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CONTENTS
LIST OF FIGURES 3
LIST OF TABLES 4
LIST OF NOTATIONS 5
1. INTRODUCTION 7
1.1. Types of CFST Members 8
1.2. Advantages of CFST Over RCC 10
2. BEHAVIOUR OF CFST ELEMENTS 11
3. CFST ARCH BRIDGES 15
4. STRUCTURAL BEHAVIOUR OF CFST ARCHES 16
4.1. Uniform Compression 16
4.1.1. Elastic buckling 16
4.1.2. Effect of residual stresses and 17
initial geometric imperfections
4.2. Combined Bending And Compression 17
4.2.1. Internal actions in elastic CFST arches 17
4.2.2. Elastic plastic behaviour 19
4.2.3. Effects of initial geometric imperfections 19
4.2.4. Full plastic moment of CFST arch section 20
4.2.5. In-plane strength of CFST arches 20
4.3. Modulus Of Elasticity And Strain 20
1
4.4. Dynamic Behaviour 23
4.4.1. Dynamic analysis of a half-through CFST 23
arch bridge
4.4.2. Study on dynamic properties of a CFST arch bridge 24
constructed in China
5. CONCLUDING REMARKS 25
REFERENCES 26
2
LIST OF FIGURES
Fig No. Title Page No.
Fig 1 Various Cross-sections of Solid and Hollow CFST 8
Composite Columns
Fig 2 Cross-section of CFST Element 9
Fig 3 Stress Condition in Steel Tube and Concrete Core at 12
Different Stages of Loading
Fig 4 Stress- Strain Relationship of a CFST Element 13
Fig 5 Distribution of Stresses in Hollow CFST Element 14
Fig 6 Stress Distribution in Concrete Core of CFST 14
Fig 7 Pictures of (a) Yangtze River Bridge at Wuxia; 15
(b) Shin-Saikai Bridge
Fig 8 CFST Arches under Uniform Radial Load 17
Fig 9 Arch and Loading 18
Fig 10 Diagrams (a) ECT–F of CT and (b) ECFST–F of CFST 22
Elements
Fig 11 The 3-D FE model of the Beichuan River Bridge. 24
3
LIST OF TABLES
Table No. Title Page No.
Table 1 Geometrical Parameters of Single- and Two-Layered 21
CFST and CT Elements.
Table 2 Natural Frequencies of CFST Arch Bridge and Steel 25
Arch Bridge.
4
LIST OF NOTATIONS
da outside diameter of steel tube
dci inner diameter of hollow concrete core
dce outer diameter of hollow concrete core
dci,n inner diameter of the nth hollow concrete layer from the exterior of the
tube
dce,n outer diameter of the nth hollow concrete layer from the exterior of the
tube
rc,i inner radius of hollow concrete core
rc,e outer radius of hollow concrete core
rc,0 radius from the centre of tube to the middle of the concrete core
ta thickness of steel tube
tc thickness of hollow concrete core
tc,n thickness of nth hollow concrete core layer from the exterior of the tube
υa Poisson’s ratio of steel (tube)
υc Poisson’s ratio of concrete (core)
σz,i vertical stress in the inner surface (corresponding to interior diameter)
of hollow concrete core
σz,e vertical stress in the outer surface (corresponding to exterior diameter)
of hollow concrete core
σz,av average vertical stress in the hollow concrete core
σt,i tangential (hoop) stress in the inner surface (corresponding to interior
diameter) of hollow concrete core
σt,e tangential (hoop) stress in the outer surface (corresponding to exterior
diameter) of hollow concrete core
σt,av average tangential (hoop) stress in the hollow concrete core
5
σt,i radial stress in the inner surface (corresponding to interior diameter) of
hollow concrete core
σt,i radial stress in the outer surface (corresponding to exterior diameter) of
hollow concrete core
σt,av average radial stress in hollow concrete core
σ’z,e, σ’’z,e additional stress ‘steps’ in each concrete layer
σa stress in steel
σc stress in concrete
σac stress in CFST element (net effect of steel and concrete)
ε strain
θ included angle of circular arch
q uniformly distributed compressive load on arch
Ea modulus of elasticity of steel
Ec modulus of elasticity of concrete
ECT modulus of elasticity of concrete tube
Eac, ECFST modulus of elasticity of CFST element (net effect of steel and
concrete)
E’ac modulus of hardening of CFST element
F applied load
N axial force in CFST arch
Ncr critical axial force (reaction) in CFST arch from classic buckling
theory
6
1. INTRODUCTION
In the ancient period, bricks and stones bonded with lime mortar were the primary
construction materials used, as they performed well under compressive loads. But the
conception of tensile and flexural members posed a challenge of finding new
materials which were good in tension. Bamboos and ropes were the main materials
used for this. Later when concrete was used to bear compression and steel to bear
tension, they were combined together to make concrete-steel composite materials like
Reinforced Cement Concrete (RCC), so as to achieve better structural performance.
Thereafter, lot of research and progress took place in the field of composite materials,
giving birth to a number of new materials like the prestressed concrete, fibre
reinforced concrete (FRC) etc.
Concrete filled steel tubular structures (CFST) are one of the modifications to
combined load-bearing steel-concrete composite structures. Unlike RCC members,
which have the entire tension reinforcement embedded within the concrete, CFST
members consist of circular, rectangular or multi-sided steel tubes, as external steel
shells, and internal concrete core. This concrete core can either be solid or hollow.
Hollow concrete core will have an interior hollow portion, like a tube, while the solid
core will not have this. Hollow CFST members can also be fabricated by more than
one concentric layer of concrete (like double layered, triple layered etc). Recently,
there have been some researches focused on using different grades of concrete for
each concentric layer so as to achieve an improved stress distribution within the core,
which may ultimately increase its load bearing capacity. Some sections are also made
with an additional inner steel tube with the concrete layer confined within the annular
space between the two steel tubes, which are known as double skin hollow CFST
members. Some of the commonly used types of CFST members are shown in fig 1.
In the case of arch bridges, the CFST technology is found to deliver better
performance and economy than other steel and concrete construction techniques.
Consequently, more than 400 CFST arch bridges have already been constructed
worldwide, with China topping the list. Further research and development of the
CFST technology is under progress in China, Japan, U.S.A., and Russia.
7
1.1. Types of CFST Members
Both the steel tube shell as well as the concrete core can be of different forms. The
change in form of steel does not significantly affect the basic properties and behaviour
of the CFST members, like the stress distribution or stress-strain relationship, while
the form of the concrete core used has an instrumental role in defining the properties
and behaviour of CFST members (Shantong and Kuranovas (2007)).
(a) (b) (c) (d)
Fig 1: Various Cross-sections of Solid and Hollow CFST Composite Columns:
(a) Rectangular, (b) Octagonal, (c) 16-sided and (d) Circular.
(Source: Shantong and Kuranovas (2007))
Thus, based on the form of concrete CFST members are classified into two types:
with solid and hollow concrete core. Solid concrete core CFST members are prepared
by placing plain concrete in the steel tube and then vibrating for compaction. Hollow
concrete core CFST members are formed by the spinning process. This spinning
process causes the compaction of the plastic wet concrete by centrifugal action
forming a highly dense concrete core with better physical and mechanical properties.
(Kuranovas and Kvedaras (2007)).
8
(a) (b)
(c) (d)
Fig 2: Photograph of the Cross-section of CFST Element with (a) Single-
and (b) Double Layered Concrete Core. Diagram Indicating the Geometric
Parameters Associated with the Cross-section of (c) Single- and (d) Double
Layered Concrete Core. (Source: Kuranovas and Kvedaras (2007))
Fig 2 shows the photographs and diagrams of the cross-sections of single and double
layered concrete cores of hollow CFST elements. The boundary between the external
and internal concrete layers is clearly indicated. The various geometric parameters
like the diameter, thickness of layer are also indicated, which are explained in the list
of notations.
9
1.2. Advantages of CFST Over RCC
Steel members have high tensile strength and ductility, while concrete members have
high compressive strength and stiffness. CFST members utilize the advantages of both
steel and concrete. The main effect of concrete is that it delays the buckling of the
tube wall and the concrete itself, in the restrained state, and is able to sustain higher
stresses and strains when unrestrained (Kuranovas and Kvedaras (2007)). Some of the
major benefits of using CFST members are listed below.
a) Steel structural hollow sections are the most efficient of all the structural
sections in resisting compression. CFST members perform better than hollow
steel sections in compression due to presence of concrete.
b) By filling concrete in these steel hollow sections either the load bearing
capacity is increased or the size of the column/ member is reduced.
c) The hollow steel tubes also act as formwork and thus there is no requirement
of additional formwork.
d) The presence of concrete delays the failure of steel section by local buckling.
e) The modulus of elasticity of CFST members is found greater than that
expected by the combined action of concrete and steel.
f) Concrete placement and compaction in many cases is unhindered by
reinforcement.
g) CFST members posses higher strength, stiffness and ductility in comparison
with the corresponding RCC members with same material properties.
h) CFST members are perfectly suitable for outside pressure resisting such as
ocean waves, ice, in seismic regions because of their high strength, high
ductility and large energy absorption properties.
i) During construction, the hollow steel tube columns can be used to support
several levels of construction prior to the filling of concrete.
j) The CFST members can be loaded before the full curing time (time for gain
of design strength). This also helps in faster construction.
k) The concrete core is protected from any probable direct mechanical damages.
l) Additional external fireproofing is not always necessary.
m) Since CFST members possess remarkable strength against compression and
buckling, slender sections compared to RCC can be chosen. This reduces the
application time and cost of applied finishes.
10
n) The choice of smaller sections for structural members helps in gaining more
useable floor area and higher visibility.
o) CFST members are more aesthetically pleasing than other types.
p) The use of steel can be minimised.
Furthermore, the hollow CFST members have some additional advantages over the
solid CFST members.
a) Hollow CFST members consume less concrete but are observed to perform
structurally better than the solid members.
b) They are preferable when a reduction in the dead load of the structure is
desired.
c) Amenities like pipelines, electrical cables and other services can be installed
and concealed within the hollow space of the concrete core.
d) Sometimes they also contribute to easier and cheaper hauling and assembling.
2. BEHAVIOUR OF CFST ELEMENTS
Extensive studies on the structural behaviour of CFST elements were carried out by
many researches. These explained the complex structural behaviour of the CFST
elements to a very high degree of accuracy through combinations of mathematical
modellings and experimental studies.
The ultimate axial resistance of a CFST column is found greater than the sum of
resistances of separately tested steel and concrete components of the column
(Kuranovas and Kvedaras (2007)). Further investigations by many researchers
revealed that the increase in the load bearing capacity of CFST elements is mainly due
to the confining effect of steel tube on concrete core. The structural behaviour of
CFST elements (both solid and hollow) are primarily influenced by the difference in
Poisson’s ratio values of steel tube and concrete core, and the change in these values
with the increase in applied load.
In the initial stage of loading, the Poisson’s ratio of concrete remains lower than that
of the steel tube. Thus, the steel does not exert any inward lateral stress (confining
effect) on the concrete core. In the initial stage, most of the load is resisted by the
11
steel tube and only a part of this is taken up by the concrete core. But as the
longitudinal strain increases, the concrete attains a higher value of Poisson’s ratio and
expands laterally at a faster rate than the steel tube. Upon reaching this state, the
concrete core becomes triaxially stressed and the steel becomes biaxially stressed as a
result of the lateral stress exerted by steel on the expanding concrete, which is called
the confining effect (fig 3).
Fig 3: Stress Condition in Steel Tube and Concrete Core at Different Stages of
Loading: (a) υa>υc , (b) υa<υc. (Source: Shantong and Kuranovas (2007))
Figure 4 shows the typical σ-ε relationship of a CFST element, which consists of
elastic (o-a), elastoplastic (a-b), and hardening stages (b-c-d). Since steel tube takes
most of the load in the initial stages it yields even before the concrete reaches its
ultimate stress (corresponding to point ‘a’ in the fig). On yielding, the load is
transferred gradually from steel tube to the concrete core. The steel tube shows a
decrease in load sharing until the concrete reaches its maximum micro-cracking
compressive strength. Upon reaching the maximum compressive stress of concrete,
12
(point b) further loading causes the redistribution of load from concrete to the steel
tube. At this stage the steel exhibits a strain hardening behaviour with the same nature
as that in uniaxial stress-strain hardening relationship of steel (b-c-d).
Fig 5 shows the typical distribution of stresses across the cross-section of a CFST
element in these 5 stages.
Fig 4: Stress- Strain Relationship of a CFST Element.
(Source: Shantong and Kuranovas (2007))
The behaviour of multilayered hollow CFST elements is more complex than single
layered hollow CFST elements owing to the additional interaction and contact forces
developed between the different concrete layers. The stresses at the contact surfaces
(steel-concrete, concrete-concrete) increase in ‘steps’ on account of the changes in
stiffness and appearance of internal forces between the different concrete layers
(Kuranovas and Kvedaras (2007)). This phenomenon is illustrated in fig 6, showing
the distribution of principal stresses across the cross-section. There are totally 3
elements shown in the fig with 1, 2 and 3 concrete layers respectively. For each
element the nature of stress distribution in each direction (vertical, radial &
tangential) are also shown.
13
Fig 5: Distribution of Stresses in Hollow CFST Element,
(a) vertical normal stress, (b) radial stress, and (c) tangential (hoop) stress.
(Source: Shantong and Kuranovas (2007)).
(a) (b) (c)
Fig 6: Stress Distribution in Concrete Core of Centrifuged
(a) Single- (b) Double- and (c) Triple-Layered CFST Elements.
(Source: Kuranovas and Kvedaras (2007))
14
3. CFST ARCH BRIDGES
Arch ribs with CFST sections are widely used in arch bridges across the world
because arches resist the in-plane external loading predominantly by axial
compression and CFST members have good structural performance under
compression (section 2). Advantages like the ease of construction, assembling;
strength, stiffness, improved ductility, delayed buckling etc have made CFST arches
as the best suitable arch ribs for long span arch bridges. Advancement of the concrete
pumping technology has served as an added advantage to the wide use of CFST
arches for long span arch bridges. More than 400 CFST bridges have been constructed
worldwide of which at least 200 bridges are in China. Japan follows China as the
second country with the most number of arch bridges. Some of the world popular
CFST bridges are: The 126m long Arco del Escudo Bridge in Spain, 240m Shinsaikai
Bridge, 430m Zhijing deck CFST arch bridge in China, and the 460 m half-through
arch CFST arch bridge over Yangtze in China (fig 7).
(a) (b)
Fig 7: (a) Yangtze River Bridge at Wuxia; (b) Shin-Saikai Bridge
Presently, CFST arch bridges are designed considering the CFST arches as curved
columns under uniform axial or eccentric compression, i.e. uniform axial compression
and bending action. They are designed by following the same procedure used for RCC
and prestressed arches, which uses the classical buckling load (Euler’s and Rankine’s
theories) of CFST columns as the reference elastic buckling load. But the structural
behaviour of CFST arches have been found quiet different from RCC arches. Yong-
Lin Pi et al. (2012) lists four main flaws associated with design of CFST arches by
considering them as columns.
15
a) Firstly, in comparison with RCC arches, CFST arches are used to build long
span arch bridges. Thus, the CFST members used in these bridges are quite
slender. This makes the failure due to buckling a significant threat to the
structure.
b) The determination of in-plane elastic buckling load of CFST arches by
considering them as columns is found ambiguous as the in-plane behaviour of
arches is found different from that of columns.
c) Also, most of the completed CFST arch bridges have an included angle less
than 90˚. Such arches were classified as shallow arches. Under traverse
loading the shallow arches may even undergo significant transverse
deformations and bending even before the elastic buckling load is reached.
d) Fourthly, slender (long span) and shallow arches are observed to resist the
loading by a non-uniform axial and bending action rather than a uniform one
as generally assumed.
4. STRUCTURAL BEHAVIOUR OF CFST ARCHES
Further experimental studies were conducted and finite element analyses were carried
out, and the results of these described the actual behavioural properties of CFST
arches in detail.
4.1. Uniform Compression
4.1.1. Elastic buckling
The classic buckling load of CFST columns given by equation (1) is referenced as the
elastic buckling load in the present design codes (based on the buckling of CFST
columns). But the buckling load of CFST arches is found to be different from that of
columns. A CFST circular arch that is subjected to a radial load q uniformly
distributed around the arch axis (fig 8) is nominally assumed to be under uniform
compression N=qR and the elastic buckling load derived by considering this
configuration can be used as a reference elastic buckling load for designing the in-
plane strength of a CFST arch. But in real cases, under these considerations, the deep
and shallow arches show different structural behaviour. Though the behaviour of deep
arches are close to that predicted by the classical elastic buckling theory, the bending
moments and radial deformations have significant effects on the buckling of shallow
16
arches (most of the CFST arches are shallow). Thus, the predictions based on the
classical elastic buckling theory may actually give an overestimate of the buckling
strength of shallow arches. Yong-Lin Pi et al (2012) introduced new equations which
also consider these effects to predict the non-linear in-plane buckling load of deep and
shallow CFST arches.
Ncr = (1)
Fig 8: CFST Arches under Uniform Radial Load.
(Source: Yong-Lin Pi et al (2012)).
4.1.2. Effect of residual stresses and initial geometric imperfections
Results have shown that the strength of arches with residual stresses is only slightly
lower than those of arches without residual stresses. This indicates that effects of
residual stresses on the strength of CFST arches are small and can be ignored. But the
research indicated that initial geometric imperfections have significant effects on the
strength of CFST arches in nominal uniform compression.
4.2. Combined Bending and Compression
4.2.1. Internal actions in elastic CFST arches
In actual practice, the CFST arches of arch bridges are subjected to a general loading
which produces combined bending and axial compression. To evaluate the relative
significance of bending and compression in the behaviour and failure of a CFST arch,
different types of loading were considered and their effects were studied by both FEM
and experimentally (Yong-Lin Pi et al (2012)).
17
The types of loadings (fig 9) considered were:
(i) A radial load uniformly distributed around the arch axis;
(ii) A central concentrated vertical load;
(iii) A quarter point concentrated vertical load;
(iv) A vertical load uniformly distributed over the horizontal projection of a
half arch;
(v) A vertical load uniformly distributed over the horizontal projection of an
entire arch.
Fig 9: Arch and Loading (Source: Yong-Lin Pi et al (2012))
And the results were:
a) Under loading (i), the compressive action of the arches is relatively high,
especially for those with a higher included angle.
b) For the loadings (ii), (iii) & (iv), the bending action is relatively high and
compressive action is relatively low. But here the compressive action is higher
for arches with smaller included angles compared to those with larger included
angles. In other words, bending action is predominant in deep arches.
c) For loading type (v) the compressive action is found relatively higher than
bending. Uniquely, the compressive action in this case is found higher in
18
arches with moderate values of included angle than those with low or high
values of included angle.
4.2.2. Elastic plastic behaviour
The experimental results (Yong-Lin Pi et al (2012)) include:
a) Though the CFST arches with very small included angle (θ= 4.6˚, shallow) do
not undergo elastic buckling, they undergo elastic-plastic buckling in
symmetric as well as asymmetric loading (loads (ii) & (iii)).
b) For slightly higher values of included angle (θ= 9.2˚, shallow), limit point
elastic buckling and elastic-plastic buckling takes place in both symmetric and
asymmetric loads (loads (ii) & (iii)).
c) In CFST arches with included angle from θ= 45˚ to θ= 135˚ (moderately
shallow to deep arches), under a central load (ii), elastic asymmetric
bifurcation buckling occurs, while under quarter point load (iii) limit point
elastic buckling takes place. However, the elastic-plastic buckling was similar
in both the loads (ii) & (iii).
d) Evidently, the effects of geometric parameters and the included angle θ on the
strengths of the CFST arches that are subjected to symmetrical loads ((ii) &
(v)) are significant, particularly for very small θ; but are less important for
CFST arches that are subjected to asymmetrical loads ((iii) & (iv)).
4.2.3. Effects of initial geometric imperfections
As mentioned in the statement (d) of section 4.2.2, under combined bending and
compression actions, the geometric parameters have significant effect on the strength
of CFST arches subjected to symmetric loads. Therefore any initial geometric
imperfection in the CFST arch results in remarkable decrease in its load-bearing
capacity and buckling resistance, especially for arches with very small included
angles. However, the effect of geometric imperfections is less severe in the case of
concentrated loads compared to uniformly distributed loads, irrespective of whether
they are symmetric or asymmetric.
19
4.2.4. Full plastic moment of CFST arch section
Experiments have shown that when a CFST section under pure bending reaches the
ultimate plastic stage, the tensile strain is quite large (on the tension side). Therefore
the concrete in this zone undergoes cracking. Thus the contribution of concrete must
be completely ignored while considering the plastic moment of a CFST section. The
actual plastic moment is greater than the value obtained from the non-linear plastic
analysis because when the maximum load carrying capacity of a CFST arch is
reached, the arch will still be only in the elastic-plastic state. That is, even at the
maximum load that could cause the failure of a CFST arch by buckling the arch
would not have reached the full plastic state but only the elastoplastic state.
4.2.5. In-plane strength of CFST arches
It is evident from the previous discussion that the strength of a CFST arch is
influenced by a number of factors such as the buckling behaviour, yielding, initial
curvature, included angle, slenderness ratio, shallowness, confinement effects, initial
in-plane geometric imperfections, residual stresses, and the type of loading and
boundary conditions.
4.3. Modulus of Elasticity and Strain
Kuranovas and Kvedaras (2007) presented the experimental study on properties of
hollow CFST elements. The study included the comparison of properties of hollow
CFST elements and concrete tubes (CT) of same total cross-sectional area. The
geometric parameters of the CFST and CT specimens (single and double layered)
used are given in table 1. From fig 10, it is clear that the modulus of elasticity of
CFST elements is found greater than that of the concrete tubes. Figure 10(a) shows
the variation of modulus of elasticity of four regions (1, 2, 3 & 4) of the CT elements
with the applied load. Figure 10 (b) shows the variation of the modulus of elasticity at
the four regions of the CFST elements with the applied load. On comparing the two
graphs it is notable that the modulus of elasticity of CFST elements has a higher value
than CT elements for the same load. This modification of the modulus of elasticity in
CFST elements is attributed to the interaction between the different layers (concrete-
concrete and concrete-steel) leading to a redistribution of stresses in the CFST
elements. This makes the CFST element more ductile and thus it resists greater
20
strains. Consequently, the modulus of elasticity of CFST elements is found
remarkably higher than its individual components steel and concrete.
Table 1: Geometrical Parameters of Single- and Two-Layered
CFST and CT Elements. (Source: Kuranovas and Kvedaras (2007))
Specime
n
Steel tube Concrete core
ta
(mm)
da
(mm)
Aa
(10-4
mm2)
dce
(mm)
tc1
(mm)
tc2
(mm)
Ac
(10-4
mm2)
1CFST1 5.0 220 33.8 210 28.5 - 162.5
1CFST2 5.0 219 33.6 209 27 - 154.4
1CFST3 4.9 220 33.1 210.2 27.1 - 155.9
2CFST1 5.1 219 34.3 208.8 16.0 15.1 173.6
2CFST2 5.1 220 34.4 209.8 15.2 15.9 174.6
2CFST3 5.1 220 34.4 209.8 16.2 15.0 175.1
1CT1 - - - 210.1 27.4 - 157.2
1CT2 - - - 211.8 26.2 - 152.9
1CT3 - - - 210.4 27.1 - 155.7
2CT1 - - - 208.8 15.5 14.0 166.2
2CT2 - - - 209.8 15.0 15.0 169.5
2CT3 - - - 209.6 15.0 13.7 163.1
21
(a)
(b)
Fig 10: Diagrams (a) ECT–F of CT and (b) ECFST–F of CFST Elements
(Source: Kuranovas and Kvedaras (2007))
22
4.4. Dynamic Behaviour
The behaviour of CFST arch ribs under dynamic loading is an important strength
criterion as the bridges are subjected to different types of dynamic loads such as wind,
movement of vehicles, earthquake etc. The behaviour of the CFST arches under
dynamic loading has gained attention only recently. Some of the important case
studies performed on actual CFST arch bridges and the discussions associated with
these are given below.
4.4.1. Dynamic analysis of a half-through CFST arch bridge
Zhou-Hong et al. (2005) presented the analytical and experimental dynamic analysis
of CFST half-through arch bridge, with a span of 90 m, located across the Beichuan
River at the centre of Xining City, Qinghai Province, China. A three-dimensional
Finite Element (FE) model was developed and an analytical modal analysis was
carried out in ANSYS to obtain natural frequencies and mode shapes. Dynamic field
testing under ambient excitations was conducted before the opening of the bridge to
traffic. Three independent but complementary output-only modal identification
techniques were used for system identification. They are a modified single-degree-of-
freedom identification (SDOFI) method and a peak picking (PP) method in the
frequency domain and the stochastic subspace identification (SSI) method in the time
domain. The 3-D FE model of the bridge is shown in fig. 11.
As mentioned in section 4.3, the CFST members possess an enhanced modulus of
elasticity. Though this results in a better strain resistance, according to the observed
results, if the modulus of elasticity of the CFST ribs increases, the frequencies in
vertical bending and torsion will also increase, but there is no change in the transverse
frequency. Hence a careful design of the CFST arch must be done and the frequency
response must be ascertained before and after construction.
23
Fig.11: The 3-D FE Model of the Beichuan River Bridge.
(Source: Zhou-Hong et al (2005))
4.4.2. Study on dynamic properties of a CFST arch bridge constructed in
China
Qingxiong Wu et al. (2003) presented the results of the dynamic analysis of a three
span CFST arch bridge constructed in China. The bridge’s main span is 251m and the
other two side spans are 60.5 m each. A 3-D finite element non-linear analysis was
carried out to obtain the dynamic properties of the CFST bridge subjected to strong
seismic excitations. Axial force fluctuation and the non-linearity of the biaxial
bending moments of the CFST arch rib were taken into account by using a fibre
model. It was observed that, in any CFST arch bridge, very large amounts of in-plane
bending moments of the arch rib were generated in addition to the out-of-plane
bending moments, when the ground motion is applied in the out-of-plane direction.
Since the in-plane and out-of-plane bending moments are strongly produced
simultaneously in CFST arch bridges, it was recommended that the analysis must
consider the biaxial bending moments of the arch rib. Thus, the strength design must
be done so as to resist the simultaneous action of the two moments.
24
The results on the dynamic behaviour of two similar arch bridges were presented; one
was a CFST arch bridge while the other was a steel arch bridge. The CFST arch was
heavier than the steel arch rib. Therefore it had a smaller natural frequency of
vibration compared to the steel arch under in-plane and seismic loading. However, the
arch action was found less effective in the out-of-plane direction. Hence the safety of
CFST bridges under seismic action in the out-of-plane direction is questionable. The
natural frequencies of the two bridges used for comparison are given in table 2.
Table 2: Natural Frequencies of CFST Arch Bridges and Steel Arch Bridge
(Source: Qingxiong Wu et al. (2005))
Bridge
Name
Span
(m)Type
Natural Frequencies of In-plane Mode (Hz)
First
anti-
symmetric
First
Symmetric
Second
anti-
symmetric
Second
symmetric
Second
Saikai
Bridge
230 CFST 0.639 0.929 1.509 1.474
Saikai
Bridge216 Steel 1.153 1.507 2.805 2.306
5. CONCLUDING REMARKS
Theoretical and experimental investigations by different researchers have revealed
that the actual structural behaviour of the CFST arches is relatively complex. Though
CFST arches are seen to possess better properties than steel and RCC members, the
detailed investigation reaffirms that there are several disadvantages involved in the
application of CFST construction. Needless to say the present methods of CFST
design needs a thorough reformation. The non-linear behaviour, combined effect of
bending and compression, effect of geometric parameters and initial geometric
imperfections, seismic behaviour etc must be given due importance while designing.
Thus, the argument that CFST is the best technology for the long span arch bridges
can not be justified in the present circumstances.
25
REFERENCES
1) Kuranovas Artiomas and Kvedaras Audronis Kazimieras (2007), “Behaviour
of Hollow Concrete-Filled Steel Tubular Composite Elements.” Journal of
Civil Engineering and Management, Vol. XIII, No. 2, 131-141.
2) Qingxiong Wu, Kazuo Takahashi, and Baochun Chen and Shozo Nakamura
(2003). “Study on Dynamic Properties of a Concrete Filled Steel Tubular
(CFT) Arch Bridge Constructed in China.” Journal of Construction Steel 11,
32-39.
3) Qingxiong Wu, Mistuhiro Yoshimura, Kazuo Takahashi, Shozo Nakamura,
and Kazuyoshi Furukawa (2005). “Vibration analysis of the Second Saikai
Bridge- a CFST arch bridge.” Journal of Sound and Vibration 290, 388–409.
4) Shantong Zhong and Kuranovas Artiomas (2007). “The Unified Theory of
Concrete-Filled Steel Tube Columns under Various Loads.” Proc. of the 9th
Int. Conference on "Modern Building Materials, Structures and Techniques".
Selected papers. Vol II, 23-30. Held on May 16-18, 2007 Vilnius, Lithuania.
5) Yong-Lin Pi, Changyong Liu, Mark Andrew Bradford, and Sumei Zhang
(2012). “In-plane Strength of Concrete-Filled Steel Tubular Circular Arches.”
Journal of Construction Steel Research 69, 77-94.
6) Zhou-Hong Zong, Bijaya Jaishi, Ji-Ping Ge, and Wei-Xin Ren (2005).
“Dynamic Analysis of a Half-Through Concrete-Filled Steel Tubular Arch
Bridge.” Journal of Engineering Structures 27, 3-15.
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