Advances in Steel Structures - part 44 docx

STRENGTH AND DUCTILITY OF HOLLOW CIRCULAR STEEL COLUMNS FILLED WITH FIBRE REINFORCED CONCRETE G.. Results of fibre reinforced composite columns were compared with those of columns fille

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Study of High Strength Concrete Filled Circular Steel Columns 411

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STRENGTH AND DUCTILITY OF HOLLOW CIRCULAR STEEL COLUMNS FILLED WITH FIBRE REINFORCED CONCRETE

G Campione, N Scibilia, G Zingone Dipartimento di Ingegneria Strutturale e Geotecnica Universitb, di Palermo, 1-90128, ITALY

ABSTRACT

The focus of the present investigation is the study of the behaviour of hollow circular steel cross- sections filled with fibre reinforced concrete (FRC), subjected to monotonic loads Using the same volume percentages of fibres, the influence of different types of fibres (steel, polyolefin) on the behaviour of the columns was investigated Results of fibre reinforced composite columns were compared with those of columns filled with plain concrete, showing the advantages of using FRC compared to plain concrete, in terms of both strength and ductility A simplified analytical model to predict load-deformation curves for composite members in compression is proposed, and the comparison between experimental and analytical results has shown good agreement Finally, a comparison is made between the bearing capacity of circular hollow steel columns filled with plain concrete evaluated according to recent European and International codes and that evaluated using the proposed model

KEYWORDS

Circular steel columns, fibre reinforced concrete, composite members, strength, ductility, active confinement

INTRODUCTION

In the design of tubular structures, after determining the shell plate thickness satisfying tensile stress, the stability of the shell should be checked for compressive stresses against buckling A thin-walled cylinder shell subjected to compression may fail either due to the instability of the shell, involving bending of the axis, or due to local instability, as shown in Figure 1, and also depends on the ratio of the thickness to the radius of the shell wall and on the length of the columns Failure of this type of structure is due to the formation of characteristic wrinkles or bulges, circular or lobed in shape To mitigate or prevent this type of failure it is common to encase or fill steel profiles with concrete The coupling of concrete and steel shapes makes it possible to obtain structural elements which, compared to the single constituent elements, ensure high performance in terms of both resistance and ductility, Cosenza & Pecce (1993)

413

414 G Campione et al

Modern codes like the Eurocode relating to composite steel-concrete structures contemplate the use of composite steel-concrete columns made using W shapes or thin-walled tubular profiles with a circular, rectangular or square section, as shown in Figure 2 Concrete, inside or outside the profile, exerts beneficial confinement action against phenomena of local or overall instability and markedly increases the resisting and dissipative capacity of steel columns, Schneider (1998) A further advantage is the increase in fire resistance, Lie (1994), Frassen et al (1998)

Figure 1: Overall and local instability Dwelling on the case of tubular steel columns, we can observe that, filled with concrete, they present deeply different behaviour from hollow ones and much better performances There is not only a higher bearing capacity, but also greater safety against flexural instability, which makes it possible to construct particularly slender columns The concrete is highly confined and hence is more ductile and has a greater bearing capacity, Shakir et al (1994) The coupling of several tubular sections makes it possible

to face very high stresses, greatly increasing the critical load values, both against local and overall buckling of composite columns

Figure 2: Typical composite members Recent applications of the structural elements concern arch road bridges in which the deck is supported

by hollow steel columns filled with concrete These very slender elements, thanks to the combined use

of the two materials, can face the high stresses induced by external loads, with the evident advantage of constituting carpentry during the construction of the structure

Recent theoretical and experimental studies have shown that if traditional reinforcement made up of bars and stirrups is added inside tubes, or these are filled with fibre reinforced concrete (FRC), their

Strength and Ductility of Hollow Circular Steel Columns 415

bearing capacity and ductility increase, Campione et al (1999) In the last few decades interest in the field of composite materials and especially in fibre reinforced concrete has led to the development of new types of fibres (carbon, polyolefin, kevlar, steel) with different shapes (hooked, crimped, deformed) Due to the bridging capacity of the fibres across the cracks, FRC behaves better than either plain concrete or plain concrete confined with traditional reinforcement in terms of energy absorption capacity, and sometimes strength, especially when a high volume fraction of fibres (1-2 %) is used, Campione et al (1999)

EXPERIMENTAL INVESTIGATION

In the present section there are briefly mentioned experimental results discussed in detail in a previous investigation by the authors, Campione et al (1998) The experimental research involved the casting of different types of composite members: steel columns, steel columns filled with normal strength concrete (NSC) and steel columns filled with fibre reinforced concrete (FRC) Different types of fibres (polyolefin straight, hooked and crimped steel fibres) were added to the fresh concrete at a dosage of 2% by volume The fibres had the characteristics shown in Table 1

TABLE 1 CHARACTERISTICS OF THE FIBRES Type of

fibres

Shape Diameter

equiv

, (ram)

Length Lf

(mm) strength Tensile

f't (MPa)

Modulus of Weight elasticity density

Ef (MPa) (kg/ m 3)

Hooked steel ~ "-" 0.50 35 1115 207000

900

7860

7860

The columns had a circular cross-section welded along their length; the yielding stress was 206 MPa and the ultimate stress 324 MPa; the internal diameter was 120 mm and the thickness 3.5 mm, the length of the entire columns was 1000 mm

Figure 3: Composite columns tested in compression The columns were tested in uniaxial compression utilising a universal testing machine operating in displacement control (Figure 3) A load cell and several LVDT's connected to a data acquisition system were used to record the load P and the vertical deformation 8 of the columns

Monotonic tests were carried out on 100x200 mm cylinders in concrete and FRC both in indirect split

tension and in compression to characterise the materials as shown in Figure 4 It is interesting to observe that adding fibres to the matrices the behaviour of the latter changes significantly, especially in

416 G Campione et al

the sottening branch, in terms of both energy absorption capacity and residual strength More details on the strength and strain values and cyclic response of the materials are given in a previous paper, Campione et al (1998) Table 2 gives the most rapresentative experimental results of the compression and indirect split tension tests, particularly the maximum compressive strength f c, and corresponding strain e0, and maximum tensile strength ft

TABLE 2 EXPERIMENTAL RESULTS FOR COMPRESSION AND INDIRECT TENSION TESTS

Types of fibres

Matrix

E;o

0.0032

f~

(A/IPa)

25.20

(*)f,

(MPa)

1.64

Hooked steel 0.0061 27.45 3.56

Polyolefin 0.0034 29.34 2.42 Crimped steel 0.0064 35.40 2.72

(*) ft=2P/0td h)

Figure 4: Monotonic tests in compression of FRC with 2% fibres Figure 5 gives load-deformation (P-8) curves in compression for steel pipes and composite members in the case of montonic loads Experimental results have shown that columns filled with FRC exhibit higher strength than those filled with plain concrete The maximum strength of composite members filled with FRC is 20 % higher than that recorded for steel pipes filled with FRC After the peak load was reached, failure was due to the crushing of concrete and to local and global buckling of the steel pipes At this point, the peak load and also the stiffness decreased By contrast, the addition of fibres ensured better softening behaviour and more available ductility

Figure 5" Load vs deformation curves for composite columns with 2% fibres

Strength and Ductility of Hollow Circular Steel Columns

STRENGTH OF COMPOSITE COLUMNS SUBJECTED TO COMPRESSIVE LOADS

417

Several European and international codes give design rules and simplified formulae to predict the bearing capacity of steel columns filled with plain concrete These are able to take into account the strength of the materials, and the buckling problems of the composite members When columns are subjected to axial forces the properties which must be taken into account in the design of members include strength of the constituent materials, local instability, and the capacity to transfer internal stresses between the steel pipe and the concrete core

For composite members having a transverse circular cross-section, EC4 allows one to neglect local buckling problems when the ratio between the diameter d and the thickness t obeys the relationship d/t<90 e 2, where e 2=235/fy, and fy is the yield stress of the steel According to EC4, the axial plastic force of the transverse cross-section of a steel column filled with plain concrete can be obtained, as a first approximation, as the sum of the contributions of the concrete core and the steel section, also taking into account the increase in strength due to the confinement in the concrete core and the reduction in steel stress due to the biaxial state of stress in the steel pipes; this is done by introducing the coefficients rl, which depend on the slenderness of the composite members The ultimate axial load capacity of the columns is obtained by multiplying the axial plastic force by the reduction coefficient ~, which depends on the slenderness ,i

~ , _ _ _ _ _ ~

TTT

~~- ~" t ~'II,"L'"ll

l~s ,t

Figure 6: Distribution of tensions in the steel pipes

It is interesting to observe that EC4 and LRFD give very conservative values of the maximum beating capacity for short columns In fact, in composite members loaded concentrically the concrete core and steel tube are subjected to a combined state of stresses (Figure 6) For this reason many researchers, like in Pecce (1993), have modelled the behaviour of composite members subjected to compression considering a triaxial state of stresses in the concrete core and a biaxial state in the steel pipes and imposing the compatibility for each loading step in terms of longitudinal and lateral strains Unfortunately, few experimental data are available for plain concrete and fibre reinforced concrete subjected to triaxial stresses, and so it is difficult to calibrate the parameters of an analytical model of concrete core subjected to a multiply stresses

In the present investigation the authors refer to a monoaxial state of stresses for concrete, in which the confinement effect due to the lateral pressure f'~ of the steel pipes, varying at each loading step, is taken into account For steel pipes a biaxial state of stresses is assumed and the relationship between longitudinal and circumferential stresses is that proposed by Von Mises up to failure condition:

418 G Campione et al

For steel in uniaxial tension an elasto-plastic stress-strain relationship was assumed, having conventional yielding stress of 300 MPa When the yielding condition occured in steel pipe due to biaxial stresses a linear reduction of longitudinal stress ~s,I up to 2 % strain was assumed, in accordance with experimental data (Figure 7-b) It was also assumed that circumferential stress ~s,t in steel pipes, evaluated using Eqn 1, increased up to the yielding value and consequently that in the concrete core the confinement effect was maximum (Figure 7-a) The longitudinal stress in the concrete core was obtained utilising the Mander et al (1988) model in which the lateral pressure fl is assumed variable The Mander et al (1988) stress-strain curve for concrete adopted is:

~c

t3~ - - - r

~ cc

r - I + ~

The r coefficient is related with the initial tangent modulus Er and with the secant modulus Eso~=o~dec~

as follow:c

E~

r = ~ ( 3 )

The maximum strength in the concrete core ~cr is:

C~cc = f~"( 1.254 + 2 2 5 4 " 1 1 + ~ 7.94.f/ 2 - ~ '1 - - ~ (4)

L' L )

The lateral pressure ft on the concrete core is:

t

The maximum longitudinal strain is:

E - 11]

Figure 7 shows the longitudinal stress-strain relationship for plain concrete (f'c=25.2 MPa) and steel obtained assuming different values of maximum longitudinal stresses

To determine the load-deformation curves (P-8) of the columns the compatibility of the lateral and longitudinal deformations was assumed and an equilibrium equation was utilised in which the stresses were calculated through constitutive relationships given before

The vertical loads of the composite members were obtained by considering of the steel pipes and concrete core to strength, without any reduction in the transverse cross-section of the concrete core, because of the effective confinement reached in concrete inside steel pipes

Strength and Ductility of Hollow Circular Steel Columns 419

Figure 7: Analytical longitudinal stress-strain curves : a) concrete; b) steel

Figure 8 shows experimental and analytical results obtained with the model proposed It is interesting to observe that the model permits one to obtain a good level of approximation in terms of maximum bearing capacity and corresponding deformation, but overestimates initial stiffness This is due perhaps

to the fact that initial deformations are affected by boundary test effects that the model does not take into account

Figure 8: Comparison between analytical and experimental results for composite members

In the case of FRC the buckling effects are reduced by the presence of fibres and failure is due essentially to plasticization of materials ensuring a good prevision of experimental results with the analytical model based on these concepts In the case of steel pipes filled with plain concrete, the buckling effects reduce the bearing capacity of the columns in the softening branch and the analytical model is not able to predict these effects

TABLE 3

ULTIMATE LOAD OF COMPOSITE MEMBERS: EXPERIMENTAL AND ANALYTICAL VALUES

Columns filled with

Plain concrete

FRC with hooked steel fibres

FRC filled with polyolefin fibres

FRC filled with crimped fibres

g g NpI, R kc Per g me~ee Nsp 0.303 0.977 678 0.640 552 876 889 0.308 0.976 700 0.650 568 966 976 0.311 0.975 718 0.660 581 1030 980 0.322 0.972 778 0.670 623 1250 1055