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Performance of concrete-filled stainless steel tubes subjected to tension: Experimental investigation
Thin–Walled Structures 148 (2020) 106602
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Performance of concrete-filled stainless steel tubes subjected to tension: Experimental investigation Yong Ye a, Wei Li b, *, Zi-Xiong Guo a a b
College of Civil Engineering, Huaqiao University, Xiamen, 361021, PR China Department of Civil Engineering, Tsinghua University, Beijing, 100084, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Concrete-filled stainless steel tubular (CFSST) member Tension Composite action Experiment Ultimate tensile strength Simplified model
Concrete-filled stainless steel tubular (CFSST) structures have attracted increasing attention from researchers and engineers in the past decade. CFSST members can be adopted as columns in buildings or chord members in trusses, which could be subjected to tension under some circumstances. This paper presents an experimental investigation on the mechanical behavior of concentrically and eccentrically loaded CFSST tensile members. The parameters considered in the tests included the steel type (stainless steel or carbon steel), load eccentricity (e ¼ 0–75 mm), steel ratio of the cross section (α ¼ 0.075 or 0.112), concrete strength (fcu ¼ 53.4 MPa or 84.7 MPa), and steel-concrete interfacial condition (lubricated or not). The test results show the concrete infill effectively works with the outer stainless steel tube, leading to tensile strength and stiffness higher than those of the cor responding hollow stainless steel tube. All the CFSST tensile members show ductile behavior and the end rotation of the eccentrically loaded members exceeds 0.1 rad. Finally, the feasibility of existing design method to predict the tensile strength of CFSST members was also evaluated.
1. Introduction The concrete-filled stainless steel tubular (CFSST) member consists of a stainless steel tube and the concrete infill. Compared with the traditional concrete-filled steel tubular (CFST) members using carbon steel as the tube material, CFSST members possess extra advantages, including better resistance to rust and corrosion, higher durability, and easier maintenance [1]. Therefore, CFSST structures have attracted increasing attention from researchers and engineers in recent years. CFSST members used in construction projects could be subjected to tension under some circumstances, such as the lower chord under sag ging moment in a truss. Besides, the CFSST component in a lattice pier, tower, or perimeter column system could also be subjected to tension during a severe earthquake [2]. For the tubular member with concrete infilled, the tensile behavior is usually different from the compressive one [3]. Extensive investigations have been conducted on the mechan ical behavior of CFST members subjected to different load conditions, while much less attention to CFST tensile members had been paid until recent years [4]. Han et al. (2011) [3] carried out experimental and numerical investigations on the behavior of CFST members subjected to axial tension. It found that the ultimate tensile strength of the
steel-concrete composite member was approximately 10% higher than that of the corresponding hollow steel tubes. Li et al. (2014) [6,7] investigated the behavior of concentrically loaded and eccentrically loaded concrete-filled double-skin steel tubular (CFDST) members. Li et al. (2015) [8] presented both experimental and numerical studies on the behavior of eccentrically loaded CFST tensile members. Wang et al. (2015) [9] investigated the tensile behavior of CFST members externally strengthened with carbon fiber reinforced polymer (CFRP) sheets. Han et al. (2016) [10] tested concrete-encased CFST members under axial tension, and a simplified model was proposed to be used to predict the ultimate tensile strength. Zhou et al. (2016) [11] carried out experi mental study on the tensile behavior of square CFST members. Chen et al. (2017) [12,13] conducted a series of concentric tension and eccentric tension tests on full-scale CFST members with reinforcing bars or angles. Xu et al. (2017) [2] developed an analytical model and design formulae for predicting the strength and stiffness of CFST members under axial tension. Han et al. (2017) [14] conducted experimental and numerical analysis to investigate the time-dependent performance of CFST tensile members subjected to coupled long-term loading and chloride corrosion. The previous investigations revealed that the tensile strength of concrete-filled members was higher than their hollow tube counterparts. As the tensile strength of concrete was low, the direct
* Corresponding author. E-mail addresses: [email protected] (Y. Ye), [email protected] (W. Li), [email protected] (Z.-X. Guo). https://doi.org/10.1016/j.tws.2020.106602 Received 19 August 2019; Received in revised form 30 November 2019; Accepted 6 January 2020 0263-8231/© 2020 Published by Elsevier Ltd.
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Notation Ac As D E0 e F Fu Fu,c Fu,e fck fcu fscy fy fu L M Mu Wsc
r t
Cross-sectional area of core concrete Cross-sectional area of steel tube Outer diameter of circular steel tube Young’s modulus of elasticity Load eccentricity Tensile load Ultimate tensile strength Calculated ultimate tensile strength Experimentally measured ultimate tensile strength Characteristic compressive strength of concrete Cube compressive strength of concrete Compressive strength of steel-concrete composite member Yield stress of steel Ultimate stress of steel Tube length Moment Ultimate flexural strength Section modulus
α γm
ε εs εs,l εs,t ε η θ θy
μ ξ
σ Mises σ 0.2 Δ Δy
Outer radius of circular steel tube Wall thickness of steel tube Steel ratio of cross section, α ¼ As/Ac Coefficient Strain Steel strain Longitudinal strain Transverse strain Average strain Elongation at break End rotation of specimen End rotation corresponding to yield moment Poisson’s ratio of steel Confinement factor, ξ¼(As⋅fy)/(Ac⋅fck) for conventional carbon-steel CFST members, or ξ¼(As⋅σ0.2)/(Ac⋅fck) for CFSST members von-Mises stress 0.2% proof stress of stainless steel Elongation of specimen Elongation corresponding to yield tensile strength
included:
strength contribution of concrete was minor. However, the tensile strength of the composite tubular member was enhanced because of the “composite action”, i.e., the interaction between steel tube and concrete, which changed the stress status of the steel tube. The tensile behavior of a CFSST member is supposed to be different from that of a CFST member using mild carbon steel, as the stainless steel and mild carbon steel possess different strengths and stress-strain re lationships from each other. The stress-strain response of stainless steel shows a “round house” shape without a notable yield plateau like that for the mild carbon steel [5]. In this circumstance, the composite action between stainless steel tube and concrete could be different from that in the member using mild steel tube. To the authors’ knowledge, research on the mechanical behavior of CFSST members subjected to tension still remains limited, resulting a lack of corresponding design formulae for the composite member, which might hinder the further application of CFSST structures. Therefore, the research on the tensile behavior of CFSST members is conducted. The purpose of this research is fourfold: (1) to provide test data related to the behavior of CFSST members under the load condition of concentric tension or eccentric tension; (2) to investigate the ultimate tensile strength of the member and composite actions between the stainless steel tube and concrete core; (3) to study the influence of different parameters on the tensile behavior of CFSST tensile members; and (4) to study the simplified method which can predict the ultimate strength of CFSST tensile members.
● Load eccentricity (e ¼ 0 mm, 25 mm, 50 mm, and 75 mm, corre sponding to e/r of 0, 0.43, 0.86, and 1.29, respectively, where r is the outer radius of steel tube); ● Steel ratio of cross section (α ¼ As/Ac ¼ 0.075 or 0.112, where As and Ac are the cross-sectional area of the steel tube and core concrete, respectively); ● Concrete compressive strength (fcu ¼ 53.4 MPa or 84.7 MPa); and ● Steel-concrete interfacial condition (lubricated or not). Detailed information of test specimens is listed in Table 1, where t is the wall thickness of the tube; fy is the yield stress of mild carbon steel; and σ0.2 is the 0.2% proof stress of stainless steel. The specimen labels in Table 1 are designated based on the following rules: (1) “C_-_” and “S_-_” stand for the hollow carbon-steel tube and stainless steel tube, respec tively; the number ahead of the hyphen stands for the wall thickness of the tube (in the unit of mm), and the number after the hyphen identifies the different specimen in the same test group. (2) “S_c_-_” stands for the concentrically loaded CFSST specimens; and the three numbers from left to right represent the wall thickness of the tube (in the unit of mm), concrete strength (in the unit of MPa), and different specimen in the same test group, respectively. (3) “S_c_e_-_” stands for the eccentrically loaded CFSST specimens; and the four numbers from left to right represent the wall thickness of the tube (in the unit of mm), concrete strength (in the unit of MPa), load eccentricity (in the unit of mm), and different specimen in the same test group, respectively. Two identical specimens were fabricated and tested for most conditions. Both stainless steel tubes and carbon-steel tubes were manufactured by rolling a flat plate into a cylindrical shell and welding the corre sponding longitudinal seam by a full-penetration groove weld. Two different wall thicknesses (t) of the steel tubes were adopted, namely, 2.07 mm and 3.00 mm, corresponding to a steel ratio (α) of 0.075 and 0.112. Two end plates with 20-mm thickness were welded at both ends of each steel tube. Six stiffeners with a thickness of 8 mm and a height of 40 mm were evenly welded between the steel tube and end plate to enhance the stiffness of the tube end. For each CFSST specimen, an 80mm-diameter hole was cut through one end plate for the concrete casting. After the welding of end plates and stiffeners, the concrete was poured into the tube and vibrated to ensure the compactness. For the
2. Experimental investigation 2.1. Experimental program 2.1.1. General information of specimens A total of 20 specimens were designed and tested, including 14 CFSST specimens, four hollow stainless steel tubes, and two hollow carbon-steel tubes. Among these 20 specimens, nine were tested under concentric tension, and the rest 11 were tested under eccentric tension. The outer diameter (D) for all steel tubes was 116 mm, and the length of all specimens was 350 mm including the thickness of two end plates and the height of stiffeners at both ends, as shown in Fig. 1. The net length of the tube between the stiffeners at both ends was 230 mm, approximately two times the outer diameters of all specimens. The test parameters 2
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Fig. 1. Schematic view of specimens (unit: mm). Table 1 Test parameters and main experimental results. No.
Specimen type
Specimen label
t (mm)
α
fy or σ0.2 (MPa)
fcu (MPa)
e (mm)
Fu (kN)
Mu (kN⋅m)
Bond condition
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
CFSST and hollow tubes under concentric tension
C2-1 C2-2 S2-1 S2-2 S2c50-1 S2c50-2 S2c50-ub S2c80-1 S2c80-2 S2e50-1 S2c50e25-1 S2c50e25-2 S2c50e50-1 S2c50e50-2 S2c50e75-1 S2c50e75-2 S3e50-1 S3c50e25-1 S3c50e50-1 S3c50e75-1
2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 3.00 3.00 3.00 3.00
0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.112 0.112 0.112 0.112
289.1 289.1 312.5 312.5 312.5 312.5 312.5 312.5 312.5 312.5 312.5 312.5 312.5 312.5 312.5 312.5 306.7 306.7 306.7 306.7
Null Null Null Null 53.4 53.4 53.4 84.7 84.7 Null 53.4 53.4 53.4 53.4 53.4 53.4 Null 53.4 53.4 53.4
0 0 0 0 0 0 0 0 0 50 25 25 50 50 75 75 50 25 50 75
190.3 201.3 198.5 203.0 192.3 229.2 215.4 212.3 231.0 116.9 167.7 159.3 154.8 117.6 111.7 104.6 191.3 217.7 196.4 154.6
Null Null Null Null Null Null Null Null Null 5.8 4.2 4.0 7.7 5.9 8.4 7.8 9.6 5.4 9.8 11.6
Null Null Null Null Bonded Bonded Unbonded Bonded Bonded Null Bonded Bonded Bonded Bonded Bonded Bonded Null Bonded Bonded Bonded
CFSST and hollow tubes under eccentric tension
3
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Two different concrete mixtures were designed in the test. The mix proportions and cube compressive strength of the concrete are listed in Table 3, and the measured average values of fcu were 53.4 MPa and 84.7 MPa during the tensile tests.
Table 2 Mechanical properties of the steel. No.
Steel type
t (mm)
fy or σ0.2 (MPa)
fu (MPa)
η
E0 (GPa)
μ
1
Carbon steel Stainless steel
2.30
289.1
360.1
0.212
202.3
0.289
2.07 3.00
312.5 306.7
678.6 690.7
0.669 0.492
178.1 173.1
0.290 0.288
2 3
2.1.3. Test setup and instrumentation A 500-kN-capacity hydraulic actuator was employed for the loading test. A schematic view of the test setup is shown in Fig. 3, where two strong hinged supports are attached at both ends of each specimen. The allowable rotation capacity of the hinged support was designed to be greater than 0.2 rad. The specimen was fastened to the hinged supports with high-strength bolts. Besides, the load eccentricity was achieved by using different holes in the hinged supports.
specimen S2c50-ub, the inner surface of the stainless steel tube was lubricated with grease lubricant before concrete placement. Besides, no stud was used between the end plate and core concrete. 2.1.2. Material properties Type 321 austenitic stainless steel according to ASTM 959-09 [15] was used in stainless steel tubes herein. The material properties of the stainless steel and carbon steel were determined by carrying out a series of coupon tests. The main measured results of the material tests are listed in Table 2, where fu is the tensile strength of steel, η is the elon gation at break, E0 is Young’s modulus of elasticity, μ is Poisson’ ratio of steel. A comparison between the stress-strain responses of stainless steel and carbon steel is shown in Fig. 2.
Table 3 Mix proportions and compressive strength of the concrete. Concrete type
Cement (kg/m3)
Water (kg/ m3)
Sand (kg/ m3)
Coarse aggregate (kg/m3)
Water reducing agent (kg/ m3)
fcu (MPa)
C50 C80
350 540
195 152
695 534
1235 1255
Null 5.4
53.4 84.7
Fig. 2. Experimentally obtained stress-strain relationships of the stainless steel and carbon steel.
Fig. 3. Test setup and instrumentation. 4
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Fig. 4. Deformation characteristics of the tested specimens.
Six strain gauges were attached to the outer surface of the steel tube on the mid-height section. The strain gauges were mounted either perpendicular to or in the lengthwise direction of the steel tube, as shown in Fig. 3. Four displacement transducers (DT1, DT2, DT3 and DT4) were placed evenly at the four corners of each specimen to mea sure the elongation and the curvature of the specimen. The load-controlled mechanism with a load interval of 1/10 of the estimated ultimate tensile strength of specimen was used before an obvious stiffness decrease in the load-deformation curve was observed. After that, the load was applied continuously with a loading rate of 0.01 mm/s. The failure of the specimen was determined as follows: (1) the fracture of weld in the steel tube occurred, or; (2) the average axial tensile strain of the specimen (εa ¼ Δ/L, where Δ is the axial elongation of specimen, and L is the tube length) attained approximately 50,000 με (corresponded Δ � 15.4 mm), which was already large enough and not allowed in practical structures. The loading was terminated when one of the situations was reached. Afterward, the load applied to the specimen was released and the corresponding load versus deformation response was also recorded. All displacement transducers, strain gauges as well as the load cell in the actuator were linked to one data acquisition system. The readings of displacement, strain and load were collected simultaneously by the data acquisition system every 3 s throughout the whole loading and unloading processes.
2.2. Experimental results and discussion 2.2.1. Deformation characteristics During the loading test, all specimens, both concentrically and eccentrically loaded, behaved in a ductile manner. Fig. 4 shows photos of specimens after tests. The uniform elongation of steel tube occurred for concentrically loaded specimens, while the elongation as well as overall bending deformation was found in eccentrically loaded ones. In order to characterize the crack pattern of the core concrete, part of the steel tube was saw-cut and removed after tests. Fig. 5 and Fig. 6 show concrete cracks for concentrically and eccentrically loaded specimens, respectively, where the maximum crack width and the crack interval were marked near the corresponding concrete cracks. For the specimens subjected to concentric tension, similar overall behavior was demonstrated in all specimens, as shown in Fig. 4(a). If the deformation characteristic of the steel tube was studied carefully, it was observed that the pinching effect of the hollow steel tube (S2-1 and C21) was more remarkable than that of the CFSST specimens, see Fig. 7(a). This is due to the lateral support from the concrete core, which reduced the radial deformation of the outer steel tube. After the unloading was finished, all steel tubes exhibited residual axial elongation but no frac ture was observed. The crack pattern of the core concrete in concentri cally loaded specimens is shown in Fig. 5, where the upper number in each photo stands for the crack width (in the unit of mm), and the lower number stands for the distance between two adjacent cracks (in the unit of mm). It was found that parallel cracks were observed in the core concrete, and the steel-concrete interfacial condition showed an obvious 5
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was not observed in the stainless steel tubes of CFSST members. Fig. 7(b) illustrates the typical deformation characteristics of hollow and infilled stainless steel tube subjected to eccentric tension. This behavior confirmed the contribution of core concrete on laterally supporting the outer stainless steel tube and delaying or even preventing the local buckling occurring in the compressive region. The crack pattern of the core concrete in eccentrically loaded specimens is shown in Fig. 6, where the numbers in each photo stand for the crack width at the tensile region (in the unit of mm). Parallel cracks were observed in the core concrete and the maximum crack width varied from 0.1 mm to 1.6 mm for most specimens. It can be found from Fig. 6 that as the load eccentricity (e) increases, the depth of the uncracked section in the core concrete in creases accordingly. 2.2.2. Axial load versus axial elongation responses The measured tensile load (F) versus average axial strain (ε) re lationships for the concentrically and eccentrically loaded specimens are shown in Figs. 8 and 9, where ε ¼ Δ/L, Δ is the elongation of specimen and is taken as the mean value of the readings obtained from the four displacement transducers. It can be seen from Fig. 8 that, the tensile load increases continuously from beginning to termination of loading for all concentrically loaded specimens. The loading section of the F-ε curve is generally composed of two ascending branches, and the slope of the second branch is much smaller than that of the first one. By comparing Fig. 8(a) and (b), it can be noted that the slope of the second branch for the stainless steel tubes (S2-1/2) is obviously greater than that for the carbon-steel tube (C2-1/2), and this is due to the more remarkable strain-hardening behavior of the stainless steel than that of the carbon steel. The average initial stiffness of the concentrically loaded specimens C2-1/2, S2-1/2, S2c50-1/2, S2c50-ub, and S2c80-1/2 was 578.2 kN/ mm, 502.3 kN/mm, 535.0 kN/mm, 505.7 kN/m, and 544.9 kN/mm, respectively. The carbon steel adopted in the present tests had a higher Young’s modulus of elasticity (E0) than the stainless steel, so the initial stiffness of the stainless steel tubes (S2-1/2) was greater than that of the carbon-steel tubes (C2-1/2). When the hollow stainless steel tube was infilled with concrete, they could work together to resist the external load, and the corresponding composite member had a greater initial stiffness than the hollow tubular one. As for the lubricated CFSST member, the outer stainless steel tube worked alone at the beginning, so the initial stiffness was not improved compared to that of the hollow stainless steel tube. Besides, the concrete strength had minor influence on the initial stiffness of the CFSST tensile specimens. After the elon gation of the specimen reached the termination limit (Δ � 15 mm), the specimen was unloaded, and the unloading stiffness was relatively smaller than the corresponding initial loading stiffness as the plastic deformation occurred. The load eccentricity (e) is a key parameter that affects the F-ε re sponses of CFSST tensile members. It can be seen from Fig. 9 that, both the tensile strength and stiffness of specimens decreases as e increases. For the eccentrically loaded hollow stainless steel tube (S2e50-1), the initial loading stiffness was lower than that of the corresponding CFSST specimens (S2c50e50-1/2). However, the hardening stiffness of the hollow stainless steel tube was nearly the same as that of the composite members. This is possibly attributed to the fact that, the cracked core concrete offers lateral support to the outer steel tube, but could not contribute to the stiffness of the whole member. Generally, the obser vations from Figs. 8 and 9 indicate that, the CFSST members possess favorable deformation capacity when subjected to either concentric or eccentric tension.
Fig. 5. Crack width and interval for core concrete in concentrically loaded CFSST members (unit: mm).
effect on the crack pattern. The average width of the main crack was 0.3 mm with an average crack interval of approximately 13.9 mm for the specimen whose tube was not lubricated (S2c50-1). While the average width of the main crack was 1.2 mm with an average crack interval of approximately 26.4 mm for the specimen with lubricated tube (S2c50ub). It is clear that the lubricated specimen had an average crack width and interval obviously greater than the unlubricated specimen. This is attributed to the fact that, the interfacial bond between the stainless steel tube and core concrete in the lubricated specimen is much smaller than that in the specimen without lubrication, resulting in a larger bonded length required to generate a crack in the core concrete. Besides, it can be found from Fig. 5(a) and (c) that, the concrete strength has minor influence on the crack pattern of core concrete in the CFSST members under concentric tension. While for the specimens subjected to eccentric tension, obvious bending deformation was observed during and after the loading test, as shown in Fig. 4(b). The ultimate rotation of all specimens before the initiation of unloading was larger than 0.1 rad. The local buckling occurred near the middle of the compressive zone for the hollow stainless steel tubes (S2e50-1 and S3e50-1), while such a phenomenon
2.2.3. Moment versus rotation responses The measured moment (M) versus rotation (θ) relationships for the eccentrically loaded specimens are shown in Fig. 10, where M ¼ F⋅e, and θ is taken as the average value of 2(Δ2-Δ1)/(h1þh2) and 2(Δ4-Δ3)/ (h3þh4), where Δ1, Δ2, Δ3, and Δ4 are the displacements measured by displacement transducers DT1, DT2, DT3, and DT4, respectively, as 6
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Fig. 6. Crack width and interval for core concrete in eccentrically loaded CFSST members (unit: mm).
Fig. 7. Schematic of deformation of hollow stainless steel tube and CFSST members under tension.
shown in Fig. 3; h1 and h2 are the distance between DT1 and DT2; h3 and h4 are the distance between DT3 and DT4. The characteristics of the M-θ relationships were similar to those of
the F-Δ relationships. The M-θ curves for both the CFSST members and hollow stainless steel tubes were characterized by an initial elastic stage followed by an inelastic response with an obviously decreased stiffness. 7
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Fig. 8. Axial load (F) versus average axial strain (ε) curves for specimens under concentric tension.
Before the termination of loading test, the end rotation of all the eccentrically loaded specimens exceeded 0.1 rad. The ultimate flexural strength (Mu) as well as the initial bending stiffness of the specimen decreased with the increase of the load eccentricity (e). Besides, the bending stiffness of the CFSST members was slightly higher than that of the hollow stainless steel tubes. The ultimate flexural strength (Mu) of the specimens was defined as the moment corresponding to the ultimate tensile strength (Fu), and the experimental values of Mu are listed in Table 1.
For the eccentrically loaded specimens (Fig. 12), the longitudinal strain was no longer uniform as the tensile load (F) increased. Both the tensile and compressive zones were developed due to the load eccen tricity. By comparing either Fig. 12(a) and (c) or Fig. 12(e) and (g), it can be seen that the longitudinal strain in the compressive section (measured by strain gauge S1) and the transverse strain in the tensile section (measured by strain gauge N2) for the composite member [Fig. 12(c) and Fig. (g)] are significantly smaller than those for the hollow stainless steel tube [Fig. 12(a) and Fig. (c)]. The core concrete directly contrib uted to the bearing of external load in the compressive section, and also provided lateral support to the outer steel tube. The contribution from core concrete led to a reduction of longitudinal strain in the compressive section of steel tube, and effectively avoided or delayed the local buckling occurring in the steel tube. For the composite tensile members, the strains in the compressive section (measure by strain gauges S1 and S2) developed faster as the load eccentricity (e) increased. The typical relationships between the tensile load (F) versus |εs,t/εs,l| (absolute value of the ratio between transverse and longitudinal strains in the stainless steel tube) are shown in Fig. 13. At the initial stage of loading, the value of |εs,t/εs,l| was close to the Poisson’s ratio of the corresponding steel (μs). As the tensile load (F) increased, the value of |εs,t/εs,l| for the composite member decreased rapidly, attributed to the supporting effect of the core concrete in restraining the development of transverse strain (εs,t). Before the termination of loading, the value of |εs, t/εs,l| for the hollow stainless steel tubes was more than 0.45, while it was less than 0.1 for the CFSST tensile members. The above observation demonstrates the core concrete in CFSST
2.2.4. Strain analysis The axial tensile load (F) versus steel strain (ε) relationships for the specimens are depicted in Figs. 11 and 12, where εs,l and εs,t denote the longitudinal and transverse strains in the steel tubes, respectively. The values of tensile strain are taken as positive and the compressive ones negative in Figs. 11 and 12. For the concentrically loaded specimens (Fig. 11), the steel strain, either longitudinal or transverse, was taken as the average reading of the four strain gauges. It can be seen that, the strain development in the longitudinal direction (εs,l) for the hollow stainless steel tube was similar to that for the composite members. Due to the lateral support from the core concrete, the radial pinching of the outer steel tube was effectively depressed, so the transverse strain (εs,t) of the composite members was significantly smaller than that of the hollow stainless steel tube under the same external load. The develop ment of longitudinal strain (εs,l) for the hollow carbon-steel tube was not significant when the tensile load (F) approximately attained 200 kN, while εs,l for the hollow stainless steel tube kept on increasing. 8
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Fig. 9. Axial load (F) versus average axial strain (ε) curves for specimens under eccentric tension.
tensile members works well with the outer stainless steel tube. This composite action contributes to the larger initial stiffness and loadbearing capacity than the hollow stainless steel tubes.
● Stage 1 (from Point O to Point A). In this stage, the tensile load in creases almost linearly with the development of elongation. For the specimens (without lubrication) subjected to concentric tension, the applied load is carried by both the steel tube and the core concrete. Once the cracking of core concrete occurs, the steel tube resists the tensile load alone. For the specimens under eccentric tension, both tensile and compressive stresses may occur in the steel tube. Point A is the start point from where the specimen behavior shows an obvious change, and the stiffness experiences decrease remarkably.
2.2.5. Characteristic behavior and ultimate tensile strength It can be noted from Figs. 8–10 that, the tensile load (F)-elongation (Δ) and moment (M)-rotation (θ) responses of CFSST members have a similar developing trend. The typical F-Δ or M-θ relationship for CFSST tensile members is shown in Fig. 14, and can be generally divided into four stages as follows: 9
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Fig. 10. Moment (M) versus rotation angle (θ) curves for specimens under eccentric tension.
● Stage 2 (from Point A to Point B). After Point A, the specimen behavior enters an elasto-plastic stage. The plastic strain develops in the stainless steel tube, and the core concrete provides lateral sup port to the outer tube. The CFSST member exhibits obvious elonga tion or bending deformation in this stage. The average longitudinal strain of the stainless steel tube reaches 5000 με at Point B. ● Stage 3 (from Point B to Point C). With the increase of tensile load or moment, the stainless steel tube shows a “pinching” phenomenon in the mid-height region, while the support provided by the core con crete restrains the increase of tube transverse strain. In this stage, the
load keeps increasing with a much smaller rate than that in Stage 1. The overall elongation or bending deformation of the CFSST tensile member increases continuously. ● Stage 4 (from Point C to Point D). The CFSST tensile member is unloaded in this stage. The elastic deformation is gradually recov ered. Meanwhile, residual overall elongation or bending deformation is observed at the end of unloading. It can be seen from Figs. 8 and 9 that, the tensile load-average axial strain (elongation) responses for all the specimens increase from 10
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3. Discussion and calculation on ultimate tensile strength 3.1. Discussion on composite effect The composite actions in a steel-concrete composite member usually refer to the interfacial interactions, including the bond-slip response and the change of stress status between contacted components [17]. Fig. 18 shows a schematic view of the patterns of concrete cracks in CFSST members and plain concrete cylinders. When the plain concrete cylinder is subjected to concentric tension, a major fracture near the mid-height can be expected. However, for the CFSST tensile members, several parallel cracks occur and distribute evenly on the surface of the concrete cylinder. This is attributed to the fact that once the concrete crack forms inside the tube, the load originally carried by the concrete will be transferred to the tube for the composite action. Meanwhile, the opening of the concrete crack could be restrained by the steel-concrete interfacial stress. For the plain concrete cylinder subjected to eccentric tension, the fracture failure near the mid-height would occur once the ultimate tensile strength is reached. While for the eccentrically loaded CFSST tensile members, several cracks initiate from the tensile region and develop towards the compressive side. With the widening and extension of cracks, the load carried by the concrete decreases while that carried by the tube increases. The development of concrete cracks could also be restrained by the steel-concrete interaction. As a result, the crack pattern for the core concrete in CFSST tensile members is similar to that of balanced-reinforced concrete members subjected to bending. It has been demonstrated by previous research [3] that, the increase of ultimate tensile strength for the CFST members compared to the corresponding hollow carbon-steel tubes derives from the change of steel-stress status. The yield locus for mild carbon-steel tube under biaxial tension obeys the following equation when neglects the radial stress: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðσ1 σ t Þ2 þ σ 21 þ σ2t σ Mises ¼ (1) 2
Fig. 11. Axial tensile load (F) versus steel strain (εs) relationships for concen trically loaded specimens.
beginning to the termination of loading. Thus, it is necessary to define a moment, at which the specimen attains its ultimate tensile strength (Fu). In some previous research on the tensile behavior of CFST members, the ultimate tensile strength was defined as the tensile load when the average axial tensile strain of the specimen (ε) reached 5000 με [3]. In Eurocode 8 [16], the ultimate strength or moment is defined as the load or moment when the deformation of member reaches 3Δy or 3θy, where Δy is the elongation corresponding to the yield tensile strength, and θy is the end rotation corresponding to yield moment. Since the stainless steel does not have an obvious yield stress like the mild carbon steel. Thus, the ultimate tensile strength of CFSST members is defined as the load when the longitudinal tensile strain of the stainless steel tube reaches 5000 με. The purposes of such definition are as follows: (1) when the average longitudinal strain attains 5000 με, the stainless steel tube has already been in the plastic stage, indicating a full utilization of its material ca pacity; and (2) the ultimate tensile strength of CFSST members can be comparable with that of CFST tensile members. Thus defined measured ultimate tensile strengths (Fu) for all speci mens are listed in Table 1. Fig. 15 gives a comparison of Fu for the concentrically loaded specimens. It can be noted that, the presence of core concrete enhances the ultimate tensile strength of hollow stainless steel tubes, and the increase ranges from 5.0% (S2c50-1/2) to 10.4% (S2c80-1/2) for the concentrically loaded CFSST tensile members. This enhancement of the tensile strength derives from the change of stress status of the steel tube. The hollow tube element is subjected to unidi rectional tension, while the tube element with infilled concrete is under bi-directional tension. Fig. 16 shows the stress status of different tube elements, where σl, σ t and p represent the longitudinal stress, transverse stress and normal stress of the tube, respectively. Moreover, the value of Fu for the lubricated specimen (S2c50-ub, Fu ¼ 215.4 kN) is similar to that of the corresponding unlubricated specimens (S2c50-1/2, Fu ¼ 210.8 kN). It shows the effect of steel-concrete interface condition is limited on the tensile strength, as the contribution of concrete is mainly restraining the deformation of the outer tube. The comparison of ultimate tensile strength (Fu) for the eccentrically loaded specimens is shown in Fig. 17. As the load eccentrically (e) in creases from zero to 75 mm (e/r ¼ 1.29), the value of Fu decreases almost linearly for both two sets of specimens. On the other hand, the core concrete in the eccentrically loaded CFSST tensile members also en hances the tensile strength of the hollow stainless steel tubes, as shown in Table 1.
in which, σ l and σ t are longitudinal and transverse stresses of the steel tube, respectively, σMises is the von-Mises stress of mild carbon steel. As for the hollow steel tubes, the von-Mises stress (σ Mises) equals to the longitudinal stress (σ l) due to the transverse stress (σ t) remains zero during the loading process. While for the composite members, the sup porting effect provided by the core concrete results in a positive value of the transverse stress (σ t), leading to the steel tube in a biaxial tensilestress state. As a result, the longitudinal stress (σ l) could grow larger than the yield stress (fy), leading to the ultimate tensile strength of the tube larger than fy⋅As. As for CFSST members, the reason for the increase in ultimate tensile strength (Fu) is like that in mild carbon-steel tubes, though the stainless steel does not have obvious yield stress. 3.2. Relevant design equations So far, there have been equations to predict the ultimate tensile strength of CFST members. The Chinese standard DBJ/T13-51-2010 [18] suggests equations for calculating the tensile load (F)-bending moment (M) interaction curve for the steel-concrete composite mem bers. A linear relationship is used for the F-M interaction curve, which is expressed as follows: F M þ �1 Fu Mu
(2)
F � Fu
(3)
where F and M are the tensile load and moment, respectively; Fu and Mu are the ultimate tensile and flexural strengths, respectively. For the ultimate tensile strength (Fu) of a CFST member, most design 11
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Thin-Walled Structures 148 (2020) 106602
Fig. 12. Axial tensile load (F) versus steel strain (εs) relationships for eccentrically loaded specimens.
12
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Fig. 16. Stress status of hollow stainless steel tubes and infilled tubes.
Fig. 13. Axial tensile load (F) versus strain ratio (|εs,t/εs,l|) relationships for concentrically loaded specimens.
Fig. 17. Comparison of ultimate tensile strength (Fu) for eccentrically loaded specimens.
codes ignore the contribution of the core concrete and consider the tensile strength of the outer steel tube only. Han et al. [3] conducted experimental and numerical investigations on the tensile strength of carbon-steel CFST member subjected to tension, and found that the existence of core concrete could effectively enhance the ultimate tensile strength of hollow steel tube by up to 10%. Furthermore, an equation for predicting the ultimate tensile strength of CFST members was proposed by Han et al. [3] as follows:
Fig. 14. Typical tensile load-elongation or moment-rotation relationship for CFSST tensile member.
Fu ¼ ð1:1
0:4αÞ ⋅ fy ⋅As
(4)
For the flexural strength (Mu) of CFST members with a circular cross section, Han [19] proposed an equation as follows: Mu ¼ γm ⋅Wsc ⋅fscy
(5)
where γ m is a coefficient, γm ¼ 1.1 þ 0.48ln(ξþ0.1), ξ is the confinement factor, ξ¼(As⋅fy)/(Ac⋅fck); Wsc is the section modulus, Wsc ¼ π⋅D3/32, D is the outer diameter of steel tube; and fscy is the comprehensive strength, fscy¼(1.14 þ 1.02ξ)⋅fck, fck is the characteristic compressive strength of concrete. 3.3. Verification and discussion The feasibility of employing the above design formulas to predict the tensile strength (Fu) of CFSST members was evaluated. While using the above equations to calculate the strength of CFSST tensile members, the yield stress (fy) of the carbon steel was replaced by the 0.2% proof stress (σ0.2) of the stainless steel. A comparison between the predicted
Fig. 15. Comparison of ultimate tensile strength (Fu) for concentrically loaded specimens. 13
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Thin-Walled Structures 148 (2020) 106602
Fig. 18. Schematic view of concrete crack patterns.
Fig. 19. Comparison between measured and predicted results.
strengths and experimentally measured ones is shown in Fig. 19. Table 4 lists detailed values. For the concentrically loaded specimens, the cur rent design code overestimates the ultimate tensile strength of CFSST members, and the ratios of predicted strength (Fu,c) to the experimental value (Fu,e) is 1.152, and the corresponding coefficient of variation (COV) is 0.087. This could be mainly due to the difference of mechanical behavior between stainless steel and carbon steel. For the CFSST tensile member, the strength development is more significant when the defor mation is large as the stainless steel has stronger hardening effect. Eq. (4) has overestimation as the strength might not be fully developed when the average elongation reaches 5000 με. For the eccentrically loaded CFSST members, the current calculation method gives a con servative prediction of the tensile strength when the moment is given. Furthermore, the difference between the predicted and measured ulti mate tensile strength increases with the increase of the load eccentricity. This is possibly due to the fact that the predicted flexural strength of CFSST member by Eq. (5) is conservative, leading to an imprecise pre diction of the strength. For the eccentrically loaded specimens, the average and COV of Fu,c/Fu,e are 0.832 and 0.117, respectively. The current design formulas for carbon-steel CFST members provides
conservative predictions on the tensile strength of CFSST members. It is suggested that further research should be carried out to improve the design formulas. 4. Conclusions A series of experiments were conducted in this research to investi gate the mechanical behavior of CFSST members subjected to concentric tension and eccentric tension. Within the range of test parameters studied herein, the following conclusions could be drawn: (1) The CFSST members subjected to either concentric tension (up to ε ¼ 50,000 με) or eccentric tension (up to θ ¼ 0.1 rad) behaved in a ductile manner. The tensile load kept on increasing from the beginning to the end of loading, and no obvious yield plateau was observed. (2) The core concrete in the CFSST tensile member worked well with the outer stainless steel tube. The composite effect between tube and concrete enhanced the ultimate tensile strength of stainless steel tube by 5–10%. 14
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Wei Li: Methodology, Writing - review & editing, Supervision. Zi-Xiong Guo: Project administration.
Table 4 Comparison of ultimate tensile strength between experimental and predicted values. Specimen label
Loading type
Fu,e (kN)
Fu,c (kN)
Fu,c/ Fu,e
Average (Fu,c/Fu, e)
Standard deviation
S2c50-1 S2c50-2 S2c80-1 S2c80-2 S2c50e251 S2c50e252 S2c50e501 S2c50e502 S2c50e751 S2c50e752 S3c50e251 S3c50e502 S3c50e751
Concentric tension
192.3 229.2 212.3 231.0 167.7
247.6 247.6 247.6 247.6 150.5
1.288 1.080 1.166 1.072 0.897
1.152
0.087
0.832
0.117
159.3
150.5
0.945
154.8
108.2
0.699
117.6
108.2
0.920
111.7
84.4
0.756
104.6
84.4
0.807
217.7
207.9
0.955
196.4
148.9
0.758
154.6
115.9
0.750
Eccentric tension
Acknowledgments This research is part of the National Natural Science Foundation of China (Grant No. 51808234). The authors would like to thank Mr. Hao Guo for his assistance in the experimental work. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.tws.2020.106602. References [1] L.H. Han, C.Y. Xu, Z. Tao, Performance of concrete filled stainless steel tubular (CFSST) columns and joints: summary of recent research, J. Constr. Steel Res. 152 (2019) 117–131. [2] L.Y. Xu, M.X. Tao, M. Zhou, Analytical model and design formulae of circular CFSTs under axial tension, J. Constr. Steel Res. 133 (2017) 214–230. [3] L.H. Han, S.H. He, F.Y. Liao, Performance and calculations of concrete filled steel tubes (CFST) under axial tension, J. Constr. Steel Res. 67 (11) (2011) 1699–1709. [4] L.H. Han, Concrete Filled Steel Tubular Structures: Theory and Practice, third ed., Science Press, Beijing, 2016 (in Chinese). [5] Y. Ye, L.H. Han, T. Sheehan, Z.X. Guo, Concrete-filled bimetallic tubes under axial compression: experimental investigation, Thin-Walled Struct. 108 (2016) 321–332. [6] W. Li, L.H. Han, T.M. Chan, Tensile behaviour of concrete-filled double-skin steel tubular members, J. Constr. Steel Res. 99 (2014) 35–46. [7] W. Li, L.H. Han, T.M. Chan, Numerical investigation on the performance of concrete-filled double-skin steel tubular member sunder tension, Thin-Walled Struct. 79 (2014) 108–118. [8] W. Li, L.H. Han, T.M. Chan, Performance of concrete-filled steel tubes subjected to eccentric tension, J. Struct. Eng.-ASCE 141 (12) (2015), 04015049. [9] Z.B. Wang, Q. Yu, Z. Tao, Behaviour of CFRP externally-reinforced circular CFST members under combined tension and bending, J. Constr. Steel Res. 106 (2015) 122–137. [10] L.H. Han, Z.B. Wang, W. Xu, Z. Tao, Behavior of concrete-encased CFST members under axial tension, J. Struct. Eng.-ASCE 142 (2) (2016), 04015149. [11] M. Zhou, J.S. Fan, M.X. Tao, J.G. Nie, Experimental study on the tensile behavior of square concrete-filled steel tubes, J. Constr. Steel Res. 121 (2016) 202–215. [12] J. Chen, J. Wang, W. Li, Experimental behaviour of reinforced concrete-filled steel tubes under eccentric tension, J. Constr. Steel Res. 136 (2017) 91–100. [13] J. Chen, J. Wang, W.L. Jin, Concrete-filled steel tubes with reinforcing bars or angles under axial tension, J. Constr. Steel Res. 133 (2017) 374–382. [14] L.H. Han, Y.X. Hua, C. Hou, Q.L. Wang, Circular concrete-filled steel tubes subjected to coupled tension and chloride corrosion, J. Struct. Eng.-ASCE 143 (10) (2017), 04017134. [15] ASTM A959-09, Standard Guide for Specifying Harmonized Standard Grade Compositions for Wrought Stainless Steels, ASTM International, West Conshohocken (PA), 2009. [16] Eurocode 8, Design of Structures for Earthquake Resistance-Part 3: Assessment and Retrofitting of Buildings, European Committee for Standardization, Brussels, Belgium, 2005. [17] L.H. Han, W. Li, R. Bjorhovde, Developments and advanced applications of concrete-filled steel tubular (CFST) structures: members, J. Constr. Steel Res. 100 (2014) 211–228. [18] Housing and urban-rural development department of Fujian Province, DBJ/T1351-2010, Fuzhou, China, in: Technical Specifications for Concrete-Filled Steel Tubular Structures (Revised Version), 2010 (in Chinese). [19] L.H. Han, Flexural behaviour of concrete-filled steel tubes, J. Constr. Steel Res. 60 (2) (2004) 313–337.
(3) The steel-concrete interfacial condition had minor effect on the load-deformation relationship and ultimate tensile strength of CFSST tensile members. The concrete strength also had minor effect on the load-carrying capacity of CFFST tensile members. (4) The ultimate tensile strength of CFSST members decreased with the increase of load eccentricity, and increased with the increase of the cross-sectional steel ratio. The composite members had higher initial loading stiffness compared with that of the corre sponding hollow stainless steel tubes. (5) By replacing the yield stress of carbon steel (fy) with 0.2% proof stress of stainless steel (σ0.2), the current design codes for carbonsteel CFST tensile members failed to precisely predict the tensile strengths of CFSST members subjected to tension. Thus, further research is needed to improve the prediction model for the tensile strength of CFSST members. It should be mentioned that, this paper only dealt with CFSST tensile members with limited types of stainless steel. As a result, further investigation is still needed to characterize the mechanical behavior of CFSST tensile members with different types of stainless steel, especially with the aid of finite element method to perform parametric analysis. Declaration of competing interest None. CRediT authorship contribution statement Yong Ye: Conceptualization, Investigation, Writing - original draft.
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Full length article
Performance of concrete-filled stainless steel tubes subjected to tension: Experimental investigation Yong Ye a, Wei Li b, *, Zi-Xiong Guo a a b
College of Civil Engineering, Huaqiao University, Xiamen, 361021, PR China Department of Civil Engineering, Tsinghua University, Beijing, 100084, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Concrete-filled stainless steel tubular (CFSST) member Tension Composite action Experiment Ultimate tensile strength Simplified model
Concrete-filled stainless steel tubular (CFSST) structures have attracted increasing attention from researchers and engineers in the past decade. CFSST members can be adopted as columns in buildings or chord members in trusses, which could be subjected to tension under some circumstances. This paper presents an experimental investigation on the mechanical behavior of concentrically and eccentrically loaded CFSST tensile members. The parameters considered in the tests included the steel type (stainless steel or carbon steel), load eccentricity (e ¼ 0–75 mm), steel ratio of the cross section (α ¼ 0.075 or 0.112), concrete strength (fcu ¼ 53.4 MPa or 84.7 MPa), and steel-concrete interfacial condition (lubricated or not). The test results show the concrete infill effectively works with the outer stainless steel tube, leading to tensile strength and stiffness higher than those of the cor responding hollow stainless steel tube. All the CFSST tensile members show ductile behavior and the end rotation of the eccentrically loaded members exceeds 0.1 rad. Finally, the feasibility of existing design method to predict the tensile strength of CFSST members was also evaluated.
1. Introduction The concrete-filled stainless steel tubular (CFSST) member consists of a stainless steel tube and the concrete infill. Compared with the traditional concrete-filled steel tubular (CFST) members using carbon steel as the tube material, CFSST members possess extra advantages, including better resistance to rust and corrosion, higher durability, and easier maintenance [1]. Therefore, CFSST structures have attracted increasing attention from researchers and engineers in recent years. CFSST members used in construction projects could be subjected to tension under some circumstances, such as the lower chord under sag ging moment in a truss. Besides, the CFSST component in a lattice pier, tower, or perimeter column system could also be subjected to tension during a severe earthquake [2]. For the tubular member with concrete infilled, the tensile behavior is usually different from the compressive one [3]. Extensive investigations have been conducted on the mechan ical behavior of CFST members subjected to different load conditions, while much less attention to CFST tensile members had been paid until recent years [4]. Han et al. (2011) [3] carried out experimental and numerical investigations on the behavior of CFST members subjected to axial tension. It found that the ultimate tensile strength of the
steel-concrete composite member was approximately 10% higher than that of the corresponding hollow steel tubes. Li et al. (2014) [6,7] investigated the behavior of concentrically loaded and eccentrically loaded concrete-filled double-skin steel tubular (CFDST) members. Li et al. (2015) [8] presented both experimental and numerical studies on the behavior of eccentrically loaded CFST tensile members. Wang et al. (2015) [9] investigated the tensile behavior of CFST members externally strengthened with carbon fiber reinforced polymer (CFRP) sheets. Han et al. (2016) [10] tested concrete-encased CFST members under axial tension, and a simplified model was proposed to be used to predict the ultimate tensile strength. Zhou et al. (2016) [11] carried out experi mental study on the tensile behavior of square CFST members. Chen et al. (2017) [12,13] conducted a series of concentric tension and eccentric tension tests on full-scale CFST members with reinforcing bars or angles. Xu et al. (2017) [2] developed an analytical model and design formulae for predicting the strength and stiffness of CFST members under axial tension. Han et al. (2017) [14] conducted experimental and numerical analysis to investigate the time-dependent performance of CFST tensile members subjected to coupled long-term loading and chloride corrosion. The previous investigations revealed that the tensile strength of concrete-filled members was higher than their hollow tube counterparts. As the tensile strength of concrete was low, the direct
* Corresponding author. E-mail addresses: [email protected] (Y. Ye), [email protected] (W. Li), [email protected] (Z.-X. Guo). https://doi.org/10.1016/j.tws.2020.106602 Received 19 August 2019; Received in revised form 30 November 2019; Accepted 6 January 2020 0263-8231/© 2020 Published by Elsevier Ltd.
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Notation Ac As D E0 e F Fu Fu,c Fu,e fck fcu fscy fy fu L M Mu Wsc
r t
Cross-sectional area of core concrete Cross-sectional area of steel tube Outer diameter of circular steel tube Young’s modulus of elasticity Load eccentricity Tensile load Ultimate tensile strength Calculated ultimate tensile strength Experimentally measured ultimate tensile strength Characteristic compressive strength of concrete Cube compressive strength of concrete Compressive strength of steel-concrete composite member Yield stress of steel Ultimate stress of steel Tube length Moment Ultimate flexural strength Section modulus
α γm
ε εs εs,l εs,t ε η θ θy
μ ξ
σ Mises σ 0.2 Δ Δy
Outer radius of circular steel tube Wall thickness of steel tube Steel ratio of cross section, α ¼ As/Ac Coefficient Strain Steel strain Longitudinal strain Transverse strain Average strain Elongation at break End rotation of specimen End rotation corresponding to yield moment Poisson’s ratio of steel Confinement factor, ξ¼(As⋅fy)/(Ac⋅fck) for conventional carbon-steel CFST members, or ξ¼(As⋅σ0.2)/(Ac⋅fck) for CFSST members von-Mises stress 0.2% proof stress of stainless steel Elongation of specimen Elongation corresponding to yield tensile strength
included:
strength contribution of concrete was minor. However, the tensile strength of the composite tubular member was enhanced because of the “composite action”, i.e., the interaction between steel tube and concrete, which changed the stress status of the steel tube. The tensile behavior of a CFSST member is supposed to be different from that of a CFST member using mild carbon steel, as the stainless steel and mild carbon steel possess different strengths and stress-strain re lationships from each other. The stress-strain response of stainless steel shows a “round house” shape without a notable yield plateau like that for the mild carbon steel [5]. In this circumstance, the composite action between stainless steel tube and concrete could be different from that in the member using mild steel tube. To the authors’ knowledge, research on the mechanical behavior of CFSST members subjected to tension still remains limited, resulting a lack of corresponding design formulae for the composite member, which might hinder the further application of CFSST structures. Therefore, the research on the tensile behavior of CFSST members is conducted. The purpose of this research is fourfold: (1) to provide test data related to the behavior of CFSST members under the load condition of concentric tension or eccentric tension; (2) to investigate the ultimate tensile strength of the member and composite actions between the stainless steel tube and concrete core; (3) to study the influence of different parameters on the tensile behavior of CFSST tensile members; and (4) to study the simplified method which can predict the ultimate strength of CFSST tensile members.
● Load eccentricity (e ¼ 0 mm, 25 mm, 50 mm, and 75 mm, corre sponding to e/r of 0, 0.43, 0.86, and 1.29, respectively, where r is the outer radius of steel tube); ● Steel ratio of cross section (α ¼ As/Ac ¼ 0.075 or 0.112, where As and Ac are the cross-sectional area of the steel tube and core concrete, respectively); ● Concrete compressive strength (fcu ¼ 53.4 MPa or 84.7 MPa); and ● Steel-concrete interfacial condition (lubricated or not). Detailed information of test specimens is listed in Table 1, where t is the wall thickness of the tube; fy is the yield stress of mild carbon steel; and σ0.2 is the 0.2% proof stress of stainless steel. The specimen labels in Table 1 are designated based on the following rules: (1) “C_-_” and “S_-_” stand for the hollow carbon-steel tube and stainless steel tube, respec tively; the number ahead of the hyphen stands for the wall thickness of the tube (in the unit of mm), and the number after the hyphen identifies the different specimen in the same test group. (2) “S_c_-_” stands for the concentrically loaded CFSST specimens; and the three numbers from left to right represent the wall thickness of the tube (in the unit of mm), concrete strength (in the unit of MPa), and different specimen in the same test group, respectively. (3) “S_c_e_-_” stands for the eccentrically loaded CFSST specimens; and the four numbers from left to right represent the wall thickness of the tube (in the unit of mm), concrete strength (in the unit of MPa), load eccentricity (in the unit of mm), and different specimen in the same test group, respectively. Two identical specimens were fabricated and tested for most conditions. Both stainless steel tubes and carbon-steel tubes were manufactured by rolling a flat plate into a cylindrical shell and welding the corre sponding longitudinal seam by a full-penetration groove weld. Two different wall thicknesses (t) of the steel tubes were adopted, namely, 2.07 mm and 3.00 mm, corresponding to a steel ratio (α) of 0.075 and 0.112. Two end plates with 20-mm thickness were welded at both ends of each steel tube. Six stiffeners with a thickness of 8 mm and a height of 40 mm were evenly welded between the steel tube and end plate to enhance the stiffness of the tube end. For each CFSST specimen, an 80mm-diameter hole was cut through one end plate for the concrete casting. After the welding of end plates and stiffeners, the concrete was poured into the tube and vibrated to ensure the compactness. For the
2. Experimental investigation 2.1. Experimental program 2.1.1. General information of specimens A total of 20 specimens were designed and tested, including 14 CFSST specimens, four hollow stainless steel tubes, and two hollow carbon-steel tubes. Among these 20 specimens, nine were tested under concentric tension, and the rest 11 were tested under eccentric tension. The outer diameter (D) for all steel tubes was 116 mm, and the length of all specimens was 350 mm including the thickness of two end plates and the height of stiffeners at both ends, as shown in Fig. 1. The net length of the tube between the stiffeners at both ends was 230 mm, approximately two times the outer diameters of all specimens. The test parameters 2
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Fig. 1. Schematic view of specimens (unit: mm). Table 1 Test parameters and main experimental results. No.
Specimen type
Specimen label
t (mm)
α
fy or σ0.2 (MPa)
fcu (MPa)
e (mm)
Fu (kN)
Mu (kN⋅m)
Bond condition
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
CFSST and hollow tubes under concentric tension
C2-1 C2-2 S2-1 S2-2 S2c50-1 S2c50-2 S2c50-ub S2c80-1 S2c80-2 S2e50-1 S2c50e25-1 S2c50e25-2 S2c50e50-1 S2c50e50-2 S2c50e75-1 S2c50e75-2 S3e50-1 S3c50e25-1 S3c50e50-1 S3c50e75-1
2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 3.00 3.00 3.00 3.00
0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.112 0.112 0.112 0.112
289.1 289.1 312.5 312.5 312.5 312.5 312.5 312.5 312.5 312.5 312.5 312.5 312.5 312.5 312.5 312.5 306.7 306.7 306.7 306.7
Null Null Null Null 53.4 53.4 53.4 84.7 84.7 Null 53.4 53.4 53.4 53.4 53.4 53.4 Null 53.4 53.4 53.4
0 0 0 0 0 0 0 0 0 50 25 25 50 50 75 75 50 25 50 75
190.3 201.3 198.5 203.0 192.3 229.2 215.4 212.3 231.0 116.9 167.7 159.3 154.8 117.6 111.7 104.6 191.3 217.7 196.4 154.6
Null Null Null Null Null Null Null Null Null 5.8 4.2 4.0 7.7 5.9 8.4 7.8 9.6 5.4 9.8 11.6
Null Null Null Null Bonded Bonded Unbonded Bonded Bonded Null Bonded Bonded Bonded Bonded Bonded Bonded Null Bonded Bonded Bonded
CFSST and hollow tubes under eccentric tension
3
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Two different concrete mixtures were designed in the test. The mix proportions and cube compressive strength of the concrete are listed in Table 3, and the measured average values of fcu were 53.4 MPa and 84.7 MPa during the tensile tests.
Table 2 Mechanical properties of the steel. No.
Steel type
t (mm)
fy or σ0.2 (MPa)
fu (MPa)
η
E0 (GPa)
μ
1
Carbon steel Stainless steel
2.30
289.1
360.1
0.212
202.3
0.289
2.07 3.00
312.5 306.7
678.6 690.7
0.669 0.492
178.1 173.1
0.290 0.288
2 3
2.1.3. Test setup and instrumentation A 500-kN-capacity hydraulic actuator was employed for the loading test. A schematic view of the test setup is shown in Fig. 3, where two strong hinged supports are attached at both ends of each specimen. The allowable rotation capacity of the hinged support was designed to be greater than 0.2 rad. The specimen was fastened to the hinged supports with high-strength bolts. Besides, the load eccentricity was achieved by using different holes in the hinged supports.
specimen S2c50-ub, the inner surface of the stainless steel tube was lubricated with grease lubricant before concrete placement. Besides, no stud was used between the end plate and core concrete. 2.1.2. Material properties Type 321 austenitic stainless steel according to ASTM 959-09 [15] was used in stainless steel tubes herein. The material properties of the stainless steel and carbon steel were determined by carrying out a series of coupon tests. The main measured results of the material tests are listed in Table 2, where fu is the tensile strength of steel, η is the elon gation at break, E0 is Young’s modulus of elasticity, μ is Poisson’ ratio of steel. A comparison between the stress-strain responses of stainless steel and carbon steel is shown in Fig. 2.
Table 3 Mix proportions and compressive strength of the concrete. Concrete type
Cement (kg/m3)
Water (kg/ m3)
Sand (kg/ m3)
Coarse aggregate (kg/m3)
Water reducing agent (kg/ m3)
fcu (MPa)
C50 C80
350 540
195 152
695 534
1235 1255
Null 5.4
53.4 84.7
Fig. 2. Experimentally obtained stress-strain relationships of the stainless steel and carbon steel.
Fig. 3. Test setup and instrumentation. 4
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Fig. 4. Deformation characteristics of the tested specimens.
Six strain gauges were attached to the outer surface of the steel tube on the mid-height section. The strain gauges were mounted either perpendicular to or in the lengthwise direction of the steel tube, as shown in Fig. 3. Four displacement transducers (DT1, DT2, DT3 and DT4) were placed evenly at the four corners of each specimen to mea sure the elongation and the curvature of the specimen. The load-controlled mechanism with a load interval of 1/10 of the estimated ultimate tensile strength of specimen was used before an obvious stiffness decrease in the load-deformation curve was observed. After that, the load was applied continuously with a loading rate of 0.01 mm/s. The failure of the specimen was determined as follows: (1) the fracture of weld in the steel tube occurred, or; (2) the average axial tensile strain of the specimen (εa ¼ Δ/L, where Δ is the axial elongation of specimen, and L is the tube length) attained approximately 50,000 με (corresponded Δ � 15.4 mm), which was already large enough and not allowed in practical structures. The loading was terminated when one of the situations was reached. Afterward, the load applied to the specimen was released and the corresponding load versus deformation response was also recorded. All displacement transducers, strain gauges as well as the load cell in the actuator were linked to one data acquisition system. The readings of displacement, strain and load were collected simultaneously by the data acquisition system every 3 s throughout the whole loading and unloading processes.
2.2. Experimental results and discussion 2.2.1. Deformation characteristics During the loading test, all specimens, both concentrically and eccentrically loaded, behaved in a ductile manner. Fig. 4 shows photos of specimens after tests. The uniform elongation of steel tube occurred for concentrically loaded specimens, while the elongation as well as overall bending deformation was found in eccentrically loaded ones. In order to characterize the crack pattern of the core concrete, part of the steel tube was saw-cut and removed after tests. Fig. 5 and Fig. 6 show concrete cracks for concentrically and eccentrically loaded specimens, respectively, where the maximum crack width and the crack interval were marked near the corresponding concrete cracks. For the specimens subjected to concentric tension, similar overall behavior was demonstrated in all specimens, as shown in Fig. 4(a). If the deformation characteristic of the steel tube was studied carefully, it was observed that the pinching effect of the hollow steel tube (S2-1 and C21) was more remarkable than that of the CFSST specimens, see Fig. 7(a). This is due to the lateral support from the concrete core, which reduced the radial deformation of the outer steel tube. After the unloading was finished, all steel tubes exhibited residual axial elongation but no frac ture was observed. The crack pattern of the core concrete in concentri cally loaded specimens is shown in Fig. 5, where the upper number in each photo stands for the crack width (in the unit of mm), and the lower number stands for the distance between two adjacent cracks (in the unit of mm). It was found that parallel cracks were observed in the core concrete, and the steel-concrete interfacial condition showed an obvious 5
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was not observed in the stainless steel tubes of CFSST members. Fig. 7(b) illustrates the typical deformation characteristics of hollow and infilled stainless steel tube subjected to eccentric tension. This behavior confirmed the contribution of core concrete on laterally supporting the outer stainless steel tube and delaying or even preventing the local buckling occurring in the compressive region. The crack pattern of the core concrete in eccentrically loaded specimens is shown in Fig. 6, where the numbers in each photo stand for the crack width at the tensile region (in the unit of mm). Parallel cracks were observed in the core concrete and the maximum crack width varied from 0.1 mm to 1.6 mm for most specimens. It can be found from Fig. 6 that as the load eccentricity (e) increases, the depth of the uncracked section in the core concrete in creases accordingly. 2.2.2. Axial load versus axial elongation responses The measured tensile load (F) versus average axial strain (ε) re lationships for the concentrically and eccentrically loaded specimens are shown in Figs. 8 and 9, where ε ¼ Δ/L, Δ is the elongation of specimen and is taken as the mean value of the readings obtained from the four displacement transducers. It can be seen from Fig. 8 that, the tensile load increases continuously from beginning to termination of loading for all concentrically loaded specimens. The loading section of the F-ε curve is generally composed of two ascending branches, and the slope of the second branch is much smaller than that of the first one. By comparing Fig. 8(a) and (b), it can be noted that the slope of the second branch for the stainless steel tubes (S2-1/2) is obviously greater than that for the carbon-steel tube (C2-1/2), and this is due to the more remarkable strain-hardening behavior of the stainless steel than that of the carbon steel. The average initial stiffness of the concentrically loaded specimens C2-1/2, S2-1/2, S2c50-1/2, S2c50-ub, and S2c80-1/2 was 578.2 kN/ mm, 502.3 kN/mm, 535.0 kN/mm, 505.7 kN/m, and 544.9 kN/mm, respectively. The carbon steel adopted in the present tests had a higher Young’s modulus of elasticity (E0) than the stainless steel, so the initial stiffness of the stainless steel tubes (S2-1/2) was greater than that of the carbon-steel tubes (C2-1/2). When the hollow stainless steel tube was infilled with concrete, they could work together to resist the external load, and the corresponding composite member had a greater initial stiffness than the hollow tubular one. As for the lubricated CFSST member, the outer stainless steel tube worked alone at the beginning, so the initial stiffness was not improved compared to that of the hollow stainless steel tube. Besides, the concrete strength had minor influence on the initial stiffness of the CFSST tensile specimens. After the elon gation of the specimen reached the termination limit (Δ � 15 mm), the specimen was unloaded, and the unloading stiffness was relatively smaller than the corresponding initial loading stiffness as the plastic deformation occurred. The load eccentricity (e) is a key parameter that affects the F-ε re sponses of CFSST tensile members. It can be seen from Fig. 9 that, both the tensile strength and stiffness of specimens decreases as e increases. For the eccentrically loaded hollow stainless steel tube (S2e50-1), the initial loading stiffness was lower than that of the corresponding CFSST specimens (S2c50e50-1/2). However, the hardening stiffness of the hollow stainless steel tube was nearly the same as that of the composite members. This is possibly attributed to the fact that, the cracked core concrete offers lateral support to the outer steel tube, but could not contribute to the stiffness of the whole member. Generally, the obser vations from Figs. 8 and 9 indicate that, the CFSST members possess favorable deformation capacity when subjected to either concentric or eccentric tension.
Fig. 5. Crack width and interval for core concrete in concentrically loaded CFSST members (unit: mm).
effect on the crack pattern. The average width of the main crack was 0.3 mm with an average crack interval of approximately 13.9 mm for the specimen whose tube was not lubricated (S2c50-1). While the average width of the main crack was 1.2 mm with an average crack interval of approximately 26.4 mm for the specimen with lubricated tube (S2c50ub). It is clear that the lubricated specimen had an average crack width and interval obviously greater than the unlubricated specimen. This is attributed to the fact that, the interfacial bond between the stainless steel tube and core concrete in the lubricated specimen is much smaller than that in the specimen without lubrication, resulting in a larger bonded length required to generate a crack in the core concrete. Besides, it can be found from Fig. 5(a) and (c) that, the concrete strength has minor influence on the crack pattern of core concrete in the CFSST members under concentric tension. While for the specimens subjected to eccentric tension, obvious bending deformation was observed during and after the loading test, as shown in Fig. 4(b). The ultimate rotation of all specimens before the initiation of unloading was larger than 0.1 rad. The local buckling occurred near the middle of the compressive zone for the hollow stainless steel tubes (S2e50-1 and S3e50-1), while such a phenomenon
2.2.3. Moment versus rotation responses The measured moment (M) versus rotation (θ) relationships for the eccentrically loaded specimens are shown in Fig. 10, where M ¼ F⋅e, and θ is taken as the average value of 2(Δ2-Δ1)/(h1þh2) and 2(Δ4-Δ3)/ (h3þh4), where Δ1, Δ2, Δ3, and Δ4 are the displacements measured by displacement transducers DT1, DT2, DT3, and DT4, respectively, as 6
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Fig. 6. Crack width and interval for core concrete in eccentrically loaded CFSST members (unit: mm).
Fig. 7. Schematic of deformation of hollow stainless steel tube and CFSST members under tension.
shown in Fig. 3; h1 and h2 are the distance between DT1 and DT2; h3 and h4 are the distance between DT3 and DT4. The characteristics of the M-θ relationships were similar to those of
the F-Δ relationships. The M-θ curves for both the CFSST members and hollow stainless steel tubes were characterized by an initial elastic stage followed by an inelastic response with an obviously decreased stiffness. 7
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Fig. 8. Axial load (F) versus average axial strain (ε) curves for specimens under concentric tension.
Before the termination of loading test, the end rotation of all the eccentrically loaded specimens exceeded 0.1 rad. The ultimate flexural strength (Mu) as well as the initial bending stiffness of the specimen decreased with the increase of the load eccentricity (e). Besides, the bending stiffness of the CFSST members was slightly higher than that of the hollow stainless steel tubes. The ultimate flexural strength (Mu) of the specimens was defined as the moment corresponding to the ultimate tensile strength (Fu), and the experimental values of Mu are listed in Table 1.
For the eccentrically loaded specimens (Fig. 12), the longitudinal strain was no longer uniform as the tensile load (F) increased. Both the tensile and compressive zones were developed due to the load eccen tricity. By comparing either Fig. 12(a) and (c) or Fig. 12(e) and (g), it can be seen that the longitudinal strain in the compressive section (measured by strain gauge S1) and the transverse strain in the tensile section (measured by strain gauge N2) for the composite member [Fig. 12(c) and Fig. (g)] are significantly smaller than those for the hollow stainless steel tube [Fig. 12(a) and Fig. (c)]. The core concrete directly contrib uted to the bearing of external load in the compressive section, and also provided lateral support to the outer steel tube. The contribution from core concrete led to a reduction of longitudinal strain in the compressive section of steel tube, and effectively avoided or delayed the local buckling occurring in the steel tube. For the composite tensile members, the strains in the compressive section (measure by strain gauges S1 and S2) developed faster as the load eccentricity (e) increased. The typical relationships between the tensile load (F) versus |εs,t/εs,l| (absolute value of the ratio between transverse and longitudinal strains in the stainless steel tube) are shown in Fig. 13. At the initial stage of loading, the value of |εs,t/εs,l| was close to the Poisson’s ratio of the corresponding steel (μs). As the tensile load (F) increased, the value of |εs,t/εs,l| for the composite member decreased rapidly, attributed to the supporting effect of the core concrete in restraining the development of transverse strain (εs,t). Before the termination of loading, the value of |εs, t/εs,l| for the hollow stainless steel tubes was more than 0.45, while it was less than 0.1 for the CFSST tensile members. The above observation demonstrates the core concrete in CFSST
2.2.4. Strain analysis The axial tensile load (F) versus steel strain (ε) relationships for the specimens are depicted in Figs. 11 and 12, where εs,l and εs,t denote the longitudinal and transverse strains in the steel tubes, respectively. The values of tensile strain are taken as positive and the compressive ones negative in Figs. 11 and 12. For the concentrically loaded specimens (Fig. 11), the steel strain, either longitudinal or transverse, was taken as the average reading of the four strain gauges. It can be seen that, the strain development in the longitudinal direction (εs,l) for the hollow stainless steel tube was similar to that for the composite members. Due to the lateral support from the core concrete, the radial pinching of the outer steel tube was effectively depressed, so the transverse strain (εs,t) of the composite members was significantly smaller than that of the hollow stainless steel tube under the same external load. The develop ment of longitudinal strain (εs,l) for the hollow carbon-steel tube was not significant when the tensile load (F) approximately attained 200 kN, while εs,l for the hollow stainless steel tube kept on increasing. 8
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Fig. 9. Axial load (F) versus average axial strain (ε) curves for specimens under eccentric tension.
tensile members works well with the outer stainless steel tube. This composite action contributes to the larger initial stiffness and loadbearing capacity than the hollow stainless steel tubes.
● Stage 1 (from Point O to Point A). In this stage, the tensile load in creases almost linearly with the development of elongation. For the specimens (without lubrication) subjected to concentric tension, the applied load is carried by both the steel tube and the core concrete. Once the cracking of core concrete occurs, the steel tube resists the tensile load alone. For the specimens under eccentric tension, both tensile and compressive stresses may occur in the steel tube. Point A is the start point from where the specimen behavior shows an obvious change, and the stiffness experiences decrease remarkably.
2.2.5. Characteristic behavior and ultimate tensile strength It can be noted from Figs. 8–10 that, the tensile load (F)-elongation (Δ) and moment (M)-rotation (θ) responses of CFSST members have a similar developing trend. The typical F-Δ or M-θ relationship for CFSST tensile members is shown in Fig. 14, and can be generally divided into four stages as follows: 9
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Fig. 10. Moment (M) versus rotation angle (θ) curves for specimens under eccentric tension.
● Stage 2 (from Point A to Point B). After Point A, the specimen behavior enters an elasto-plastic stage. The plastic strain develops in the stainless steel tube, and the core concrete provides lateral sup port to the outer tube. The CFSST member exhibits obvious elonga tion or bending deformation in this stage. The average longitudinal strain of the stainless steel tube reaches 5000 με at Point B. ● Stage 3 (from Point B to Point C). With the increase of tensile load or moment, the stainless steel tube shows a “pinching” phenomenon in the mid-height region, while the support provided by the core con crete restrains the increase of tube transverse strain. In this stage, the
load keeps increasing with a much smaller rate than that in Stage 1. The overall elongation or bending deformation of the CFSST tensile member increases continuously. ● Stage 4 (from Point C to Point D). The CFSST tensile member is unloaded in this stage. The elastic deformation is gradually recov ered. Meanwhile, residual overall elongation or bending deformation is observed at the end of unloading. It can be seen from Figs. 8 and 9 that, the tensile load-average axial strain (elongation) responses for all the specimens increase from 10
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3. Discussion and calculation on ultimate tensile strength 3.1. Discussion on composite effect The composite actions in a steel-concrete composite member usually refer to the interfacial interactions, including the bond-slip response and the change of stress status between contacted components [17]. Fig. 18 shows a schematic view of the patterns of concrete cracks in CFSST members and plain concrete cylinders. When the plain concrete cylinder is subjected to concentric tension, a major fracture near the mid-height can be expected. However, for the CFSST tensile members, several parallel cracks occur and distribute evenly on the surface of the concrete cylinder. This is attributed to the fact that once the concrete crack forms inside the tube, the load originally carried by the concrete will be transferred to the tube for the composite action. Meanwhile, the opening of the concrete crack could be restrained by the steel-concrete interfacial stress. For the plain concrete cylinder subjected to eccentric tension, the fracture failure near the mid-height would occur once the ultimate tensile strength is reached. While for the eccentrically loaded CFSST tensile members, several cracks initiate from the tensile region and develop towards the compressive side. With the widening and extension of cracks, the load carried by the concrete decreases while that carried by the tube increases. The development of concrete cracks could also be restrained by the steel-concrete interaction. As a result, the crack pattern for the core concrete in CFSST tensile members is similar to that of balanced-reinforced concrete members subjected to bending. It has been demonstrated by previous research [3] that, the increase of ultimate tensile strength for the CFST members compared to the corresponding hollow carbon-steel tubes derives from the change of steel-stress status. The yield locus for mild carbon-steel tube under biaxial tension obeys the following equation when neglects the radial stress: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðσ1 σ t Þ2 þ σ 21 þ σ2t σ Mises ¼ (1) 2
Fig. 11. Axial tensile load (F) versus steel strain (εs) relationships for concen trically loaded specimens.
beginning to the termination of loading. Thus, it is necessary to define a moment, at which the specimen attains its ultimate tensile strength (Fu). In some previous research on the tensile behavior of CFST members, the ultimate tensile strength was defined as the tensile load when the average axial tensile strain of the specimen (ε) reached 5000 με [3]. In Eurocode 8 [16], the ultimate strength or moment is defined as the load or moment when the deformation of member reaches 3Δy or 3θy, where Δy is the elongation corresponding to the yield tensile strength, and θy is the end rotation corresponding to yield moment. Since the stainless steel does not have an obvious yield stress like the mild carbon steel. Thus, the ultimate tensile strength of CFSST members is defined as the load when the longitudinal tensile strain of the stainless steel tube reaches 5000 με. The purposes of such definition are as follows: (1) when the average longitudinal strain attains 5000 με, the stainless steel tube has already been in the plastic stage, indicating a full utilization of its material ca pacity; and (2) the ultimate tensile strength of CFSST members can be comparable with that of CFST tensile members. Thus defined measured ultimate tensile strengths (Fu) for all speci mens are listed in Table 1. Fig. 15 gives a comparison of Fu for the concentrically loaded specimens. It can be noted that, the presence of core concrete enhances the ultimate tensile strength of hollow stainless steel tubes, and the increase ranges from 5.0% (S2c50-1/2) to 10.4% (S2c80-1/2) for the concentrically loaded CFSST tensile members. This enhancement of the tensile strength derives from the change of stress status of the steel tube. The hollow tube element is subjected to unidi rectional tension, while the tube element with infilled concrete is under bi-directional tension. Fig. 16 shows the stress status of different tube elements, where σl, σ t and p represent the longitudinal stress, transverse stress and normal stress of the tube, respectively. Moreover, the value of Fu for the lubricated specimen (S2c50-ub, Fu ¼ 215.4 kN) is similar to that of the corresponding unlubricated specimens (S2c50-1/2, Fu ¼ 210.8 kN). It shows the effect of steel-concrete interface condition is limited on the tensile strength, as the contribution of concrete is mainly restraining the deformation of the outer tube. The comparison of ultimate tensile strength (Fu) for the eccentrically loaded specimens is shown in Fig. 17. As the load eccentrically (e) in creases from zero to 75 mm (e/r ¼ 1.29), the value of Fu decreases almost linearly for both two sets of specimens. On the other hand, the core concrete in the eccentrically loaded CFSST tensile members also en hances the tensile strength of the hollow stainless steel tubes, as shown in Table 1.
in which, σ l and σ t are longitudinal and transverse stresses of the steel tube, respectively, σMises is the von-Mises stress of mild carbon steel. As for the hollow steel tubes, the von-Mises stress (σ Mises) equals to the longitudinal stress (σ l) due to the transverse stress (σ t) remains zero during the loading process. While for the composite members, the sup porting effect provided by the core concrete results in a positive value of the transverse stress (σ t), leading to the steel tube in a biaxial tensilestress state. As a result, the longitudinal stress (σ l) could grow larger than the yield stress (fy), leading to the ultimate tensile strength of the tube larger than fy⋅As. As for CFSST members, the reason for the increase in ultimate tensile strength (Fu) is like that in mild carbon-steel tubes, though the stainless steel does not have obvious yield stress. 3.2. Relevant design equations So far, there have been equations to predict the ultimate tensile strength of CFST members. The Chinese standard DBJ/T13-51-2010 [18] suggests equations for calculating the tensile load (F)-bending moment (M) interaction curve for the steel-concrete composite mem bers. A linear relationship is used for the F-M interaction curve, which is expressed as follows: F M þ �1 Fu Mu
(2)
F � Fu
(3)
where F and M are the tensile load and moment, respectively; Fu and Mu are the ultimate tensile and flexural strengths, respectively. For the ultimate tensile strength (Fu) of a CFST member, most design 11
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Fig. 12. Axial tensile load (F) versus steel strain (εs) relationships for eccentrically loaded specimens.
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Fig. 16. Stress status of hollow stainless steel tubes and infilled tubes.
Fig. 13. Axial tensile load (F) versus strain ratio (|εs,t/εs,l|) relationships for concentrically loaded specimens.
Fig. 17. Comparison of ultimate tensile strength (Fu) for eccentrically loaded specimens.
codes ignore the contribution of the core concrete and consider the tensile strength of the outer steel tube only. Han et al. [3] conducted experimental and numerical investigations on the tensile strength of carbon-steel CFST member subjected to tension, and found that the existence of core concrete could effectively enhance the ultimate tensile strength of hollow steel tube by up to 10%. Furthermore, an equation for predicting the ultimate tensile strength of CFST members was proposed by Han et al. [3] as follows:
Fig. 14. Typical tensile load-elongation or moment-rotation relationship for CFSST tensile member.
Fu ¼ ð1:1
0:4αÞ ⋅ fy ⋅As
(4)
For the flexural strength (Mu) of CFST members with a circular cross section, Han [19] proposed an equation as follows: Mu ¼ γm ⋅Wsc ⋅fscy
(5)
where γ m is a coefficient, γm ¼ 1.1 þ 0.48ln(ξþ0.1), ξ is the confinement factor, ξ¼(As⋅fy)/(Ac⋅fck); Wsc is the section modulus, Wsc ¼ π⋅D3/32, D is the outer diameter of steel tube; and fscy is the comprehensive strength, fscy¼(1.14 þ 1.02ξ)⋅fck, fck is the characteristic compressive strength of concrete. 3.3. Verification and discussion The feasibility of employing the above design formulas to predict the tensile strength (Fu) of CFSST members was evaluated. While using the above equations to calculate the strength of CFSST tensile members, the yield stress (fy) of the carbon steel was replaced by the 0.2% proof stress (σ0.2) of the stainless steel. A comparison between the predicted
Fig. 15. Comparison of ultimate tensile strength (Fu) for concentrically loaded specimens. 13
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Fig. 18. Schematic view of concrete crack patterns.
Fig. 19. Comparison between measured and predicted results.
strengths and experimentally measured ones is shown in Fig. 19. Table 4 lists detailed values. For the concentrically loaded specimens, the cur rent design code overestimates the ultimate tensile strength of CFSST members, and the ratios of predicted strength (Fu,c) to the experimental value (Fu,e) is 1.152, and the corresponding coefficient of variation (COV) is 0.087. This could be mainly due to the difference of mechanical behavior between stainless steel and carbon steel. For the CFSST tensile member, the strength development is more significant when the defor mation is large as the stainless steel has stronger hardening effect. Eq. (4) has overestimation as the strength might not be fully developed when the average elongation reaches 5000 με. For the eccentrically loaded CFSST members, the current calculation method gives a con servative prediction of the tensile strength when the moment is given. Furthermore, the difference between the predicted and measured ulti mate tensile strength increases with the increase of the load eccentricity. This is possibly due to the fact that the predicted flexural strength of CFSST member by Eq. (5) is conservative, leading to an imprecise pre diction of the strength. For the eccentrically loaded specimens, the average and COV of Fu,c/Fu,e are 0.832 and 0.117, respectively. The current design formulas for carbon-steel CFST members provides
conservative predictions on the tensile strength of CFSST members. It is suggested that further research should be carried out to improve the design formulas. 4. Conclusions A series of experiments were conducted in this research to investi gate the mechanical behavior of CFSST members subjected to concentric tension and eccentric tension. Within the range of test parameters studied herein, the following conclusions could be drawn: (1) The CFSST members subjected to either concentric tension (up to ε ¼ 50,000 με) or eccentric tension (up to θ ¼ 0.1 rad) behaved in a ductile manner. The tensile load kept on increasing from the beginning to the end of loading, and no obvious yield plateau was observed. (2) The core concrete in the CFSST tensile member worked well with the outer stainless steel tube. The composite effect between tube and concrete enhanced the ultimate tensile strength of stainless steel tube by 5–10%. 14
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Wei Li: Methodology, Writing - review & editing, Supervision. Zi-Xiong Guo: Project administration.
Table 4 Comparison of ultimate tensile strength between experimental and predicted values. Specimen label
Loading type
Fu,e (kN)
Fu,c (kN)
Fu,c/ Fu,e
Average (Fu,c/Fu, e)
Standard deviation
S2c50-1 S2c50-2 S2c80-1 S2c80-2 S2c50e251 S2c50e252 S2c50e501 S2c50e502 S2c50e751 S2c50e752 S3c50e251 S3c50e502 S3c50e751
Concentric tension
192.3 229.2 212.3 231.0 167.7
247.6 247.6 247.6 247.6 150.5
1.288 1.080 1.166 1.072 0.897
1.152
0.087
0.832
0.117
159.3
150.5
0.945
154.8
108.2
0.699
117.6
108.2
0.920
111.7
84.4
0.756
104.6
84.4
0.807
217.7
207.9
0.955
196.4
148.9
0.758
154.6
115.9
0.750
Eccentric tension
Acknowledgments This research is part of the National Natural Science Foundation of China (Grant No. 51808234). The authors would like to thank Mr. Hao Guo for his assistance in the experimental work. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.tws.2020.106602. References [1] L.H. Han, C.Y. Xu, Z. Tao, Performance of concrete filled stainless steel tubular (CFSST) columns and joints: summary of recent research, J. Constr. Steel Res. 152 (2019) 117–131. [2] L.Y. Xu, M.X. Tao, M. Zhou, Analytical model and design formulae of circular CFSTs under axial tension, J. Constr. Steel Res. 133 (2017) 214–230. [3] L.H. Han, S.H. He, F.Y. Liao, Performance and calculations of concrete filled steel tubes (CFST) under axial tension, J. Constr. Steel Res. 67 (11) (2011) 1699–1709. [4] L.H. Han, Concrete Filled Steel Tubular Structures: Theory and Practice, third ed., Science Press, Beijing, 2016 (in Chinese). [5] Y. Ye, L.H. Han, T. Sheehan, Z.X. Guo, Concrete-filled bimetallic tubes under axial compression: experimental investigation, Thin-Walled Struct. 108 (2016) 321–332. [6] W. Li, L.H. Han, T.M. Chan, Tensile behaviour of concrete-filled double-skin steel tubular members, J. Constr. Steel Res. 99 (2014) 35–46. [7] W. Li, L.H. Han, T.M. Chan, Numerical investigation on the performance of concrete-filled double-skin steel tubular member sunder tension, Thin-Walled Struct. 79 (2014) 108–118. [8] W. Li, L.H. Han, T.M. Chan, Performance of concrete-filled steel tubes subjected to eccentric tension, J. Struct. Eng.-ASCE 141 (12) (2015), 04015049. [9] Z.B. Wang, Q. Yu, Z. Tao, Behaviour of CFRP externally-reinforced circular CFST members under combined tension and bending, J. Constr. Steel Res. 106 (2015) 122–137. [10] L.H. Han, Z.B. Wang, W. Xu, Z. Tao, Behavior of concrete-encased CFST members under axial tension, J. Struct. Eng.-ASCE 142 (2) (2016), 04015149. [11] M. Zhou, J.S. Fan, M.X. Tao, J.G. Nie, Experimental study on the tensile behavior of square concrete-filled steel tubes, J. Constr. Steel Res. 121 (2016) 202–215. [12] J. Chen, J. Wang, W. Li, Experimental behaviour of reinforced concrete-filled steel tubes under eccentric tension, J. Constr. Steel Res. 136 (2017) 91–100. [13] J. Chen, J. Wang, W.L. Jin, Concrete-filled steel tubes with reinforcing bars or angles under axial tension, J. Constr. Steel Res. 133 (2017) 374–382. [14] L.H. Han, Y.X. Hua, C. Hou, Q.L. Wang, Circular concrete-filled steel tubes subjected to coupled tension and chloride corrosion, J. Struct. Eng.-ASCE 143 (10) (2017), 04017134. [15] ASTM A959-09, Standard Guide for Specifying Harmonized Standard Grade Compositions for Wrought Stainless Steels, ASTM International, West Conshohocken (PA), 2009. [16] Eurocode 8, Design of Structures for Earthquake Resistance-Part 3: Assessment and Retrofitting of Buildings, European Committee for Standardization, Brussels, Belgium, 2005. [17] L.H. Han, W. Li, R. Bjorhovde, Developments and advanced applications of concrete-filled steel tubular (CFST) structures: members, J. Constr. Steel Res. 100 (2014) 211–228. [18] Housing and urban-rural development department of Fujian Province, DBJ/T1351-2010, Fuzhou, China, in: Technical Specifications for Concrete-Filled Steel Tubular Structures (Revised Version), 2010 (in Chinese). [19] L.H. Han, Flexural behaviour of concrete-filled steel tubes, J. Constr. Steel Res. 60 (2) (2004) 313–337.
(3) The steel-concrete interfacial condition had minor effect on the load-deformation relationship and ultimate tensile strength of CFSST tensile members. The concrete strength also had minor effect on the load-carrying capacity of CFFST tensile members. (4) The ultimate tensile strength of CFSST members decreased with the increase of load eccentricity, and increased with the increase of the cross-sectional steel ratio. The composite members had higher initial loading stiffness compared with that of the corre sponding hollow stainless steel tubes. (5) By replacing the yield stress of carbon steel (fy) with 0.2% proof stress of stainless steel (σ0.2), the current design codes for carbonsteel CFST tensile members failed to precisely predict the tensile strengths of CFSST members subjected to tension. Thus, further research is needed to improve the prediction model for the tensile strength of CFSST members. It should be mentioned that, this paper only dealt with CFSST tensile members with limited types of stainless steel. As a result, further investigation is still needed to characterize the mechanical behavior of CFSST tensile members with different types of stainless steel, especially with the aid of finite element method to perform parametric analysis. Declaration of competing interest None. CRediT authorship contribution statement Yong Ye: Conceptualization, Investigation, Writing - original draft.
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