Title of paper · Concrete filled steel tube (CFST) members have become of widespread structural...

Bauhaus Summer School in Forecast Engineering: From Past Design to Future Decision

22 August – 2 September 2016, Weimar, Germany

Experimental assessment of the flexural behaviour of rubberized

concrete-filled steel tubes

SILVA António, JIANG Yadong1, CASTRO José Miguel

Department of Civil Engineering, Faculty of Engineering, University of Porto 1 Istituto Universitario di Studi Superiori di Pavia, Italy

Abstract

This paper describes an experimental campaign in which a total of 36 concrete-filled steel tube

specimens were tested under flexural loading. The test campaign was conceived with a number of

parameters in mind, namely rubberized and standard concrete infills, cross-section type and

slenderness, aggregate replacement ratio, axial load level and lateral loading type. A special device

was developed as part of an innovative testing setup, aimed at reducing both the cost and preparation

time of the specimens. The members were tested under both monotonic and cyclic lateral loading, with

different levels of applied axial loading. The test results show that the bending behaviour of CFST

elements is highly dependent on the steel tube properties and that the type of infill does not have a

significant influence on the flexural behaviour of the member. It was also found that Eurocode 4 is

conservative in predicting the flexural capacity of the specimens

1. Introduction

Concrete filled steel tube (CFST) members have become of widespread structural use in high

seismicity areas. These members can be fabricated through precast or cast-in-place concrete infill of

steel tubes, with a wide assortment of cross-section types. CFST members exhibit unique advantages

over typical steel or reinforced concrete solutions. The synergy that results from an efficient

combination of the two materials is reflected in an increase of strength and, importantly, ductility, over

steel and reinforced concrete members. The behaviour of the concrete core is improved through

confinement effect provided by the steel tube, with gains in material strength and ductility. On the

other hand, the concrete core prevents inwards local buckling of the steel tube and outwards local

buckling is delayed to higher levels of deformation. CFST members are seen as an attractive structural

solution to adopt in seismic resisting structures due to their ductility and energy dissipation properties,

and excellent hysteresis behaviour under cyclic loading (Hajjar 2000).

Extensive research has been carried out in the past aiming at the characterisation of the behaviour of

axially loaded CFSTs (e.g., Schneider 1998, Han 2002, Sakino et al. 2004, Liu and Gho 2005 and

Ellobody et al. 2006). Regarding the flexural behaviour of these elements, several research studies

(Elchalakani et al. 2001, Varma et al. 2002a, Varma et al. 2002b, Han et al. 2003, Elchalakani et al.

2004, Han 2004, Han and Yang 2005, Han et al. 2006, Zhang et al. 2007, Elchalakani and Zhao 2008

and Jiang et al. 2013) reported ductile behaviour of CFSTs subjected to flexural loading conditions.

Additionally, some work has been developed in recent years in the field of CFST members with

sustainable infill materials. The use of recycled rubber particles in CFST concrete infill was

experimentally studied by (Duarte et al. 2016a) for stub columns under compression and (Duarte et al.

2016b) for stub columns under cyclic bending. The authors highlighted the enhanced ductility of

rubberized concrete in comparison to standard concrete.

From a European design perspective, (Eurocode 4 2004a) provides methods for the calculation of the

capacity of composite members under different loading conditions. Additionally, the code

prescriptions aim at preventing the development of local buckling mechanisms in CFST members,

before the ultimate loads of the structural system is reached. This is achieved by imposing cross-

SILVA António, JIANG Yadong, CASTRO José Miguel / FE 2016 2

section slenderness limits, as shown in Table 1. Moreover, The European seismic code, (Eurocode 8

2004b), makes use of this parameter for the definition of the ductility class requirements for

dissipative elements, as shown in Table 1, where 𝑓𝑦 is the yield strength of the steel tube.

Table 1. EC4 and EC8 𝑑/𝑡 limits for CFSTs

Cross-section type Eurocode 4

Eurocode 8

DCM DCM DCH

1.5 < 𝑞 ≤ 2 2 < 𝑞 ≤ 4 𝑞 > 4

𝑑 𝑡⁄ ≤ 90 ×235

𝑓𝑦

𝑑 𝑡⁄ ≤ 90 ×235

𝑓𝑦

𝑑 𝑡⁄ ≤ 85 ×235

𝑓𝑦

𝑑 𝑡⁄ ≤ 80 ×235

𝑓𝑦

ℎ 𝑡⁄ ≤ 52 × √235

𝑓𝑦

ℎ 𝑡⁄ ≤ 52 × √235

𝑓𝑦

ℎ 𝑡⁄ ≤ 38 × √235

𝑓𝑦

ℎ 𝑡⁄ ≤ 24 × √235

𝑓𝑦

This paper mainly focuses on: 1) the experimental assessment of the influence of rubberized concrete

(RuC) in CFST members under monotonic and cyclic bending; 2) the comparison of the experimental

results with expected design capacities according to Eurocode 4.

2. Description of the test campaign

2.1. Specimen definition and material properties

In order to characterize the behaviour of CFST columns under simple and combined bending, a

comprehensive experimental campaign was carried out. A total of 16 CFST members of circular cross-

section type, 12 of which made with rubberized concrete (RuC) and the remaining with standard

concrete (StdC), 16 CFST members of square cross-section type, 12 RuCFST and 4 StdCFST, and 4

RuCFST members of rectangular cross section were selected and tested. For the definition of the test

specimens a number of parameters were considered, namely the cross-section slenderness ratio 𝑑/𝑡 or

ℎ/𝑡, the concrete aggregate replacement ratio 𝛽, the axial load level 𝑛 and the lateral load type. The

total length of each specimen was set to 2 meters.

Regarding the parameter 𝑑/𝑡 or ℎ/𝑡, the former applied to circular tubes and the latter to square and

rectangular tubes, 𝑑 and ℎ are the maximum external dimension of the encasing steel tube, and 𝑡 the

corresponding thickness. In order to assess the influence of this parameter on member ductility, both

high and low values were considered for circular and square specimens, taking into account the

requirements of Eurocode 8 for high and medium ductility class CFSTs. Therefore, two different

circular cross-sections (219mm in diameter with a nominal tube thickness of 3mm and 5mm), two

square cross-sections (180mm and 200mm of width, with 3mm and 10mm of thickness, respectively)

and a single rectangular member (250mm by 150mm and a nominal thickness of 12mm). Henceforth,

the circular steel tubes will be referred to as C219x3 and C219x5, the square steel tubes as S180x3 and

S200x10, and the rectangular tube as R250x150x12. All steel tubes used in the test campaign were

cold-formed, and, considering that each tube of the same steel section type comes from a single lot,

tensile testing of steel coupons taken from a set of specimens was performed. In Table 2 a comparison

between the steel mechanical properties before and after the cold-forming process, namely in terms of

steel yield strength, 𝑓𝑦, and ultimate strength, 𝑓𝑢, is shown. Additionally, an assessment of the steel

tube thickness was carried out. To this end, a total of 8 thickness measurements per specimen were

performed. Table 2 summarizes the average results obtained for each steel section type, in terms of

mean values, 𝜇, and the corresponding standard deviation, 𝜎.


Table 2. Steel tube material properties

Steel Section

Steel tube thickness Before cold-forming After cold-forming

𝜇

[mm] 𝜎

𝑓𝑦

[MPa]

𝑓𝑢

[MPa]

𝑓𝑦

[MPa]

𝑓𝑢

[MPa]

C219x3 2.96 0.07 343 414 308 373

C219x5 4.72 0.11 341 405 393 485

S180x3 2.97 0.10 353 421 320 403

S200x10 9.75 0.15 411 519 425 525

R250x150x12 11.93 0.09 560 581 550 585

Concerning the concrete used to infill the steel tubes, a reference concrete StdC (𝛽 = 0%) and two

rubberized concrete mixtures, RuC5% (𝛽 = 5%) and RuC15% (𝛽 = 15%), were used in the test

campaign. Although theoretically based on the reference concrete by simple material quantity

substitution, the rubberized concrete mixtures have a modified mixing ratio in order to have the same

European slump class S3 of 125 ± 15mm across all concrete types. The obtained mixing ratios for

each concrete type are summarized in Table 3, as well as the average cube compressive strength of the

three concrete types. The tested concrete cubes were taken during the steel tube column pouring

process, and were tested at 28 days of age.

Table 3. Concrete mixing ratios and material properties

Concrete type

Mixing component 𝑓𝑐

[MPa] Water

[l/m3]

Cement

[kg/m3]

0/4 GF85

[kg/m3]

4/10 GC85/20

[kg/m3]

Rubber

[kg/m3]

StdC 216 420 551 1072 - 53

RuC5% 216 420 551 1019 54 39

RuC15% 227 420 542 896 158 20

All concrete mixtures had a similar formulation as those considered in the experimental work carried

out by (Duarte et al. 2016a) and (Duarte et al. 2016b). As reported by the authors, RuC concrete

mixtures were associated with larger ductility in comparison with StdC concrete.

Since one of the main purposes of the research project is to gauge the difference between the ductile

behaviour of RuCFST and CFST, the likelihood of steel tube buckling at a relative low bending

deformation should be avoided. Consequently, steel tubes C219×3, S180x3 and R250x150x12 were

only casted with concrete type RuC15%, as opposed to tubes C219×5 and S200x10 which are infilled

with the three concrete types.

As to the axial load level 𝑛, this factor was considered as 𝑁0 𝑁𝑢⁄ , where 𝑁0 is the axial load applied to

the specimen and 𝑁𝑢 is the cross-section bearing capacity, simply defined as 𝑁𝑢 = 𝐴𝑐𝑓𝑐 + 𝐴𝑎𝑓𝑢,

where 𝐴𝑐 and 𝐴𝑎 are the concrete core and steel tube cross-section area, respectively, 𝑓𝑐 and 𝑓𝑢 are the

concrete compressive strength and steel ultimate strength, respectively. Additionally, two levels of

axial load of the composite column were targeted, namely 𝑛 = 0% (simple bending) and 𝑛 = 15%

(combined compression with bending) for circular members, and 𝑛 = 0% and 𝑛 = 10% for square

and rectangular members. The lateral loading protocol adopted for the cyclic tests was based on a

specimen drift 𝜃 dependent loading protocol (SAC 1997), in which six cycles are imposed for

𝜃 = 0.375%, 𝜃 = 0.50% and 𝜃 = 0.75%, four cycles at 𝜃 = 1%, and two cycles for the remaining

levels of 𝜃 with 1% increment.

All tested specimens are listed in Table 4, in which the specimen name is a simple concatenation of

cross-section type, concrete type, steel tube geometrical properties, axial load level and lateral load

type. Therefore, a specimen designated by SR-RuC5%-180-3-10%-C refers to a square RuCFST

member, with 𝛽 = 5%, with the steel tube S180×3, under cyclic lateral loading with a constant axial

load level 𝑛 = 10%.


Table 4. Specimen list

Designation 𝛽 Steel tube Axial load

[kN] Lateral load type

Circular

CR-RuC15%-219-3-0%-M

15%

C219×3

- Monotonic

CR-RuC15%-219-3-15%-M 222

CR-RuC15%-219-3-0%-C - Cyclic

CR-RuC15%-219-3-15%-C 222

CR-RuC15%-219-5-0%-M

C219×5

- Monotonic

CR-RuC15%-219-5-15%-M 290


CR-RuC15%-219-5-15%-C 290

CR-RuC5%-219-5-0%-M

5% C219×5

- Monotonic

CR-RuC5%-219-5-15%-M 359


CR-RuC5%-219-5-15%-C 359

CR-StdC-219-5-0%-M

0% C219×5

- Monotonic

CR-StdC-219-5-15%-M 380

CR-StdC-219-5-0%-C - Cyclic

CR-StdC-219-5-15%-C 380

Square

SR-RuC15%-180-3-0%-M

15%

S180x3

- Monotonic

SR-RuC15%-180-3-10%-M 141

SR-RuC15%-180-3-0%-C - Cyclic

SR-RuC15%-180-3-10%-C 141

SR-RuC15%-200-10-0%-M

S200x10

- Monotonic

SR-RuC15%-200-10-10%-M 391


SR-RuC15%-200-10-10%-C 391

SR-RuC5%-200-10-0%-M

5% S200x10

- Monotonic

SR-RuC5%-200-10-10%-M 391


SR-RuC5%-200-10-10%-C 391

SR-StdC-200-10-0%-M

0% S200x10

- Monotonic

SR- StdC -200-10-10%-M 391

SR- StdC -200-10-0%-C - Cyclic

SR- StdC -200-10-10%-C 391

Rectangular

RR-RuC15%-250-150-12-0%-M

15% R250x150x12

- Monotonic

RR-RuC15%-250-150-12-10%-C 391

RR-RuC15%-250-150-12-0%-M - Cyclic

RR-RuC15%-250-150-12-10%-C 391

2.2. Test setup

In order to combine the benefits of different experimental test setups of CFST members, e.g.

(Han et al. 2004 and Varma et al. 2004), as well as to minimise the costs and also the preparation time

associated with the test campaign, a steel box testing device was developed, as shown in Figure 1 and

Figure 2.

The testing device consists of a 1400×1400×60 mm steel base plate, anchored to the strong floor with

four Ø25mm rods using holes near the corners, and four steel walls with a height of 500mm and a

thickness of 50 mm welded to each other and to the base plate. In order to stiffen their connection to

the base plate, five stiffeners are welded on the exterior of each box wall. As a result, the internal


dimensions of the steel box are 750×750mm. The internal part of the box has custom made high

strength steel bolts and nuts, with four sets of bolts and nuts in each of the walls. Each steel bolt has a

length of 100mm with an additional Ø110mm hexagon cap on one end, to increase the contact area

between the bolt and the test specimen. For each bolt, two Ø110mm steel nuts are used, one with a

thickness of 70mm welded to the steel wall, to connect with the bolt end, whilst another with 25mm is

placed between the previous nut and the bolt hexagon cap, to prevent any movement of the bolt during

the test. After the placement of the specimen inside the box, one should unscrew the bolts until the bolt

caps are in full contact with the specimen, and finally, move the 25mm nut along the screw until it

reaches the steel wall nut. As shown in Figure 2, additional filling steel plates are placed between the

bolt hexagon caps and the specimen in order to provide proper basal restraint and load transfer. Due to

the layout of the bolts, only rectangular and square specimens can be installed and laterally restrained

without any member adaptation. For circular steel tubes, additional steel plates are welded to the

bottom part of the tube in order to allow a similar boundary connection to the box constraint

mechanism, as shown in Figure 3 In the test campaign, all the specimens are desired to have a fully

restrained base, with a vertical and lateral force applied at the top, as illustrated in Figure 4.

Figure 1. Overview of the designed steel box Figure 2. Specimen placement in the steel box

Figure 3. Base adaptation for circular specimens Figure 4. Test setup

Due to the characteristics of the test setup, the specimen free length is approximately 1.35m.

Additionally, and due to strength limitations of the testing device, the bending tests of the rectangular

cross-section columns were performed imposing a moment in turn of the minor-axis. Testing was

conducted either up to specimen failure or until reaching the range limits of the actuator. All test

specimens exhibited very ductile behaviour and testing proceeded in a smooth and controlled manner.


3. Experimental Bending Tests

3.1. Test results

The main results obtained during the tests are presented in the following paragraphs. Figure 5 to

Figure 12 show the test results of the circular composite columns, Figure 13 to Figure 20 depict the

results of the square columns, and, finally, Figure 21 and Figure 22 show the results of the rectangular

elements. The test results are presented for both monotonic and cyclic loading cases, in the form of

charts showing the applied lateral force and the corresponding specimen’s drift ratio, 𝜃, obtained by

dividing the top lateral displacement imposed to the specimen by the effective height of the specimen,

equal to 1.35m. It is important to note that the maximum values of drift ratio, 𝜃, are different between

specimens due to fact that rigid-body rotations, resulting from the flexibility of the testing device,

were taken into account in the calculation of the imposed lateral displacement.

Figure 5. Experimental behaviour of specimens CR-

RuC15%-219-3-0%


RuC15%-219-3-15%


RuC15%-219-5-0%


RuC15%-219-5-15%


RuC5%-219-5-0%

Figure 10. Experimental behaviour of specimens

CR-RuC5%-219-5-15%



CR-StdC-219-5-0%


CR-StdC-219-5-15%

Figure 13. Experimental behaviour of specimens SR-

RuC15%-180-3-0%


RuC15%-180-3-10%


RuC15%-200-10-0%


RuC15%-200-10-10%


RuC5%-200-10-0%


RuC5%-200-10-10%



StdC-200-10-0%


StdC-200-10-10%


RR-RuC15%-250-150-12-0%


RR-RuC15%-250-150-12-10%

It is important to note that specimen CR-RuC15%-219-3-0% tested under cyclic lateral loading

(Figure 3.1) exhibited noticeable unsymmetrical hysteresis. It is possible that this asymmetry may be

due to a single or a combination of different factors, namely some misalignment of the centre of the

specimen and the lateral load actuator (which may have resulted in some unexpected bi-axial bending

of the specimen during the test), the orientation of the longitudinal weld of the steel tube, the

variability of the steel tube thickness along the perimeter of the cross-section, damage of the steel tube

to one of the sides of the plastic hinge zone, and significant asymmetry of the aggregate particles

(normal and rubber) in the cross section of the plastic hinge zone.

In general, all monotonically tested composite columns, in simple bending or combined bending with

compression, developed outwards local buckling shapes at the specimen’s plastic hinge region.

Nonetheless, this proved to have a negligible influence on the overall behaviour of the member, as

denoted by the absence of strength degradation in practically all specimens. However, the cyclic

loaded specimens exhibited larger levels of local buckling, with a clear effect in the global behaviour

of the specimen, as denoted by the presence of strength degradation during the tests. Interestingly, the

cyclic loaded thin-walled test columns, i.e. using steel tubes C219x3, C219x5 and S180x3, exhibited

fracture of the steel tube at the plastic hinge region, after very pronounced local buckling. The

remaining steel tube types, i.e. S200x10 and R250x150x12, did not display the same phenomenon,

mainly due to a much higher steel tube thickness than the aforementioned specimen types. Figure 23

and Figure 24 show some examples of the global and local deformation of the composite members at

the final stages of each cyclic bending test.


Figure 23. Global and local deformation of circular

specimen CR-RuC5%-219-5-0%-C

Figure 24. Global and local deformation of square

specimen SR-RuC15%-180-3-0%-C

3.2. Influence of steel section infill with concrete

One key benefit of CFST members is the combination of the advantages of both the encased and

encasing materials, significantly changing the overall behaviour of the individual concrete and steel

parts. Therefore, and with the objective of determining the improvement in bending behaviour

prompted by simple steel section concrete infill, one circular C219x3 hollow steel tube specimen was

tested under monotonic bending without axial force. Thus, a comparison of the overall behaviour can

be made using a compatible composite specimen CR-RuC15%-219-3-0%-M, as shown in Figure 25.

Figure 26 shows the local buckling shape of the corresponding specimens at the final stage of the

experiment.

C219x3

CR-RuC15%-219-3-0%-M

Figure 25. Comparison of steel and CFST

bending behaviour

Figure 26. Comparison of steel and CFST local buckling

shapes

As expected, significant improvements in member behaviour, in terms of strength and ductility, are

accomplished when the tube is infilled with concrete, with an increase of 43% in peak lateral force.

Moreover, the analysis of the local buckling shapes shown in Figure 26 reveals that both are similar in

terms of amplitude, though it was clear during the test that the maximum amplitude was reached for

lower level of bending deformation in the case of the hollow steel tube specimen. However, a

comparison of the global force-deformation curves shows that only in the case of the steel specimen

does this local buckling mechanism governs beam-column behaviour. As shown in Figure 25,

specimen C219x3 develops significant strength degradation, whereas composite specimen CR-

RuC15%-219-3-0%-M exhibits a very stable post-elastic behaviour. Thus, the influence of steel

section infill by concrete not only increases the capacity of the member, but also substantially

enhances its ductility.


3.3. Influence of the type of infill concrete

One key objective of the test campaign was to assess the behaviour of sustainable CFST structural

members, by comparing both standard and rubberized concrete infill types. To this end, only two steel

sections, circular C219x5 and square S200x10, were considered to have all concrete types. Thus, all

other test results are disregarded in the following discussion. Figure 27 and Figure 28 show a

comparison of the different types on infill concrete used in the tested composite columns, for both

monotonic and cyclic loading cases, for the selected circular and square specimens, respectively.

Simple bending Combined bending with compression

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Figure 27. Influence of concrete type on the behaviour of circular specimens


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Figure 28. Influence of concrete type on the behaviour of square specimens


In general, the test results show that, despite 20% and 60% reductions in concrete strength, column

behaviour is not significantly different between the three concrete types (StdC, RuC5% and RuC15%).

It is important to note that the square specimen SR-200-10 filled with RuC5% tested in simple bending

(Figure 3.24), is noticeably different from the remaining specimens (StdC and RuC15%), namely in

terms of stiffness and flexural capacity. This could be explained either by the reported high variability

of the steel tube thickness, as well as some differences on the material properties of the steel between

this and the remaining members. Disregarding this specimen, and for the limited range of CFST

members for this assessment, it is possible to conclude that despite slight variations in terms of

member strength, the overall behaviour and ductility of the member is similar between specimens.

This shows that the bending performance of this type of composite element is highly dependent on the

steel tube, as fundamentally different concrete cores have little to no influence on the specimen’s

behaviour. This conclusion is applicable to both circular and square cross-section type members, as

demonstrated by the test results.

3.4. Influence of cross-section slenderness

Cross-section slenderness has a considerable influence on the behaviour of a structural member,

regardless of the type of loading or material. In the case of CFSTs, this parameter is defined as the

ratio 𝑑/𝑡 for circular specimens, or ℎ/𝑡 for square and rectangular specimens, and has an influence on

a number of behavioural characteristics such has member capacity, local buckling mechanism,

ductility and cyclic load degradation. In the context of the test campaign, only specimens of circular

steel sections C219x3 and C219x5, and square steel sections S180x3 and S200x10, with concrete infill

RuC15%, have compatible test results. Thus, in Figure 29 and Figure 30, this assessment is carried out

for both monotonic and cyclic loading cases, for the selected circular and square test specimens,

respectively. In the plots, the normalized lateral force was obtained by dividing, for every point of the

curve, the lateral force by the corresponding maximum absolute value, i.e. the maximum between the

maximum positive value and maximum absolute negative value of applied lateral force during the test.


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Figure 29. Influence of cross-section slenderness on the behaviour of circular specimens



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Figure 30. Influence of cross-section slenderness on the behaviour of square specimens

For circular composite specimens, the test results indicate a slight influence of cross-section

slenderness on the overall behaviour of the member, in addition to an expected increase in member

strength and stiffness, as the 𝑑/𝑡 ratio decreases. The monotonic test results do not indicate large

differences between both members, in spite of a 40% reduction in 𝑑/𝑡 from specimen CR-219-3 to

CR-219-5. As for the cyclically loaded specimens, the same conclusion can be withdrawn, if the

asymmetric behaviour of some tests is taken into account. Conversely, the square columns show a

more noticeable influence of ℎ/𝑡 on the behaviour of the member. In the monotonic tests, the ductility

of the member is lower for the thin-walled column, SR-180-3, as the peak bending capacity is reached

and some strength degradation is observed during the test. Moreover, in the cyclic tests, a lower value

of ℎ/𝑡 was associated with lower cyclic strength degradation, as is the case of specimen SR-200-10.

Particularly, for the case of combined bending with compression, significant differences in cyclic load

degradation and member ductility can be seen between both SR-180-3 and SR-200-10 specimens, with

the thin-walled column exhibiting a much poor behaviour. Generally, significant variations in member

behaviour and ductility are only substantial for square cross-section members, possibly validating the

considerable performance of circular CFST members in comparison with other cross-section types, in

line with the prescriptions of Eurocode 4 and Eurocode 8 for composite columns.

3.5. Comparisons with Eurocode 4

Taking into account the procedures for the calculation of the bending capacity of CFST columns

presented in Eurocode 4, it is possible to evaluate the accuracy of the code in predicting the strength of

the composite specimens that were tested in the experimental campaign described before in this paper.

Since no consideration is given in the code to the type of loading, i.e. monotonic or cyclic, these

comparisons can only be carried out for the monotonic cases. Therefore, in the following paragraphs a

comparison is shown between the obtained peak bending moment in the test 𝑀𝑢𝑇𝐸𝑆𝑇, given by the

multiplication of the peak applied lateral force 𝐹𝑢𝑇𝐸𝑆𝑇 by the free length of the column (1.35m), and the

corresponding value calculated with EC4 𝑀𝑅𝐸𝐶4, using the yield steel strength 𝑓𝑦 and average steel tube

thickness listed in Table 2, and the concrete compressive strength 𝑓𝑐 listed in Table 3. The calculation

procedure of 𝑀𝑅𝐸𝐶4 for CFSTs of circular and square/rectangular members is provided by Silva et al.


2016a and Silva et al. 2016b, respectively. No material partial safety factors were considered in the

application of EC4, either for the steel tube or for the concrete core. Table 5 shows a summary of this

comparison, separated between circular and square/rectangular columns, where 𝜇 is the average

difference of 𝑀𝑅𝐸𝐶4/𝑀𝑢

𝑇𝐸𝑆𝑇 and 𝜎 the corresponding standard deviation.

Table 5. EC4 design comparisons for monotonically tested specimens

Specimen 𝐹𝑢

𝑇𝐸𝑆𝑇

[kN]

𝑀𝑢𝑇𝐸𝑆𝑇

[kNm]

𝑀𝑅𝐸𝐶4

[kNm] 𝑀𝑅

𝐸𝐶4/𝑀𝑢𝑇𝐸𝑆𝑇 𝜇 𝜎

Circular

CR-RuC15%-219-3-0%-M 47.8 64.5 48.7 0.75

0.76 0.023

CR-RuC15%-219-3-15%-M 54.8 74.0 53.8 0.73

CR-RuC15%-219-5-0%-M 89.1 120.2 93.9 0.78

CR-RuC15%-219-5-15%-M 99.7 134.6 98.3 0.73

CR-RuC5%-219-5-0%-M 94.8 127.9 99.0 0.77

CR-RuC5%-219-5-15%-M 103.1 139.1 106.9 0.77

CR-StdC-219-5-0%-M 96.5 130.3 101.6 0.78

CR-StdC-219-5-15%-M 105.3 142.2 111.4 0.78

Square and Rectangular

SR-RuC15%-180-3-0%-M 40.3 54.4 51.6 0.95

0.87 0.170

SR-RuC15%-180-3-10%-M 31.0 41.9 54.5 1.30

SR-RuC15%-200-10-0%-M 229.8 310.2 237.2 0.76

SR-RuC15%-200-10-10%-M 210.0 283.5 239.4 0.84

SR-RuC5%-200-10-0%-M 268.0 361.9 245.3 0.68

SR-RuC5%-200-10-10%-M 231.8 312.9 250.6 0.80

SR-StdC-200-10-0%-M 236.9 319.8 249.9 0.78

SR-StdC-200-10-10%-M 226.0 305.2 256.3 0.84

RR-RuC15%-250-12-0%-M 260.5 351.7 286.4 0.81

RR-RuC15%-250-12-10%-M 231.0 311.9 287.2 0.92

The analysis of the obtained results reveals that EC4 is conservative in predicting the bending capacity

of the tested CFST and RuCFST specimens. Average differences of 24% and 13% between the code

and the experimental results were identified for circular and square/rectangular specimens,

respectively. More importantly, one can conclude that no significant differences were found between

𝑀𝑅𝐸𝐶4/𝑀𝑢

𝑇𝐸𝑆𝑇 for StdC and RuC type members. This confirms once again the reduced influence of

concrete type on the bending behaviour of a CFST member.

As the confinement effect of the concrete core is only significant for the case of circular members

under compression loading, one can disregard this concrete behavioural enhancement as the main

reason for the conservativeness of EC4. However, other factors may justify the differences observed,

namely the multiaxial stress state in the steel tube or the fact that the EC4 calculations were performed

based on the yield stress of the material. According to EC4, the steel yield properties must be used,

meaning that any material hardening is not taken into account. However, if the ultimate steel strength

𝑓𝑢 was used instead of 𝑓𝑦, the average difference would change to a conservative 9% for circular and

non-conservative 3% for square and rectangular specimens. Finally, the high variability of the test

specimen’s real steel tube thickness can also play a key role in these conclusions, particularly in the

case of monotonic tests, where cross-section asymmetry can amplify the member bending capacity

compared to a specimen with an average thickness, if the right alignment conditions are met.

Finally, in spite of the differences between the material properties of the standard and rubberized

concrete types, the comparison of the test results with the code shows that the design assumptions of

EC4 in the context of RuCFST columns are still valid, confirming in this way the applicability of the

European code to the design of both CFST and RuCFST members subjected to bending.


4. Conclusions

In this paper, an experimental assessment of the flexural behaviour of rubberized CFST members was

achieved. The following conclusions can be withdrawn:

The developed box testing device performed very well throughout the test campaign, proving to

be a noteworthy alternative to the traditional test setup;

Infill of the steel tube by concrete has the ability to significantly enhance the ductility of the

member, in addition to an expected increase in bending capacity;

The tested CFST and RuCFST columns showed a very ductile behaviour, for both monotonic and

cyclic bending conditions;

Concrete type effect in specimen behaviour is negligible, as different concrete types do not have a

great influence in the member’s bending behaviour;

Eurocode 4 is conservative in the prediction of the flexural capacity of the tested specimens.

Nonetheless, the code proves to be applicable to the design of both CFST and RuCFST members;

Eurocode 8 has very restrictive cross-section slenderness requirements concerning dissipative

square and rectangular CFSTs, in comparison to circular members.

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