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.

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REFERENCES

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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,

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3) Qingxiong Wu, Mistuhiro Yoshimura, Kazuo Takahashi, Shozo Nakamura,

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Bridge- a CFST arch bridge.” Journal of Sound and Vibration 290, 388–409.

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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.

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Bridge.” Journal of Engineering Structures 27, 3-15.

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