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Experimental investigation on concrete-filled stainless steel stiffened tubular stub columns
Engineering Structures 31 (2009) 300–307
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Experimental investigation on concrete-filled stainless steel stiffened tubular stub columns M.A. Dabaon, M.H. El-Boghdadi, M.F. Hassanein ∗ Department of Structural Engineering, Tanta University, Tanta, Egypt
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Article history: Received 28 February 2008 Received in revised form 26 August 2008 Accepted 27 August 2008 Available online 24 September 2008 Keywords: Composite columns Normal-strength concrete High-strength concrete Cold-formed Experimental investigation Stainless steel tubes Slender plates Stiffeners Square hollow sections Rectangular hollow sections Compressive strength Structural design
a b s t r a c t This paper presents an experimental investigation on concrete-filled normal-strength stainless steel stiffened tubular stub columns using the austenitic stainless steel grade EN 1.4301 (304). The stiffened stainless steel tubes were fabricated by welding four lipped angles or two lipped channels at the lips. Therefore, the stiffeners were formed at the mid-depth of the sections. In total, five hollow columns and ten concrete-filled columns were tested. The longitudinal stiffener of the column plate was formed to avoid shrinkage of the concrete and to behave as a continuous connector between the concrete core and the stainless steel tube. The behavior of the columns was investigated using two different nominal concrete cubic strengths of 30 and 60 MPa. A series of tests was performed to investigate the effects of cross-section shape and concrete strength on the behavior and strength of concrete-filled stainless steel stiffened tubular stub columns. The measured average overall depth-to-width ratios (aspect ratio) varied from 1.0 to 1.8. The depth-to-plate thickness ratio of the tube sections varied from 60 to 90. Different lengths of columns were selected to fix the length-to-depth ratio to a constant value of 3. The concrete-filled stiffened stainless steel tubular columns were subjected to uniform axial compression over the concrete core and the stainless steel tube to force the entire section to undergo the same deformations by blocking action. The column strengths, load–axial strain relationships and failure modes of the columns are presented. Several comparisons were made to evaluate the test results. The results of the experimental study showed that the design rules, as specified in the European specifications and the ASCE, are highly conservative for square and rectangular cold-formed concrete-filled normal-strength stainless steel stiffened stub columns. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction The general term ‘‘composite column’’ refers to any compression member in which a steel element acts compositely with a concrete element, so that both elements resist compressive force. There is a wide variety of composite columns of varying crosssection, but the most commonly used and studied types are encased I-section and concrete-filled steel tubes. In contrast to the encased composite column, the concrete-filled column has the advantage that it does not need any formwork or reinforcement. The concrete-filled column offers several advantages, related to its structural behaviour, over pure steel, reinforced concrete or encased composite column. The location of the steel and the concrete in the cross-section optimizes the strength and stiffness of the section. The steel lies at the outer perimeter where it performs most effectively in tension and in resisting bending moments. Also, the
∗
Corresponding author. E-mail addresses: [email protected] (M.A. Dabaon), [email protected] (M.H. El-Boghdadi), [email protected] (M.F. Hassanein). 0141-0296/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.engstruct.2008.08.017
stiffness of the concrete-filled column is greatly enhanced because the steel is situated farthest from the centroid, where it makes the greatest contribution to the moment of inertia. The concrete forms an ideal core to withstand compressive loading and it delays and often prevents local buckling of the steel tube. The lateral confinement provided by the steel tube improves the strength, ductility and deformability of the concrete. The steel tube also prevents the spalling of concrete and minimizes the accumulation of reinforcement in connection zones. It can be said that a concrete-filled column delivers the economies of a concrete column with the speed of construction and the constructability of a steel column which results in significant economies in the overall structure of a building project [1]. In recent years, cold-formed stainless steel sections have been increasingly used in architectural and structural applications, e.g. curtain wall panels, roofing and siding, mullions, railings, columns, etc., due to their superior corrosion resistance, ease of maintenance, attractive appearance and high strength. However, there are limited test data on concrete-filled stainless steel tube columns. The behaviour of stainless steel sections is different from that of carbon steel sections. Stainless steel sections have a
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rounded stress–strain curve with no yield plateau and have a low proportional limit stress compared to that of carbon steel sections. Young and Ellobody [2] investigated experimentally concretefilled cold-formed high-strength stainless steel tube columns. In this study, concentrically loaded rectangular hollow section columns were tested. The depth-to-plate thickness ratio of the tube sections varied from 25.7 for compact sections to 55.8 for relatively slender sections. Generally the local buckling failure mode of the high-strength stainless steel tubes was observed for specimens with relatively slender sections. A concrete crushing failure mode together with local buckling of the high-strength stainless steel tubes for specimens with compact sections was also observed. However, no experimental test data were found in the literature on concrete-filled stainless steel stiffened tubular stub columns. On the other hand, several methods were used for column stiffening. The first method is to use longitudinal stiffeners. The second method is to fill the cross-section of the column with concrete, which avoids the inward buckling of the stainless steel plates. The third one is to use the previous two methods together. In this paper, an experimental series of tests to investigate the behaviour and strength of concrete-filled hollow-section stainless steel stiffened stub columns is reported. The principle aim was to improve the current knowledge of the mechanical behaviour of concrete-filled stainless steel stiffened stub columns, which would lead to a more efficient use of concrete with higher compressive strength. A series of tests was conducted on square and rectangular hollow-sections by using two different in-filled concrete strengths with no use of discrete mechanical shear connectors to improve the bond at the stainless steel interface or additional reinforcement besides the stainless steel tubes. The stiffened stainless steel tubular stub sections were fabricated by welding four lipped angles or two lipped channels at the lips. Therefore, the stiffeners were formed at the mid-depth of the sections. The longitudinal stiffener of the column plate was formed to avoid shrinkage of the concrete and to behave as a continuous connector between the concrete core and the stainless steel tube. The dimensions of the stainless steel tubes were chosen to include relatively slender sections [3]. 2. Experimental program 2.1. Problem statement The strength enhancement in excess of uniaxial strength and the deformation improvement of concrete can be observed when concrete is subjected to passive confinement. Common examples of passively confined concrete can be seen in unstiffened concrete-filled stainless steel tubular columns. Concrete-filled hollow-section stainless steel columns, with a high value of depth-to-thickness ratio, provide inadequate confinement for the concrete core due to the local buckling of the stainless steel tubes. On the other hand, concrete-filled hollow-section stainless steel columns with a small value of depth-to-thickness ratio provide remarkabley good confinement for the concrete core. In addition, stainless steel structural shapes are becoming increasingly complex as cold-forming techniques are advancing. This leads to the ability to generate several different stainless steel cross-section shapes; see Dabaon et al. [4]. Considering the stainless steel cross-sections, as shown in Fig. 1, the value of the lateral confining pressure of the stainless steel tube is expected to increase for those stiffened longitudinally. This means that the confinement of concrete will increase and affect the strength of concrete-filled hollow-section stainless steel columns. This idea encouraged the authors to experimentally investigate this type of stainless steel column. Accordingly, this paper presents the experimental results of concrete-filled stainless steel stiffened tubular stub columns. The work described in this paper is extended
301
Fig. 1. Definition of symbols for concrete-filled stainless steel stiffened tubular stub columns.
by the same authors in [5], where a comparative study between stiffened and unstiffened concrete-filled stainless steel tubular stub columns is conducted. In paper [5], further discussions, mainly concerning the confinement effect for both types of columns, are presented. 2.2. Test specimens Concrete-filled normal-strength tubes using square hollow sections (SHS) and rectangular hollow sections (RHS) were tested. The tubes were cold-formed from flat strips of stainless steel material. The stainless steel strips were all of constant thickness equal to 2 mm. Each test specimen was prepared from two or four parts welded together by argon welds, as presented in Fig. 1. The test program consisted of five test series, including two series of concrete-filled SHS tubes (SHS1 and SHS2) and three series of concrete-filled RHS tubes (RHS1, RHS2 and RHS3). Table 1 summarizes the dimensions of the specimens. The cross-section dimensions D and B are outside measurement. The lengths were chosen so that the length-to-depth ratio (L/D) generally remained at a constant value of 3 to prevent flexural buckling. The specimens were tested using nominal concrete cubic compressive strength of 30 and 60 MPa. The depth of the stiffeners was fixed at a constant value of 30 mm for all test specimens. The classification of crosssections and the effective cross-sectional area of the test series are calculated according the EN 1993-1-4 [6] and the ASCE [7]. The concrete-filled stainless steel stiffened tube column test specimens are labelled such that the shape of stainless steel tube and concrete strength can be identified from the label. For example, the label ‘‘SHS1C30’’ defines the specimen with a square hollow section that belonged to test series SHS1, and the letter ‘‘C’’ indicates the concrete strength followed by its value in MPa (30 MPa). Five tests were conducted on stainless steel hollow stiffened tubular columns (without in-filled concrete) denoted by ‘‘C0’’ for each series. 2.3. Stainless steel properties The austenitic stainless steel grade EN 1.4301 (304) was used in this experimental investigation. The properties of the stainless steel used for tube specimens were determined by tensile coupon tests. The stress–strain relationship obtained from tensile coupon tests reflected the average behaviour of the material through its thickness. For this reason, the tensile coupon test specimens were taken from the centre of the longitudinal direction of the flat portion of the cross-section depth from an untested rectangular specimen, where the stainless steel columns were formed using the same coil of stainless steel sheet, as shown in Fig. 2. It is important to note that the coupons were taken away from the longitudinal weld. Therefore, the bending residual stresses arising from cold-forming are included in the coupon results, while the welding residual stresses are not taken into account, as previously discussed by the same authors in [4].
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Table 1 Measured test specimen dimensions Group
Specimen
Depth D (mm)
Width B (mm)
Thickness t (mm)
D/t
Length L (mm)
L/D
Area of stainless steel (mm2 )
Area of concrete (mm2 )
SHS1
SHS1C0 SHS1C30 SHS1C60 SHS2C0 SHS2C30 SHS2C60 RHS1C0 RHS1C30 RHS1C60 RHS2C0 RHS2C30 RHS2C60 RHS3C0 RHS3C30 RHS3C60
120 120 120 160 160 160 140 140 140 170 170 170 180 180 180
120 120 120 160 160 160 80 80 80 120 120 120 100 100 100
2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
60 60 60 80 80 80 70 70 70 85 85 85 90 90 90
360 360 360 480 480 480 420 420 420 510 510 510 540 540 540
3
1351 1351 1351 1671 1671 1671 1061 1061 1061 1341 1341 1341 1301 1301 1301
– 12 985 12 985 – 23 865 23 865 – 10 096 10 096 – 19 016 19 016 – 16 656 16 656
SHS2
RHS1
RHS2
RHS3
3
3
3
3
Fig. 2. Location of tensile coupon test specimen.
The coupon dimensions conformed to the Australian Standard AS 1391 for the tensile testing of metals [8] using 12.5 mm wide coupons of gauge length 50 mm, as detailed in Fig. 3. The coupons were tested according to AS 1391 in a 30 ton capacity UTM displacement controlled testing machine using friction grips. A calibrated extensometer of 50 mm gauge length was used to measure the longitudinal strain. In addition, two linear strain gauges were attached to each coupon at the center, one for each face. The readings of strain gauges were used to determinate the initial Young’s modulus. A data acquisition system was used to record the load and the strain readings at regular intervals during the tests. The tensile coupons were tested till fracture. The measured static 0.2% proof stress (σ0.2 ) was 285 MPa for the tested specimens. The measured elongation after fracture (εf ), based on a gauge length of 50 mm, was 41%. On the other hand, the calculated Ramberg–Osgood parameter was (n = 4.67) which describes the degree of roundness of the curve [9]. The complete stress–strain curves obtained from the tensile coupon tests for the stainless steel tubes are shown in Fig. 4. 2.4. Concrete properties The concrete properties were determined from standard cube tests. The tests were conducted using the procedure conforming the Egyptian Code of Practice for Concrete Design ECP 203-2001 [10]. The concretes were produced using commercially available materials with normal mixing and curing techniques. The concrete mix design is shown in Table 2 using a super-plasticizer for Mix Π . Twelve concrete cube tests were conducted. At the same time as the concrete-filled stainless steel stiffened tubular column tests,
Fig. 4. Stress-strain curve of stainless-steel.
the mean compressive strengths of the concrete were determined as 34.8 and 61.9 MPa, with the corresponding coefficients of variation (COV) of 0.157 and 0.115, respectively, for nominal concrete cubic strengths of C30 and C60. Table 3 summarizes the measured concrete cubic strengths and the number of tests. 2.5. Instrumentations Three mechanical dial gauges were used to measure the axial shortening of the columns, as shown in Figs. 5–7. The axial shortening was obtained from the average readings of the dial gauges for each specimen. Six strain gauges were fixed on the square columns while four strain gauges were used for the rectangular columns to monitor the axial strain and plate deformations of each specimen, as shown in Figs. 6 and 7. All strain gauges were placed on the outside surface of the stainless steel tubes at mid-length of each column. 2.6. Column test procedure The set-up used for testing the concrete-filled stainless steel tube stub column is shown in Fig. 5. A hydraulic testing machine of 300 ton capacity was used to apply axial compressive force to the column specimens. Prior to testing, the top level of the
Fig. 3. Dimensions of test coupon.
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Table 2 Concrete mix design Mix.
I II
Nominal concrete cube strength (MPa)
30 60
Water/cement ratio
0.47 0.27
Mix proportions (relative to the weight of cement) Cement
Water
Fine aggregate
Coarse aggregate
Super-plasticizer
1.0 1.0
0.47 0.27
1.545 1.09
3.09 1.77
– 0.02
Table 3 Measured concrete cube strengths
Fig. 5. Test set-up of stainless steel tubular stub columns.
columns was treated mechanically to allow an even distribution of the axial load in both concrete core and stainless steel tube. Also, the column ends were strengthened by stainless steel brackets to prevent failure at the ends so that the column strength would not be influenced by end effects (elephant foot buckling). Therefore, the columns were ground flat and the ends were covered with thick rigid steel plates in full contact with the concrete. The load was axially applied to the columns over the concrete and stainless steel tube to force the entire section to undergo the same deformations by blocking action, as shown in Fig. 5. Thus, the bond strength was of minor importance. During loading, the readings of the longitudinal strain gauges near the corners of the columns were carefully monitored to ensure that the load was applied concentrically to the columns. Displacement control was used to drive the hydraulic actuator at a constant speed of 0.5 mm/min to allow continuation of the test in the post-ultimate stage. 3. Test results and evaluation The main objective of this research was to investigate the effect of the concrete core in the stainless steel stiffened tubular stub column. However, the columns in this investigation were stiffened by two methods; using longitudinal stiffeners in addition to infilled concrete. The main variables were the cross-section shape
Nominal concrete cube strength (MPa)
Measured mean value of concrete strength (MPa)
Coefficient of variation (COV)
Number of tested cubes
30 60
34.8 61.9
0.157 0.115
5 7
of the stainless steel tube and in-filled concrete strength. The test strengths and load–axial strain relationships were measured for each column specimen. The test strengths (PTest ) of the concretefilled stainless steel stiffened tubular columns of the SHS and RHS are shown in Table 5. Three comparisons were made in this section to evaluate the test results. First, comparison of test strengths with the design specifications was made. Then the comparison was made between the stiffened hollow sections and the concretefilled test specimens. Finally, the effect of the value of the concrete strength on the strength of the concrete-filled stainless steel stiffened stub columns was discussed. 3.1. Comparison with design specifications The test strength (PTest ) for each hollow test specimen was compared to the design rules specified by EN 1993-1-4 [6] and the ASCE [7], as shown in Table 4. On the other hand, the test strength (PTest ) for each concrete-filled test specimen was compared to the design rules specified by ENV 1994-1-1 [11]. In general, the predicted strengths were calculated using equation (6.30) of the ENV 1994-1-1 [11]. In this equation, the plastic resistance (Ppl,Rd ) of the concrete-filled stainless steel stiffened tubular columns should be calculated by adding the plastic resistance of its components. In the case where no reinforcement was used, as in the current test specimens, it is as follows: Ppl,Rd = Aa fyd + 0.85Ac fcd
(1)
where: Aa is the cross-sectional area of the structural steel section. Ac is the cross-sectional area of concrete. fyd is the design value of the yield strength of structural steel. fcd is the design value of the cylinder compressive strength of concrete. However, for concrete filled sections, the coefficient 0.85 due to long-term effect may be replaced by 1.0, since the development
Fig. 6. Locations of strain gauges and dial gauges on square specimens.
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Table 4 Comparison of test strengths with unfactored design strengths according to the EN 1993-1-4 [6] and ASCE standard [7] for hollow stiffened columns Group
Specimen
PTest (kN)
Aeff ,EC 3 (mm2 )
PPred,1 (kN)
PTest /PPred,1
Aeff ,ASCE (mm2 )
PPred,2 (kN)
PTest /PPred,2
SHS1 SHS2 RHS1 RHS2 RHS3
SHS1C0 SHS2C0 RHS1C0 RHS2C0 RHS3C0
425 350 258 263 254
1275 1429 953 1014 1013
363 407 272 289 289
1.17 0.86 0.95 0.91 0.88
1346 1647 1053 1187 1184
384 469 300 338 337
1.11 0.75 0.86 0.78 0.75
Table 5 Comparison of test strengths with unfactored design strengths according to the ENV 1994-1-1 [11] for concrete-filled stiffened columns Group Stiffened specimen
PTest (kN)
Aeff ,EC 3 (mm2 )
PPred,1 (kN)
PTest /PPred,1
0 PPred ,1 (kN)
0 PTest /PPred ,1
At (mm2 )
PPred,3 (kN)
PTest /PPred,3
0 PPred ,3 (kN)
0 PTest /PPred ,3
SHS1
870 1090 1422 1706 875 928 1225 1444 1006 1488
1275 1275 1429 1429 953 953 1014 1014 1013 1013
671 910 972 1412 511 697 739 1089 683 990
1.30 1.20 1.46 1.21 1.71 1.33 1.66 1.33 1.47 1.50
725 1006 1072 1589 553 772 818 1231 752 1114
1.20 1.08 1.33 1.07 1.58 1.20 1.50 1.17 1.34 1.34
1351 1351 1671 1671 1061 1061 1341 1341 1301 1301
692 932 1041 1481 541 727 832 1183 765 1072
1.26 1.17 1.37 1.15 1.62 1.28 1.47 1.22 1.32 1.39
747 1028 1141 1658 584 802 912 1324 835 1196
1.17 1.06 1.25 1.03 1.50 1.16 1.34 1.09 1.21 1.24
SHS2 RHS1 RHS2 RHS3
SHS1C30 SHS1C60 SHS2C30 SHS2C60 RHS1C30 RHS1C60 RHS2C30 RHS2C60 RHS3C30 RHS3C60
Fig. 7. Locations of strain gauges and dial gauges on rectangular specimens.
of concrete strength is improved because of protection against the environment [11]. Here, the comparison is made using four predicted values, as given in Eqs. (2)–(5). In the first predicted strength (PPred,1 ), the effective cross-sectional area of the stainless steel tube as well as the coefficient of the long term effect were considered, while the coefficient of long term effect was neglected 0 in the second predicted strength (PPred ,1 ). On the other hand, the whole cross-sectional area of the stainless steel tube as well as the coefficient of the long term effect were considered in the third predicted strength (PPred,3 ), while the coefficient of the long term 0 effect was neglected in the fourth predicted strength (PPred ,3 ). The comparisons of test strengths with design strengths for test series are given in Table 5. PPred,1 = Aeff fyd + 0.85Ac fcd 0
(3)
PPred,3 = At fyd + 0.85Ac fcd
(4)
PPred,3 = At fyd + Ac fcd
ρ=
(5)
where: Aeff is the effective cross-sectional area of the structural steel section. At is the whole cross-sectional area of the structural steel section. However, Table 5.2 in the EN 1993-1-4 [6] was used to classify the cross-section type of the test specimens. By applying the limitations of Table 5.2, the test specimens were all of Class 4.
0.772
0.125
−
λp
λ2p
but ≤1.
(6)
Welded outstand elements, which are used with longitudinal stiffeners, as shown in Fig. 1:
(2)
PPred,1 = Aeff fyd + Ac fcd 0
As is well known, the effective widths may be used in order to reduce the resistance of the cross-section due to the effect of local buckling. For this reason, clause 5.2.3 was in use during the calculations of the effective widths in Class 4 cross-sections forming this research. A reduction factor ρ should be taken as follows: For Cold formed or welded internal elements, which are used with flat portions, as shown in Fig. 1:
ρ=
1
λp
−
0.242
λ2p
but ≤1
(7)
where λp is the element slenderness defined as:
λp =
b/t 28.4ε
√ kσ
(8)
where; t is the relevant thickness. kσ is the buckling factor corresponding to the stress ratio ψ and boundary conditions from Table 4.1 or 4.2 in EN 19931-5 [12] as appropriate: it is 4.0 for flat portions and 0.43 for longitudinal stiffeners. These values mean that the case of
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Table 6 Comparison of test strengths with unfactored design strengths according to the ASCE standard [7] for concrete-filled stiffened columns Group
Stiffened specimen
PTest (kN)
Aeff ,ASCE (mm2 )
PPred,2 (kN)
PTest /PPred,2
SHS1
SHS1C30 SHS1C60 SHS2C30 SHS2C60 RHS1C30 RHS1C60 RHS2C30 RHS2C60 RHS3C30 RHS3C60
870 1090 1422 1706 875 928 1225 1444 1006 1488
1346 1346 1647 1647 1053 1053 1187 1187 1184 1184
691 930 1034 1474 539 725 788 1139 732 1039
1.26 1.17 1.37 1.16 1.62 1.28 1.55 1.27 1.38 1.43
SHS2 RHS1 RHS2 RHS3
simple support is considered. The assumptions about simple support when determining kσ are normally unfavourable. The assumption is therefore normally a simplification intended for manual calculation. b is the relevant width as follows: b = flat element width while b = c for outstand flanges ε is the material factor defined in Table 5.2 in EN 1993-1-4 [6]. The comparison of the test strength (PTest ) for each concretefilled test specimen was also made by using the predicted value according to the ASCE Standard [7]. The comparisons of test strengths with design strengths (PPred,2 ) according to the ASCE Standard [7] for test series are given in Table 6. Clause 3.4 in the ASCE Standard [7] was used to classify the cross-section type of the test specimens. The resistance of a stainless steel crosssection subjected to compression with a resultant acting through the centroid of the effective section shall be calculated as follows: PPred,2 = Ae Fn + 0.85Ac fc
(9)
where: Ae is the effective area calculated at stress Fn . A reduction factor ρ should be taken as follows: For uniformly compressed stiffened elements, this was used with flat portions:
ρ=
1 − 0.22/λ
if λ =
λ 1.052 w f ≥ 0.673. √ t
k
E◦
(10) (11)
The uniform compressed unstiffened elements, which were used with longitudinal stiffeners, used the same equations as for stiffened elements with the exception that k = 0.5. Fn is the flexural buckling stress for doubly symmetric sections, closed cross sections, and any other sections which are not subjected to torsional or torsional–flexural buckling, the flexural buckling stress. Fn , is determined as follows: Fn = Et =
π 2 Et ≤ Fy (KL/r )2
(12)
E◦ Fy Fy + 0.002nE◦ Fn /Fy
n−1
(13)
E◦ is the initial elastic modulus. n is Ramberg–Osgood parameter which is the strain hardening exponent that defines the degree of roundness of the curve (n = 4.67). Ac is the cross-sectional area of concrete. fc = 0.8fcu is the design value of the cylinder compressive strength of concrete. The determination of flexural buckling stress and then the design of axial strength of concentrically loaded cold-formed stainless steel compression members, according to the ASCE Standard [7], requires an iterative process. In general, the ASCE
Standard [7] requires a significantly greater calculation effort than that required by ENV 1994-1-1 [11]. From Tables 5 and 6, it can be noticed that there is a slight difference between the five predicted values in the case of concretefilled stainless steel stiffened tubular stub columns. But still the 0 predicted value (PPred ,3 ), according to the ENV 1994-1-1 [11], by using the whole cross-sectional area of the stainless steel tube and neglecting the coefficient of long term effect, is more relevant to the test strength (PTest ) for concrete-filled stainless steel stiffened tubular stub columns. On the other hand, the predicted value (PPred,1 ) is close to the test strengths (PTest ) for the stiffened hollowsection stub columns, as can be seen in Table 4. Further results dealing with the stainless steel stiffened hollow section columns can be found in [4]. 3.2. Comparison between hollow-section and concrete in-filled stiffened stub columns From Tables 4 and 5, which tabulate the experimental results of both the hollow-sections and concrete-filled sections, it can be seen that the column strength of the concrete-filled stainless steel stiffened tubular stub columns was considerably higher than that of the stiffened slender stainless steel hollow tubular stub columns. The average values of the column strength of columns in-filled with a concrete of 34.8 MPa mean compressive strength divided by that of the stiffened slender stainless steel hollow tubular stub columns is equal to 3.63. This ratio was about 4.48 in case of the columns in-filled by a concrete of 61.9 MPa mean compressive strength. However, different failure modes were observed from the test series. The mode of failure of the stiffened slender stainless steel hollow tubular stub columns was due to local buckling. Once local buckling has occurred, in concrete-filled stainless steel stiffened tubular stub columns, by the time of reaching the test strength, the stainless steel tube was not able to provide confinement for the concrete. The column capacity, at this stage, was governed by local buckling failure mode. By other words, the occurrence of local buckling was the reason for the descending parts, as can be seen in Figs. 9 and 10. Another observation that can be noticed from the experimental tests is that the stiffeners contributed largely to the test strength (PTest ) of columns even when the stiffeners’ rigidities were small, because the local buckling of longitudinal stiffeners was prevented by the concrete. Fig. 8 represents the deformed shape of the Group SHS1. The load–axial shortening behaviour of the stainless steel stiffened hollow section columns and concrete-filled stainless steel stiffened tubular stub columns was compared. Figs. 9 and 10 show the load versus axial shortening curves of the stiffened square stub columns for Groups SHS1 and RHS1, respectively. The ultimate load of the stiffened column SHS1C0 was 425 kN with an axial shortening at the ultimate load of 1.86 mm. For the concrete-filled specimens, the ultimate load was 870 and 1090 kN for SHS1C30 and SHS1C60, respectively. The axial shortening corresponding to the ultimate load was 2.85 and 1.13 mm for SHS1C30 and
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Fig. 8. Deformed shape of Group SHS1.
SHS1C60, respectively. On the other hand, the ultimate load of the stiffened column RHS1C0 was 258 kN with an axial shortening at the ultimate load of 0.80 mm. For the concrete-filled rectangular specimens, the ultimate load was 875 and 928 kN for RHS1C30 and RHS1C60, respectively. The axial shortening corresponding to the ultimate load was 2.41 and 2.0 mm for RHS1C30 and RHS1C60, respectively. The column strengths of the stiffened square concrete-filled columns SHS1C60 and SHS1C30 were increased by 156% and 105% over that of the stiffened hollow-section column due to the existence of concrete. The increase in the column strength was 259% and 239% for the stiffened rectangular concrete-filled columns RHS1C60 and RHS1C30 over the RHS1C0. It could be seen that the in-filled stiffened column had a considerably higher stiffness than that of the stiffened hollow-section column resulting in a sharper load–axial shortening behaviour.
Fig. 9. Load–shortening relationship of SHS1.
3.3. Effect of nominal concrete strength Two different nominal concrete strengths were used in the tested stub columns, as summarized in Table 3. The mean concrete cubic strengths were 34.8 and 61.9 MPa. From Tables 5 and 6, it can be seen that the test strengths (PTest ) increased with the increase of the nominal value of the in-filled concrete. For instance, the test strength (PTest ) increased from 870 kN to 1090 kN for SHS1C30 and SHS1C60 by increasing the mean concrete cubic strength by 27.1 MPa. The average values of the column strength (PTest ) of the concrete-filled stiffened columns of the 61.9 MPa concrete cubic strength divided by that of 34.8 MPa concrete cubic strength is equal to 1.23. However, the failure mode was of the same form. It was by stainless steel yielding and crushing of concrete. The deformed shape for SHS1C30 and SHS1C60 is shown in Fig. 8. From Figs. 9 and 10, it can be seen that the higher the strength of in-filled concrete, the sharper the load–axial shortening behaviour. 4. Conclusions In this paper, a test program of concrete-filled normal-strength stainless steel stiffened tubular stub columns was presented. The stainless steel grade 1.4301 (304) was used. Results for five concentrically loaded slender stainless steel stiffened columns and ten concrete-filled stainless steel stiffened columns have been presented. The results of the experimental study showed that the design rules specified in European specifications as well as the American one are highly conservative for concrete-filled stainless steel stiffened stub columns. Here, the comparison with the European specifications was made by using four predicted values, while the comparison with the ASCE was made using only one predicted 0 value. However, the predicted values (PPred ,3 ), according to ENV 1994-1-1 which considers the whole cross-sectional area of the
Fig. 10. Load–shortening relationship of RHS1.
stainless steel tube and neglects the long term effect, was more relevant to the test strength (PTest ) of the concrete-filled stainless steel stiffened stub columns than any other predicted value. The tests showed that to increase the capacity of slender stainless steel stiffened tubular stub columns, in-filled concrete may be used. However, the test strengths (PTest ) increased with the increase of the nominal in-filled concrete. Thus, increasing the nominal compressive strength of the in-filled concrete leads to smaller column size which accordingly increases the amount of usable floor space in a structure. Once local buckling had occurred by the time of reaching the test strength, the steel tube was not able to provide confinement to the concrete. The column capacity, at this stage, was governed by local buckling failure mode. The failure was achieved by the local buckling of the stainless tube and the crushing of concrete. The stiffeners contributed largely to the test strength (PTest ) of the columns even when the rigidity of the stiffeners was small, because the local buckling of longitudinal stiffeners is prevented by the concrete. However, the mode of failure of the tests is shown in Figs. 11 and 12.
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Bernt Johansson at LTU; Lulea; Sweden for his support, help and belief in the third author. The third author owes you, thanks. Last but not the least, special thanks to Prof. Milan Veljkovic at LTU; Lulea; Sweden who guided the third author in order to learn more and more. References
Fig. 11. Modes of failure of stainless steel square stub columns.
Fig. 12. Modes of failure of stainless steel rectangular stub columns.
Acknowledgements The authors would like to acknowledge Faculty of Engineering; Tanta University for its support. They are grateful to the Concrete and Heavy Structures Laboratory staff, Faculty of Engineering; Tanta University for their intensive help and technical support. Further thanks are due to the materials properties Laboratory staff, The Housing and Building Research Center; Cairo for their help. Especially the authors would like to thank very much Professor
[1] Johansson M. Composite action and confinement effects in tubular steelconcrete columns. Ph.D. thesis. Goteborg (Sweden): Chalmers University of Technology; 2002. [2] Young B, Ellobody E. Experimental investigation of concrete-filled coldformed high strength stainless steel tube columns. J Constr Steel Res 2006; 62:484–92. [3] Hassanein M. Behavior of composite columns: Concrete-filled hollowsection stiffened steel columns. Ph.D. thesis. Tanta (Egypt): Tanta University [in preparation]. [4] Dabaon M, El-Boghdadi M, Hassanein M. A comparative experimental study between stiffened and unstiffened stainless steel hollow tubular stub columns. Thin-Walled Structures [in press]. [5] Dabaon M, El-Khoriby S, El-Boghdadi M, Hassanein M. Confinement effect of stiffened and unstiffened concrete-filled stainless steel hollow tubular stub columns [in preparation]. [6] EN 1993-1-4. Eurocode 3: Design of steel structures - Part 1-4: General rulessupplementary rules for stainless steel. CEN; 2006. [7] ASCE. Specification for the design of cold-formed stainless steel structural members (SEI/ASCE 8-02). American Society of Civil Engineers; 2002. [8] Australian Standard. Methods for tensile testing of metals, AS 1391. Sydney (Australia): Standards Association of Australia; 1991. [9] Ramberg W, Osgood WR. Description of stress–strain curves by three parameters. NACA technical note no. 902, July; 1942. [10] ECP 203-2001: Egyptian code of practice for design and constructing concrete structures, Code No. 203, second ed. Cairo (Egypt). 2001. [11] ENV 1994-1-1. Eurocode 4: Design of composite steel and concrete structures — Part 1-1: General rules and rules of building. CEN; 2004. [12] EN 1993-1-5. Eurocode 3: Design of steel structures — Part 1-5: Plated structural elements. CEN; 2007.
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Engineering Structures journal homepage: www.elsevier.com/locate/engstruct
Experimental investigation on concrete-filled stainless steel stiffened tubular stub columns M.A. Dabaon, M.H. El-Boghdadi, M.F. Hassanein ∗ Department of Structural Engineering, Tanta University, Tanta, Egypt
article
info
Article history: Received 28 February 2008 Received in revised form 26 August 2008 Accepted 27 August 2008 Available online 24 September 2008 Keywords: Composite columns Normal-strength concrete High-strength concrete Cold-formed Experimental investigation Stainless steel tubes Slender plates Stiffeners Square hollow sections Rectangular hollow sections Compressive strength Structural design
a b s t r a c t This paper presents an experimental investigation on concrete-filled normal-strength stainless steel stiffened tubular stub columns using the austenitic stainless steel grade EN 1.4301 (304). The stiffened stainless steel tubes were fabricated by welding four lipped angles or two lipped channels at the lips. Therefore, the stiffeners were formed at the mid-depth of the sections. In total, five hollow columns and ten concrete-filled columns were tested. The longitudinal stiffener of the column plate was formed to avoid shrinkage of the concrete and to behave as a continuous connector between the concrete core and the stainless steel tube. The behavior of the columns was investigated using two different nominal concrete cubic strengths of 30 and 60 MPa. A series of tests was performed to investigate the effects of cross-section shape and concrete strength on the behavior and strength of concrete-filled stainless steel stiffened tubular stub columns. The measured average overall depth-to-width ratios (aspect ratio) varied from 1.0 to 1.8. The depth-to-plate thickness ratio of the tube sections varied from 60 to 90. Different lengths of columns were selected to fix the length-to-depth ratio to a constant value of 3. The concrete-filled stiffened stainless steel tubular columns were subjected to uniform axial compression over the concrete core and the stainless steel tube to force the entire section to undergo the same deformations by blocking action. The column strengths, load–axial strain relationships and failure modes of the columns are presented. Several comparisons were made to evaluate the test results. The results of the experimental study showed that the design rules, as specified in the European specifications and the ASCE, are highly conservative for square and rectangular cold-formed concrete-filled normal-strength stainless steel stiffened stub columns. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction The general term ‘‘composite column’’ refers to any compression member in which a steel element acts compositely with a concrete element, so that both elements resist compressive force. There is a wide variety of composite columns of varying crosssection, but the most commonly used and studied types are encased I-section and concrete-filled steel tubes. In contrast to the encased composite column, the concrete-filled column has the advantage that it does not need any formwork or reinforcement. The concrete-filled column offers several advantages, related to its structural behaviour, over pure steel, reinforced concrete or encased composite column. The location of the steel and the concrete in the cross-section optimizes the strength and stiffness of the section. The steel lies at the outer perimeter where it performs most effectively in tension and in resisting bending moments. Also, the
∗
Corresponding author. E-mail addresses: [email protected] (M.A. Dabaon), [email protected] (M.H. El-Boghdadi), [email protected] (M.F. Hassanein). 0141-0296/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.engstruct.2008.08.017
stiffness of the concrete-filled column is greatly enhanced because the steel is situated farthest from the centroid, where it makes the greatest contribution to the moment of inertia. The concrete forms an ideal core to withstand compressive loading and it delays and often prevents local buckling of the steel tube. The lateral confinement provided by the steel tube improves the strength, ductility and deformability of the concrete. The steel tube also prevents the spalling of concrete and minimizes the accumulation of reinforcement in connection zones. It can be said that a concrete-filled column delivers the economies of a concrete column with the speed of construction and the constructability of a steel column which results in significant economies in the overall structure of a building project [1]. In recent years, cold-formed stainless steel sections have been increasingly used in architectural and structural applications, e.g. curtain wall panels, roofing and siding, mullions, railings, columns, etc., due to their superior corrosion resistance, ease of maintenance, attractive appearance and high strength. However, there are limited test data on concrete-filled stainless steel tube columns. The behaviour of stainless steel sections is different from that of carbon steel sections. Stainless steel sections have a
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rounded stress–strain curve with no yield plateau and have a low proportional limit stress compared to that of carbon steel sections. Young and Ellobody [2] investigated experimentally concretefilled cold-formed high-strength stainless steel tube columns. In this study, concentrically loaded rectangular hollow section columns were tested. The depth-to-plate thickness ratio of the tube sections varied from 25.7 for compact sections to 55.8 for relatively slender sections. Generally the local buckling failure mode of the high-strength stainless steel tubes was observed for specimens with relatively slender sections. A concrete crushing failure mode together with local buckling of the high-strength stainless steel tubes for specimens with compact sections was also observed. However, no experimental test data were found in the literature on concrete-filled stainless steel stiffened tubular stub columns. On the other hand, several methods were used for column stiffening. The first method is to use longitudinal stiffeners. The second method is to fill the cross-section of the column with concrete, which avoids the inward buckling of the stainless steel plates. The third one is to use the previous two methods together. In this paper, an experimental series of tests to investigate the behaviour and strength of concrete-filled hollow-section stainless steel stiffened stub columns is reported. The principle aim was to improve the current knowledge of the mechanical behaviour of concrete-filled stainless steel stiffened stub columns, which would lead to a more efficient use of concrete with higher compressive strength. A series of tests was conducted on square and rectangular hollow-sections by using two different in-filled concrete strengths with no use of discrete mechanical shear connectors to improve the bond at the stainless steel interface or additional reinforcement besides the stainless steel tubes. The stiffened stainless steel tubular stub sections were fabricated by welding four lipped angles or two lipped channels at the lips. Therefore, the stiffeners were formed at the mid-depth of the sections. The longitudinal stiffener of the column plate was formed to avoid shrinkage of the concrete and to behave as a continuous connector between the concrete core and the stainless steel tube. The dimensions of the stainless steel tubes were chosen to include relatively slender sections [3]. 2. Experimental program 2.1. Problem statement The strength enhancement in excess of uniaxial strength and the deformation improvement of concrete can be observed when concrete is subjected to passive confinement. Common examples of passively confined concrete can be seen in unstiffened concrete-filled stainless steel tubular columns. Concrete-filled hollow-section stainless steel columns, with a high value of depth-to-thickness ratio, provide inadequate confinement for the concrete core due to the local buckling of the stainless steel tubes. On the other hand, concrete-filled hollow-section stainless steel columns with a small value of depth-to-thickness ratio provide remarkabley good confinement for the concrete core. In addition, stainless steel structural shapes are becoming increasingly complex as cold-forming techniques are advancing. This leads to the ability to generate several different stainless steel cross-section shapes; see Dabaon et al. [4]. Considering the stainless steel cross-sections, as shown in Fig. 1, the value of the lateral confining pressure of the stainless steel tube is expected to increase for those stiffened longitudinally. This means that the confinement of concrete will increase and affect the strength of concrete-filled hollow-section stainless steel columns. This idea encouraged the authors to experimentally investigate this type of stainless steel column. Accordingly, this paper presents the experimental results of concrete-filled stainless steel stiffened tubular stub columns. The work described in this paper is extended
301
Fig. 1. Definition of symbols for concrete-filled stainless steel stiffened tubular stub columns.
by the same authors in [5], where a comparative study between stiffened and unstiffened concrete-filled stainless steel tubular stub columns is conducted. In paper [5], further discussions, mainly concerning the confinement effect for both types of columns, are presented. 2.2. Test specimens Concrete-filled normal-strength tubes using square hollow sections (SHS) and rectangular hollow sections (RHS) were tested. The tubes were cold-formed from flat strips of stainless steel material. The stainless steel strips were all of constant thickness equal to 2 mm. Each test specimen was prepared from two or four parts welded together by argon welds, as presented in Fig. 1. The test program consisted of five test series, including two series of concrete-filled SHS tubes (SHS1 and SHS2) and three series of concrete-filled RHS tubes (RHS1, RHS2 and RHS3). Table 1 summarizes the dimensions of the specimens. The cross-section dimensions D and B are outside measurement. The lengths were chosen so that the length-to-depth ratio (L/D) generally remained at a constant value of 3 to prevent flexural buckling. The specimens were tested using nominal concrete cubic compressive strength of 30 and 60 MPa. The depth of the stiffeners was fixed at a constant value of 30 mm for all test specimens. The classification of crosssections and the effective cross-sectional area of the test series are calculated according the EN 1993-1-4 [6] and the ASCE [7]. The concrete-filled stainless steel stiffened tube column test specimens are labelled such that the shape of stainless steel tube and concrete strength can be identified from the label. For example, the label ‘‘SHS1C30’’ defines the specimen with a square hollow section that belonged to test series SHS1, and the letter ‘‘C’’ indicates the concrete strength followed by its value in MPa (30 MPa). Five tests were conducted on stainless steel hollow stiffened tubular columns (without in-filled concrete) denoted by ‘‘C0’’ for each series. 2.3. Stainless steel properties The austenitic stainless steel grade EN 1.4301 (304) was used in this experimental investigation. The properties of the stainless steel used for tube specimens were determined by tensile coupon tests. The stress–strain relationship obtained from tensile coupon tests reflected the average behaviour of the material through its thickness. For this reason, the tensile coupon test specimens were taken from the centre of the longitudinal direction of the flat portion of the cross-section depth from an untested rectangular specimen, where the stainless steel columns were formed using the same coil of stainless steel sheet, as shown in Fig. 2. It is important to note that the coupons were taken away from the longitudinal weld. Therefore, the bending residual stresses arising from cold-forming are included in the coupon results, while the welding residual stresses are not taken into account, as previously discussed by the same authors in [4].
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Table 1 Measured test specimen dimensions Group
Specimen
Depth D (mm)
Width B (mm)
Thickness t (mm)
D/t
Length L (mm)
L/D
Area of stainless steel (mm2 )
Area of concrete (mm2 )
SHS1
SHS1C0 SHS1C30 SHS1C60 SHS2C0 SHS2C30 SHS2C60 RHS1C0 RHS1C30 RHS1C60 RHS2C0 RHS2C30 RHS2C60 RHS3C0 RHS3C30 RHS3C60
120 120 120 160 160 160 140 140 140 170 170 170 180 180 180
120 120 120 160 160 160 80 80 80 120 120 120 100 100 100
2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
60 60 60 80 80 80 70 70 70 85 85 85 90 90 90
360 360 360 480 480 480 420 420 420 510 510 510 540 540 540
3
1351 1351 1351 1671 1671 1671 1061 1061 1061 1341 1341 1341 1301 1301 1301
– 12 985 12 985 – 23 865 23 865 – 10 096 10 096 – 19 016 19 016 – 16 656 16 656
SHS2
RHS1
RHS2
RHS3
3
3
3
3
Fig. 2. Location of tensile coupon test specimen.
The coupon dimensions conformed to the Australian Standard AS 1391 for the tensile testing of metals [8] using 12.5 mm wide coupons of gauge length 50 mm, as detailed in Fig. 3. The coupons were tested according to AS 1391 in a 30 ton capacity UTM displacement controlled testing machine using friction grips. A calibrated extensometer of 50 mm gauge length was used to measure the longitudinal strain. In addition, two linear strain gauges were attached to each coupon at the center, one for each face. The readings of strain gauges were used to determinate the initial Young’s modulus. A data acquisition system was used to record the load and the strain readings at regular intervals during the tests. The tensile coupons were tested till fracture. The measured static 0.2% proof stress (σ0.2 ) was 285 MPa for the tested specimens. The measured elongation after fracture (εf ), based on a gauge length of 50 mm, was 41%. On the other hand, the calculated Ramberg–Osgood parameter was (n = 4.67) which describes the degree of roundness of the curve [9]. The complete stress–strain curves obtained from the tensile coupon tests for the stainless steel tubes are shown in Fig. 4. 2.4. Concrete properties The concrete properties were determined from standard cube tests. The tests were conducted using the procedure conforming the Egyptian Code of Practice for Concrete Design ECP 203-2001 [10]. The concretes were produced using commercially available materials with normal mixing and curing techniques. The concrete mix design is shown in Table 2 using a super-plasticizer for Mix Π . Twelve concrete cube tests were conducted. At the same time as the concrete-filled stainless steel stiffened tubular column tests,
Fig. 4. Stress-strain curve of stainless-steel.
the mean compressive strengths of the concrete were determined as 34.8 and 61.9 MPa, with the corresponding coefficients of variation (COV) of 0.157 and 0.115, respectively, for nominal concrete cubic strengths of C30 and C60. Table 3 summarizes the measured concrete cubic strengths and the number of tests. 2.5. Instrumentations Three mechanical dial gauges were used to measure the axial shortening of the columns, as shown in Figs. 5–7. The axial shortening was obtained from the average readings of the dial gauges for each specimen. Six strain gauges were fixed on the square columns while four strain gauges were used for the rectangular columns to monitor the axial strain and plate deformations of each specimen, as shown in Figs. 6 and 7. All strain gauges were placed on the outside surface of the stainless steel tubes at mid-length of each column. 2.6. Column test procedure The set-up used for testing the concrete-filled stainless steel tube stub column is shown in Fig. 5. A hydraulic testing machine of 300 ton capacity was used to apply axial compressive force to the column specimens. Prior to testing, the top level of the
Fig. 3. Dimensions of test coupon.
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Table 2 Concrete mix design Mix.
I II
Nominal concrete cube strength (MPa)
30 60
Water/cement ratio
0.47 0.27
Mix proportions (relative to the weight of cement) Cement
Water
Fine aggregate
Coarse aggregate
Super-plasticizer
1.0 1.0
0.47 0.27
1.545 1.09
3.09 1.77
– 0.02
Table 3 Measured concrete cube strengths
Fig. 5. Test set-up of stainless steel tubular stub columns.
columns was treated mechanically to allow an even distribution of the axial load in both concrete core and stainless steel tube. Also, the column ends were strengthened by stainless steel brackets to prevent failure at the ends so that the column strength would not be influenced by end effects (elephant foot buckling). Therefore, the columns were ground flat and the ends were covered with thick rigid steel plates in full contact with the concrete. The load was axially applied to the columns over the concrete and stainless steel tube to force the entire section to undergo the same deformations by blocking action, as shown in Fig. 5. Thus, the bond strength was of minor importance. During loading, the readings of the longitudinal strain gauges near the corners of the columns were carefully monitored to ensure that the load was applied concentrically to the columns. Displacement control was used to drive the hydraulic actuator at a constant speed of 0.5 mm/min to allow continuation of the test in the post-ultimate stage. 3. Test results and evaluation The main objective of this research was to investigate the effect of the concrete core in the stainless steel stiffened tubular stub column. However, the columns in this investigation were stiffened by two methods; using longitudinal stiffeners in addition to infilled concrete. The main variables were the cross-section shape
Nominal concrete cube strength (MPa)
Measured mean value of concrete strength (MPa)
Coefficient of variation (COV)
Number of tested cubes
30 60
34.8 61.9
0.157 0.115
5 7
of the stainless steel tube and in-filled concrete strength. The test strengths and load–axial strain relationships were measured for each column specimen. The test strengths (PTest ) of the concretefilled stainless steel stiffened tubular columns of the SHS and RHS are shown in Table 5. Three comparisons were made in this section to evaluate the test results. First, comparison of test strengths with the design specifications was made. Then the comparison was made between the stiffened hollow sections and the concretefilled test specimens. Finally, the effect of the value of the concrete strength on the strength of the concrete-filled stainless steel stiffened stub columns was discussed. 3.1. Comparison with design specifications The test strength (PTest ) for each hollow test specimen was compared to the design rules specified by EN 1993-1-4 [6] and the ASCE [7], as shown in Table 4. On the other hand, the test strength (PTest ) for each concrete-filled test specimen was compared to the design rules specified by ENV 1994-1-1 [11]. In general, the predicted strengths were calculated using equation (6.30) of the ENV 1994-1-1 [11]. In this equation, the plastic resistance (Ppl,Rd ) of the concrete-filled stainless steel stiffened tubular columns should be calculated by adding the plastic resistance of its components. In the case where no reinforcement was used, as in the current test specimens, it is as follows: Ppl,Rd = Aa fyd + 0.85Ac fcd
(1)
where: Aa is the cross-sectional area of the structural steel section. Ac is the cross-sectional area of concrete. fyd is the design value of the yield strength of structural steel. fcd is the design value of the cylinder compressive strength of concrete. However, for concrete filled sections, the coefficient 0.85 due to long-term effect may be replaced by 1.0, since the development
Fig. 6. Locations of strain gauges and dial gauges on square specimens.
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Table 4 Comparison of test strengths with unfactored design strengths according to the EN 1993-1-4 [6] and ASCE standard [7] for hollow stiffened columns Group
Specimen
PTest (kN)
Aeff ,EC 3 (mm2 )
PPred,1 (kN)
PTest /PPred,1
Aeff ,ASCE (mm2 )
PPred,2 (kN)
PTest /PPred,2
SHS1 SHS2 RHS1 RHS2 RHS3
SHS1C0 SHS2C0 RHS1C0 RHS2C0 RHS3C0
425 350 258 263 254
1275 1429 953 1014 1013
363 407 272 289 289
1.17 0.86 0.95 0.91 0.88
1346 1647 1053 1187 1184
384 469 300 338 337
1.11 0.75 0.86 0.78 0.75
Table 5 Comparison of test strengths with unfactored design strengths according to the ENV 1994-1-1 [11] for concrete-filled stiffened columns Group Stiffened specimen
PTest (kN)
Aeff ,EC 3 (mm2 )
PPred,1 (kN)
PTest /PPred,1
0 PPred ,1 (kN)
0 PTest /PPred ,1
At (mm2 )
PPred,3 (kN)
PTest /PPred,3
0 PPred ,3 (kN)
0 PTest /PPred ,3
SHS1
870 1090 1422 1706 875 928 1225 1444 1006 1488
1275 1275 1429 1429 953 953 1014 1014 1013 1013
671 910 972 1412 511 697 739 1089 683 990
1.30 1.20 1.46 1.21 1.71 1.33 1.66 1.33 1.47 1.50
725 1006 1072 1589 553 772 818 1231 752 1114
1.20 1.08 1.33 1.07 1.58 1.20 1.50 1.17 1.34 1.34
1351 1351 1671 1671 1061 1061 1341 1341 1301 1301
692 932 1041 1481 541 727 832 1183 765 1072
1.26 1.17 1.37 1.15 1.62 1.28 1.47 1.22 1.32 1.39
747 1028 1141 1658 584 802 912 1324 835 1196
1.17 1.06 1.25 1.03 1.50 1.16 1.34 1.09 1.21 1.24
SHS2 RHS1 RHS2 RHS3
SHS1C30 SHS1C60 SHS2C30 SHS2C60 RHS1C30 RHS1C60 RHS2C30 RHS2C60 RHS3C30 RHS3C60
Fig. 7. Locations of strain gauges and dial gauges on rectangular specimens.
of concrete strength is improved because of protection against the environment [11]. Here, the comparison is made using four predicted values, as given in Eqs. (2)–(5). In the first predicted strength (PPred,1 ), the effective cross-sectional area of the stainless steel tube as well as the coefficient of the long term effect were considered, while the coefficient of long term effect was neglected 0 in the second predicted strength (PPred ,1 ). On the other hand, the whole cross-sectional area of the stainless steel tube as well as the coefficient of the long term effect were considered in the third predicted strength (PPred,3 ), while the coefficient of the long term 0 effect was neglected in the fourth predicted strength (PPred ,3 ). The comparisons of test strengths with design strengths for test series are given in Table 5. PPred,1 = Aeff fyd + 0.85Ac fcd 0
(3)
PPred,3 = At fyd + 0.85Ac fcd
(4)
PPred,3 = At fyd + Ac fcd
ρ=
(5)
where: Aeff is the effective cross-sectional area of the structural steel section. At is the whole cross-sectional area of the structural steel section. However, Table 5.2 in the EN 1993-1-4 [6] was used to classify the cross-section type of the test specimens. By applying the limitations of Table 5.2, the test specimens were all of Class 4.
0.772
0.125
−
λp
λ2p
but ≤1.
(6)
Welded outstand elements, which are used with longitudinal stiffeners, as shown in Fig. 1:
(2)
PPred,1 = Aeff fyd + Ac fcd 0
As is well known, the effective widths may be used in order to reduce the resistance of the cross-section due to the effect of local buckling. For this reason, clause 5.2.3 was in use during the calculations of the effective widths in Class 4 cross-sections forming this research. A reduction factor ρ should be taken as follows: For Cold formed or welded internal elements, which are used with flat portions, as shown in Fig. 1:
ρ=
1
λp
−
0.242
λ2p
but ≤1
(7)
where λp is the element slenderness defined as:
λp =
b/t 28.4ε
√ kσ
(8)
where; t is the relevant thickness. kσ is the buckling factor corresponding to the stress ratio ψ and boundary conditions from Table 4.1 or 4.2 in EN 19931-5 [12] as appropriate: it is 4.0 for flat portions and 0.43 for longitudinal stiffeners. These values mean that the case of
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Table 6 Comparison of test strengths with unfactored design strengths according to the ASCE standard [7] for concrete-filled stiffened columns Group
Stiffened specimen
PTest (kN)
Aeff ,ASCE (mm2 )
PPred,2 (kN)
PTest /PPred,2
SHS1
SHS1C30 SHS1C60 SHS2C30 SHS2C60 RHS1C30 RHS1C60 RHS2C30 RHS2C60 RHS3C30 RHS3C60
870 1090 1422 1706 875 928 1225 1444 1006 1488
1346 1346 1647 1647 1053 1053 1187 1187 1184 1184
691 930 1034 1474 539 725 788 1139 732 1039
1.26 1.17 1.37 1.16 1.62 1.28 1.55 1.27 1.38 1.43
SHS2 RHS1 RHS2 RHS3
simple support is considered. The assumptions about simple support when determining kσ are normally unfavourable. The assumption is therefore normally a simplification intended for manual calculation. b is the relevant width as follows: b = flat element width while b = c for outstand flanges ε is the material factor defined in Table 5.2 in EN 1993-1-4 [6]. The comparison of the test strength (PTest ) for each concretefilled test specimen was also made by using the predicted value according to the ASCE Standard [7]. The comparisons of test strengths with design strengths (PPred,2 ) according to the ASCE Standard [7] for test series are given in Table 6. Clause 3.4 in the ASCE Standard [7] was used to classify the cross-section type of the test specimens. The resistance of a stainless steel crosssection subjected to compression with a resultant acting through the centroid of the effective section shall be calculated as follows: PPred,2 = Ae Fn + 0.85Ac fc
(9)
where: Ae is the effective area calculated at stress Fn . A reduction factor ρ should be taken as follows: For uniformly compressed stiffened elements, this was used with flat portions:
ρ=
1 − 0.22/λ
if λ =
λ 1.052 w f ≥ 0.673. √ t
k
E◦
(10) (11)
The uniform compressed unstiffened elements, which were used with longitudinal stiffeners, used the same equations as for stiffened elements with the exception that k = 0.5. Fn is the flexural buckling stress for doubly symmetric sections, closed cross sections, and any other sections which are not subjected to torsional or torsional–flexural buckling, the flexural buckling stress. Fn , is determined as follows: Fn = Et =
π 2 Et ≤ Fy (KL/r )2
(12)
E◦ Fy Fy + 0.002nE◦ Fn /Fy
n−1
(13)
E◦ is the initial elastic modulus. n is Ramberg–Osgood parameter which is the strain hardening exponent that defines the degree of roundness of the curve (n = 4.67). Ac is the cross-sectional area of concrete. fc = 0.8fcu is the design value of the cylinder compressive strength of concrete. The determination of flexural buckling stress and then the design of axial strength of concentrically loaded cold-formed stainless steel compression members, according to the ASCE Standard [7], requires an iterative process. In general, the ASCE
Standard [7] requires a significantly greater calculation effort than that required by ENV 1994-1-1 [11]. From Tables 5 and 6, it can be noticed that there is a slight difference between the five predicted values in the case of concretefilled stainless steel stiffened tubular stub columns. But still the 0 predicted value (PPred ,3 ), according to the ENV 1994-1-1 [11], by using the whole cross-sectional area of the stainless steel tube and neglecting the coefficient of long term effect, is more relevant to the test strength (PTest ) for concrete-filled stainless steel stiffened tubular stub columns. On the other hand, the predicted value (PPred,1 ) is close to the test strengths (PTest ) for the stiffened hollowsection stub columns, as can be seen in Table 4. Further results dealing with the stainless steel stiffened hollow section columns can be found in [4]. 3.2. Comparison between hollow-section and concrete in-filled stiffened stub columns From Tables 4 and 5, which tabulate the experimental results of both the hollow-sections and concrete-filled sections, it can be seen that the column strength of the concrete-filled stainless steel stiffened tubular stub columns was considerably higher than that of the stiffened slender stainless steel hollow tubular stub columns. The average values of the column strength of columns in-filled with a concrete of 34.8 MPa mean compressive strength divided by that of the stiffened slender stainless steel hollow tubular stub columns is equal to 3.63. This ratio was about 4.48 in case of the columns in-filled by a concrete of 61.9 MPa mean compressive strength. However, different failure modes were observed from the test series. The mode of failure of the stiffened slender stainless steel hollow tubular stub columns was due to local buckling. Once local buckling has occurred, in concrete-filled stainless steel stiffened tubular stub columns, by the time of reaching the test strength, the stainless steel tube was not able to provide confinement for the concrete. The column capacity, at this stage, was governed by local buckling failure mode. By other words, the occurrence of local buckling was the reason for the descending parts, as can be seen in Figs. 9 and 10. Another observation that can be noticed from the experimental tests is that the stiffeners contributed largely to the test strength (PTest ) of columns even when the stiffeners’ rigidities were small, because the local buckling of longitudinal stiffeners was prevented by the concrete. Fig. 8 represents the deformed shape of the Group SHS1. The load–axial shortening behaviour of the stainless steel stiffened hollow section columns and concrete-filled stainless steel stiffened tubular stub columns was compared. Figs. 9 and 10 show the load versus axial shortening curves of the stiffened square stub columns for Groups SHS1 and RHS1, respectively. The ultimate load of the stiffened column SHS1C0 was 425 kN with an axial shortening at the ultimate load of 1.86 mm. For the concrete-filled specimens, the ultimate load was 870 and 1090 kN for SHS1C30 and SHS1C60, respectively. The axial shortening corresponding to the ultimate load was 2.85 and 1.13 mm for SHS1C30 and
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Fig. 8. Deformed shape of Group SHS1.
SHS1C60, respectively. On the other hand, the ultimate load of the stiffened column RHS1C0 was 258 kN with an axial shortening at the ultimate load of 0.80 mm. For the concrete-filled rectangular specimens, the ultimate load was 875 and 928 kN for RHS1C30 and RHS1C60, respectively. The axial shortening corresponding to the ultimate load was 2.41 and 2.0 mm for RHS1C30 and RHS1C60, respectively. The column strengths of the stiffened square concrete-filled columns SHS1C60 and SHS1C30 were increased by 156% and 105% over that of the stiffened hollow-section column due to the existence of concrete. The increase in the column strength was 259% and 239% for the stiffened rectangular concrete-filled columns RHS1C60 and RHS1C30 over the RHS1C0. It could be seen that the in-filled stiffened column had a considerably higher stiffness than that of the stiffened hollow-section column resulting in a sharper load–axial shortening behaviour.
Fig. 9. Load–shortening relationship of SHS1.
3.3. Effect of nominal concrete strength Two different nominal concrete strengths were used in the tested stub columns, as summarized in Table 3. The mean concrete cubic strengths were 34.8 and 61.9 MPa. From Tables 5 and 6, it can be seen that the test strengths (PTest ) increased with the increase of the nominal value of the in-filled concrete. For instance, the test strength (PTest ) increased from 870 kN to 1090 kN for SHS1C30 and SHS1C60 by increasing the mean concrete cubic strength by 27.1 MPa. The average values of the column strength (PTest ) of the concrete-filled stiffened columns of the 61.9 MPa concrete cubic strength divided by that of 34.8 MPa concrete cubic strength is equal to 1.23. However, the failure mode was of the same form. It was by stainless steel yielding and crushing of concrete. The deformed shape for SHS1C30 and SHS1C60 is shown in Fig. 8. From Figs. 9 and 10, it can be seen that the higher the strength of in-filled concrete, the sharper the load–axial shortening behaviour. 4. Conclusions In this paper, a test program of concrete-filled normal-strength stainless steel stiffened tubular stub columns was presented. The stainless steel grade 1.4301 (304) was used. Results for five concentrically loaded slender stainless steel stiffened columns and ten concrete-filled stainless steel stiffened columns have been presented. The results of the experimental study showed that the design rules specified in European specifications as well as the American one are highly conservative for concrete-filled stainless steel stiffened stub columns. Here, the comparison with the European specifications was made by using four predicted values, while the comparison with the ASCE was made using only one predicted 0 value. However, the predicted values (PPred ,3 ), according to ENV 1994-1-1 which considers the whole cross-sectional area of the
Fig. 10. Load–shortening relationship of RHS1.
stainless steel tube and neglects the long term effect, was more relevant to the test strength (PTest ) of the concrete-filled stainless steel stiffened stub columns than any other predicted value. The tests showed that to increase the capacity of slender stainless steel stiffened tubular stub columns, in-filled concrete may be used. However, the test strengths (PTest ) increased with the increase of the nominal in-filled concrete. Thus, increasing the nominal compressive strength of the in-filled concrete leads to smaller column size which accordingly increases the amount of usable floor space in a structure. Once local buckling had occurred by the time of reaching the test strength, the steel tube was not able to provide confinement to the concrete. The column capacity, at this stage, was governed by local buckling failure mode. The failure was achieved by the local buckling of the stainless tube and the crushing of concrete. The stiffeners contributed largely to the test strength (PTest ) of the columns even when the rigidity of the stiffeners was small, because the local buckling of longitudinal stiffeners is prevented by the concrete. However, the mode of failure of the tests is shown in Figs. 11 and 12.
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Bernt Johansson at LTU; Lulea; Sweden for his support, help and belief in the third author. The third author owes you, thanks. Last but not the least, special thanks to Prof. Milan Veljkovic at LTU; Lulea; Sweden who guided the third author in order to learn more and more. References
Fig. 11. Modes of failure of stainless steel square stub columns.
Fig. 12. Modes of failure of stainless steel rectangular stub columns.
Acknowledgements The authors would like to acknowledge Faculty of Engineering; Tanta University for its support. They are grateful to the Concrete and Heavy Structures Laboratory staff, Faculty of Engineering; Tanta University for their intensive help and technical support. Further thanks are due to the materials properties Laboratory staff, The Housing and Building Research Center; Cairo for their help. Especially the authors would like to thank very much Professor
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