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Experimental study of concrete-filled CHS stub columns with inner FRP tubes
Thin-Walled Structures 122 (2018) 606–621
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Full length article
Experimental study of concrete-filled CHS stub columns with inner FRP tubes ⁎
Yue-Ling Longa,c, Wen-Tao Lia, Jian-Guo Daib, , Leroy Gardnerc, a b c
T
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Department of Civil Engineering, Guangdong University of Technology, Guangzhou 510006, China Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China Department of Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, UK
A R T I C L E I N F O
A B S T R A C T
Keywords: Axial compression Composite Concrete-filled steel tube Ductility Experiments Inner FRP tube Load bearing capacity
An experimental study into the axial compressive behaviour of concrete-filled circular hollow section (CHS) steel columns with internal fibre reinforced polymer (FRP) tubes is presented in this paper. A total of 17 concretefilled steel tubular (CFST) columns were tested, 15 with an inner FRP tube and 2 with no inner tube. Complementary material tests and tests on 15 FRP-confined concrete (FCC) columns were also carried out. The varied test parameters included the concrete strength, the ratio of the diameter of the steel tube to that of the FRP tube, the diameter to wall thickness ratio of the inner FRP tube and the type (influencing principally the rupture strain) of the FRP. It was found that the presence of the inner FRP tube led to considerably improved axial compressive behaviour due to the greater levels of confinement afforded to the ‘doubly-confined’ inner concrete core; the load-bearing capacity was increased by between about 10% and 50% and the ductility was also enhanced. Greater benefits arose with (1) increasing diameter of the inner FRP tube due to the increased portion of the cross-section that is doubly-confined and (2) increasing wall thickness of the inner FRP tube due to the increased level of confinement afforded to the inner concrete core. The load-deflection responses of all tested specimens were reported, revealing that failure was generally gradual with no sharp loss in load-bearing capacity, implying that the embedment of the inner FRP tube within the concrete enables it to continue to provide a reasonable degree of confinement even after the initiation of fibre rupture; this is different to the sudden loss of confinement typically observed in FRP externally jacketed concrete columns.
1. Introduction Concrete-filled steel tubular (CFST) columns are being extensively used in tall buildings, long-span bridges and other mega structures due to their advantages such as high strength, high ductility and large energy absorption capacity. In addition, the use of CFST columns can bring convenience to construction due to the absence of formwork. In the past few decades, a significant number of experimental and analytical studies into the structural behaviour of CFST columns have been carried out [1–17]. In these studies, it was shown that confinement to the concrete in circular CFST columns brings substantial benefit in terms of load-bearing capacity, but this effect degrades dramatically upon yielding or local buckling of the steel tube. In recent years, fibre reinforced polymer (FRP) jackets have been used to enhance the structural performance of circular CFST columns by providing additional confinement to the concrete and delaying the occurrence of local buckling of the steel tube (Fig. 1(a)). Following the initial work of Xiao [18], a number of studies have been carried out to
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investigate the behaviour of circular CFST columns externally confined by FRP jackets [18–25]. Besides FRP-confined circular CFST columns, another structural form of column featuring the combined use of FRP and steel tubes is the FRP-concrete-steel double-skin tubular column (DSTC) as displayed in Fig. 1(b), which was originally proposed by Teng et al. [26]. This column has an outer FRP tube and an inner steel tube. A number of experimental studies has been carried out on DSTCs by Teng et al. [27–31], Han et al. [32] and Ozbakkaloglu et al. [33–36]. The above-described types of composite column have demonstrated that the combined use of FRP jackets/tubes and steel tubes can offer substantially improved performance over circular concrete columns. However, the external FRP jackets/tubes may not be ideally suited to building construction due to limitations on their fire resistance arising from the rapid degradation of the mechanical properties of FRP and possible smoke or toxic gas generation during a fire. This paper is concerned with a new type of circular CFST column with an inner FRP tube, as shown in Fig. 1(c). These cross-sections, referred to hereafter as ‘FRP-CFST columns’, are expected to have the
Corresponding authors. E-mail addresses: [email protected] (J.-G. Dai), [email protected] (L. Gardner).
http://dx.doi.org/10.1016/j.tws.2017.10.046 Received 22 April 2017; Received in revised form 24 October 2017; Accepted 31 October 2017 Available online 08 November 2017 0263-8231/ © 2017 Elsevier Ltd. All rights reserved.
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Nomenclature Ds Df ts tf As Af Ac Acc Es EFRP fFRP
f′co fy εau εFRP εal
cylinder strength of concrete yield strength of steel tube average axial strain at ultimate load ultimate rupture strain of FRP tube average axial strain corresponding to initiation of local buckling Nue ultimate load-bearing capacity of specimens Nue,CFST ultimate load-bearing capacity of CFST specimens Nue,FCC ultimate load-bearing capacity of FCC specimens Nue,FRP-CFST ultimate load-bearing capacity of FRP-CFST specimens Nre residual load-bearing capacity of specimens Nle axial load corresponding to initiation of local buckling
diameter of steel tube diameter of FRP tube wall thickness of steel tube wall thickness of FRP tube cross-sectional area of steel tube cross-sectional area of FRP tube cross-sectional area of concrete cross-sectional area of core concrete within the FRP tube elastic modulus of steel tube elastic modulus of FRP tube ultimate tensile stress of FRP tube
Fig. 1. Different forms of concrete-filled steel and FRP tubular columns.
target concrete strength, with the symbols “L” and “H” representing the lower and higher strength concrete, respectively. The specimen designation system also describes the number of tubes of each type (S = steel and F = FRP), as well as the diameter and thickness of the FRP tube. For instance, specimen 1S1FH-100-2 represents a CFST specimen with an outer steel tube (1S), an inner FRP tube (1F), high strength concrete, Df = 100 mm and tf = 2 mm. Among the 15 FRP-CFST columns, 13 of the inner tubes were made from glass FRP (GFRP) tubes and 2 from high rupture strain (HRS) FRP (i.e. polyethylene terephthalate (PET) FRP and polyethylene naphthalate (PEN) FRP) tubes. High rupture strain FRP composites usually possess a rupture strain greater than 5% [39,40] and have been recently studied as jacket material for reinforced concrete members [41–44]. The fibres were oriented in the hoop direction resulting in the FRP tubes having high hoop stiffness but low axial stiffness. All the FRP-CFST specimens and CFST specimens were 273 mm in diameter and 820 mm in height. The height was chosen to be three times the specimen diameter to avoid global buckling and end effects. The steel tubes used in all the specimens had a nominal thickness of 6 mm, leading to diameter to thickness ratio Ds/ts of 45.5. The following parameters were considered in the test programme: (i) concrete cylinder compressive strength (i.e. normal strength f′co = 36.5 MPa and high strength f′co = 54.7 MPa), (ii) diameter of inner FRP tube (100 mm, 150 mm and 200 mm), resulting in three different diameter ratios of outer steel tube to inner FRP tube (i.e. Ds/Df = 2.73, 1.82 and 1.37) and (iii) wall thickness of inner FRP tube (i.e. tf = 2 mm, 3 mm and 4 mm). As a result, diameter to wall thickness ratios of the inner FRP tube varied between 25 and 100 (i.e. Df /tf = 25, 33.3, 37.5, 50,
following distinct features: (1) the inner FRP tube is expected to provide continuous and additional confinement to the core concrete, further improving the ductility and strength of the columns even after yielding of the steel tube; (2) failure of the FRP tubes will be less brittle due to their embedment in concrete, avoiding a sudden loss of the FRP contribution upon rupture; and (3) the existence of the inner FRP tube will restrict the lateral expansion of the concrete, reducing the hoop strains and hence delaying yielding in the steel tube. A limited number of tests have been performed on square CFST columns with inner FRP tubes by Feng et al. [37,38], demonstrating higher ultimate strengths and better ductility compared to conventional square CFST columns. However, no tests have been conducted on circular FRP-CFST columns, as studied herein. The aim of the present paper is therefore to study experimentally the axial compressive behaviour of circular FRP-CFST columns and to advance the understanding of this new structural form. 2. Experimental programme 2.1. Test specimens A total of 32 circular composite stub columns, including 15 FRPCFST columns, 2 conventional CFST columns (without an inner FRP tube) and 15 FRP-confined concrete (FCC) columns were manufactured and tested under axial compression. The measured geometrical and material details of the 15 FRP-CFST specimens (depicted in Fig. 2(a)) and the 2 CFST specimens (depicted in Fig. 2(b)) are listed in Table 1. In the table, the specimens are divided into two groups according to the 607
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Fig. 2. Cross-sections of (a) FRP-CFST, (b) CFST and (c) FCC specimens.
prepared and tested in compression following ASTM C39/C39M [45] for each batch to determine the concrete strengths, achieving average measured cylinder strengths of 36.5 MPa and 54.7 MPa at 28 days. The stub column specimens were also tested at 28 days. The outer steel tubes were cut from the same batch of two hot-rolled seamless steel tubes. For each tube, tensile tests on three steel coupons were conducted in accordance with BS18 [46]. The average yield stress from these coupon tests was 300 MPa, with little variation. For the inner FRP tubes made of glass fibres, PET fibres and PEN fibres, hoop tensile tests on 5 FRP rings for each type of FRP material were conducted following ASTM D2290-08 [47]. The average measured ultimate tensile strength, hoop rupture strain and elastic modulus of the FRP tubes obtained from these coupon tests are summarised in Tables 1 and 2.
66.6, 75 and 100) as indicated in Table 1. At the specific diameter of inner FRP tube of 150 mm and in the case of high strength concrete, a range of FRP materials – GFRP, PET FRP and PEN FRP – were employed to assess the influence of rupture strain on the column behaviour. All three types of FRP were designed to have approximately the same tensile stiffness (i.e. Ef tf) in the transverse direction. In parallel to the above-mentioned 15 FRP-CFST column tests, 15 counterpart FCC specimens with the same FRP tube diameter and tensile stiffness were also prepared and tested under axial compression. The purpose of these complementary tests was to investigate how the behaviour of the FRP tubes differs when acting as an outer tube (FCC) and when embedded as an inner tube (FRP-CFST) within the concrete. The FCC specimens are shown in Fig. 2(c). The specimen designation follows the notation “0S1FL-Df-tf” and their key measured properties are given in Table 2.
2.3. Preparation of FRP-CFST specimens 2.2. Material properties As shown in Fig. 3, a simple timber-steel composite formwork was developed to fix the inner FRP tube and the outer steel tube in position (i.e. concentric) during concrete casting. At the base, a wooden spacer was used to hold the bottom of the steel tube in place and nails were
Two separate batches of commercial self-compacting concrete with different strengths were used to fill the test specimens. Three plain concrete cylinders (152.5 mm in diameter and 305 mm in height) were
Table 1 Properties of FRP-CFST and CFST columns. Specimen
1S0FL 1S1FL-100-2 1S1FL-100-3 1S1FL-100-4 1S1FL-150-2 1S1FL-150-3 1S1FL-150-4 1S1FL-200-2 1S1FL-200-3 1S1FL-200-4 1S0FH 1S1FH-150-3 1S1FH-150-4 1S1FH-200-3 1S1FH-200-4 1S1TH-200-30 1S1NH-200-18
L (mm)
820 820 820 820 820 820 820 820 820 820 820 820 820 820 820 820 820
Ds (mm)
273 273 273 273 273 273 273 273 273 273 273 273 273 273 273 273 273
Df (mm)
– 100 100 100 150 150 150 200 200 200 – 150 150 200 200 200 200
ts (mm)
6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1
tf (mm)
– 2.0 3.0 4.0 2.0 3.0 4.0 2.0 3.0 4.0 – 3.0 4.0 3.0 4.0 30.0 18.0
Ds/Df
– 2.73 2.73 2.73 1.82 1.82 1.82 1.37 1.37 1.37 – 1.82 1.37 1.82 1.37 1.37 1.37
Df/tf
– 50 33.3 25 75 50 37.5 100 66.7 50 – 50 37.5 66.7 50 6.7 11.1
Note: '–' indicates value not available.
608
Concrete
Steel tube
FRP properties
f′co (MPa)
fy (MPa）
Es (GPa)
Type
EFRP (GPa)
fFRP (MPa)
εFRP
36.5 36.5 36.5 36.5 36.5 36.5 36.5 36.5 36.5 36.5 54.7 54.7 54.7 54.7 54.7 54.7 54.7
300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4
193 193 193 193 193 193 193 193 193 193 193 193 193 193 193 193 193
– GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP – GFRP GFRP GFRP GFRP PET FRP PEN FRP
– 87 87 87 87 87 87 87 87 87 – 87 87 87 87 8.7 13.9
– 2714 2714 2714 2714 2714 2714 2714 2714 2714 – 2714 2714 2714 2714 760 856
– 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 – 0.0312 0.0312 0.0312 0.0312 0.0873 0.0616
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Table 2 Properties of FCC specimens. Specimen
0S1FL-100-2 0S1FL-100-3 0S1FL-100-4 0S1FL-150-2 0S1FL-150-3 0S1FL-150-4 0S1FL-200-2 0S1FL-200-3 0S1FL-200-4 0S1FH-150-3 0S1FH-150-4 0S1FH-200-3 0S1FH-200-4 0S1TH-200-30 0S1NH-200-18
L (mm)
300 300 300 450 450 450 600 600 600 450 450 600 600 600 600
Df (mm)
100 100 100 150 150 150 200 200 200 150 150 200 200 200 200
tf (mm)
2.0 3.0 4.0 2.0 3.0 4.0 2.0 3.0 4.0 3.0 4.0 3.0 4.0 30.0 18.0
Df/tf
50 33.3 25 75 50 37.5 100 66.7 50 50 37.5 66.7 50 6.7 11.1
Concrete
FRP properties
f′co (MPa)
Type
EFRP (GPa)
fFRP (MPa)
εFRP
36.5 36.5 36.5 36.5 36.5 36.5 36.5 36.5 36.5 54.7 54.7 54.7 54.7 54.7 54.7
GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP PET-FRP PEN-FRP
87 87 87 87 87 87 87 87 87 87 87 87 87 8.7 13.9
2714 2714 2714 2714 2714 2714 2714 2714 2714 2714 2714 2714 2714 760 856
0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0873 0.0616
Fig. 5. Test setup. Fig. 3. Support system employed to maintain position of tubes during casting of FRPCFST specimens.
Fig. 4. FRP-CFST specimen and instrumentation (dimensions in mm).
Fig. 6. FCC specimen and instrumentation.
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Fig. 7. Load-average axial strain curves of FRP-CFST, CFST and FCC specimens.
2.4. Instrumentation and testing
used to maintain the position of the FRP tube relative to the steel tube. At the top, two steel arms were used to maintain the concentric position of the inner and outer tubes. The concrete was poured simultaneously into both the FRP tube and the annular space between the steel tube and the FRP tube to form the FRP-CFST columns.
2.4.1. FRP-CFST and CFST specimens As shown in Fig. 4, for each FRP-CFST or CFST specimen, six hoop strain gauges (i.e. at 60° intervals) and six vertical strain gauges were uniformly placed on the exterior surface of the steel tube at mid-height. For each FRP-CFST specimen, six hoop strain gauges and four vertical
Table 3 Key test results of FRP-CFST and CFST specimens. Specimen
εal
εau
Nle (kN)
Nue (kN)
Nue,FRP-CFST/ Nue,CFST
Nre (kN)
Nre/Nue
Nre / Nue,CFST
φ
SI
DI
1S0FL 1S1FL-100-2 1S1FL-100-3 1S1FL-100-4 1S1FL-150-2 1S1FL-150-3 1S1FL-150-4 1S1FL-200-2 1S1FL-200-3 1S1FL-200-4 1S0FH 1S1FH-150-3 1S1FH-150-4 1S1FH-200-3 1S1FH-200-4 1S1TH-200-30 1S1NH-200-18
0.0310 0.0417 0.0520 0.0568 0.0360 0.0405 0.0435 0.0336 0.0382 0.0395 0.0038 0.0210 0.0260 0.0197 0.0202 0.0475 0.0419
0.0439 0.0501 0.0607 0.0684 0.0426 0.0499 0.0581 0.0376 0.0450 0.0492 0.0038 0.0259 0.0295 0.0228 0.0259 0.1291 0.0928
4167 4609 4729 4970 4973 5183 5490 5327 5597 6147 5573 6107 6676 6507 6863 5373 5451
4238 4734 4877 5154 5156 5422 5849 5519 5811 6392 5573 6252 6721 6624 7190 6601 6467
– 1.12 1.15 1.22 1.22 1.28 1.38 1.30 1.37 1.51 – 1.12 1.21 1.19 1.29 1.18 1.16
4167 4404 4527 4747 4669 4831 4889 4885 4892 5151 4594 5304 5577 5601 5731 – –
0.98 0.93 0.93 0.92 0.91 0.89 0.84 0.88 0.84 0.81 0.82 0.85 0.83 0.85 0.80 – –
– 1.04 1.07 1.12 1.1 1.14 1.15 1.15 1.15 1.21 – 0.95 1.00 1.00 1.03 – –
– 0.96 0.92 0.92 0.97 0.96 0.94 1.00 0.98 0.94 – 0.93 0.93 0.97 0.97 0.80 1.00
1.22 1.37 1.41 1.49 1.49 1.57 1.69 1.60 1.68 1.85 1.26 1.41 1.52 1.50 1.62 1.49 1.46
21.9 25.1 30.3 34.2 21.3 24.9 29.1 18.8 22.5 24.6 1.9 13.0 14.8 11.4 12.9 64.5 46.4
Note: '–' indicates value not available.
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3. Experimental results and discussion
strain gauges were also attached on the exterior surface of the inner FRP tube. Two linear variable differential transducers (LVDTs) (LVDT1 and LVDT2) placed on opposite sides of the specimen, as shown in Fig. 4, were installed between the top and bottom platens of the testing machine to monitor the overall specimen shortening and to ensure that uniform compression was applied the specimen. Three other LVDTs (LVDT3, LVDT4 and LVDT5) placed at 120° intervals were employed to measure axial shortening overall the central 500 mm height of the test specimens to avoid the influence of local end effects, as shown in Fig. 4. All the FRP-CFST and CFST specimens were tested using a 10,000 kN universal loading machine under displacement control. The loading rate was constant at 0.5 mm/min. All test data were recorded using a data logger at 1.0 s intervals. The loading process of the FRPCFST and CFST specimens was terminated manually when the overall axial displacement reached around 85–100 mm. A photograph of the test setup is shown in Fig. 5.
In this section, the failure modes, load-deformation responses, ultimate loads and ductility of the tested specimens are reported. The results and the influence of the key varied parameters are also analysed and discussed. 3.1. Load-axial deformation responses The load-axial deformation responses of the tested FRP-CFST, CFST and FCC specimens are presented in Fig. 7, where the axial deformation is presented in normalised form (i.e. average axial strain). The average axial strain was calculated by normalising the average axial displacement from the central LVDTs (see Figs. 4 and 6) by the lengths over which they recorded, equal to 500 mm for the FRP-CFST and CFST specimens and 2/3L for the FCC specimens. It can be seen from Fig. 7 that the load-bearing capacity Nue,FRP-CFST of all the tested FRP-CFST specimens was greater than that of the counterpart CFST specimens Nue,CFST, indicating the positive contribution of the inner FRP tube (see Fig. 7(a) and (b)). Furthermore, it can also be observed that the residual load-bearing capacity Nre (defined as the minimum recorded post-peak resistance) of all FRP-CFSTs except Specimen 1S1FH-150-3 were greater than the peak loads of the corresponding CFSTs. The key results from the FRP-CFST and CFST stub column tests are summarised in Table 3, including the ultimate load-bearing capacity
2.4.2. FCC specimens For each FCC specimen, six hoop strain gauges and six vertical strain gauges were placed on the exterior surface of the FRP tube wall at midheight, as indicated in Fig. 6. Three LVDTs placed at 120° intervals were employed to monitor the axial deformation of the specimens within the central 2/3 height. Each FCC and its FRP-CFST counterpart were tested on the same day.
Fig. 8. Influence of key parameters on the load-axial deformation responses of FRP-CFST and CFST specimens.
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Fig. 8. (continued)
load at which local buckling occurred was determined approximately by both monitoring of the axial and hoop strains in the steel tube and by visual observation. The key test results of FCC specimens, including the load-bearing capacity Nue (Nue,FCC specifically) and the axial strain εau and hoop strain εhu corresponding to Nue, are summarised in Table 4.
Table 4 Key test results of FCC specimens. Specimen
Df (mm)
tf (mm)
Df/tf
FRP type
Nue (kN)
εau
εhu
0S1FL-100-2 0S1FL-100-3 0S1FL-100-4 0S1FL-150-2 0S1FL-150-3 0S1FL-150-4 0S1FL-200-2 0S1FL-200-3 0S1FL-200-4 0S1FH-150-3 0S1FH-150-4 0S1FH-200-3 0S1FH-200-4 0S1TH-200-30 0S1NL-200-18
100 100 100 150 150 150 200 200 200 150 150 200 200 200 200
2.0 3.0 4.0 2.0 3.0 4.0 2.0 3.0 4.0 3.0 4.0 3.0 4.0 30.0 18.0
50 33.3 25 75 50 37.5 100 66.7 50 50 37.5 66.7 50 50 50
GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP PET FRP PEN FRP
999 1323 1671 1718 2080 2657 2370 2866 3676 2141 2650 2955 3546 4360 2480
0.0472 0.0604 0.0636 0.0362 0.0453 0.0563 0.0286 0.0354 0.0449 0.0185 0.0203 0.0135 0.0186 0.0917 0.0422
0.0146 0.0160 0.0181 0.0125 0.0139 0.0167 0.0110 0.0116 0.0133 0.0130 0.0138 0.0122 0.0125 0.0527 0.0407
3.2. Influence of key varied parameters on load-axial deformation responses The influence of the key varied parameters on the load-axial deformation response of the tested FRP-CFST and CFST specimens is assessed in Fig. 8. The influence of the thickness of the FRP tube tf on the load-axial strain curves is shown in Fig. 8(a) to (e), where it may be seen that increasing tf leads to delayed local buckling and substantially higher ultimate loads and corresponding deformations (Nue and εau, respectively). This is attributed to the greater confinement afforded to the inner concrete core. The influence of the diameter of the FRP tube Df is shown in Fig. 8(f)-(j), where it may be seen that increasing Df results in higher ultimate loads, despite local buckling generally occurring earlier; this is attributed to the increased area that benefits from the ‘double-confinement’ (i.e. from both the outer steel tube and the inner FRP tube) as the diameter of the FRP tube is increased. The effect on the average axial strain at ultimate load εau is marginal. The influence of the steel-to-FRP tube diameter ratio Ds/Df on the load-axial strain curves is shown in Fig. 8(k)-(l), in which the Df/tf ratios are constant. From the figures, it may be seen that decreasing the
Nue (Nue,FRP-CFST for the FRP-CFST specimens and Nue,CFST for the CFST specimens), the average axial strain εau corresponding to Nue, the approximate axial load at which local buckling occurred Nle, which is also illustrated in Fig. 8, the average axial strain εal corresponding to Nle, the residual load-bearing capacity Nre and the Nre/Nue ratio. Note that the 612
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Fig. 9. Influence of concrete strength on the load-axial strain response of tested FPR-CFST and CFST specimens.
made of three different fibre materials (i.e., GFRP, PET and PEN fibres) but with approximately the same tensile stiffness in the hoop direction (i.e., EFRPtf) were compared. It is seen that the ultimate load-bearing capacity Nue of the FRP-CFST stub column with the inner GFRP tube (i.e., 1S1FH-200-3) was slightly higher than that of the other two specimens, but the average axial strain at failure εau was significantly lower due to earlier rupture of the FRP tube. These preliminary results
Ds/Df ratio has little influence on the occurrence of local buckling in the steel tube or the value of εau, but results in significant increases in loadbearing capacity, due to a greater portion of the concrete being doublyconfined. The influence of the type of FRP material employed for the inner tube is assessed in Fig. 8(m). Three FRP-CFST stub columns (1S1FH200-3, 1S1FH-200-30 and 1S1FH-200-18) with the inner FRP tubes 613
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Fig. 10. Load-axial/hoop strain curves for the FRP tubes in the FRP-CFST and FCC specimens.
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Fig. 10. (continued)
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Fig. 11. Hoop strain-axial strain curves for the FRP tube in the FRP-CFST and FCC specimens.
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Fig. 11. (continued)
3.3. Contribution of inner FRP tube to axial capacity
indicate that the ductility, but not necessarily the load-bearing capacity, benefits from the use of high rupture strain FRP for the inner tube in the investigated system. The influence of concrete strength on the load-axial deformation response of the tested FRP-CFST and CFST stub columns is assessed in Fig. 9, in which the axial load has been normalised by the corresponding peak load of each specimen. The comparisons highlight the earlier loss of stiffness, but more ductile response of the tubes filled with the lower strength concrete.
The contribution of the inner FRP tube to the axial load-bearing resistance of the FRP-CFST stub columns is assessed in this sub-section by examination of the load-axial strain and load-hoop strain responses of the inner FRP tubes in the FRP-CFST specimens and their FCC counterparts. The results are shown in Fig. 10. It may be seen that the peak hoop strain (i.e. corresponding to Nue) in the inner FRP tube of the FRP-CFST specimens was generally less than that in the corresponding FCC specimens, 617
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Fig. 12. Typical failure modes of CFST and FRP-CFST specimens.
confining effect of the inner FRP tube. Transverse expansion expediates yielding of the steel tube and cracking of the concrete, and both of these effects promote local buckling of the steel tube. At the end of the tests, the outer steel tubes of the FRP-CFST specimens were opened using a cutter to expose the concrete and inner FRP tube, as shown in Fig. 13(a). The infilled concrete was typically found to be severely crushed, while the FRP tubes were seen to have localised/ partial rupturing, generally adjacent to the point of local buckling of the steel tube – see Fig. 13(a). For the FCC specimens, full rupture of the FRP tubes was observed – see Fig. 13(b).
indicating that the material strength of the inner FRP tube in the FRPCFST sections may not have been fully utilized. This is consistent with the failure mode of inner FRP tubes described in Section 3.4, and may also explain the higher residual load-bearing capacity achieved in the FRP-CFST specimens with an inner FRP tube. The lower hoop strains in the inner FRP tubes of the FRP-CFST specimens than the FCC tubes also implies that the load-bearing capacity of the FRP-CFST cross-sections will not be accurately predicted based on the superimposition of the CFST and FCC specimen resistances, as discussed further in Section 3.5. The hoop strain-axial strain relationship for the inner FRP tube in each of the tested FRP-CFST specimens is compared with that of the corresponding FCC in Fig. 11. It may be seen that the hoop strain development in the inner FRP tube of the FRP-CFST specimens was consistently slower than that in the corresponding FCC specimen for a given axial strain. This implies that the concrete in the FRP tubes of FRP-CFST and FCC specimens may exhibit different dilation properties, which will influence the load-carrying capacity. This finding is in line with observations from previous studies [41,48] that the dilation of confined concrete (reflected by the ratio of the hoop strain to the axial strain) is related to the lateral confinement stiffness [41,48].
3.5. Ultimate load-carrying capacity of FRP-CFST cross-sections The ultimate load-bearing capacities of the tested FRP-CFST stub columns Nue,FRP-CFST and those of the corresponding CFST stub columns with no inner FRP tube Nue,CFST are reported in Table 3. The Nue,FRPCFST/Nue,CFST ratios are also presented to demonstrate the strength benefit arising from the addition of the inner FRP tube, which may be seen to have ranged between 12% and 51%. As discussed in Section 3.2, the greatest benefits were achieved when the area of the doubly-confined concrete was increased (i.e. when the Ds/Df ratio was reduced) and when the level of confinement to the inner core was increased (i.e. when the thickness of the FRP tube was increased). The cross-section load-bearing performance relative to the full plastic compression resistance of the tested specimens can be assessed through a strength index (SI), as employed by Han [49] and McCann et al. [50]. The strength index is expressed as follows:
3.4. Failure modes Typical failures modes for the tested CFST and FRP-CFST stub columns are shown in Fig. 12. Both specimen types exhibited outward only local buckling of the steel tube, but this occurred at considerably lower strains (see Fig. 8) for the CFST specimens (Fig. 12(a)) than the FRPCFST specimens (Fig. 12(b)–(d)). Slightly different patterns of local buckling may be seen for differing FRP-CFST cross-section geometries and material properties, as shown in Fig. 12(b)–(d). The delayed local buckling in the FRP-CFST specimens is attributed to the reduced transverse expansion of the concrete core due to the additional
SI =
Nue Npl, Rd
(1)
where Npl,Rd is the plastic compressive resistance of the column crosssection, which is defined in EN 1994-1-1 [51], in the absence of steel 618
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Fig. 13. Typical failure modes of FRP-CFST and FCC specimens.
seen that the strength index of the FRP-CFST specimens increased with (1) increasing Df/Ds ratio due to the increased portion of the crosssection that is doubly-confined and (2) increasing wall thickness of the inner FRP tube due to the increased level of confinement afforded to the inner concrete core. There are three main contributory components to the axial loadcarrying capacity of FRP-CFST cross-sections: (1) the steel tube, (2) the
reinforcement, as:
Npl, Rd = Aa f y + Ac fco′
(2)
in which Aa and Ac are the cross-sectional areas of the steel tube and concrete, respectively, and fy and f’co are the yield strength of steel tube and cylinder strength of the concrete, respectively. The calculated SI values for the tested FRP-CFST specimens are listed in Table 3 and plotted against the Df/Ds ratio in Fig. 14. It may be
Fig. 16. Variation of reduction coefficient φ with Df/Ds for tested FRP-CFST specimens.
Fig. 14. Comparison of strength indices for FRP-CFST and CFST specimens.
Fig. 15. Illustration of contributory components to axial loadcarrying capacities of FRP-CFST, CFST and FCC columns.
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specimens 1S1TH-200-30 with the PET FRP inner tube and S1NH-20018 with the PEN FRP inner tube were 64.5 and 46.4, respectively, which are the largest and second largest among all the values. This clearly demonstrates that the ductility of FRP-CFST cross-sections can be significantly improved by using an inner FRP tube made of high rupture strain FRP. The DI of the FRP-CFST specimens with inner GFRP tubes are plotted in Fig. 17 against the Df/Ds ratio. It may be seen that the DI value exhibits a decreasing trend with increasing Df/Ds ratio and reducing FRP tube thickness. Also, the ductility of the specimens filled with high strength concrete is clearly inferior to those filled with normal strength concrete.
sandwiched concrete between the FRP tube and the steel tube and (3) the doubly-confined inner concrete core, as illustrated in Fig. 15. Similarly, the components contributing to the axial load-bearing capacity of CFST and FCC cross-sections are also illustrated in Fig. 15. It would be anticipated that the ultimate axial load-bearing capacity of the FRP-CFST cross-sections Nue,FRP-CFST could be accurately predicted as the sum of the ultimate load-bearing capacities of the corresponding CFST cross-section (Nue,CFST) and FCC (Nue,FCC) cross-section, minus (to avoid double-counting) the contribution of the FRP tube and the inner concrete core i.e. (Accf′co+Aff′co) where Acc is cross-sectional area of the inner core concrete, Af is the cross-sectional area of the FRP tube and f′co is concrete compressive strength. However, while this was found to provide reasonable results, the predicted values were in fact consistently larger than the experimental values Nue,FRP-CFST for all tested FRP-CFST cross-sections, indicating that the material strength of the inner FRP tube was not fully utilized in the FRP-CFST specimens. Hence, a reduction coefficient φ is proposed, as defined by Eq. (3):
φ=
Nue, CFST
Nue, FRP − CFST + Nue, FCC−Acc fco′ −Af fco′
3.7. Residual load-bearing capacity of FRP-CFST cross-sections with GFRP tubes The minimum residual load-bearing capacities Nre of each of the tested FRP-CFST specimens are reported in Table 4, together with the ratios Nre/Nue to assess the post-peak drop-off in capacity with increasing deformations. The residual load-bearing capacities of the FRPCFST specimens with the high rupture strain inner FRP tubes are not presented in the table since there was no drop in load throughout the load-deformation history of the tests. The Nre/Nue ratio of the FRP-CFST specimens varied between 0.80 and 0.93 depending on the properties of the inner GFRP tube. The Nre/Nue ratios for the tested FRP-CFST crosssections are plotted against the Df/Ds ratios in Fig. 18. It may be seen that the Nre/Nue ratios show a deceasing trend with increasing FRP tube diameter and thickness. Therefore, although increasing the diameter and thickness of the FRP tube leads to the greatest benefit in terms of ultimate load-carrying capacity, it does also result in the greatest postpeak drop-off in load.
(3)
The calculated values of φ for all the tested FRP-CFST specimens are listed in Table 3, and may be seen to vary between 0.8 and 1.0 depending on the properties of inner FRP tubes; for the GFRP inner tubes, φ varied between 0.92 and 1.0. The values of φ are also plotted against the Df/Ds ratio for the FRP-CFST specimens with GFRP inner tubes in Fig. 16, where it may be seen that there is a trend of increasing φ with increasing Df /Ds ratios, while the influence of tf is less distinct. Further research is required to develop a more in-depth understanding of the above phenomena, but as a preliminary recommendation, a safe-sided value of φ = 0.9 appears appropriate for FRP-CFST cross-sections with GFRP inner tubes, while a value of φ = 0.8 appears suitable for FRPCFST cross-sections with HRS FRP inner tubes.
4. Conclusions 3.6. Ductility of FRP-CFST cross-sections
An experimental study into the axial compressive behaviour of circular CFST columns with an inner FRP tubes (termed FRP-CFST cross-sections herein) has been presented. A total of 32 circular composite stub columns including 15 FRP-CFST stub columns, 2 conventional CFST stub columns (without an inner FRP tube) and 15 FRPconfined concrete (FCC) stub columns were prepared and tested. Based on the results and analysis presented in this paper, the following conclusions can be drawn:
The ductility of the tested FRP-CFST and CFST specimens can been evaluated through the ductility index DI proposed by Ge and Usami [52] and defined as follows:
DI = δu/ δ y = εu/ εy
(4)
where δu and εu are axial displacement and average axial strain, respectively, corresponding to the ultimate capacity and δy and εy are the axial displacement and average axial strain, respectively, at the yield point. In cases where the yield point was not sharply defined, based on the proposals of Schneider [2], the yield strain εy was taken as 0.2%. The DI values of all the FRP-CFST specimens and CFST specimens are listed in Table 3. As presented in the table, the DI values of the
(1) All tested FRP-CFST stub columns exhibited superior performance in terms of delayed local buckling, increased load-bearing capacity and improved ductility over their CFST counterparts without inner FRP tubes. (2) The axial load-bearing capacity of the FRP-CFST cross-sections
Fig. 18. Variation of Nre/Nue ratios for tested FRP-CFST specimens with Df/Ds.
Fig. 17. Ductility index for FRP-CFST specimens with inner GFRP tubes.
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increased with increasing thickness of the inner FRP tube due to the greater level of confinement afforded to the inner concrete core and with increasing diameter (and FRP-to-steel tube diameter ratio) of the inner FRP tube due to the greater area of doubly-confined concrete. (3) The ultimate load-bearing capacity of the tested FRP-CFST crosssections was slightly over-predicted by a simple superposition of the capacities of equivalent CFST and FCC specimens minus the overlapping portions, and some preliminary insights into a possible design treatment were made. (4) The material type of the inner FRP tube significantly influenced the ductility of FRP-CFST stub columns, with high rupture strain FRP material showing the best performance. The ductility index also showed a decreasing trend with increasing FRP tube diameter and reducing FRP tube thickness. (5) The post-peak residual capacity of the tested FRP-CFST specimens lay between 80% and 93% of the ultimate load-carrying capacity, and showed a deceasing trend with increasing FRP tube diameter and thickness. Acknowledgments This research was funded by the National Natural Science Foundation of China (Grant No. 51008085 and Grant No. 11472084), Guangzhou Pearl River New Star of Science & Technology Project (Grant No. 2012J2200100), China Scholarship Council (Grant No. 201608440006) and China Postdoctoral Science Foundation (Grant No. 2012M511810 and No. 2014T70807). The paper was prepared during the first author's stay at Imperial College London as a visiting academic. References [1] H.B. Ge, T. Usami, Strength of concrete-filled thin-walled steel box column: experiment, J. Struct. Eng. ASCE 118 (11) (1992) 3036–3054. [2] S.P. Schneider, Axially loaded concrete-filled steel tubes, J. Struct. Eng. ASCE 124 (10) (1998) 1125–1138. [3] B. Uy, Strength of concrete filled steel box columns incorporating local buckling, J. Struct. Eng. ASCE 126 (3) (2000) 341–352. [4] B. Uy, Strength of short concrete filled high strength steel box columns, J. Constr. Steel Res. 57 (2) (2001) 114–134. [5] T. Fujimoto, A. Mukai, I. Nishiyama, K. Sakino, Behavior of eccentrically loaded concrete-filled steel tubular columns, J. Struct. Eng. ASCE 130 (2) (2004) 203–212. [6] K. Sakino, H. Nakahara, S. Morino, I. Nishiyama, Behaviour of centrally loaded concrete-filled steel-tube short columns, J. Struct. Eng. ASCE 130 (2) (2004) 180–188. [7] D. Liu, W.M. Gho, Axial load behaviour of high strength rectangular concrete-filled steel tubular stub columns, Thin-Walled Struct. 43 (8) (2005) 1131–1142. [8] L.H. Han, G.H. Yao, X.L. Zhao, Tests and calculations for hollow structural steel (HSS) stub columns filled with self-consolidating concrete (SCC), J. Constr. Steel Res. 61 (9) (2005) 1241–1269. [9] C.W. Roeder, D.E. Lehman, R. Thody, Composite action in CFST components and connections, AISC Eng. J. 47 (4) (2009) 229–242. [10] Z. Ou, B. Chen, K.H. Hsieh, M.W. Halling, P.J. Barr, Experimental and analytical investigation of concrete-filled steel tubular columns, J. Struct. Eng. ASCE 137 (6) (2011) 635–645. [11] X.H. Dai, D. Lam, Shape effect on the behaviour of axially loaded concrete filled steel tubular stub columns at elevated temperature, J. Constr. Steel Res. 73 (2012) 117–127. [12] Z. Tao, Z.B. Wang, Q. Yu, Finite element modelling of concrete-filled steel stub columns under axial compression, J. Constr. Steel Res. 89 (2013) 121–131. [13] Y.L. Long, J. Cai, Stress–strain relationship of concrete confined by rectangular steel tubes with binding bars, J. Constr. Steel Res. 88 (2013) 1–14. [14] J.G. Nie, Y.H. Wang, J.S. Fan, Experimental research on concrete filled steel tube columns under combined compression-bending-torsion cyclic load, Thin-Walled Struct. 67 (2013) 1–14. [15] Y. Yang, Y. Wang, F. Fu, Effect of reinforcement stiffeners on square concrete-filled steel tubular columns subjected to axial compressive load, Thin-Walled Struct. 82 (2014) 132–144. [16] Y.L. Long, J. Wan, J. Cai, Theoretical study on local buckling of rectangular CFST columns under eccentric compression, J. Constr. Steel Res. 120 (2016) 70–80. [17] Z. Lai, A.H. Varma, Effective stress-strain relationships for analysis of noncompact and slender filled composite (CFST) members, Eng. Struct. 124 (2016) 457–472. [18] Y. Xiao, Applications of FRP composites in concrete columns, Adv. Struct. Eng. 7 (4) (2004) 335–343. [19] Y. Xiao, W. He, K.K. Choi, Confined concrete-filled tubular columns, J. Struct. Eng. ASCE 131 (3) (2005) 488–497.
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Full length article
Experimental study of concrete-filled CHS stub columns with inner FRP tubes ⁎
Yue-Ling Longa,c, Wen-Tao Lia, Jian-Guo Daib, , Leroy Gardnerc, a b c
T
⁎
Department of Civil Engineering, Guangdong University of Technology, Guangzhou 510006, China Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China Department of Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, UK
A R T I C L E I N F O
A B S T R A C T
Keywords: Axial compression Composite Concrete-filled steel tube Ductility Experiments Inner FRP tube Load bearing capacity
An experimental study into the axial compressive behaviour of concrete-filled circular hollow section (CHS) steel columns with internal fibre reinforced polymer (FRP) tubes is presented in this paper. A total of 17 concretefilled steel tubular (CFST) columns were tested, 15 with an inner FRP tube and 2 with no inner tube. Complementary material tests and tests on 15 FRP-confined concrete (FCC) columns were also carried out. The varied test parameters included the concrete strength, the ratio of the diameter of the steel tube to that of the FRP tube, the diameter to wall thickness ratio of the inner FRP tube and the type (influencing principally the rupture strain) of the FRP. It was found that the presence of the inner FRP tube led to considerably improved axial compressive behaviour due to the greater levels of confinement afforded to the ‘doubly-confined’ inner concrete core; the load-bearing capacity was increased by between about 10% and 50% and the ductility was also enhanced. Greater benefits arose with (1) increasing diameter of the inner FRP tube due to the increased portion of the cross-section that is doubly-confined and (2) increasing wall thickness of the inner FRP tube due to the increased level of confinement afforded to the inner concrete core. The load-deflection responses of all tested specimens were reported, revealing that failure was generally gradual with no sharp loss in load-bearing capacity, implying that the embedment of the inner FRP tube within the concrete enables it to continue to provide a reasonable degree of confinement even after the initiation of fibre rupture; this is different to the sudden loss of confinement typically observed in FRP externally jacketed concrete columns.
1. Introduction Concrete-filled steel tubular (CFST) columns are being extensively used in tall buildings, long-span bridges and other mega structures due to their advantages such as high strength, high ductility and large energy absorption capacity. In addition, the use of CFST columns can bring convenience to construction due to the absence of formwork. In the past few decades, a significant number of experimental and analytical studies into the structural behaviour of CFST columns have been carried out [1–17]. In these studies, it was shown that confinement to the concrete in circular CFST columns brings substantial benefit in terms of load-bearing capacity, but this effect degrades dramatically upon yielding or local buckling of the steel tube. In recent years, fibre reinforced polymer (FRP) jackets have been used to enhance the structural performance of circular CFST columns by providing additional confinement to the concrete and delaying the occurrence of local buckling of the steel tube (Fig. 1(a)). Following the initial work of Xiao [18], a number of studies have been carried out to
⁎
investigate the behaviour of circular CFST columns externally confined by FRP jackets [18–25]. Besides FRP-confined circular CFST columns, another structural form of column featuring the combined use of FRP and steel tubes is the FRP-concrete-steel double-skin tubular column (DSTC) as displayed in Fig. 1(b), which was originally proposed by Teng et al. [26]. This column has an outer FRP tube and an inner steel tube. A number of experimental studies has been carried out on DSTCs by Teng et al. [27–31], Han et al. [32] and Ozbakkaloglu et al. [33–36]. The above-described types of composite column have demonstrated that the combined use of FRP jackets/tubes and steel tubes can offer substantially improved performance over circular concrete columns. However, the external FRP jackets/tubes may not be ideally suited to building construction due to limitations on their fire resistance arising from the rapid degradation of the mechanical properties of FRP and possible smoke or toxic gas generation during a fire. This paper is concerned with a new type of circular CFST column with an inner FRP tube, as shown in Fig. 1(c). These cross-sections, referred to hereafter as ‘FRP-CFST columns’, are expected to have the
Corresponding authors. E-mail addresses: [email protected] (J.-G. Dai), [email protected] (L. Gardner).
http://dx.doi.org/10.1016/j.tws.2017.10.046 Received 22 April 2017; Received in revised form 24 October 2017; Accepted 31 October 2017 Available online 08 November 2017 0263-8231/ © 2017 Elsevier Ltd. All rights reserved.
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Nomenclature Ds Df ts tf As Af Ac Acc Es EFRP fFRP
f′co fy εau εFRP εal
cylinder strength of concrete yield strength of steel tube average axial strain at ultimate load ultimate rupture strain of FRP tube average axial strain corresponding to initiation of local buckling Nue ultimate load-bearing capacity of specimens Nue,CFST ultimate load-bearing capacity of CFST specimens Nue,FCC ultimate load-bearing capacity of FCC specimens Nue,FRP-CFST ultimate load-bearing capacity of FRP-CFST specimens Nre residual load-bearing capacity of specimens Nle axial load corresponding to initiation of local buckling
diameter of steel tube diameter of FRP tube wall thickness of steel tube wall thickness of FRP tube cross-sectional area of steel tube cross-sectional area of FRP tube cross-sectional area of concrete cross-sectional area of core concrete within the FRP tube elastic modulus of steel tube elastic modulus of FRP tube ultimate tensile stress of FRP tube
Fig. 1. Different forms of concrete-filled steel and FRP tubular columns.
target concrete strength, with the symbols “L” and “H” representing the lower and higher strength concrete, respectively. The specimen designation system also describes the number of tubes of each type (S = steel and F = FRP), as well as the diameter and thickness of the FRP tube. For instance, specimen 1S1FH-100-2 represents a CFST specimen with an outer steel tube (1S), an inner FRP tube (1F), high strength concrete, Df = 100 mm and tf = 2 mm. Among the 15 FRP-CFST columns, 13 of the inner tubes were made from glass FRP (GFRP) tubes and 2 from high rupture strain (HRS) FRP (i.e. polyethylene terephthalate (PET) FRP and polyethylene naphthalate (PEN) FRP) tubes. High rupture strain FRP composites usually possess a rupture strain greater than 5% [39,40] and have been recently studied as jacket material for reinforced concrete members [41–44]. The fibres were oriented in the hoop direction resulting in the FRP tubes having high hoop stiffness but low axial stiffness. All the FRP-CFST specimens and CFST specimens were 273 mm in diameter and 820 mm in height. The height was chosen to be three times the specimen diameter to avoid global buckling and end effects. The steel tubes used in all the specimens had a nominal thickness of 6 mm, leading to diameter to thickness ratio Ds/ts of 45.5. The following parameters were considered in the test programme: (i) concrete cylinder compressive strength (i.e. normal strength f′co = 36.5 MPa and high strength f′co = 54.7 MPa), (ii) diameter of inner FRP tube (100 mm, 150 mm and 200 mm), resulting in three different diameter ratios of outer steel tube to inner FRP tube (i.e. Ds/Df = 2.73, 1.82 and 1.37) and (iii) wall thickness of inner FRP tube (i.e. tf = 2 mm, 3 mm and 4 mm). As a result, diameter to wall thickness ratios of the inner FRP tube varied between 25 and 100 (i.e. Df /tf = 25, 33.3, 37.5, 50,
following distinct features: (1) the inner FRP tube is expected to provide continuous and additional confinement to the core concrete, further improving the ductility and strength of the columns even after yielding of the steel tube; (2) failure of the FRP tubes will be less brittle due to their embedment in concrete, avoiding a sudden loss of the FRP contribution upon rupture; and (3) the existence of the inner FRP tube will restrict the lateral expansion of the concrete, reducing the hoop strains and hence delaying yielding in the steel tube. A limited number of tests have been performed on square CFST columns with inner FRP tubes by Feng et al. [37,38], demonstrating higher ultimate strengths and better ductility compared to conventional square CFST columns. However, no tests have been conducted on circular FRP-CFST columns, as studied herein. The aim of the present paper is therefore to study experimentally the axial compressive behaviour of circular FRP-CFST columns and to advance the understanding of this new structural form. 2. Experimental programme 2.1. Test specimens A total of 32 circular composite stub columns, including 15 FRPCFST columns, 2 conventional CFST columns (without an inner FRP tube) and 15 FRP-confined concrete (FCC) columns were manufactured and tested under axial compression. The measured geometrical and material details of the 15 FRP-CFST specimens (depicted in Fig. 2(a)) and the 2 CFST specimens (depicted in Fig. 2(b)) are listed in Table 1. In the table, the specimens are divided into two groups according to the 607
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Fig. 2. Cross-sections of (a) FRP-CFST, (b) CFST and (c) FCC specimens.
prepared and tested in compression following ASTM C39/C39M [45] for each batch to determine the concrete strengths, achieving average measured cylinder strengths of 36.5 MPa and 54.7 MPa at 28 days. The stub column specimens were also tested at 28 days. The outer steel tubes were cut from the same batch of two hot-rolled seamless steel tubes. For each tube, tensile tests on three steel coupons were conducted in accordance with BS18 [46]. The average yield stress from these coupon tests was 300 MPa, with little variation. For the inner FRP tubes made of glass fibres, PET fibres and PEN fibres, hoop tensile tests on 5 FRP rings for each type of FRP material were conducted following ASTM D2290-08 [47]. The average measured ultimate tensile strength, hoop rupture strain and elastic modulus of the FRP tubes obtained from these coupon tests are summarised in Tables 1 and 2.
66.6, 75 and 100) as indicated in Table 1. At the specific diameter of inner FRP tube of 150 mm and in the case of high strength concrete, a range of FRP materials – GFRP, PET FRP and PEN FRP – were employed to assess the influence of rupture strain on the column behaviour. All three types of FRP were designed to have approximately the same tensile stiffness (i.e. Ef tf) in the transverse direction. In parallel to the above-mentioned 15 FRP-CFST column tests, 15 counterpart FCC specimens with the same FRP tube diameter and tensile stiffness were also prepared and tested under axial compression. The purpose of these complementary tests was to investigate how the behaviour of the FRP tubes differs when acting as an outer tube (FCC) and when embedded as an inner tube (FRP-CFST) within the concrete. The FCC specimens are shown in Fig. 2(c). The specimen designation follows the notation “0S1FL-Df-tf” and their key measured properties are given in Table 2.
2.3. Preparation of FRP-CFST specimens 2.2. Material properties As shown in Fig. 3, a simple timber-steel composite formwork was developed to fix the inner FRP tube and the outer steel tube in position (i.e. concentric) during concrete casting. At the base, a wooden spacer was used to hold the bottom of the steel tube in place and nails were
Two separate batches of commercial self-compacting concrete with different strengths were used to fill the test specimens. Three plain concrete cylinders (152.5 mm in diameter and 305 mm in height) were
Table 1 Properties of FRP-CFST and CFST columns. Specimen
1S0FL 1S1FL-100-2 1S1FL-100-3 1S1FL-100-4 1S1FL-150-2 1S1FL-150-3 1S1FL-150-4 1S1FL-200-2 1S1FL-200-3 1S1FL-200-4 1S0FH 1S1FH-150-3 1S1FH-150-4 1S1FH-200-3 1S1FH-200-4 1S1TH-200-30 1S1NH-200-18
L (mm)
820 820 820 820 820 820 820 820 820 820 820 820 820 820 820 820 820
Ds (mm)
273 273 273 273 273 273 273 273 273 273 273 273 273 273 273 273 273
Df (mm)
– 100 100 100 150 150 150 200 200 200 – 150 150 200 200 200 200
ts (mm)
6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1
tf (mm)
– 2.0 3.0 4.0 2.0 3.0 4.0 2.0 3.0 4.0 – 3.0 4.0 3.0 4.0 30.0 18.0
Ds/Df
– 2.73 2.73 2.73 1.82 1.82 1.82 1.37 1.37 1.37 – 1.82 1.37 1.82 1.37 1.37 1.37
Df/tf
– 50 33.3 25 75 50 37.5 100 66.7 50 – 50 37.5 66.7 50 6.7 11.1
Note: '–' indicates value not available.
608
Concrete
Steel tube
FRP properties
f′co (MPa)
fy (MPa）
Es (GPa)
Type
EFRP (GPa)
fFRP (MPa)
εFRP
36.5 36.5 36.5 36.5 36.5 36.5 36.5 36.5 36.5 36.5 54.7 54.7 54.7 54.7 54.7 54.7 54.7
300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4
193 193 193 193 193 193 193 193 193 193 193 193 193 193 193 193 193
– GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP – GFRP GFRP GFRP GFRP PET FRP PEN FRP
– 87 87 87 87 87 87 87 87 87 – 87 87 87 87 8.7 13.9
– 2714 2714 2714 2714 2714 2714 2714 2714 2714 – 2714 2714 2714 2714 760 856
– 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 – 0.0312 0.0312 0.0312 0.0312 0.0873 0.0616
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Table 2 Properties of FCC specimens. Specimen
0S1FL-100-2 0S1FL-100-3 0S1FL-100-4 0S1FL-150-2 0S1FL-150-3 0S1FL-150-4 0S1FL-200-2 0S1FL-200-3 0S1FL-200-4 0S1FH-150-3 0S1FH-150-4 0S1FH-200-3 0S1FH-200-4 0S1TH-200-30 0S1NH-200-18
L (mm)
300 300 300 450 450 450 600 600 600 450 450 600 600 600 600
Df (mm)
100 100 100 150 150 150 200 200 200 150 150 200 200 200 200
tf (mm)
2.0 3.0 4.0 2.0 3.0 4.0 2.0 3.0 4.0 3.0 4.0 3.0 4.0 30.0 18.0
Df/tf
50 33.3 25 75 50 37.5 100 66.7 50 50 37.5 66.7 50 6.7 11.1
Concrete
FRP properties
f′co (MPa)
Type
EFRP (GPa)
fFRP (MPa)
εFRP
36.5 36.5 36.5 36.5 36.5 36.5 36.5 36.5 36.5 54.7 54.7 54.7 54.7 54.7 54.7
GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP PET-FRP PEN-FRP
87 87 87 87 87 87 87 87 87 87 87 87 87 8.7 13.9
2714 2714 2714 2714 2714 2714 2714 2714 2714 2714 2714 2714 2714 760 856
0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0312 0.0873 0.0616
Fig. 5. Test setup. Fig. 3. Support system employed to maintain position of tubes during casting of FRPCFST specimens.
Fig. 4. FRP-CFST specimen and instrumentation (dimensions in mm).
Fig. 6. FCC specimen and instrumentation.
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Fig. 7. Load-average axial strain curves of FRP-CFST, CFST and FCC specimens.
2.4. Instrumentation and testing
used to maintain the position of the FRP tube relative to the steel tube. At the top, two steel arms were used to maintain the concentric position of the inner and outer tubes. The concrete was poured simultaneously into both the FRP tube and the annular space between the steel tube and the FRP tube to form the FRP-CFST columns.
2.4.1. FRP-CFST and CFST specimens As shown in Fig. 4, for each FRP-CFST or CFST specimen, six hoop strain gauges (i.e. at 60° intervals) and six vertical strain gauges were uniformly placed on the exterior surface of the steel tube at mid-height. For each FRP-CFST specimen, six hoop strain gauges and four vertical
Table 3 Key test results of FRP-CFST and CFST specimens. Specimen
εal
εau
Nle (kN)
Nue (kN)
Nue,FRP-CFST/ Nue,CFST
Nre (kN)
Nre/Nue
Nre / Nue,CFST
φ
SI
DI
1S0FL 1S1FL-100-2 1S1FL-100-3 1S1FL-100-4 1S1FL-150-2 1S1FL-150-3 1S1FL-150-4 1S1FL-200-2 1S1FL-200-3 1S1FL-200-4 1S0FH 1S1FH-150-3 1S1FH-150-4 1S1FH-200-3 1S1FH-200-4 1S1TH-200-30 1S1NH-200-18
0.0310 0.0417 0.0520 0.0568 0.0360 0.0405 0.0435 0.0336 0.0382 0.0395 0.0038 0.0210 0.0260 0.0197 0.0202 0.0475 0.0419
0.0439 0.0501 0.0607 0.0684 0.0426 0.0499 0.0581 0.0376 0.0450 0.0492 0.0038 0.0259 0.0295 0.0228 0.0259 0.1291 0.0928
4167 4609 4729 4970 4973 5183 5490 5327 5597 6147 5573 6107 6676 6507 6863 5373 5451
4238 4734 4877 5154 5156 5422 5849 5519 5811 6392 5573 6252 6721 6624 7190 6601 6467
– 1.12 1.15 1.22 1.22 1.28 1.38 1.30 1.37 1.51 – 1.12 1.21 1.19 1.29 1.18 1.16
4167 4404 4527 4747 4669 4831 4889 4885 4892 5151 4594 5304 5577 5601 5731 – –
0.98 0.93 0.93 0.92 0.91 0.89 0.84 0.88 0.84 0.81 0.82 0.85 0.83 0.85 0.80 – –
– 1.04 1.07 1.12 1.1 1.14 1.15 1.15 1.15 1.21 – 0.95 1.00 1.00 1.03 – –
– 0.96 0.92 0.92 0.97 0.96 0.94 1.00 0.98 0.94 – 0.93 0.93 0.97 0.97 0.80 1.00
1.22 1.37 1.41 1.49 1.49 1.57 1.69 1.60 1.68 1.85 1.26 1.41 1.52 1.50 1.62 1.49 1.46
21.9 25.1 30.3 34.2 21.3 24.9 29.1 18.8 22.5 24.6 1.9 13.0 14.8 11.4 12.9 64.5 46.4
Note: '–' indicates value not available.
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3. Experimental results and discussion
strain gauges were also attached on the exterior surface of the inner FRP tube. Two linear variable differential transducers (LVDTs) (LVDT1 and LVDT2) placed on opposite sides of the specimen, as shown in Fig. 4, were installed between the top and bottom platens of the testing machine to monitor the overall specimen shortening and to ensure that uniform compression was applied the specimen. Three other LVDTs (LVDT3, LVDT4 and LVDT5) placed at 120° intervals were employed to measure axial shortening overall the central 500 mm height of the test specimens to avoid the influence of local end effects, as shown in Fig. 4. All the FRP-CFST and CFST specimens were tested using a 10,000 kN universal loading machine under displacement control. The loading rate was constant at 0.5 mm/min. All test data were recorded using a data logger at 1.0 s intervals. The loading process of the FRPCFST and CFST specimens was terminated manually when the overall axial displacement reached around 85–100 mm. A photograph of the test setup is shown in Fig. 5.
In this section, the failure modes, load-deformation responses, ultimate loads and ductility of the tested specimens are reported. The results and the influence of the key varied parameters are also analysed and discussed. 3.1. Load-axial deformation responses The load-axial deformation responses of the tested FRP-CFST, CFST and FCC specimens are presented in Fig. 7, where the axial deformation is presented in normalised form (i.e. average axial strain). The average axial strain was calculated by normalising the average axial displacement from the central LVDTs (see Figs. 4 and 6) by the lengths over which they recorded, equal to 500 mm for the FRP-CFST and CFST specimens and 2/3L for the FCC specimens. It can be seen from Fig. 7 that the load-bearing capacity Nue,FRP-CFST of all the tested FRP-CFST specimens was greater than that of the counterpart CFST specimens Nue,CFST, indicating the positive contribution of the inner FRP tube (see Fig. 7(a) and (b)). Furthermore, it can also be observed that the residual load-bearing capacity Nre (defined as the minimum recorded post-peak resistance) of all FRP-CFSTs except Specimen 1S1FH-150-3 were greater than the peak loads of the corresponding CFSTs. The key results from the FRP-CFST and CFST stub column tests are summarised in Table 3, including the ultimate load-bearing capacity
2.4.2. FCC specimens For each FCC specimen, six hoop strain gauges and six vertical strain gauges were placed on the exterior surface of the FRP tube wall at midheight, as indicated in Fig. 6. Three LVDTs placed at 120° intervals were employed to monitor the axial deformation of the specimens within the central 2/3 height. Each FCC and its FRP-CFST counterpart were tested on the same day.
Fig. 8. Influence of key parameters on the load-axial deformation responses of FRP-CFST and CFST specimens.
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Fig. 8. (continued)
load at which local buckling occurred was determined approximately by both monitoring of the axial and hoop strains in the steel tube and by visual observation. The key test results of FCC specimens, including the load-bearing capacity Nue (Nue,FCC specifically) and the axial strain εau and hoop strain εhu corresponding to Nue, are summarised in Table 4.
Table 4 Key test results of FCC specimens. Specimen
Df (mm)
tf (mm)
Df/tf
FRP type
Nue (kN)
εau
εhu
0S1FL-100-2 0S1FL-100-3 0S1FL-100-4 0S1FL-150-2 0S1FL-150-3 0S1FL-150-4 0S1FL-200-2 0S1FL-200-3 0S1FL-200-4 0S1FH-150-3 0S1FH-150-4 0S1FH-200-3 0S1FH-200-4 0S1TH-200-30 0S1NL-200-18
100 100 100 150 150 150 200 200 200 150 150 200 200 200 200
2.0 3.0 4.0 2.0 3.0 4.0 2.0 3.0 4.0 3.0 4.0 3.0 4.0 30.0 18.0
50 33.3 25 75 50 37.5 100 66.7 50 50 37.5 66.7 50 50 50
GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP PET FRP PEN FRP
999 1323 1671 1718 2080 2657 2370 2866 3676 2141 2650 2955 3546 4360 2480
0.0472 0.0604 0.0636 0.0362 0.0453 0.0563 0.0286 0.0354 0.0449 0.0185 0.0203 0.0135 0.0186 0.0917 0.0422
0.0146 0.0160 0.0181 0.0125 0.0139 0.0167 0.0110 0.0116 0.0133 0.0130 0.0138 0.0122 0.0125 0.0527 0.0407
3.2. Influence of key varied parameters on load-axial deformation responses The influence of the key varied parameters on the load-axial deformation response of the tested FRP-CFST and CFST specimens is assessed in Fig. 8. The influence of the thickness of the FRP tube tf on the load-axial strain curves is shown in Fig. 8(a) to (e), where it may be seen that increasing tf leads to delayed local buckling and substantially higher ultimate loads and corresponding deformations (Nue and εau, respectively). This is attributed to the greater confinement afforded to the inner concrete core. The influence of the diameter of the FRP tube Df is shown in Fig. 8(f)-(j), where it may be seen that increasing Df results in higher ultimate loads, despite local buckling generally occurring earlier; this is attributed to the increased area that benefits from the ‘double-confinement’ (i.e. from both the outer steel tube and the inner FRP tube) as the diameter of the FRP tube is increased. The effect on the average axial strain at ultimate load εau is marginal. The influence of the steel-to-FRP tube diameter ratio Ds/Df on the load-axial strain curves is shown in Fig. 8(k)-(l), in which the Df/tf ratios are constant. From the figures, it may be seen that decreasing the
Nue (Nue,FRP-CFST for the FRP-CFST specimens and Nue,CFST for the CFST specimens), the average axial strain εau corresponding to Nue, the approximate axial load at which local buckling occurred Nle, which is also illustrated in Fig. 8, the average axial strain εal corresponding to Nle, the residual load-bearing capacity Nre and the Nre/Nue ratio. Note that the 612
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Fig. 9. Influence of concrete strength on the load-axial strain response of tested FPR-CFST and CFST specimens.
made of three different fibre materials (i.e., GFRP, PET and PEN fibres) but with approximately the same tensile stiffness in the hoop direction (i.e., EFRPtf) were compared. It is seen that the ultimate load-bearing capacity Nue of the FRP-CFST stub column with the inner GFRP tube (i.e., 1S1FH-200-3) was slightly higher than that of the other two specimens, but the average axial strain at failure εau was significantly lower due to earlier rupture of the FRP tube. These preliminary results
Ds/Df ratio has little influence on the occurrence of local buckling in the steel tube or the value of εau, but results in significant increases in loadbearing capacity, due to a greater portion of the concrete being doublyconfined. The influence of the type of FRP material employed for the inner tube is assessed in Fig. 8(m). Three FRP-CFST stub columns (1S1FH200-3, 1S1FH-200-30 and 1S1FH-200-18) with the inner FRP tubes 613
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Fig. 10. Load-axial/hoop strain curves for the FRP tubes in the FRP-CFST and FCC specimens.
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Fig. 10. (continued)
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Fig. 11. Hoop strain-axial strain curves for the FRP tube in the FRP-CFST and FCC specimens.
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Fig. 11. (continued)
3.3. Contribution of inner FRP tube to axial capacity
indicate that the ductility, but not necessarily the load-bearing capacity, benefits from the use of high rupture strain FRP for the inner tube in the investigated system. The influence of concrete strength on the load-axial deformation response of the tested FRP-CFST and CFST stub columns is assessed in Fig. 9, in which the axial load has been normalised by the corresponding peak load of each specimen. The comparisons highlight the earlier loss of stiffness, but more ductile response of the tubes filled with the lower strength concrete.
The contribution of the inner FRP tube to the axial load-bearing resistance of the FRP-CFST stub columns is assessed in this sub-section by examination of the load-axial strain and load-hoop strain responses of the inner FRP tubes in the FRP-CFST specimens and their FCC counterparts. The results are shown in Fig. 10. It may be seen that the peak hoop strain (i.e. corresponding to Nue) in the inner FRP tube of the FRP-CFST specimens was generally less than that in the corresponding FCC specimens, 617
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Fig. 12. Typical failure modes of CFST and FRP-CFST specimens.
confining effect of the inner FRP tube. Transverse expansion expediates yielding of the steel tube and cracking of the concrete, and both of these effects promote local buckling of the steel tube. At the end of the tests, the outer steel tubes of the FRP-CFST specimens were opened using a cutter to expose the concrete and inner FRP tube, as shown in Fig. 13(a). The infilled concrete was typically found to be severely crushed, while the FRP tubes were seen to have localised/ partial rupturing, generally adjacent to the point of local buckling of the steel tube – see Fig. 13(a). For the FCC specimens, full rupture of the FRP tubes was observed – see Fig. 13(b).
indicating that the material strength of the inner FRP tube in the FRPCFST sections may not have been fully utilized. This is consistent with the failure mode of inner FRP tubes described in Section 3.4, and may also explain the higher residual load-bearing capacity achieved in the FRP-CFST specimens with an inner FRP tube. The lower hoop strains in the inner FRP tubes of the FRP-CFST specimens than the FCC tubes also implies that the load-bearing capacity of the FRP-CFST cross-sections will not be accurately predicted based on the superimposition of the CFST and FCC specimen resistances, as discussed further in Section 3.5. The hoop strain-axial strain relationship for the inner FRP tube in each of the tested FRP-CFST specimens is compared with that of the corresponding FCC in Fig. 11. It may be seen that the hoop strain development in the inner FRP tube of the FRP-CFST specimens was consistently slower than that in the corresponding FCC specimen for a given axial strain. This implies that the concrete in the FRP tubes of FRP-CFST and FCC specimens may exhibit different dilation properties, which will influence the load-carrying capacity. This finding is in line with observations from previous studies [41,48] that the dilation of confined concrete (reflected by the ratio of the hoop strain to the axial strain) is related to the lateral confinement stiffness [41,48].
3.5. Ultimate load-carrying capacity of FRP-CFST cross-sections The ultimate load-bearing capacities of the tested FRP-CFST stub columns Nue,FRP-CFST and those of the corresponding CFST stub columns with no inner FRP tube Nue,CFST are reported in Table 3. The Nue,FRPCFST/Nue,CFST ratios are also presented to demonstrate the strength benefit arising from the addition of the inner FRP tube, which may be seen to have ranged between 12% and 51%. As discussed in Section 3.2, the greatest benefits were achieved when the area of the doubly-confined concrete was increased (i.e. when the Ds/Df ratio was reduced) and when the level of confinement to the inner core was increased (i.e. when the thickness of the FRP tube was increased). The cross-section load-bearing performance relative to the full plastic compression resistance of the tested specimens can be assessed through a strength index (SI), as employed by Han [49] and McCann et al. [50]. The strength index is expressed as follows:
3.4. Failure modes Typical failures modes for the tested CFST and FRP-CFST stub columns are shown in Fig. 12. Both specimen types exhibited outward only local buckling of the steel tube, but this occurred at considerably lower strains (see Fig. 8) for the CFST specimens (Fig. 12(a)) than the FRPCFST specimens (Fig. 12(b)–(d)). Slightly different patterns of local buckling may be seen for differing FRP-CFST cross-section geometries and material properties, as shown in Fig. 12(b)–(d). The delayed local buckling in the FRP-CFST specimens is attributed to the reduced transverse expansion of the concrete core due to the additional
SI =
Nue Npl, Rd
(1)
where Npl,Rd is the plastic compressive resistance of the column crosssection, which is defined in EN 1994-1-1 [51], in the absence of steel 618
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Fig. 13. Typical failure modes of FRP-CFST and FCC specimens.
seen that the strength index of the FRP-CFST specimens increased with (1) increasing Df/Ds ratio due to the increased portion of the crosssection that is doubly-confined and (2) increasing wall thickness of the inner FRP tube due to the increased level of confinement afforded to the inner concrete core. There are three main contributory components to the axial loadcarrying capacity of FRP-CFST cross-sections: (1) the steel tube, (2) the
reinforcement, as:
Npl, Rd = Aa f y + Ac fco′
(2)
in which Aa and Ac are the cross-sectional areas of the steel tube and concrete, respectively, and fy and f’co are the yield strength of steel tube and cylinder strength of the concrete, respectively. The calculated SI values for the tested FRP-CFST specimens are listed in Table 3 and plotted against the Df/Ds ratio in Fig. 14. It may be
Fig. 16. Variation of reduction coefficient φ with Df/Ds for tested FRP-CFST specimens.
Fig. 14. Comparison of strength indices for FRP-CFST and CFST specimens.
Fig. 15. Illustration of contributory components to axial loadcarrying capacities of FRP-CFST, CFST and FCC columns.
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specimens 1S1TH-200-30 with the PET FRP inner tube and S1NH-20018 with the PEN FRP inner tube were 64.5 and 46.4, respectively, which are the largest and second largest among all the values. This clearly demonstrates that the ductility of FRP-CFST cross-sections can be significantly improved by using an inner FRP tube made of high rupture strain FRP. The DI of the FRP-CFST specimens with inner GFRP tubes are plotted in Fig. 17 against the Df/Ds ratio. It may be seen that the DI value exhibits a decreasing trend with increasing Df/Ds ratio and reducing FRP tube thickness. Also, the ductility of the specimens filled with high strength concrete is clearly inferior to those filled with normal strength concrete.
sandwiched concrete between the FRP tube and the steel tube and (3) the doubly-confined inner concrete core, as illustrated in Fig. 15. Similarly, the components contributing to the axial load-bearing capacity of CFST and FCC cross-sections are also illustrated in Fig. 15. It would be anticipated that the ultimate axial load-bearing capacity of the FRP-CFST cross-sections Nue,FRP-CFST could be accurately predicted as the sum of the ultimate load-bearing capacities of the corresponding CFST cross-section (Nue,CFST) and FCC (Nue,FCC) cross-section, minus (to avoid double-counting) the contribution of the FRP tube and the inner concrete core i.e. (Accf′co+Aff′co) where Acc is cross-sectional area of the inner core concrete, Af is the cross-sectional area of the FRP tube and f′co is concrete compressive strength. However, while this was found to provide reasonable results, the predicted values were in fact consistently larger than the experimental values Nue,FRP-CFST for all tested FRP-CFST cross-sections, indicating that the material strength of the inner FRP tube was not fully utilized in the FRP-CFST specimens. Hence, a reduction coefficient φ is proposed, as defined by Eq. (3):
φ=
Nue, CFST
Nue, FRP − CFST + Nue, FCC−Acc fco′ −Af fco′
3.7. Residual load-bearing capacity of FRP-CFST cross-sections with GFRP tubes The minimum residual load-bearing capacities Nre of each of the tested FRP-CFST specimens are reported in Table 4, together with the ratios Nre/Nue to assess the post-peak drop-off in capacity with increasing deformations. The residual load-bearing capacities of the FRPCFST specimens with the high rupture strain inner FRP tubes are not presented in the table since there was no drop in load throughout the load-deformation history of the tests. The Nre/Nue ratio of the FRP-CFST specimens varied between 0.80 and 0.93 depending on the properties of the inner GFRP tube. The Nre/Nue ratios for the tested FRP-CFST crosssections are plotted against the Df/Ds ratios in Fig. 18. It may be seen that the Nre/Nue ratios show a deceasing trend with increasing FRP tube diameter and thickness. Therefore, although increasing the diameter and thickness of the FRP tube leads to the greatest benefit in terms of ultimate load-carrying capacity, it does also result in the greatest postpeak drop-off in load.
(3)
The calculated values of φ for all the tested FRP-CFST specimens are listed in Table 3, and may be seen to vary between 0.8 and 1.0 depending on the properties of inner FRP tubes; for the GFRP inner tubes, φ varied between 0.92 and 1.0. The values of φ are also plotted against the Df/Ds ratio for the FRP-CFST specimens with GFRP inner tubes in Fig. 16, where it may be seen that there is a trend of increasing φ with increasing Df /Ds ratios, while the influence of tf is less distinct. Further research is required to develop a more in-depth understanding of the above phenomena, but as a preliminary recommendation, a safe-sided value of φ = 0.9 appears appropriate for FRP-CFST cross-sections with GFRP inner tubes, while a value of φ = 0.8 appears suitable for FRPCFST cross-sections with HRS FRP inner tubes.
4. Conclusions 3.6. Ductility of FRP-CFST cross-sections
An experimental study into the axial compressive behaviour of circular CFST columns with an inner FRP tubes (termed FRP-CFST cross-sections herein) has been presented. A total of 32 circular composite stub columns including 15 FRP-CFST stub columns, 2 conventional CFST stub columns (without an inner FRP tube) and 15 FRPconfined concrete (FCC) stub columns were prepared and tested. Based on the results and analysis presented in this paper, the following conclusions can be drawn:
The ductility of the tested FRP-CFST and CFST specimens can been evaluated through the ductility index DI proposed by Ge and Usami [52] and defined as follows:
DI = δu/ δ y = εu/ εy
(4)
where δu and εu are axial displacement and average axial strain, respectively, corresponding to the ultimate capacity and δy and εy are the axial displacement and average axial strain, respectively, at the yield point. In cases where the yield point was not sharply defined, based on the proposals of Schneider [2], the yield strain εy was taken as 0.2%. The DI values of all the FRP-CFST specimens and CFST specimens are listed in Table 3. As presented in the table, the DI values of the
(1) All tested FRP-CFST stub columns exhibited superior performance in terms of delayed local buckling, increased load-bearing capacity and improved ductility over their CFST counterparts without inner FRP tubes. (2) The axial load-bearing capacity of the FRP-CFST cross-sections
Fig. 18. Variation of Nre/Nue ratios for tested FRP-CFST specimens with Df/Ds.
Fig. 17. Ductility index for FRP-CFST specimens with inner GFRP tubes.
620
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increased with increasing thickness of the inner FRP tube due to the greater level of confinement afforded to the inner concrete core and with increasing diameter (and FRP-to-steel tube diameter ratio) of the inner FRP tube due to the greater area of doubly-confined concrete. (3) The ultimate load-bearing capacity of the tested FRP-CFST crosssections was slightly over-predicted by a simple superposition of the capacities of equivalent CFST and FCC specimens minus the overlapping portions, and some preliminary insights into a possible design treatment were made. (4) The material type of the inner FRP tube significantly influenced the ductility of FRP-CFST stub columns, with high rupture strain FRP material showing the best performance. The ductility index also showed a decreasing trend with increasing FRP tube diameter and reducing FRP tube thickness. (5) The post-peak residual capacity of the tested FRP-CFST specimens lay between 80% and 93% of the ultimate load-carrying capacity, and showed a deceasing trend with increasing FRP tube diameter and thickness. Acknowledgments This research was funded by the National Natural Science Foundation of China (Grant No. 51008085 and Grant No. 11472084), Guangzhou Pearl River New Star of Science & Technology Project (Grant No. 2012J2200100), China Scholarship Council (Grant No. 201608440006) and China Postdoctoral Science Foundation (Grant No. 2012M511810 and No. 2014T70807). The paper was prepared during the first author's stay at Imperial College London as a visiting academic. References [1] H.B. Ge, T. Usami, Strength of concrete-filled thin-walled steel box column: experiment, J. Struct. Eng. ASCE 118 (11) (1992) 3036–3054. [2] S.P. Schneider, Axially loaded concrete-filled steel tubes, J. Struct. Eng. ASCE 124 (10) (1998) 1125–1138. [3] B. Uy, Strength of concrete filled steel box columns incorporating local buckling, J. Struct. Eng. ASCE 126 (3) (2000) 341–352. [4] B. Uy, Strength of short concrete filled high strength steel box columns, J. Constr. Steel Res. 57 (2) (2001) 114–134. [5] T. Fujimoto, A. Mukai, I. Nishiyama, K. Sakino, Behavior of eccentrically loaded concrete-filled steel tubular columns, J. Struct. Eng. ASCE 130 (2) (2004) 203–212. [6] K. Sakino, H. Nakahara, S. Morino, I. Nishiyama, Behaviour of centrally loaded concrete-filled steel-tube short columns, J. Struct. Eng. ASCE 130 (2) (2004) 180–188. [7] D. Liu, W.M. Gho, Axial load behaviour of high strength rectangular concrete-filled steel tubular stub columns, Thin-Walled Struct. 43 (8) (2005) 1131–1142. [8] L.H. Han, G.H. Yao, X.L. Zhao, Tests and calculations for hollow structural steel (HSS) stub columns filled with self-consolidating concrete (SCC), J. Constr. Steel Res. 61 (9) (2005) 1241–1269. [9] C.W. Roeder, D.E. Lehman, R. Thody, Composite action in CFST components and connections, AISC Eng. J. 47 (4) (2009) 229–242. [10] Z. Ou, B. Chen, K.H. Hsieh, M.W. Halling, P.J. Barr, Experimental and analytical investigation of concrete-filled steel tubular columns, J. Struct. Eng. ASCE 137 (6) (2011) 635–645. [11] X.H. Dai, D. Lam, Shape effect on the behaviour of axially loaded concrete filled steel tubular stub columns at elevated temperature, J. Constr. Steel Res. 73 (2012) 117–127. [12] Z. Tao, Z.B. Wang, Q. Yu, Finite element modelling of concrete-filled steel stub columns under axial compression, J. Constr. Steel Res. 89 (2013) 121–131. [13] Y.L. Long, J. Cai, Stress–strain relationship of concrete confined by rectangular steel tubes with binding bars, J. Constr. Steel Res. 88 (2013) 1–14. [14] J.G. Nie, Y.H. Wang, J.S. Fan, Experimental research on concrete filled steel tube columns under combined compression-bending-torsion cyclic load, Thin-Walled Struct. 67 (2013) 1–14. [15] Y. Yang, Y. Wang, F. Fu, Effect of reinforcement stiffeners on square concrete-filled steel tubular columns subjected to axial compressive load, Thin-Walled Struct. 82 (2014) 132–144. [16] Y.L. Long, J. Wan, J. Cai, Theoretical study on local buckling of rectangular CFST columns under eccentric compression, J. Constr. Steel Res. 120 (2016) 70–80. [17] Z. Lai, A.H. Varma, Effective stress-strain relationships for analysis of noncompact and slender filled composite (CFST) members, Eng. Struct. 124 (2016) 457–472. [18] Y. Xiao, Applications of FRP composites in concrete columns, Adv. Struct. Eng. 7 (4) (2004) 335–343. [19] Y. Xiao, W. He, K.K. Choi, Confined concrete-filled tubular columns, J. Struct. Eng. ASCE 131 (3) (2005) 488–497.
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