral axis
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
Polythene concrete-filled tubes after test (l = 600 mm) and before test (l=1200 mm)
Polythene concrete-filled tubular beam-columns (l =1800 mm and l = 2400 mm)
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
Polythene concrete-filled tubes after a simple bending test (l = 1800 mm)
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
FULL SCALE TESTING OF LATICE COLUMNS
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
APPLICATION (1)
• 1250
D=340
CROSS SECTION OF COLUMN
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
APPLICATION (2)
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
APPLICATION (3)
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas
1
2
3
APPLICATION (4)
METNET Seminar, March 13, 2013, VGTU, Vilnius, Lithuania // A.K.Kvedaras & A.Šapalas