6

CHAPTER 2

LITERATURE REVIEW

2.1 GENERAL

This Chapter presents a review of studies performed on concrete-

filled steel tubular members. The discussion is mainly focused on flexural

members at static loading condition, cyclic loading condition and analytical

consideration. Emphasis is given on ultimate strength, ductility, energy

absorption capacity, stiffness degradation and damages indices of the

composite members. Interface bond between steel and concrete is also

reviewed.

The studies discussed in this chapter are divided mainly in two

parts. The first part reviews the experimental studies conducted by different

researchers on concrete-filled steel tubular sections that includes beams,

columns, beam –to- column connections and seismic performance of

concrete-filled sections. In the other part, the studies on analysis and design of

the composite sections are reviewed and discussed. The design

recommendations for the analysis of the steel-concrete composite sections

provided in the different codes of practices are also discussed.

2.2 CONCRETE-FILLED HOLLOW STEEL FLEXURAL

MEMBERS

2.2.1 Initial Stiffness

Furlong (1968) conducted an experiment on fifty-two concrete-

filled steel tubes to observe the stiffness and capacity. Twenty-six columns

7

were made from square tubes and other twenty-six were circular tubes of

different diameters varying from 4.50 in. to 6 in. For the columns in eccentric

load tests, the sum of axial forces was maintained virtually constant while the

moments were increased until failure. From these test results basic equation

for stiffness was developed, and results were in favorable agreement with the

test measurements.

Ge and Usami (1992) conducted an experimental study on

concrete-filled square box stub-columns. Six specimens of concrete-filled

composite columns were tested under cyclic compressive loads and four

specimens of steel columns were tested to failure for the purpose of

comparison. From the test results, it was concluded that hysteretic loops in the

steel tube were very narrow even after the peak load. On the other hand, the

hysteretic loops in the concrete-filled columns were relatively narrow in

initial cycle and then became wider at later cycles. This phenomenon implied

that the filled in concrete had been damaged more or less before the peak

load. It was also observed that the stiffness decreased rapidly in case of

concrete-filled column.

Lu and Kennedy (1994) conducted a pure bending investigation on

rectangular and square hollow steel and concrete-filled column sections. From

this test, it was observed that concrete-filled sections have more initial

stiffness than hollow steel sections.

2.2.2 Ultimate Strength

Hunaiti (1997) conducted a research on hollow steel sections filled

with foamed and lightweight aggregate concrete. This study covers eight

numbers of square and circular columns and eight numbers of square simply

supported beams. It aimed to calculate the ultimate moment capacity of filled

8

specimens. Unfilled specimens of similar sections were also tested for

comparison purpose. A theoretical model also was developed to find out the

squash load and to estimate the ultimate moment capacity of in-filled beams.

From this test, the following conclusions are formulated. Column specimen

filled with lightweight aggregate concrete has developed the ultimate axial

capacity and there is a significant enhancement of the strength compared to

the hollow steel sections. Beam specimens filled with foamed and lightweight

aggregate concrete behave flexurally and developed excess moments of the

theoretical values. The test results show that foamed and lightweight

aggregate concrete can be used in composite construction to increase the

flexural capacity of the hollow steel sections.

Shakir-khalil and Al-Rawan (2006) studied the behaviour of

asymmetrically loaded concrete-filled tubular columns. Tests have been

carried out on eight full-scale composite columns and beam-column

connections. The specimens were connected to either one or two concrete-

filled rectangular hollow sections columns. The load on columns were

increased proportionately upto failure. From this test it was concluded that

concrete filling increases the strength of composite members.

Naithani and Gupta (2001) conducted a survey on composite

construction, which is a viable alternative material for buildings. The aim of

this study is to popularize composite construction in India. It was carried out

by Central Building Research Institute for Steel Authority of India Limited.

The report highlights the mode of construction, its various advantages, its

design and typical connection details. The economical advantage of this

system is summarized in the study.

Assi et al (2002) conducted an experimental investigation on

partially encased composite beams. The study was conducted both

9

theoretically and experimentally. Tests were conducted on a total number of

twelve beams of four different sizes with length of 2.0m. The beam specimens

were loaded by two point loading. Eight of the simply supported beams were

partially encased in lightweight concrete and normal concrete and three were

bare steel sections. Three groups of four beams were tested to investigate the

contribution of different types of concrete to the ultimate moment capacity of

partially encased sections. The results of this study showed that, the use of

normal mix concrete showed insignificant enhancement to the flexural

strength of the tested composite sections when compared to lightweight

concrete. The tested beams were capable of reaching moments in excess of

the theoretical predictions and thus lightweight concrete can be considered as

a reliable component to be used in composite construction.

Assi et al (2003) reported the results of an experimental

investigation on lightweight aggregate and foamed concrete-filled hollow

steel beams. Thirty-four simply supported beam specimens of rectangular and

square specimens of different d/t ratios were used at a span length of

1000 mm. Normal mix concrete specimen and hollow steel sections also were

tested for comparison purposes. Theoretical values of the ultimate moment

capacity of the beam specimens were also calculated in this study. From this

study, the following conclusions were drawn. Beams filled with foamed and

lightweight aggregate concrete behaved flexurally and were capable of

developing the full flexural strength of their sections. Moreover the loads

supported by the tested beams were in close agreement with the theoretical

predictions. The investigation demonstrated that predominant failure

mechanism of the beam specimen was due to an excessive deflection with no

lateral distortions or other form of instability. The load-deflection was

obtained during the test with specimens filled with foamed and lightweight

aggregate concrete. The bare steel sections showed similar behaviour except

an increase in ductility. Finally the test conducted in this investigation

10

confirms that foamed and lightweight aggregate concrete can be used in

composite construction to increase the flexural capacity of the steel section.

Ramana Gopal and Devadas Manoharan (2004) conducted a test on

reinforced concrete-filled steel tubular columns. From this research strength

and deformation of both short and slender concrete-filled steel tubular

columns under the combined actions of axial compression and bending

moment were studied. Sixteen specimens were tested to investigate the effect

of fiber reinforced concrete on the ultimate strength and behaviour of the

composite column. The primary test parameter was column slenderness.

Comparison tests were also undertaken on eight numbers of similar hollow

steel tubes to highlight the synergistic effects of composite column. Based on

these test results the following conclusions were drawn. The use of fiber

reinforced concrete moderately improves upon the behaviour of concrete-

filled steel tubular columns subjected to eccentric loading. The ductility

behaviour of fiber reinforced concrete-filled specimens is found to be slightly

better than the plain concrete-filled specimens. The load-deflection curves

showed that fiber reinforced concrete-filled steel tubular columns have

relatively more stiffness than the plain concrete-filled steel tubular columns as

it undergoes less deflection. Concrete-filled steel tubular columns show large

enhancement of load carrying capacity as compared to hollow steel tubular

columns and sustain large strains and deformations.

Han et al (2004a) developed a mathematical model for concrete-

filled CHS (Circular Hollow Section) beam-columns. A unified theory was

described where a confinement factor was introduced to describe the

composite action between the steel tube and the in-filled concrete. The

theoretical model was used to investigate the influence of important

parameters like calculating the section capacity, member capacity and

moment capacity of the concrete-filled steel CHS beam-columns. A complied

11

interaction curve was derived for concrete-filled steel CHS beam-columns.

Comparison was also made with predicted column strengths using LRFD

(1999), AIJ (1997), BS 5400 (1979) and EC4 (1994). The codes are found to

be conservative in general. The capacities predicted by the simplified model

was about 4% to 10% lower than that of the tests.

Han (2004b) conducted an experiment on sixteen concrete-filled

steel RHS and SHS beam specimens. The width-to-depth ratio of the tube

ranges from 20 to 50. All specimens are 1100 mm in length. The average

Yield Strength and Modulus of Elasticity of the steel specimen is

approximately 300 MPa and 200000 MPa. The compressive strength of the

in-filled concrete is 30 MPa. This experiment helps not only to determine

ultimate moment capacity but also to determine the failure pattern beyond the

yield load. From this test the following conclusions are obtained. Because of

infill of concrete, the steel RHS and SHS section behave in a ductile manner.

The Load Vs Deflection curves for the composite beams have been found in

good agreement with experimental values. The in-filled concrete also

increases the ultimate moment value.

Mahasnesh et al (2005) conducted a research on steel tubes filled

with fiber polymer. In this study, five specimens with lengths of

100,150,200,250 and 300mm were selected. A lean mix concrete (15MPa)

was used in this test. The axial load was applied directly to a thick steel disk

that is smaller in diameter than the steel pipe to assure that the load is applied

to the concrete core. The steel cover is subjected to a confined hoop stress

only. This study presents the effect of both composite and confinement of

concrete for the in-filled steel tubes.

Kang et al (2005) presents experimental study of the behaviour of

circular and square stub columns filled with high strength concrete and

12

polymer cement concrete under concentric compressive load. Twenty-four

specimens were tested to investigate the effects of variations in the tube shape

(circular, square) wall thickness and concrete type on the axial strength of

stub column. From the experimental results, the following conclusions are

obtained. The circular steel tube specimens filled with high strength polymer

concrete do not show a sudden decline in load-carrying capacity beyond

ultimate load. One of the key findings in this study is that the circular section

members filled with PCC retain their structural resistance without any

reduction far beyond the ultimate capacity.

Jane Helena and Samuel Knight (2005) conducted an experimental

investigation of axial load on columns and flexural load on beams of

concrete-filled cold-formed hollow steel sections. Forty-eight columns and six

beams are filled with different grade concrete. The sizes of the hollow steel

sections have a different d/t ratio and the strength of concrete used in this

experiment is 24.94MPa (low grade) and 30 MPa (high grade). From the

above test the following conclusions are drawn. All the axially loaded

columns filled with high grade concrete have failed by overall buckling

followed by local buckling. The resistance to flexural buckling increases the

strength of the filled concrete. Provisions of infill have increased the load

carrying capacity to one and half times to two times .The failure of the beam

is indicated by the overall bending of the beam followed by localized

buckling of the beam under the concentrated load in the case of hollow steel

sections and by overall bending in the case of concrete-filled beams.

Dalin Liu (2005) carried out an experimental programme on high

strength rectangular concrete-filled hollow steel section column. A total of

twenty-two specimens were tested under the concentric loading. The

parameters include material strength, cross-sectional aspect ratio and

volumetric steel-to-concrete ratio. The test result manifests the favorable

13

ductility performance of the high-strength composite columns. The strength

improvement is adversely affected by the cross-sectional aspect ratio. A

comparison of failure loads between the test and the design codes has been

presented. Results show that provisions in EC4, ACI and AISC conservatively

estimate the ultimate capacities of the specimens by 1, 9 and 11%. The EC4

approach can be used for estimating the ultimate capacity of high-strength

rectangular CFSHS columns subjected to concentric loading.

Han and Yang (2005a) conducted an experiment on eight concrete-

filled steel CHS and two hollow steel sections to find out the flexural loading

capacity. The specimens were subjected to constant axial load and cyclically

increasing flexural load. The filler material used was the normal mix concrete.

The length of the test specimen was 1500mm. The flexural loading was

applied by imposing cyclically lateral loading in the middle of the specimen.

From the above test it was found that after steel reached its yield strain, an

outward indent or bulge was formed close to the stub at the compression face

of the composite column on both sides of the stub. The bulge was also formed

on other face of the specimen when the lateral load was reversed. Local

buckling of the empty CHS happened earlier than that of the specimens of

concrete infill. Because of infill concrete, the circumferential deformation and

ovalization of the composite cross sections were prevented resulting in higher

stiffness, larger moment capacity and richer ductility.

Han et al (2005b) conducted an experimental investigation to find

out the flexural behaviour of concrete-filled steel tubes after exposing to

standard fire. A theoretical model was been developed to predict the post-fire

load versus deformation relationship of CFST stub columns and beams.

Totally four hollow steel sections of different sizes were used to conduct this

test and the strength of the concrete used was 43MPa. From this experimental

investigation following conclusions were drawn. The load tested concrete-

14

filled SHS and RHS stub columns and beams after the exposure to fire

behaved in a ductile manner. The residual strengths predicted by the

theoretical model were also in good agreement with the test results.

Zeghiche and Chaoui (2005) conducted an experimental research to

study the behaviour of concrete-filled tubular columns. In this study the tests

were conducted on twenty-seven concrete-filled tubular columns. The test

parameters were the column slenderness, the load eccentricity covering

axially and eccentricity loaded columns with single and double curvature

bending and compressive strength of the concrete core. The test result

demonstrated that the strength behaviour of concrete-filled columns was

based on this influencing parameters. A comparison of experimental failure

loads with the predicted failure loads in accordance with the method

described in Eurocode 4 part 1.1 showed good agreement for axially and

eccentrically loaded columns with single curvature bending. In case of

columns with double curvature bending the Eurocode loads were found to be

higher and on the unsafe side.

Angeline Prabhavathy and Samuel Knight (2006) investigated a

series of tests on cold-formed rectangular hollow steel and concrete in-filled

beam to column connections and frames. Twelve experiments were conducted

on cold-formed steel, directly welded tubular beam to column connections

and twelve experiments on columns in-filled with concrete subjected to

monotonic loading. From the above test the following conclusions were

obtained. Having connected with hollow steel beam, the in-filled columns

with square and rectangular sections loaded along the major axis, increased

the initial stiffness, moment carrying capacity and ductility ratio marginally.

When the columns were loaded along the major axis the ductility was

reduced. In case of in-filled frames, when they were loaded along the major

and minor axis the load deflection behaviour was the same as that of hollow

15

steel frames and concrete in-filled frames. But there was an increase in load

carrying capacity.

Oh et al (2006) studied the structural performance of steel-concrete

composite column subjected to axial and flexural loading. In this

experimental study the concrete was filled in the empty space of H-shaped

steel flanges. Based on the results of this experiment, it was reported that

AISC-LRFD provisions evaluated the load-carrying capacity of the composite

column conservatively. AIJ and EC4 code provisions were considered

desirable for the use in evaluating the capacity of the axial force and bending

moment of steel-concrete composite columns with non-compact steel section.

Lue et al (2007) carried out an experimental investigation of

rectangular CFST columns filled with high strength concrete. Twenty-two,

1855 mm long rectangular hollow steel sections of 1501004.5mm were

used with mean yield strength of 379.8 MPa and the strengths of filled

concrete was 70 and 84MPa. Gypsum was used to compensate the minor

shrinkage of concrete and to make the top end surface of CFST specimen in

full contact with the bearing plate of the MTS machine. It was ensured that

the applied load was transferred to the concrete core and hollow steel section

uniformly during the test. The aim of the test on LRFD CFST column was to

determine the strength of long rectangular columns filled with high strength

concrete. From the test results of this study it was reported that the design

CFST strength (Pu) predicted by the AISC-LRFD formulae and the test results

(Ptest) were found to be in good agreement.

Huang et al (2008) proposed a method to estimate the ultimate

strength of rectangular concrete-filled steel tubular (CFST) stub columns

under axial compression. The ultimate strength of concrete core was

determined by using the conception of the effective lateral confining pressure

16

and failure criterion of concrete under true triaxial compression. It took into

account the difference between the lateral confining pressure provided by the

broad faces of the steel tube. This research paper proposed a method for

calculating the ultimate strength of rectangular CFST stub columns under

axial compression. Difference in the longitudinal steel strength between broad

faces and narrow faces and the difference in lateral confining pressure on the

concrete core were considered. The experimental results of fifty-six

rectangular CFST specimens were compared with the calculated values by

ACI (2005), GJB4142-2000, AISC (1999) and the proposed method. It was

found from comparison that the proposed method showed a good agreement

with the test results.

2.2.3 Width to Thickness Ratio (Aspect Ratio)

Vijayarangan and Joyce (1992) conducted an experimental

investigation of eccentrically loaded slender steel tubular columns filled with

high-strength concrete. The test parameters were the slenderness ratio and the

eccentricity of axial thrust. In this paper a simple method to calculate the

strength of column was presented. It was based on the assumption that failure

load was reached when the maximum moment at mid height of the column

was equal to the ultimate bending strength of the cross section at that location.

In this method the deflection of the column due to creep and imperfections in

the steel sheath was to be treated as an additional eccentricity.

Shakir-khalil (1993a) conducted experiments on the connection of

Rolled steel beams to concrete-filled tubes. The test was carried out on

twenty-eight connected specimens. The columns were made of rectangular

hollow steel sections. The fin-plate was welded to the middle of the sidewall

of the hollow steel section. In each test the load was applied to both columns

and beams in order to simulate actual column loading in structure. From this

17

test results it was observed that in small eccentricities, failure resulted from

overall collapse of the upper part of column section. The concrete core was

preventing the fully developed yield line type of failure, by preventing the

tube corners from moving inwards closer to each other. The connections

exhibited larger rotations when compared to the circular hollow section

column.

Schneider (1998) presented an experimental report on short,

concentrically loaded, concrete-filled steel tubular columns. Parameters for

this study included the shape of the steel tube and the d/t ratio. From this

investigation was concluded that circular steel tubes offer much more post-

yield axial ductility than the square and rectangular tube sections.

Calado et al (2000) studied cyclic behviour of steel and composite

beam-to-column joints. In this test, two series of full-scale specimens were

tested, namely, a fully welded series and a top and seat with web angle

series.Cyclic tests were carried out with constant and step-wise increasing

amplitude displacement histories. From this investigation it was shown that

for welded steel joints, the behaviour of the connection was strongly affected

by the panel zone, which was directly related to the column size.

Elchalakani et al (2001) investigated a series of test on CFST

subjected to large deformation pure bending where d/t ratio of the hollow

steel section ranges from 12 to 110.The filler material used was normal mix

concrete. This investigation compared the behaviour of empty and void-filled,

cold-formed circular hollow sections under pure elastic bending. From these

test it was concluded that in the range of d/t 40, void filling prevented local

buckling for very large rotations, whereas multiple plastic ripples were

formed in the inelastic range for specimens with 74 d/t 110. It was also

18

reported that void fillings of the steel tube increased flexural strength,

ductility and energy absorption capacity especially in thinner sections.

Dalin Liu et al (2003) conducted a study on high strength concrete-

filled rectangular stub column. In this experimental programme twenty-two

CFSHS specimens with cross-sectional aspect ratio of 1.0, 1.5 and 2.0 were

used for testing purposes for failure under axial concentric loading. All the

specimens were fabricated from high strength materials. The average yield

strength of the hollow steel sections was 550 MPa and average compressive

strength for concrete was 70.8 and 82.1 MPa. The test results revealed that

these high strength columns could have similar failure behaviour as normal

strength CFSHS columns. A high axial load capacity and a good ductility

performance were obtained. The comparison of the results showed that the

codes (EC4, AISC and ACI) have underestimated the ultimate capacity of the

test specimens. EC4 predicted with a difference of 6% while AISC and ACI

underestimated the critical loads by 16% and 14% respectively. The primary

reason for the discrepancy of results was that the strength enhancement of

CFSHS columns due to the confinement of core concrete by hollow steel

section was considered in EC4 but not in AISC and ACI. It was also reported

that the strength of the specimens was decreased with an increase of cross-

sectional ratios.

Hsu and Yu (2003) conducted an investigation on eighteen

specimens of concrete-filled sections with various tube width/thickness

ratios.The steel tubes were fabricarted by welding cold-bent JIS SS-400 thin

plates with different thickness. The average yield strength of the plates was

321 MPa. The compressive strength of the filler material was 34 MPa. This

study focused on the improved performance of CFST members by utilizing

pair of tie rods at possible plastic hinge location to restrain relative plate

deformation after the members were locally buckled. The post buckling

19

performance of concrete-filled tube columns under earthquake load was

governed by the deterioration of the tube plates in the plastic hinge region.

The results of this combined axial and the lateral load tests showed that the tie

rods effectively restrained the development of excessive plate deformation

and delayed local buckling.

Elchalakani et al (2004) studied the cyclic inelastic flexural

behaviour of the cold-formed circular hollow section (CHS) beams. The tests

were conducted on different sizes of compact CHS with section slenderness,

with the d/t value ranging from 13 to 39. It was observed that with continuous

cycles, the growth of ovalization caused a progressive reduction in the

bending rigidity of the steel tube. The CHS beam exhibited stable hysteresis

behaviour upto local buckling and then showed considerable degradation in

the strength and ductility depending upon the d/t ratio.

2.2.4 Bond Strength

Prion and Boehme (1994) conducted an investigation on concrete-

filled steel tubes in bending. The results indicated that specimen dissipated a

significant amount of energy with only a slight decrease in strength when the

loading cycle progressed. The strength of CFSTs during subsequent cycles

was not greatly affected by the slip between the two materials. The beam

specimens showed a loss of stiffness due to a lack of bond and the cracking of

the concrete after the first cycle.

Hunaiti (1994) investigated on fifteen battened composite

specimens to find the bond strength between steel and concrete at the age of

five years. The result of this investigation showed that the bond strength at the

age of five years was about two and half times greater than of that the age of

one year. This was mainly due to rusting of steel at the surface of contact with

20

concrete. It resulted in the increase of the mechanical keying due to micro-

irregularities and thus enhanced the bond between the two materials.

Hunaiti (1996) conducted an experimental investigation on

composite action of foamed and lightweight aggregate concrete. Thirty-six

push out tests were performed on concrete-filled hollow steel sections in

square and circular shapes. It was found that the strength of bond in

composite sections was significantly affected by the type of concrete.

However, it appeared that the type of concrete did not influence the load-slip

behaviour as all the tested specimens produced similar load-slip curves.

Lightweight aggregate concrete showed higher resistance to push-out loads

and thus had better composite action. More over, bond reduction due to age in

normal concrete specimens is higher than that of lightweight aggregate

concrete specimens.

Roeder et al (1999) studied the composite action in concrete-filled

steel tubes. CFST applications in buildings, the importance of bond stress and

the behaviour of interface conditions were noted. It was shown that shrinkage

could be very detrimental to bond stress capacity and the importance of

shrinkage depended on the characteristics of the concrete, the diameter of the

tube and the surface condition of the inside tube. The bond capacity was

smaller in tubes with large diameter and large d/t ratios. The bond capacity

was interrelated with slip at the steel concrete surface.

Shanmugam and Lakshmi (2001) presented a literature review of

the past research carried out on composite columns with an emphasis on

experimental and analytical work. The review also included the research work

that was carried out to account for the effects of local buckling, bond strength,

seismic loading, confinement of concrete and secondary stresses on the

behaviour of steel-concrete composite columns. Intensive research is required

21

on the interaction between steel and concrete, the effect of concrete

restraining local buckling of steel plate elements, effect of steel section,

confining concrete etc.

Hussain (2003) studied the experimental and theoretical behaviour

of thin walled composite beams. The in-filled material used in this work was

normal and lightweight volcanic pumice concrete. The strength and failure

modes of the beams were found to depend on the interface connections. The

effect of the various modes of interface connections was co-related to the

generation of shear bond between the sheeting and concrete. Analytical

models for the design of beams were developed and their performance was

validated through the experimental results using both full and partial

connections. A comparison of series of tests revealed that the performance of

various modes of interface connections had an impact on the strength

enhancement of beams. The strength of the beams was limited by the

compression buckling capacity of the steel plate at the top of the open box

sections. It was also shown that enhancement of the strength could be possible

by stiffening the compression steel plates at the open end of box section with

various modes of interface connections.

Amir Fam et al (2004) conducted an experimental investigation on

bonded CFST beam columns. From this study it was found that CFST beam

columns showed a better ductile behaviour than the unbonded specimens.

They also had experienced rapid flexural strength deterioration after initiation

of local buckling of the tube. Both unbonded and bonded beam columns were

failed by fracture of the steel tubes in the local buckling zone as a result of

successive buckling and straightening.

Dennis Lam and Williams (2004) performed a series of tests on

eighteen short composite columns under axial compressive loading. The steel

22

sections covered a range of S275 and S355 grade steel. The square hollow

sections were filled with normal and high strength concrete. The test showed

that the peak load was achieved at small shortening in the composite columns

with low constraining factor. In the composite column with high constraining

factor, the ultimate load was maintained with large displacement. As the

concrete strength increased the effects of the bond of the concrete, the steel

tube became more critical. For normal strength concrete, reduction of the

axial capacity of the column due to bonding was negligible.

Ahu Hyung-Joon et al (2007) studied the behaviour of modular

composite profiled beams. In this concept, prefabrication was applied to the

existing composite profiled beams. The prefabrication concept produced a

beam of desired size having two profiles, i.e. side module and bottom module.

As for shear connections, full shear, partial shear and no shear connections

were considered. Six types of specimens were used in this experiment. The

thickness, the width and the height of the profile steel sheet were 1.6mm,

200mm and 300 mm respectively. Concrete was used as the filler material

whose strength was 23 MPa. The specimen was simply supported and tested

under two point loading. From the above test it was observed that all the

specimens showed similar stiffness initially. Module section improved the

construction and the composite profile beam reduced the deflection due to

creep and shrinkage.

2.2.5 Deflection and Local Buckling

Tsuji (1992) studied on buckling and post buckling strength of

concrete-filled square tube. In this study, stub the column compression tests

and buckling tests of concrete-filled square steel were performed. Maximum

strength was compared with the tangent modulus buckling load. The major

conclusion obtained from this study was that, for short column with large

23

width-to-thickness ratio, local buckling appeared first, then overall buckling.

In short in the column with small with-to-thickness ratio, overall buckling

occurred first, then local buckling occurred.

Oethlers (1994) studied the flexural strength of the profiled beams.

An experimental study on large-scale profiled beams showed that the side

steel decking subsequently increased the flexural strength, the ductility, and

shear strength of the beam. Theoretical research showed that the side steel

decking could substantially reduce the deflections due to the creep and

shrinkage and could increase the span/depth ratios to 20%. However, local

buckling of the steel decking and the strength of the shear bond at the

interface between the decking and concrete can affect the behaviour of this

form of composite construction.

Uy (2000) investigated on the strength of concrete-filled steel box

columns. An extensive set of experiments were carried out and a numerical

model was developed to calibrate with these results. The experiments were

conducted on columns and beams to ascertain the axial and flexural behaviour

of these members as well as the combined effects of bending and

compression. A simple numerical model for the determination of the strength-

interaction diagram was also developed to verify both the tests. The

experimental results showed that local buckling was significant in thin walled

composite column with large plate slenderness values particularly for large

values of axial force.

Morino et al (2001) studied the advantages of concrete-filled steel

tubular column systems in comparison with ordinary steel and reinforced

concrete system. It was found that one of the main advantages in the

interaction between steel tubes and concrete was the occurrence of local

buckling of steel tube which was delayed by the restraint of concrete. The

24

strength of the concrete was increased by the confining effect provided from

the steel tube.

Ghannam et al (2004) conducted a test on steel tubular columns of

square, rectangular and circular sections filled with normal and lightweight

concrete. It was conducted to investigate the failure modes of the composite

columns. Thirty-six full-scale columns filled with lightweight and normal

weight aggregate concrete, each with eighteen specimens were tested under

axial loads. Nine hollow steel sections of similar specimens were also tested

and results were compared to those of filled sections. The test result showed

that both types of filled columns failed due to overall buckling, while the

hollow steel columns failed due to bulging at their ends (local buckling). It

was also concluded that higher deflection reflects higher ductility.

Fujimoto (2004) conducted an experimental work on CFST

columns. From this test it was concluded that there was an increase in bending

strength due to the confinement effect. Moreover, the effect of local buckling

must be considered when evaluating the bending strength in square CFST

columns with proportionately large width-to-thickness ratios. Local buckling

in square CFST columns mainly causes the reduction in ultimate strength.

Liang et al (2005) studied the properties of the concrete-filled steel

box columns. A nonlinear fiber element analysis method was presented for

this study for predicting the ultimate strength and behaviour of short concrete-

filled thin-walled steel box columns with local buckling effects. The

confinement effects on the ductility of the encased concrete in concrete-filled

steel box columns were considered. From this study it was demonstrated that

the fiber element analysis programme predicted the ultimate strengths and

behaviour of concrete-filled steel box columns with local buckling effects. It

25

was suggested that the fiber element analysis method could also be employed

in the advanced analysis of composite frames.

Lapko et al (2005) studied an experimental and numerical analysis

of flexural capacity and deformability of structural concrete beams. In this

experiment the composite specimen consisted of two concrete layers made of

reinforced normal concrete and high performance concrete (HPC). The

reinforced concrete composite beams in the tests were prepared in full scale

with the cross-section of 120200 mm and the effective span of 2950 mm.

From the test it was concluded that bending tests conformed significantly to

reduce the compressive strains and the deflections measured in composite

beams in comparison with the strains and deflections measured in the control

beams made of normal concrete. The results of the analysis showed that the

applications of such composite flexural structures can be used in

strengthening the structural concrete members in rehabilitation and

reconstruction works.

Lee (2007) studied the relationship of capacity and moment-

curvature of high-strength concrete-filled steel tube columns under eccentric

loads. This study included a series of tests of different width-to-thickness ratio

and buckling length–sectional width ratio and eccentricity ratio of the HCFST

columns. This study reported that the ductility of high strength CFST columns

was decreased significantly with an increase in the width-to-thickness ratio of

the steel tube. The axial load capacity of HCFST columns was increased in

the buckling length-sectional width ratio or width-to-thickness ratio of the

steel tube. In the case of strain hardening range via the elasto-plastic area, the

trend was towards the decrease in bending moment and the ultimate axial load

capacity was towards an increment of curvature. Therefore it was concluded

that the appropriate selection of width-to-thickness ratio improved the

ultimate axial load capacity. Deformation efficiency is also improved

26

considerably in the high-strength concrete CFST columns. The deflection and

buckling of compressive flange and that of the web were minimized.

2.3 SEISMIC PERFORMANCE OF CONCRETE-FILLED

BEAM, COLUMN MEMBERS

2.3.1 Ductility, Stiffness Degradation and Energy Absorption

Capacity

Knowles and Park (1969) studied the steel tubes filled with

concrete and loaded axially. The buckling load of the long columns was

accurately predicted by summing the tangent modulus loads for the steel tube

and concrete core acting as independent columns. It was shown that the

concrete was suddenly increased in volume at a certain value of longitudinal

compressive strain. This caused an internal pressure inside the steel tube

which in turn caused the steel tube to an extra confining stress on the concrete

and thus increased the longitudinal compressive stress.

Usami and Ge (1994) conducted an experimental investigation on

the ductility of concrete-filled steel box columns under cyclic loading. In this

study eleven unstiffened and stiffened box columns made of SS400 steel of

yield stress 235MPa were used. The filler material concrete had an average

strength of 35MPa. The load was applied at the lower end plate of a test

specimen bolted to the column base, which was anchored to the floor. The

horizontal lateral load and vertical axial compressive load were applied by an

MTS servo controlled hydraulic actuator. From this study it was proved that a

steel box column, partially filled with concrete upto 0.3 or 0.5 times of the

column height, showed a very effective earthquake-resistant capacity. The

concrete-filled specimens significantly increased both ductility and energy

27

absorption capacity, when it was compared to hollow steel specimens. The

rate of increase was about two to four times. When the inward local-plate-

buckling displacements were delayed and prevented by filled concrete, it

resulted in the increase of ductility and energy absorption capacity. However,

it was observed that the longer columns due to the larger plate width-

thickness ratio had smaller ductility and energy absorption capacity.

Uy and Bradford (1995a) conducted an experimental and analytical

ductility study on profiled composite beams under sustained service loads.

The experiments included two-profiled composite beams and two reinforced

concrete beams. From the experiments the following conclusions were

reported. The profiled composite beams provided an innovative construction

method in the construction. They proved to be more advantageous for the

construction industry and other structural application. The profiled composite

beam system provided an increased long-term stiffness, improved ductility

and reduced construction times. The flexural experiments provided

benchmark data for profiled composite beam that deflected less than the

reinforced concrete beams under long-term loads when designed for the same

flexural strength. A numerical model based on a simple cross-sectional

analysis was shown in good agreement with the experimental data for the

short-term Moment-Curvature response. The Load-Deflection characteristics

of the model were also shown to agree well with the experimental results. A

Finite–Strip model for the calculation of the local buckling was also

formulated and the results of the analysis were in agreement with the

experiments.

Uy and Bradford (1995b) studied an analytical investigation on

ductility of profiled composite beams. A simple theoretical model was

developed to study the Moment-Curvature response of the ductility of a

profiled composite beam. The material nonlinearities of the cold-formed steel

28

and the concrete are used to find the accurate description of the Moment-

Curvature response. Parametric studies were made of material strength of

various cross-sections. The developed theoretical model was verified against

experimental data. The model developed was shown to agree well with the

linear and nonlinear ranges of full-scale testing. It showed that increase the

strength of the cross-section may affect the ductility.

Ge and Usami (1996) conducted an investigation on twelve

concrete-filled steel box columns. The specimens modeling for steel bridge

piers were tested under a constant axial load and cyclic lateral load. The

purpose of the experimental investigation was to study the influences of the

in-filled concrete strength, ductility and energy absorption capacity of the

column. Moreover the effect of plate width-thickness ratio and length of the

in-filled concrete were chosen as main parameters. The effect of the

diaphragm over the concrete and on the column behaviour was also examined.

From the above test results it was reported that when filling was carried out,

light improvement of ductility and energy absorption capacity of steel bridge

piers was observed. It was also concluded that concrete-filled steel box

columns could be effectively used as bridge substructures in strong

earthquake areas.

Zhao et al (1998) carried out an investigation an a void-filled SHS

beams subjected to high amplitude cyclic loading. From this experiments it

was indicated that local buckling and plastic hinges may occur in the

compressive flange of square hollow section (SHS) beams under high

amplitude cyclic bending due to earthquakes, serve storms and heavy traffic.

Once local mechanism was formed, the residual strength was reduced within a

few cycles. Due to the filler material in the SHS section, there was an increase

in the ductility and energy absorption capacity.

29

Elchalakani et al (2002) conducted a series of test on concrete-filled

double-skin (CHS outer and SHS inner) composite short columns under axial

compression. For the specimens used in this study annulus was filled with

micro high strength concrete having a cylinder with compressive strength of

64 MPa. The outer skin was made of Circular Hollow Sections (CHS), while

the inner skin is made of Square Hollow Sections (SHS). It was concluded

that the CFDT construction was found to have significant increase in strength,

ductility and energy absorption capacity.

Han and Huo (2003) had done the experiments on concrete-filled

hollow structural steel columns after an exposure to standard fire. Twelve

concrete-filled HSS columns with or without fire protection were exposed to

ISO-834 fire standard and subjected to the axial or eccentric loads for

experimental investigation. A mechanics model was also developed for

concrete-filled column standard fire by an analysis used for ambient

condition. It is reported that the tested concrete-filled HSS columns after

exposure to ISO 834 fire standard behaved in a ductile manner. Fire exposure

increases the deflections and decreases the strength of concrete-filled HSS

columns. There is no strength reduction due to fire exposure where the

duration is limited to less than 10 min.

Elchalakani et al (2003) conducted a test on nine circular hollow

sections with different diameter-to-thickness ratio. The CHS braces were

subjected to cyclic concentric axial loading. From the above test it was

concluded that CHS braces exhibited stable hysterics behaviour upto local

buckling and showed considerable degradation in strength and ductility

depending on the KL/r and D/t ratios. The structural response factor (ductility

index) was also examined and it was used to derive a new section slenderness

limits for seismic design.

30

Varma et al (2004) conducted research on cyclic loading of high

strength concrete-filled beam column. From this investigation the following

conclusions were obtained. The cyclic loading did not have a significant

influence on the flexural stiffness and moment capacity of CFST beam

column. However, the post peak moment resistance decreased more rapidly

under cyclic loading. The stiffness of the steel tube alone was decreased

rapidly due to extensive tension cracking of the concrete infill. The cyclic

curvature ductility was decreased significantly with an increase in the axial

load level.

Gho and Dalin Liu (2004) conducted experiments on twelve,

1600 mm long high strength rectangular concrete-filled hollow steel section

beams. The specimens tested to failure under pure bending. In this test, three

different sizes of hollow steel sections, filled with high-strength concrete were

used. From the above test, the following conclusions were obtained. A good

ductility performance of the specimens was observed in all tests. Local

buckling was specifically noted on the compression face of the specimens. A

comparison of the moment capacities with the values calculated from the

formulae in the codes showed that EC4, ACI and AISC underestimated the

flexural strength of the specimens by 11, 15 and 18% respectively. However,

it should also be noted that the formulae in the codes derived were based on

normal strength hollow steel section and concrete.

Broderick et al (2004) presented an experimental work to study the

cyclic behaviour of hollow steel and filled axially loaded bracing members. In

this study the rectangular and square sections with three different d/t ratios

were tested under monotonic and cyclic load. The length of the specimen used

for first two series of tests was 1.1m and that of the third series of tests was

3.3 m. In this research work, numerical analysis work was also carried out

based on the Finite Element software package of LUSAS to study the local

31

inelastic behaviour. The filler material used was cement mortar. From the

above test it was observed that under monotonic tension loading, the presence

of the infill could lead to a reduction in ductile capacity. Numerical analysis

had shown that the prevention of ductile necking in the infill causes non-

negligible hoop stresses to arise in the steel section and they were increased

rapidly after yield. Comparing the performance of hollow steel and filled

specimens under cyclic loading it was evident that mortar infill prevented

inward local buckling in most cases whereas it was delayed in others. The in-

filled sections showed an improvement of ductility and energy absorption

capacity.

Chitawadagi et al (2007) conducted a research work on strength

deformations behaviour of concrete-filled steel tube columns. It was reported

that a considerable progress was made during the last two decade in the

investigation of steel-concrete composite column and beams. The available

information was summarized in this paper. A fundamental knowledge on

composite construction system such as ultimate strength was already obtained

by the research so far. It was also stated that an intensive research is required

between the steel and concrete restraining the local buckling of steel plate

elements, the effect of steel section confining concrete etc., while a larger

focus on investigating circular CFSTs studies are required to understand the

structural performance of rectangular CFSTs.

2.3.2 Moment Carrying Capacity

Shakir-Khalil (1993b) studied the pushout strength of concrete-

filled hollow steel sections. Pushout tests were carried out on 40 short

concrete-filled hollow steel section tubes. The shapes of the tubes were

square, rectangular and circular and the concrete mix was of normal strength.

The lengths of the specimens were 200 mm, 400 mm and 600 mm. The same

32

concrete mix was used throughout the test and the load was applied at the top

of the specimen on the concrete core and was resisted by steel section on its

base. From the above tests it was found that the resistance of the specimens to

the pushout load was a function of the shape and size of the hollow steel

sections.

Zhao et al (2002) conducted an experimental investigation of void-

filled cold-formed rectangular hollow steel section braces subjected to large

deformation cyclic axial loading. The special characteristics of this program

were the selection of thin-walled section, high depth–to–thickness ratio, high

strength of steel section (yield stress upto 481 MPa), two different types of

filler materials (normal concrete and light weight concrete), and the two

different loading system (cyclic direct and cyclic incremental). First the cycle

buckling load was compared with the design loads, predicting the usage of

various National standards. The effects of filler material and section

slenderness on the Load-Deflection response, the first cycle peak load, the

post peak residual strength, the ductility and energy absorption capacity of

void –filled RHS braces were studied. The effect of loading scheme was also

discussed. From this study it was concluded that the concrete filling increased

energy absorption capacity and residual load capacity especially when the

axial displacement became larger. When the compressive strength is higher,

the ductility index becomes larger, especially for thinner sections.

Zhao and Grzebieta (1999) conducted an experimental investigation

of void–filled SHS beams subjected to a large deformation cyclic bending.

The specimens used in the tests were of different d/t ratios. The filler

materials were of normal strength concrete, polyurethane and lightweight

concrete. The cyclic test rig was developed. Each specimen was bent to a

maximum angle of 20°, so that the results should be compared with the

previous test results of empty SHS beams. The results indicated that an

33

increase in rotation angle at ultimate moment was 300% for all filler materials

tested. The increase in the ultimate moment capacity mainly depended on the

strength of the filler material. It was also observed that lightweight concrete

or other lighter filler materials may be used for compact section and non-

compact section to achieve acceptable mechanism. A failure pattern is also

studied. Void-filled SHS beams survive a limited number of multiple bending

cycles without the tube being cracked.

2.3.3 Damage Index

Satish Kumar and Usami (1996) conducted experiments on ten

hollow steel beam columns, modeling piers, using a constant axial loading

and different lateral loadings. It was observed that the columns were

susceptible to be damaged during a severe seismic event. From the above

tests, it was found that the degree of damage sustained depended on the

parameters of the structures and loading history. Since residual strength was

important, the physical measure of the damage sustained and the approximate

expressions for correlating the residual strength and the damage index were

suggested. The damage model developed in this test was intended to quantify

the damage sustained under earthquake loading.

Usami and Satish Kumar (1996) conducted pseudodynamic

experiments on five steel box columns. The specimens were designed in

accordance with the code of the Japan Road Association (JPA). The

earthquake accelerograms used were those prescribed by Japan public works

research institute (manual-1993) for level-2. The scaled model for the steel

bridge piers of the box section was tested under a constant axial load, using

site-specific spectrum –compatible accelerograms. From the above test it was

observed that the columns sustained a damage in the form of local buckling,

due to the large deformations and low-cycle fatigue. The overall damage

34

sustained was quantified by means of a damage index developed by the

theoretical considerations and cyclic loading tests. The damage index was

meant to assess the safety against the collapse and did not consider the

serviceability requirements.

Lakshmanan et al (2007) studied seismic damage through the

dynamic characteristics. They studied ductility based reinforced concrete

structures. The damage undergone by a structure needed to be quantified

along with the post-seismic reparability and functionality of the structure.

They studied an analytical method of quantification and location of seismic

damage. A time period based damage identification model was used in this

study, which is suitably calibrated with classic damage models. Multi

resolution analysis using wavelets was also utilized to localize damage

identification for soft storey columns.

Sreekala et al (2007) conducted an experimental study on using a

simple technique to evaluate the damage indices for R.C beams under cyclic

loading. Applicability of damage indices for concrete was limited to the

limited calibration of the indices against the observed damage in laboratory

tests or earthquake investigations. Here the experiment was made to find a

simple and realistic indicator, in order to give a reliable measure of the

structural damage. This indicator can be made use of in the design stage

calculation of medium and long period structures. A simple set of

relationships, connecting damage index to the failure cyclic ductility ratio,

shear span to depth ratio and cyclic amplitude was derived. The beam was

suitably designed to meet the above requirements.

35

2.3.4 Pinching Effect

Shao et al (2005) studied the cyclic analysis of concrete-filled fiber

reinforced polymer tubes (CFFT). Six CFFT beam columns were tested with

2.4 m length and 0.3 m core diameter under constant axial loading and reverse

lateral loading in four-point flexure. Three of FRP (Fiber Reinforced Plastic)

tubes were made of glass and the other three were of filament wound with

5mm thickness. One specimen of each type of tube had no internal

reinforcement while the other two incorporated approximately. From the

above study it was observed that two types of tubes represented two different

failure modes, a brittle compression failure for the thick tubes with the

majority of the fibers in the longitudinal direction and a ductile tension failure

for the thin tubes with off-axis fibers. Also from the hysteretic loop it was

observed that the pinching effect of linear CFFT could be improved. The

nonlinear FRP provided much wider and more stable hysteretic loops. While

nonlinearity of FRP slightly improved the pinching effects in the absence of

internal steel, nonlinear FRP with internal steel appeared to shed its pinching

effects totally. Pinching did not seem to be affected much by the L/D ratio. It

appeared that the higher deformation capacity of nonlinear FRP improved

crack closure in concrete. On comparing strength ratios to CFST, it was very

close to CFFT tubes. Study showed that CFFT beam column can be designed

with a ductile behaviour comparable to RC members.

2.4 ANALYSIS AND DESIGN

Ge and Usami (1994) conducted a research on strength analysis of

concrete-filled thin walled steel box columns. An elasto-plastic finite element

displacement analysis of concrete-filled thin walled steel stub-columns of box

shape was presented in this study. In the analysis a hardening–softening

model was used to describe rationally the elasto-plastic behaviour of concrete.

36

A contact element for the interface combined with a bilinear constrained shell

element for the plate and a cubic element for the concrete was employed.

Both initial, geometrical imperfections and residual stresses were also

considered in the plate elements. Analytical results were compared with the

previous experimental results. The conclusion of the study was that the

ultimate strength of concrete-filled columns obtained from the analysis

generally agrees well with the experimental results.

Tryland et al (2001) carried out a finite element a numerical study

on aluminum and steel beams subjected to concentrated loading and

compared with data available in the past literature. The modeling test

specimens were simply supported beams. The concentrated load was applied

either mid-span and at the support and the influence of varying bending

moment and beam overhang was investigated. The contact between the beam

specimens and the loading bars was modeled with a contact algorithm and the

problem was solved by an explicit code. The correlation between

experimental and numerical results was quite good especially for ultimate

moment capacity. The results showed that small elements were necessary for

predicting the correct mode of failure and the development of instability was

dependent on the mass scaling and assumed imperfection field. Furthermore,

brick elements seemed to indicate the effect of hydrostatic pressure and the

adopted fracture model seemed to predict the crack development.

Johansson et al (2001) conducted an experimental and analytical

study on the structural behaviour of slender circular steel-concrete composite

columns. Eleven specimens were tested and the load was applied eccentrically

to the concrete section, to the steel section or to the entire section. Three-

dimensional nonlinear finite element models were established in order to

verify the experimental results. The analytical model was used to study the

behaviour of column and the bond strength between steel and concrete core.

37

The results obtained from tests and FEA on the circular steel-concrete

composite columns presented the following conclusion. For all the columns,

the maximum load capacity was determined by global buckling with no sign

of local buckling of the steel tube at the critical cross-section. The effect of

composite behaviour and confinement had increased the strength of concrete.

Mursi et al (2002) conducted a study on non-linear analysis of

slender concrete-filled steel columns incorporating local buckling. This study

presented numerical sections incorporating material non-linearity combined

with geometric nonlinearties. The equilibrium of the member was investigated

in the deformed state using the idealized stress-strain relationship for both

materials of steel and concrete considering elastic and plastic range. The

effect of the confined concrete core was addressed, showing good agreement

with the experimental results of the concrete-filled columns with compact

sections. The effect of elastic and inelastic local buckling was also

investigated. The analysis in the post-buckling range will be very useful for

implementation in advanced analysis programme.

Lakshmi and Shanmugam (2002) reported a research study on

behaviour of in-filled column using a semi analytical method to predict the.

Moment-Curvature-thrust relationships. Nonlinear equilibrium equations

resulting geometric and material nonlinearities were solved by an

incremental-iterative numerical scheme based on the generalized

displacement control method. In this analysis, square, rectangular and circular

shaped compact steel tubes were filled with concrete. The columns were pin-

ended and subjected to uniaxial or biaxial loading. Based on the results,

moment capacity of columns was found to decrease with an increase in

applied axial load. For the eccentrically loaded columns, the load carrying

capacity was found to drop significantly with an increase of eccentricity.

38

Al-Rodan and Al-Tarawnah (2003) carried out a finite element

analysis of rectangular tubular sections filled with high-strength concrete

using ABACUS software package. This research paper presented a three-

dimensional and materially nonlinear Finite Element (FE) model for RHS

filled with HSC (High Strength Concrete) which was used as a beam. The

comparison between the numerical and the experimental results demonstrated

the validity of the finite element model. The experimental results were also

compared with the Eurocode 4 predictions. It showed that, the design method

of Eurocode 4 for high strength concrete beams appeared generally on the

safe side.

Nassif (2004) conducted a research on finite element thermal

analysis of concrete-filled hollow steel sections to investigate the fire rating

and the transient behaviour during fire. Fire structural design for various fire

scenarios was explored with particular reference to the emerging Eurocodes.

The thermal modeling was based on dry and wet values for the thermal

properties of concrete. Change in thermal conductivity of concrete at elevated

temperature plays a significant part in the thermal analysis. Isothermals and

transient temperature gradients in steel filled tubes have been established.

Valivonis (2006) conducted an investigation on the contact between

the profiled steel sheeting and the concrete. In designing the composite

structures it was necessary to verify the following three sections namely

normal, diagonal and horizontal. They were distinguished stages in the

behaviour of contact between the concrete and profile sheet steel. A

composite action was provided by friction and anchors until the chemical

bond was effective in the first stage, after the failure of chemical bond in the

second stage and after the failure of mechanical bond in the third stage. It was

reported that deformation and contact strength was substantially influenced by

the shape of the profiled sheeting.

39

Han (2007) conducted an analytical investigation on concrete-filled

steel tubes to find out the torsional capacity. To date, such a problem has not

been addressed satisfactorily by design codes. This study is an attempt to

bring the torsional behaviour of concrete-filled thin-walled steel tubes.

ABACUS software is used for the Finite Element Analysis (FEA) of CFST

subjected to torsion. The FEA modeling was used to investigate the influence

of important parameters that determine the ultimate torsional capacity. The

comparisons of results calculated using this modeling were in good agreement

with the test results. The parametric studies provide information for the

development of formulae to calculate the ultimate torsional strength.

2.5 REVIEW OF THE DESIGN CODES

Design guidance is provided for concrete-filled steel tubes in some

codes of practices, particularly in Eurocode 4:1994, ACI–318:1989. The

design methods described in these two codes are different in concept. The

Eurocode 4 method is similar to that of steel section members while the

concept of ACI is the traditional reinforced concrete approach.

A comparative study of the design codes with the experimental

studies (Elchalakani et al 2001; Gho et al 2004; Han 2004b) shows the

methods for deriving the ultimate strength in Eurocode 4. It also provides

more accurate predictions than other procedures. In the following sections a

review of design methods provided in different codes of practices on steel-

concrete composite section is presented. Review mainly concentrates on four

design codes. They are Eurocode 4:1994, ACI–318:1989 and AISC-LRFD:

1999 and CIDECT.

40

2.5.1 Eurocode 4-1994

2.5.1.1 Ultimate strength

Among the major assumptions in the analysis of the ultimate

moment of resistance of the composite beams, the steel portion is considered

to be perfectly plastic. Analysis also assumes full strain compatibility that

exists between steel and concrete at the interface. The design concrete

strength is considered as 0.85fck where fck is the characteristic compressive

strength of concrete. The concrete portion in the tensile zone is assumed to

contribute no strength in the tensile zone.

2.5.1.2 Local buckling

The width-to-thickness ratio for the rectangular hollow section in

the composite members is kept as

52 235h f yt (2.1)

where h is the greater overall dimension of the section parallel to a

principal axis

t is the thickness of the wall of a concrete-filled hollow steel

section,

fy is the yield strength of the steel in MPa.

2.5.1.3 Bond strength

It is assumed that the shear resistance shall be provided by bond

stresses and friction at the interface, so that no significance slip occurs. The

design shear strength bond and friction, for concrete-filled hollow steel

41

sections are given by 0.4MPa. However this assumption of bond strength is

only valid for normal strength concrete with compressive strength upto

50MPa.

2.5.1.4 Properties of concrete

One of the major limitations of the current Eurocode 4 is the limit

on the concrete strength, which is not more than 50MPa. The secant modulus

of elasticity for 50MPa concrete is 37000MPa as mentioned in Eurocode. The

nominal value of Poisson’s ratio for elastic strains is assumed as 0.2. The

Poisson’s ratio may be assumed to be zero when concrete in tension is

assumed to be cracked.

2.5.1.5 Elastic flexural stiffness

The elastic flexural stiffness of the composite section is given in the

Eurocode as,

0.6EI E I E Is s ccd (2.2)

where Is and Ic are the second moment of areas of structural steel and the

concrete respectively.

Es is the elastic modulus for the structural steel and

EcmEcd c

where Ecm is the secant modulus of elasticity of concrete and

γc = 1.35 is the safety factor for stiffness.

42

2.5.1.6 Moment of resistance

The moment of resistance of a HSS beams in EC4 was determined

based on the plastic stress distribution as well as the full strain compatibility

in the cross section for both steel and concrete.

2maxM R w f w f w fpa ps pcyd sdd cd (2.3)

where wpa - Plastic section modulus of the structural steel

wps - Plastic section modulus of the reinforcement

wpc - Plastic section modulus of the concrete part of section

(for the calculation of wpc the concrete is assumed to be

uncracked)

fyd - Design strength for the structural steel

fsd - Design strength for the reinforcement

fcd - Design strength for the concrete

2.5.2 ACI –318:1989

2.5.2.1 Ultimate strength

The ACI design recommendations are used in the reinforced

concrete design approach. For the analysis of the ultimate moment of the

composite section, a full bond interaction is considered between steel and

concrete. For the concrete stress distribution in the overall section, a

uniformly distributed concrete stress of 0.85fck is considered over an

equivalent compression zone bounded by the edges of the cross-section and a

straight line parallel to the neutral axis at a distance d from the extreme fiber

concrete, fck is the characteristic compressive cylinder strength of concrete.

The factor d depends on the concrete strength. It is 0.85 for concrete strength

43

fck ≤ 30MPa and is reduced continuously at a rate of 0.05 for each 7 MPa of

strength in excess of 30 MPa, but must not be taken to be less than 0.65. It is

also assumed that tensile concrete carries no strength.

The stress in the steel below the yield strength fy is taken as Es

(elastic modulus of steel) times of steel strain while the steel stress is

considered as fy when the strains are greater than yield strain.

2.5.2.2 Properties of concrete

In the ACI design method, no consideration is given for high

strength concrete. The secant modulus of elasticity of concrete is calculated

from the formula:

1.5 0.043E fc cc (2.4)

This formula is not suitable for high strength concrete, because it

overestimates the modulus of elasticity for concrete strength above 40 MPa.

No special considerations are included for high strength concrete for

shrinkage and creep

2.5.2.3 Elastic flexural stiffness

The elastic flexural stiffness of the steel-concrete composite section

in the ACI is given as,

0.75 0.2 c c s sEI E E II (2.5)

44

where Ic is the second moment of area of the gross composite cross-section

ignoring steel

Is is the second moment area of the steel

Es is the modulus of elasticity of the steel and

Ec is the elastic modulus of concrete.

2.5.2.4 Moment of resistance

ACI codes suggested the following equation for finding the

ultimate moment of the sections. Similar to EC4, the strength of the concrete

in tension was ignored in ACI.

0.85 /2maxM Z f Z fckp y con (2.6)

where Zp - Plastic section modulus of steel tube

Zcon - Plastic section modului of the concrete part of section

fy - Yield strength of structural steel

fck - Characteristic strength of concrete

2.5.3 AISC-LRFD: 1999

Similar to EC4 , AISC predicted the flexural strength of a concrete-

filled section (CFS) by assuming the plastic stress distribution in the cross-

section for both the steel and concrete. The steel and concrete were assumed

to have the strength of fy and fck respectively at the moment capacity.

However, the flexural strength of the CFS column was determined based only

on the hollow steel section.

45

In AISC-LRFD method the equation for moment of resistance of a

HSS beam is as follows:

2 2 11/ 3( 2 ) ( / 2 /1.7 )n y r r yr w c w yM Z f h C A f h A fy f h A f (2.7)

where Mn - Ultimate moment of composite cross-section

Aw - Web area of reinforcing steel

Ar - Area of steel reinforcing steel

Z - Plastic section modulus of the hollow steel tube

Cr - Average distance of the reinforcement

h1 - Width of the member perpendicular to the plane for

bending

h2 - Width of the member parallel to the plane of bending

fc - Concrete cylinder strength

fyr - Yield strength of reinforcement steel

fy - Yield strength of the steel tube.

2.5.4 CIDECT Standard

The ultimate moment capacity for concrete-filled SHS sections can

be expressed as

22

y(ratio)u CIDECT

D B- D-2 B-2 fM =M

4t t

(2.8)

where M ratio - Ratio of the bending capacity of composite hollow steel

section to that of hollow steel section.

D - Depth of the section

B - Breadth of the section

t - Thickness of the section

fy - Yield stress of the section.

46

2.6 Conclusion of the review and scope of the investigation

From the exhaustive review of literature presented the following

conclusions are made.

1. It is reported that many research works have been carried out

pertaining to composite columns. It was shown that concrete

filling increases the strength of composite members.

2. The filling materials like light weight aggregate concrete,

foamed concrete, normal mix concrete, high strength concrete

and low strength concrete can be used in composite

construction to increase the flexural capacity of hollow steel

section. The hollow steel section shows similar behaviour

except in increase in ductility. Void filling prevented the local

buckling for very large rotations. Concrete filled sections have

more initial stiffness than hollow sections and at peak load

stage its stiffness decrease rapidly.

3. Fiber reinforced concrete filled steel tubes moderately

improves upon the behaviour of concrete filled steel tublar

column subjected to eccentric loading. The ductility behaviour

of fiber reinforced concrete-filled specimens is found to be

slightly better than the plain concrete filled tubular columns.

These columns have relatively more stiffness than the plain

concrete filled column and undergoes less deflection and also

sustained large strains and deformations.

4. Some parameter study also included the shape of the steel

tube and d/t ratio. From these investigation it is concluded that

circular steel tubes offer much more post-yield axial ductility

47

than the square and rectangular tube sections. Therefore

appropriate selection of width-to-thickness ratio is required in

calculating the ultimate axial load capacity.

5. It is also observed that the bond in composite section is

significantly affected by the type of concrete. Lightweight

aggregate concrete showed higher resistance to push-out loads

and thus had better composite action. Moreover bond

reduction due to age in normal concrete specimens is higher

than the lightweight concrete specimens.

6. The concrete filled section under cyclic loading significantly

increases both ductility and energy absorption capacity when

compared to hollow steel sections. In columns inward local-

plate buckling displacements are prevented by filled concrete,

the local displacements are delayed and results increase of

ductility and energy absorption capacity.

7. For beams filled with concrete, subjected to high amplitude

cycle loading, once local mechanism forms, residual strength

reduces within a few cycles. Due to filler material increase in

ductility and energy absorption capacity were observed.

8. While considering pinching behaviour effect , due to filling of

hollow sections pinching effect of CFST could be improved.

9. A comparison of failure loads between the experimental and

international standard design codes has been presented in

some works. The design codes are Euro code (EC-4), ACI,

AISC-LRFD and CIDECT. Based on these results it is

reported that codal provisions evaluate the load-carrying

capacity of the composite sections conservatively.

48

From the detailed review of research works already carried out it

can be seen that, the behaviour of CFST tubes as compression members were

well understood and the codal provisions are also available. But the works

related to the study of flexural behaviour of concrete filled steel tubular

sections are only limited. Hence in this investigation it is planned to study the

flexural behaviour of CFST-Beams under static and cyclic reversal load

conditions.