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Axial compressive behaviours of square CFST stub columns at low temperatures
Journal of Constructional Steel Research 164 (2020) 105812
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Axial compressive behaviours of square CFST stub columns at low temperatures Jia-Bao Yan a, Xin Dong a, Tao Wang b, * a b
School of Civil Engineering / Key Laboratory of Coast Civil Structure Safety of Ministry of Education, Tianjin University, Tianjin, 300350, China Key Laboratory of Earthquake Engineering and Engineering Vibration, Institute of Engineering Mechanics, CEA, Harbin, 150080, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 May 2019 Received in revised form 14 October 2019 Accepted 15 October 2019 Available online xxx
This paper evaluates the low-temperature compressive behaviours of square CFST stub columns (SCFSTSCs) in steel-concrete composite structures. 21 compressive tests were firstly performed on 14 SCFST-SCs and seven rectangular CFST stub columns (RCFST-SCs) at low temperatures of 20, -30, 60, and 80 C. The key parameters in this testing program are low temperatures, wall-thickness of steel tube, and shape of the cross section. Concrete crushing, outward local buckling of steel tube, and tensile fracture at the corner of steel tube were three observed failure modes in the compressive tests. The SCFST-SCs and RCFST-SCs exhibited linear, elasto-plastic, and recession working stages. The test results showed that the ultimate compressive resistance and initial stiffness were increased, but the ductility was decreased for the square CFST stub columns as the temperature decreases from 20 to 80 C. The wall-thickness of steel tube showed equivalent contributions on the improvements of ultimate compressive resistance due to different low temperature, but its influences on ductility tends to be weakened as temperature decreases. Modified Eurocode 4 code equations have been proposed for the predictions on ultimate compressive resistances of SCFST-SCs at low temperatures, and their accuracies were checked by 21 reported test results. © 2019 Elsevier Ltd. All rights reserved.
Keywords: CFST column Composite structure Low temperature Compressive tests Compressive resistance Arctic structures
1. Introduction Concrete-filled steel tube (CFST) columns, consisting of an external steel tube and an in-filled concrete core, have been extensively studied in last three decades [1]. This type of structure exhibits many advantages that include high resistance, high stiffness, high ductility, excellent seismic resistance, easy construction for members and joints, and savings on site labour as well as formwork [2e4]. Due to these advantages, square or circular CFST columns (CFSTCs) have been extensively applied as columns in high-rise buildings, bridge piers, compressive members in arch bridges, and jacket leg [2e4]. More recently, due to the increasing infrastructures built in the cold regions, square CFST columns have been used in the composite bridges for high speed railway in Tibet, China and the Arctic offshore platforms, e.g., Lasa river railway composite bridge as shown in Fig. 1. Previous extensive studies have been reported on square CFSTCs at ambient and elevated temperatures. Skair-Khalil and Zeghiche
Abbreviations: CFST, concrete-filled steel tube; RCFST-SC, rectangular CFST stub column; LVDT, linear varying displacement transducer; SCFSTC, square CFST columns; SCFST-SC, square CFST stub column; COV, coefficient of variation. * Corresponding author. E-mail address: [email protected] (T. Wang). https://doi.org/10.1016/j.jcsr.2019.105812 0143-974X/© 2019 Elsevier Ltd. All rights reserved.
[5] experimentally investigated the compressive behaviour of hotrolled rectangular CFSTCs. Susantha et al. [6] experimentally studied the influence of the confinement by different shapes of external steel tube on the compressive stress-strain behaviour of the infilled concrete core. Zhang et al. [5] and Guo et al. [7] studied the axial compressive behaviour of the concrete-filled or bare steel tube through full-scale tests. Uy [8] extended the compressive tests on square CFST stub columns with high strength concrete. Han and Yao [9] focused on the concrete compaction on the axial compressive behaviour of rectangular CFST columns. Nonlinear analytical models have been developed by Liang et al. [10] on compressive behaviour of CFST columns that considered the effect of local buckling in the steel tube. Tao et al. [11] studied compressive behaviours of stiffened CFSTCs. Yang et al. [12] investigated the compressive behaviours of square CFSTCs subjected to local compression. Lee et al. [13] contributed to the compressive behaviour of square CFST columns (SCFSTCs) with welded built up ~ ez et al. [14] investigated the shape effect external steel tube. Iban on compressive behaviour of high strength CFSTCs. Xue et al. [15] studied the seismic behaviour of SCFSTCs. Chen and Huang [16] further extended the seismic behaviour of CFST column under attack of acid rain. Hua et al. [17] studied the long-term performance of square CFST columns under compression and corrosions. Zhou and Han [18], Ritchie et al. [19], and Yang et al. [20]
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Nomenclature Ac As B D DI Es L Ke Pu P45% P0 T fcT fyT r t
D D45% εc
x
Cross sectional area of concrete Cross sectional area of steel tube Length of shorter side of rectangular steel tube Length of longer side of rectangular steel tube Ductility index The elastic modulus of steel at low temperature Height of the SCFST-SC Initial stiffness of SCFST-SCs subjected to compression Ultimate compressive resistance of SCFST-SC 45% ultimate compressive resistance Generalized compressive resistance Temperature level Concrete compressive strength at T Steel tube's yield strength at temperature T Radius of square steel tube at corner Thickness of steel tube in SCFST-SC Axial shortening of SCFST-SC Shortening at P45% Axial compressive strain at ultimate compressive resistance Confinement coefficient
contributed to the studies on structural behaviours of square CFST columns at high temperatures. Thus, it can be found that most of these previous studies focused on compressive behaviours of square CFST stub columns (SCFST-SCs) at ambient or high temperatures. The information on the axial compressive behaviour of SCFST-SCs at low temperatures is still very limited, and this topic is reserved for detailed investigations. The lowest temperatures in the cold region of Tibet and Northern China could be about 60 C in winter [21]. In the Arctic, the recorded lowest temperature was about 70 C [22]. Previous extensive studies exhibited the mechanical properties of constructional materials used in the steel-concrete composite structures were significantly influenced by low temperatures. Elices et al. [23], Lahlou et al. [24], Yan and Xie [25], and Xie et al. [26] reported that the mechanical properties of mild steel reinforcements were significantly influenced by the low temperature. Further studies by Yan et al. [27] revealed that mechanical properties of steel materials used in the composite structures were significantly influenced by the Arctic low temperatures. Including the mechanical properties of steel materials, the compressive strengths of the normal weight concrete (NWC) [29,30] and lightweight concrete (LWC) [31,32] were also influenced by the low temperatures. Including the influences of the low temperature on mechanical properties of constructional materials, more recent studies by Yan and Xie [33] and Yan et al. [34] revealed that the bonding strength at steel-concrete interface were also significantly influenced by the Arctic low temperatures. It can be concluded from these previous studies that most of them only focused on the materials level or component level, the information on the steelconcrete composite structural members is still very limited. With such transparent influences on the material properties and bonding strength, the structural behaviours of important SCFST-SCs at low temperatures need to be well understood. It is necessary to perform corresponding experimental investigations to better understand the axial compressive behaviours of SCFST-SCs. This paper aims to study axial compressive behaviours of SCFSTSCs at different low temperatures. 21 axial large scale SCFST-SCs at different low temperatures of 20, -30, 60, and 80 C were firstly
tested subjected to axial compression. The test results offered detailed information on typical failure modes and axial load versus displacement behaviour. The influences of different parameters, e.g., low temperatures, wall-thickness of steel tube, and shape of the cross section on axial compressive behaviours of SCFST-SCs were analysed, discussed, and reported. Including the experimental studies, this paper also made efforts to develop analytical models to predict the ultimate compressive resistances of SCFSTSCs. Their accuracies were also validated by the reported 21 test results. With these reported experimental and analytical studies, some conclusions were finally drawn at the end of the paper. 2. Testing program 2.1. Details of square and rectangular CFST stub columns Fourteen square CFST (SCFST) and seven rectangular CFST (RCFST) stub columns have been prepared for this testing program. Each stub column measures 350 mm in height. The width for the square cross section of the SCFST stub column measures 120 mm. The length and width for the rectangular cross section in the RCFST stub columns are 150 mm and 100 mm, respectively. Fig. 2 illustrates the general geometric details of the SCFST or RCFST stub columns, and more details are given in Table 1. For the SCFST, the studied parameters in this testing program are low temperature level (T) and wall-thickness of the steel tube (t). Two types of wall thickness for square steel tube, i.e., 3.2 mm and 5.8 mm, were used in SCFSTs. Thus, these 14 SCFSTs can be categorized into two groups with seven specimens in each group. SCFSTs in each group were tested at varying low temperatures of 20, -30, 60, and 80 C, respectively that simulates the low temperature environment in cold regions. For RCFSTs, only 4.2 mm-thick rectangular tube was used, and they were also tested at varying low temperatures of 20, -30, 60, and 80 C, respectively. For these SCFSTs and RCFSTs, two identical specimens were tested at each low temperature level except one specimen was tested at 30 C. More details of geometric and testing parameters can be found in Table 1. 2.2. Materials Two types of materials were involved in this experimental study, i.e., normal weight concrete for core and mild steel for steel tube. Since these materials were exposed to low-temperature environment, their low-temperature mechanical properties need to be required. Steel coupons were firstly prepared through cutting from the square steel tubes, and their geometries are shown in Fig. 3. Followed tensile tests were performed in a cooling chamber that utilized liquid nitrogen gas (LNG) to cool down the coupons. Finally, the obtained elastic yield and ultimate strength as well as Young's modulus of the steel tubes at different low temperatures are given in Table 1. Including tensile tests, compressive tests on concrete cubes with side width of 100 mm were also performed at low temperatures. Finally, the obtained mechanical properties of normal weight concrete that include modulus of elasticity and compressive strengths at different low temperatures are given in Table 1. 2.3. Test setup and measurements Compressive tests on SCFST-SCs were performed in a 500-ton MTS testing machine with a cooling chamber as shown in Fig. 4. Following the specifications in Chinese code GB51081-2015 [35], all the SCFST-SCs were firstly put in a cold storage for 48 h and cooled down to the target temperature before testing. During this cooling process, the PT100 type of thermal sensors embedded in the specimens and attached to the external surface of steel tube assist
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Fig. 1. CFST column in steel-concrete composite bridges.
Fig. 2. Geometric details of circular CFST stub columns.
measuring the temperatures in the SCFST-SCs. Fig. 4 shows the representative compression-test setup for the SCFST-SC at low temperatures. After the specimens were cooled down to the target low temperatures in the cold storage, the SCFST-SC specimen was then moved into the testing frame with a cooling chamber. After
that, LNG was injected to maintain the low temperature during the process of compression testing. All the SCFST-SCs were firstly installed to a bottom rigid support, and displacement-controlled loading was applied to their top surfaces. The shortening of SCFST-SCs was instrumented by four linear varying displacement transducers (LVDTs) that locates at four corner of the loading platen as shown in Fig. 4. Vertical and horizontal strains developed in the external surfaces of steel tube were measured by the linear strain gauges as shown in Fig. 2. The reaction forces of SCFST-SC during the testing were measured by a load cell between the loading platen and actuator as shown in Fig. 4. Finally, the shortening of column, reaction forces, and the vertical and horizontal strains during the testing process were recorded by a data logger.
3. Test results 3.1. Failure modes Fig. 5 depicts the typical failure modes occurred to 21 specimens. In this figure, three types of failure modes are observed that
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Table 1 Details and results of CFST columns subjected to axial compression at low temperatures. Item
D (mm)
B (mm)
R (mm)
T (mm)
D/t Ratio
L/B Ratio
T (ºC)
fcT (MPa)
EcT (GPa)
fyT (MPa)
fuT (MPa)
EsT (GPa)
As (mm2)
Ac (mm2)
x
St3T20-1 St3T20-2 St3T-30 St3T-60-1 St3T-60-2 St3T-80-1 St3T-80-2 St5T20-1 St5T20-2 St5T-30 St5T-60-1 St5T-60-2 St5T-80-1 St5T-80-2 Rt4T20-1 Rt4T20-2 Rt4T-30 Rt4T-60-1 Rt4T-60-2 Rt4T-80-1 Rt4T-80-2
119.7 119.7 119.7 119.7 119.7 119.7 119.7 119.4 119.4 119.4 119.4 119.4 119.4 119.4 149.7 149.7 149.7 149.7 149.7 149.7 149.7
120.2 120.2 120.2 120.2 120.2 120.2 120.2 120.4 120.4 120.4 120.4 120.4 120.4 120.4 100.2 100.2 100.2 100.2 100.2 100.2 100.2
2.99 2.99 2.99 2.99 2.99 2.99 2.99 3.02 3.02 3.02 3.02 3.02 3.02 3.02 3.01 3.01 3.01 3.01 3.01 3.01 3.01
3.1 3.1 3.1 3.1 3.1 3.1 3.1 4.8 4.8 4.8 4.8 4.8 4.8 4.8 4.2 4.2 4.2 4.2 4.2 4.2 4.2
38.8 38.8 38.8 38.8 38.8 38.8 38.8 25.1 25.1 25.1 25.1 25.1 25.1 25.1 23.9 23.9 23.9 23.9 23.9 23.9 23.9
2.9 2.9 2.9 2.9 2.9 2.9 2.9 2.9 2.9 2.9 2.9 2.9 2.9 2.9 3.5 3.5 3.5 3.5 3.5 3.5 3.5
20 20 30 60 60 80 80 20 20 30 60 60 80 80 20 20 30 60 60 80 80
34.0 34.0 37.4 48.8 48.8 59.2 59.2 34.0 34.0 37.4 48.8 48.8 59.2 59.2 34.0 34.0 37.4 48.8 48.8 59.2 59.2
27 27 29 33 33 36 36 27 27 29 33 33 36 36 27 27 29 33 33 36 36
337 337 355 369 369 379 379 347 347 366 380 380 391 391 377 377 402 420 420 434 434
408 408 420 428 428 434 434 443 443 457 467 467 475 475 467 467 496 517 517 534 534
200 200 208 215 215 229 229 210 210 209 226 226 231 231 204 204 206 217 217 227 227
1448 1448 1448 1448 1448 1448 1448 2210 2210 2210 2210 2210 2210 2210 2029 2029 2029 2029 2029 2029 2029
12929 12929 12929 12929 12929 12929 12929 12168 12168 12168 12168 12168 12168 12168 12971 12971 12971 12971 12971 12971 12971
1.11 1.11 1.07 0.85 0.85 0.72 0.72 1.85 1.85 1.78 1.41 1.41 1.20 1.20 1.73 1.73 1.68 1.35 1.35 1.15 1.15
Item
DI Ratio
Е ( m)
Ke kN/mm
Pu (kN)
Pu,ACI (kN)
Pu,ACI/Pu
Pu,AI (kN)
Pu,AI/Pu
Pu,EC4 (kN)
Pu,EC4/Pu
Pu,GB (kN)
Pu,GB/Pu
St3T20-1 St3T20-2 St3T-30 St3T-60-1 St3T-60-2 St3T-80-1 St3T-80-2 St6T20-1 St6T20-2 St6T-30 St6T-60-1 St6T-60-2 St6T-80-1 St6T-80-2 Rt4T20-1 Rt4T20-2 Rt4T-30 Rt4T-60-1 Rt4T-60-2 Rt4T-80-1 Rt4T-80-2 Mean Cov
1.75 1.99 1.39 1.64 1.81 1.54 1.62 e 7.01 8.63 2.85 3.98 2.00 2.11 3.02 2.99 2.51 1.68 1.38 1.70 1.56
3500 3677 3362 2721 2116 2971 2430 4993 4587 3189 3353 3149 3283 3361 3655 3522 3127 3654 3326 3514 3706
1840 1840 1922 2103 2103 2284 2284 2279 2279 2318 2569 2569 2716 2716 2198 2198 2259 2475 2475 2656 2656
1044 1000 1202 1299 1261 1423 1484 1304 1314 1416 1578 1620 1773 1773 1308 1299 1398 1585 1632 1732 1714
862 862 925 1071 1071 1200 1200 1118 1118 1195 1345 1345 1476 1476 1139 1139 1227 1390 1390 1534 1534
0.83 0.86 0.77 0.82 0.85 0.84 0.81 0.86 0.85 0.84 0.85 0.83 0.83 0.83 0.87 0.88 0.88 0.88 0.85 0.89 0.90 0.85 0.03
857 857 920 1064 1064 1192 1192 1113 1113 1189 1337 1337 1468 1468 1131 1131 1217 1379 1379 1521 1521
0.82 0.86 0.77 0.82 0.84 0.84 0.80 0.85 0.85 0.84 0.85 0.83 0.83 0.83 0.86 0.87 0.87 0.87 0.84 0.88 0.89 0.84 0.03
927 927 998 1165 1165 1315 1315 1180 1180 1263 1434 1434 1584 1584 1206 1206 1299 1485 1485 1649 1649
0.89 0.93 0.83 0.90 0.92 0.92 0.89 0.91 0.90 0.89 0.91 0.88 0.89 0.89 0.92 0.93 0.93 0.94 0.91 0.95 0.96 0.91 0.03
1013 1013 1095 1309 1309 1501 1501 1207 1207 1293 1503 1503 1695 1695 1249 1249 1347 1574 1574 1779 1779
0.97 1.01 0.91 1.01 1.04 1.05 1.01 0.93 0.92 0.91 0.95 0.93 0.96 0.96 0.95 0.96 0.96 0.99 0.96 1.03 1.04 0.97 0.04
D is side length of square tube; B is length of shorter side of rectangular tube; r is radius at corner of square tube; t denotes thickness of steel tube; T is low temperature; fcuT and fcT are compressive strengths of concrete cubes and cylinders at T, respectively; EcT denotes modulus of elasticity of concrete at T; fyT, fuT, and EsT denotes yield strength, ultimate strength, and elastic modulus of steel tube, respectively; εsF denotes fracture strain; Ac and As denote cross-sectional area of concrete and steel tube, respectively; Ke and Ka denote experimental and analytical initial stiffness of CFST column, respectively; εu denotes compressive strain at ultimate compressive resistance for circular CFST column; DI denotes ductility index for square CFST column and rectangular CFST column; Pu experimental ultimate compressive resistance of CFST column; Pu, ACI, Pu, AI, Pu, EC4, and Pu, GB denotes predicted ultimate compressive resistance by ACI, AISC, EC4, and GB50936, respectively. Notes: the height of the specimen is all 350 mm, and the slenderness ratio (L/B Ratio) is listed above.
include outward local buckling of the external steel tube, concrete crushing, and tensile fracture at the corner position of the steel tube. Outward buckling occurred to the external steel tube in all tested SCFST-SCs. Concrete crushing in the core was also observed in most of specimens, and Fig. 5(l) shows the typical concrete crushing mode of SCFST-SCs. Tensile fracture of external steel tube occurred to all specimens due to excessive expansion of the concrete core. In addition, this tensile fracture becomes more serious for SCFST-SCs tested at low temperatures. This is because that the excessive expansion of the concrete core resulted in concentrated stress at the corner of steel tube. Moreover, the mild steel tends to be more brittle at low temperatures [25].
3.2. Load-shortening and load-strain behaviours Fig. 6(a)~(c) depicts the average load-shortening (P-D) curves of
SCFST-SCs with 3.1 mm- and 4.8 mm-thick steel tube and RCFSTSCs at different low temperatures, respectively. Fig. 7(a)~(c) shows the load versus vertical axial compressive strain and hoop strain for SCFST-SCs with 3.1 mm- and 4.8 mm-thick steel tube and RCFST-SCs at different low temperatures, respectively. These figures show that there are three working stages for the SCFST-SCs and RCFST-SCs under axial compression at low temperatures that include elastic linear, elasto-plastic, and post-peak recession working stage. The first elastic stage starts at the beginning of loading and terminates at about 70e90% ultimate axial compressive resistance. As shown in Fig. 6(a)~(c), the limit of the first working stage tends to increase as the low temperature decreases from 20 to 80 C. After the elastic working stage, the SCFST-SCs and RCFST-SCs behave nonlinearly due to the nonlinearity of inside concrete core. However, the low temperature tends to shorten this working stage due to the reduced ductility of concrete
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3.3. Ultimate compressive resistance, initial stiffness, and ductility ratios The ultimate compressive resistance (Pu) can be obtained from the P-D curves of SCFST-SCs and RCFST-SCs. Since the P-D curves of SCFST-SCs and RCFST-SCs at different temperature levels behave linearly till to 70e90% their ultimate axial compressive resistances, the initial stiffness (Ke) is determined by the load at 45% peak resistance to its corresponding shortening ratio as specified in ACI318M-05 [36], i.e.,
Ke ¼ P45% =D45%
(1)
where, P45% and D45% denotes 45% ultimate compressive resistance and its corresponding shortening, respectively. The ductility ratio is defined as D85% to Du ratio as the following [28];
DI ¼ D85% =Du
Fig. 3. Details of steel coupons and corresponding tensile-test setup.
materials at low temperatures [29]. At the end of elasto-plastic working stage outward local buckling took place and the SCFSTSCs or RCFST-SCSs achieved their ultimate compressive resistances. Through the load-strain curves in Fig. 7, they show that the vertical buckling strains in the steel tube are close to their yielding strains, which implies elasto-plastic or plastic buckling occurred to SCFST-SCs or RCFST-SCSs. After the peak load, the SCFST-SCs or RCFST-SCs at low temperatures exhibit much sharper reductions in their compressive resistance during the recession stage than those at ambient temperatures. This is probably caused by the reduced ductility of both steel tube and concrete core [25].
(2)
where D85% is the shortening at 85% ultimate compressive resistance at recession stage; Du is the shortening at ultimate compressive resistance. However, this method is only applicable for the specimens exhibiting sharp drops after their peak resistances. Thus, all these determined Pu, Ke, and DI ratios are given in Table 1. 3.4. Discussions 3.4.1. Effect of low temperatures Fig. 8 shows the influences of low temperature (T) on strength, stiffness and ductility of SCFST-SCs and RCFST-SCs. Fig. 8(a) shows that the Pu value of the SCFST-SCs generally increased as the low temperature decreases from 20 C to 80 C. For SCFST-SCs with
Fig. 4. Setup for compression tests on CCFST-SC or RCFST-SC at low temperatures.
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Fig. 5. Failure mode of CCFST-SC at low temperatures (CC¼concrete crushing, OB¼Outward buckling, TF¼Tensile fracture at the welding seam).
3.1 mm-thick steel tube, the Pu value is averagely increased by 18%, 25% and 42% as the T value decreases from 20 to 30, 60 to 80 C, respectively; meanwhile, these increments in Pu value are 8%, 22%, and 35% for SCFST-SCs with 4.8 mm-thick steel tube. For RCFST-SCs with steel-tube wall-thickness of 4.2 mm, the Pu value was averagely increased by 7%, 23%, and 32% as the temperature decreases from 20 to 30, 60 to 80 C, respectively. These significant increases in ultimate compressive resistances of SCFSTSCs or RCFST-SCs are mainly caused by the improved strengths of both steel and concrete materials [27,31]. As reported in Table 1, the increments of compressive strength of the normal weight concrete are 10%, 44%, and 74% whilst the increments of yield strength for steel tube are 10%, 18%, and 24%, respectively. Fig. 8(b) shows the influences of low temperature on initial stiffness (Ke) of the CFST stub columns. It shows that the initial stiffness of both SCFST-SCs and RCFST-SCs generally is positive linear relationship with the decreasing temperature. As the low temperature decreases from 20 to 80 C, the Ke of SCFST-SCs with 3.1 mm- and 4.8 mm-thick steel tube and RCFST-SC with 4.2 mm-thick steel tube were averagely increased by 35%, 60%, and 22%, respectively. This is due to that the low temperatures lead to increased elastic modulus of both mild steel materials and concrete materials. Previous experimental studies on mild steel [27] and concrete materials [29,31,32] showed that the strengths and elastic modulus of mild steel and concrete all increased with the decreasing temperature. Fig. 8(c) shows that decreasing the temperature level from 20 C to 80 C generally
decreases the ductility of SCFST-SCs, but exhibits marginal influence on the RCFST-SCs. The DI ratios of SCFST-SC with 3.1 mm- and 4.8 mm-thick steel tube are averagely decreased by 25% and 30%, respectively. However, the DI ratio of RCFST-SC is only decreased by 13%, 3%, and 0% as T decreases from 20 to 30, 60, and 80 C, respectively. Thus, the influences of the low temperature on behaviours of the CFST stub columns subjected to compression need to be well considered in the developments of prediction equations. 3.4.2. Effect of wall thickness of steel tube Fig. 9 plots the influences of wall-thickness of steel tube, t, on strengths and ductility of SCFST-SCs. Fig. 9 shows that Pu, Ke, and DI ratio increase as the t value increases from 3.1 mm to 4.8 mm. Fig. 9(a) shows that as t increases from 3.1 mm to 4.8 mm that corresponds to 55% increments in cross sectional area, the Pu value is averagely increased by 28%, 18%, 25%, and 22% for SCFST-SCs tested at 20 to 30, 60, and 80 C, respectively; meanwhile, the Ke value is averagely increased by 16%, 16%, 5%, and 38% for SCFST-SCs tested at 20 to 30, 60, and 80 C, respectively; and the DI ratio is averagely increased by 275%, 521%, 98%, and 30% for SCFST-SCs tested at 20 to 30, 60, and 80 C, respectively. This can be explained by that increasing the t value in SCFST-SCs actually resulted in the increased steel content of cross section, reduced radius-to-thickness ratio (D/t), and improved hoop confining effect. All these improvements resulted in the improved strengths and
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shows that as the t value increases from 3.1 mm to 4.8 mm, the confinement factor of the SCFST-SC was averagely increased by 67%. These increased confinement factors partially improve the ductility and compressive strength and of concrete, which improved compressive behaviours of the SCFST-SCs.
3.4.3. Effect of cross sectional shape of the CFST stub columns Since the SCFST-SCs with 4.2 mm-thick steel tube were designed with the close cross sectional area with those RCFST-SCs, the effect of cross-sectional shape on their compressive behaviours can be investigated. As reflected in Fig. 6(b)~(c), the RCFST-SCs exhibit sharper drops in their P-D curves at recession stages, which iss due to more serious local buckling occurred to the RCFST-SCs (see Fig. 5). In order to eliminate the influences of the differences in the cross sectional area of the steel tube and concrete core, the generalized compressive resistance ratio was used as the following;
P0 ¼
Pu fyT As þ fcT Ac
(4)
where, Pu denotes ultimate compressive resistance of CFST column. Table 1 lists all the calculated P 0 values for both SCFST-SCs and RCFST-SCs. It shows that the P 0 value for SCFST-SCs at different low temperatures varies from 1.10 to 1.13, which are averagely 5% larger than those of RCFST-SCs varying from 1.04 to 1.09. These reduced P 0 values of RCFST-SCs are due to that with the equivalent cross sectional area the rectangular shape of steel tube increases the radius-to-thickness ratio (D/t ratio) of the side from 25 to 36, which compromises the compressive resistance of steel tube and further reduced the compressive resistance of SCFST-SCs.
4. Prediction equations on ultimate compressive resistance of SCFST-SCs and RCFST-SCs at low temperatures 4.1. Code provisions This section makes efforts to modify the equations in different codes to evaluate the ultimate compressive resistance of SCFST-SCs and RCFST-SCs. In this section, ACI318 code [36], AISC360-10 [37], Eurocode 4 [38], and Chinese code [39]. ACI318 [36] specifies the ultimate compressive resistance of SCFST-SCs and RCFST-SCs as the following;
Pu;ACI ¼ fyT As þ 0:85fcT Ac
(5)
AISC360-10 [37] defines the ultimate compressive resistance of SCFST-SCs and RCFST-SCs as follows;
Pu;AI ¼ Fig. 6. Load versus shortening curves of the SCFST-SCs and RCFST-SCs at different low temperatures.
ductility of the SCFST-SCs. The confinement factor (x) of the steel tube on CFST columns is specified as the following;
x ¼ fyT As
.
0:658P0 =Pcr P0 ; 0:877Pcr ;
P0 ¼ fyT As þ C2 fcT Ac
Pcr ¼
p2 KL
EIeff
P0 2:25Pcr P0 > 2:25Pcr
(6)
(7)
(8)
where, C2 is adopted as 0.85 for rectangular cross section; EIeff ¼ Es
ðfcT Ac Þ
(3)
All the calculated confinement factors are listed in Table 1. It
s Is þ C1Ec Ic; C1 ¼ 0:1 þ 2 AsAþA 0:3. c
According to Eurocode 4 [38], the ultimate compressive strength of SCFST-SCs and RCFST-SCs are determined as follows;
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Fig. 7. Load versus strain curves of the CCFST-SCs and RCFST-SCs at different low temperatures.
8 > < h fyT As þ fcT Ac 1 þ h tfyT a c DfcT Pu;EC4 ¼ > : fyT As þ fcT Ac
ha ¼ 0:25ð3 þ 2lÞ
(9)
sffiffiffiffiffiffiffiffiffiffiffi Ppl;Rk l¼ Pcr
(12)
(10) Ppl;Rk ¼ As fyT þ 0:85Ac fcT 2
hc ¼ 4:9 18:5l þ 17l
(11)
(13)
where, Pcr is the elastic critical normal force that can be determined by the effective flexural stiffness (EI)eff as the following;
J.-B. Yan et al. / Journal of Constructional Steel Research 164 (2020) 105812
9
Fig. 8. Effect of low temperature on strength and ductility of the SCFST-SCs and RCFST-SCs.
4.2. Validations
ðEIÞeff ¼ EsT Is þ Ke EcT Ic
(14)
where, Ke equals to 0.6; Ic and Is denote the second moment of area for concrete core and steel tube, respectively; EsT and EcT denote elastic modulus of steel tube and concrete at low temperature level T, respectively. According to GB 50936-2014 [39], the compressive resistance of SCFST-SCs and RCFST-SCs at low temperatures can be determined as follows;
0 2 Pu;GB ¼ ðAs þ Ac Þ, 1:212 þ mq þ nq f cT
(15)
. m ¼ 0:131fyT 213 þ 0:723
(16)
. 0 n ¼ 0:070f cT 14:4 þ 0:026
(17)
q¼
fyT As 0
f cT Ac 0
(18)
where, f cT denotes compressive strength of concrete prisms at low temperature level T. For prediction purpose, the partial safety factors in Eqn. (5)~(18) are taken as units.
The ultimate compressive resistances of 14 SCFST-SCs and seven RCFST-SCs at different low temperatures estimated by different code equations are tabulated in Table 1, and Fig. 10 compares the prediction-to-test ratios by different modified code equations. These table and figure show that the average prediction-to-test ratios for ACI318, AIJ, and AISC are 0.85, 0.85, and 0.84 with the same Coefficient of Variation (Cov) of 0.03 for the reported 21 tests. These three codes offer the most conservative estimations on the ultimate compressive resistances of SCFST-SCs and RCFST-SCs at different low temperatures. Eurocode 4 equations offer the second most conservative predictions on Pu of SCFST-SCs and RCFST-SCs at different low temperatures. It averagely underestimates the Pu values by 9% with a Cov of 0.03 for the 21 compression tests on SCFST-SCs and RCFST-SCs. GB50936 offers the most accurate predictions on the ultimate compressive resistances of SCFST-SCs and RCFST-SCs. It averagely underestimates the ultimate compressive resistances of SCFST-SCs and RCFST-SCs by 3% with a Cov of 0.03 for the 21 test results. However, as reflected in Fig. 10, it offers seven unsafe predictions for the 21 tests. Thus, considering both accuracy and reliability of the predictions, the modified Eurocode 4 equations were highly recommended to predict the ultimate compressive resistances of the SCFST-SCs and RCFST-SCs at low temperatures. However, these validations were based limited test results, further validations are still required.
10
J.-B. Yan et al. / Journal of Constructional Steel Research 164 (2020) 105812
Fig. 9. Effect of t on strength and ductility of the SCFST-SCs.
thickness of external steel tube, and shape of the cross section were discussed and analysed. Finally, including the reported test results, equations in different design codes were modified to estimate the ultimate compressive resistance of SCFST-SCs and RCFST-SCs at low temperatures. Based on these investigation, the conclusions are given as follows;
Fig. 10. Scatters of prediction-to-test ratios by different code equations.
5. Conclusions Firstly, this paper performed axial compressive tests on 14 SCFST-SCs and seven RCFST-SCs at different low temperatures of 20~-80 C. Then, the test results reported detailed information on the failure modes and axial compression force versus shortening/ strain behaviour. Thirdly, the influences of low temperature,
(1) At low temperatures, three types of typical failure modes occurred to the SCFST-SCs and RCFST-SCs, i.e., concrete crushing, outward local buckling of the external steel tube, and tensile fracture of the steel tubes at their corner positions. (2) Three working stages of SCFST-SCs and RCFST-SCs subjected to compression at low temperatures were observed that include elastic, elasto-plastic, and after peak recession stages. The low temperatures shortened the second working stage and results in sharper P-D curves at the recession stage. Elasto-plastic or plastic buckling occurred to the SCFST-SCs or RCFST-SCs when they achieved their ultimate compressive resistances. (3) Low temperatures significantly increase the ultimate compressive resistances of SCFST-SCs or RCFST-SCs. The increments of the ultimate compressive resistance of CFST stub columns due to the low temperatures decreases with the thickness of the steel tube. The low temperatures also significantly increase the initial stiffness, but reduces the ductility of SCFST-SCs or RCFST-SCs subjected compression.
J.-B. Yan et al. / Journal of Constructional Steel Research 164 (2020) 105812
(4) Increasing the thickness of steel tube of about 55% resulted in 28%, 18%, 25%, and 22% increases in ultimate compressive resistance of SCFST-SCs at 20 to 30, 60, and 80 C, respectively; meanwhile, the Ke value is averagely increased by 16%, 16%, 5%, and 38% for SCFST-SCs tested at 20 to 30, 60, and 80 C, respectively; and the DI ratio is averagely increased by 275%, 521%, 98%, and 30% for SCFSTSCs tested at 20 to 30, 60, and 80 C, respectively. The low temperature reduces the increments of the DI ratio receiving from the increased thickness of steel tube. (5) With the close cross sectional area, the square CFST-SCs exhibit slight larger compressive resistance at different low temperature compared with the rectangular CFST stub columns. (6) The modified ACI318 and AISC code equations offer the most conservative estimations among the four codes. Eurocode 4 offers the second most conservative predictions on ultimate compressive resistances of SCFST-SCs and RCFST-SCs at low temperatures. GB50936 offers the most accurate predictions on the ultimate compressive resistances of SCFST-SCs and RCFST-SCs, but offers the most unsafe predictions. Considering both accuracy and reliability of the predictions, modified Eurocode 4 equations were recommended to predict the ultimate compressive resistances of the SCFST-SCs and RCFST-SCs at low temperatures. However, these validations were based limited test results, further validations are still required.
[12] [13] [14] [15]
[16]
[17]
[18]
[19]
[20]
[21] [22] [23] [24] [25]
[26]
Acknowledgment The authors would like to acknowledge the research grant 51608358 received from National Natural Science Foundation of China and Peiyang Scholar Foundation (grant no. 2019XRX-0026) under Reserved Academic Program from Tianjin University for the works reported herein. The authors gratefully express their gratitude for the financial supports.
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Contents lists available at ScienceDirect
Journal of Constructional Steel Research journal homepage: www.elsevier.com/locate/ijcard
Axial compressive behaviours of square CFST stub columns at low temperatures Jia-Bao Yan a, Xin Dong a, Tao Wang b, * a b
School of Civil Engineering / Key Laboratory of Coast Civil Structure Safety of Ministry of Education, Tianjin University, Tianjin, 300350, China Key Laboratory of Earthquake Engineering and Engineering Vibration, Institute of Engineering Mechanics, CEA, Harbin, 150080, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 May 2019 Received in revised form 14 October 2019 Accepted 15 October 2019 Available online xxx
This paper evaluates the low-temperature compressive behaviours of square CFST stub columns (SCFSTSCs) in steel-concrete composite structures. 21 compressive tests were firstly performed on 14 SCFST-SCs and seven rectangular CFST stub columns (RCFST-SCs) at low temperatures of 20, -30, 60, and 80 C. The key parameters in this testing program are low temperatures, wall-thickness of steel tube, and shape of the cross section. Concrete crushing, outward local buckling of steel tube, and tensile fracture at the corner of steel tube were three observed failure modes in the compressive tests. The SCFST-SCs and RCFST-SCs exhibited linear, elasto-plastic, and recession working stages. The test results showed that the ultimate compressive resistance and initial stiffness were increased, but the ductility was decreased for the square CFST stub columns as the temperature decreases from 20 to 80 C. The wall-thickness of steel tube showed equivalent contributions on the improvements of ultimate compressive resistance due to different low temperature, but its influences on ductility tends to be weakened as temperature decreases. Modified Eurocode 4 code equations have been proposed for the predictions on ultimate compressive resistances of SCFST-SCs at low temperatures, and their accuracies were checked by 21 reported test results. © 2019 Elsevier Ltd. All rights reserved.
Keywords: CFST column Composite structure Low temperature Compressive tests Compressive resistance Arctic structures
1. Introduction Concrete-filled steel tube (CFST) columns, consisting of an external steel tube and an in-filled concrete core, have been extensively studied in last three decades [1]. This type of structure exhibits many advantages that include high resistance, high stiffness, high ductility, excellent seismic resistance, easy construction for members and joints, and savings on site labour as well as formwork [2e4]. Due to these advantages, square or circular CFST columns (CFSTCs) have been extensively applied as columns in high-rise buildings, bridge piers, compressive members in arch bridges, and jacket leg [2e4]. More recently, due to the increasing infrastructures built in the cold regions, square CFST columns have been used in the composite bridges for high speed railway in Tibet, China and the Arctic offshore platforms, e.g., Lasa river railway composite bridge as shown in Fig. 1. Previous extensive studies have been reported on square CFSTCs at ambient and elevated temperatures. Skair-Khalil and Zeghiche
Abbreviations: CFST, concrete-filled steel tube; RCFST-SC, rectangular CFST stub column; LVDT, linear varying displacement transducer; SCFSTC, square CFST columns; SCFST-SC, square CFST stub column; COV, coefficient of variation. * Corresponding author. E-mail address: [email protected] (T. Wang). https://doi.org/10.1016/j.jcsr.2019.105812 0143-974X/© 2019 Elsevier Ltd. All rights reserved.
[5] experimentally investigated the compressive behaviour of hotrolled rectangular CFSTCs. Susantha et al. [6] experimentally studied the influence of the confinement by different shapes of external steel tube on the compressive stress-strain behaviour of the infilled concrete core. Zhang et al. [5] and Guo et al. [7] studied the axial compressive behaviour of the concrete-filled or bare steel tube through full-scale tests. Uy [8] extended the compressive tests on square CFST stub columns with high strength concrete. Han and Yao [9] focused on the concrete compaction on the axial compressive behaviour of rectangular CFST columns. Nonlinear analytical models have been developed by Liang et al. [10] on compressive behaviour of CFST columns that considered the effect of local buckling in the steel tube. Tao et al. [11] studied compressive behaviours of stiffened CFSTCs. Yang et al. [12] investigated the compressive behaviours of square CFSTCs subjected to local compression. Lee et al. [13] contributed to the compressive behaviour of square CFST columns (SCFSTCs) with welded built up ~ ez et al. [14] investigated the shape effect external steel tube. Iban on compressive behaviour of high strength CFSTCs. Xue et al. [15] studied the seismic behaviour of SCFSTCs. Chen and Huang [16] further extended the seismic behaviour of CFST column under attack of acid rain. Hua et al. [17] studied the long-term performance of square CFST columns under compression and corrosions. Zhou and Han [18], Ritchie et al. [19], and Yang et al. [20]
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Nomenclature Ac As B D DI Es L Ke Pu P45% P0 T fcT fyT r t
D D45% εc
x
Cross sectional area of concrete Cross sectional area of steel tube Length of shorter side of rectangular steel tube Length of longer side of rectangular steel tube Ductility index The elastic modulus of steel at low temperature Height of the SCFST-SC Initial stiffness of SCFST-SCs subjected to compression Ultimate compressive resistance of SCFST-SC 45% ultimate compressive resistance Generalized compressive resistance Temperature level Concrete compressive strength at T Steel tube's yield strength at temperature T Radius of square steel tube at corner Thickness of steel tube in SCFST-SC Axial shortening of SCFST-SC Shortening at P45% Axial compressive strain at ultimate compressive resistance Confinement coefficient
contributed to the studies on structural behaviours of square CFST columns at high temperatures. Thus, it can be found that most of these previous studies focused on compressive behaviours of square CFST stub columns (SCFST-SCs) at ambient or high temperatures. The information on the axial compressive behaviour of SCFST-SCs at low temperatures is still very limited, and this topic is reserved for detailed investigations. The lowest temperatures in the cold region of Tibet and Northern China could be about 60 C in winter [21]. In the Arctic, the recorded lowest temperature was about 70 C [22]. Previous extensive studies exhibited the mechanical properties of constructional materials used in the steel-concrete composite structures were significantly influenced by low temperatures. Elices et al. [23], Lahlou et al. [24], Yan and Xie [25], and Xie et al. [26] reported that the mechanical properties of mild steel reinforcements were significantly influenced by the low temperature. Further studies by Yan et al. [27] revealed that mechanical properties of steel materials used in the composite structures were significantly influenced by the Arctic low temperatures. Including the mechanical properties of steel materials, the compressive strengths of the normal weight concrete (NWC) [29,30] and lightweight concrete (LWC) [31,32] were also influenced by the low temperatures. Including the influences of the low temperature on mechanical properties of constructional materials, more recent studies by Yan and Xie [33] and Yan et al. [34] revealed that the bonding strength at steel-concrete interface were also significantly influenced by the Arctic low temperatures. It can be concluded from these previous studies that most of them only focused on the materials level or component level, the information on the steelconcrete composite structural members is still very limited. With such transparent influences on the material properties and bonding strength, the structural behaviours of important SCFST-SCs at low temperatures need to be well understood. It is necessary to perform corresponding experimental investigations to better understand the axial compressive behaviours of SCFST-SCs. This paper aims to study axial compressive behaviours of SCFSTSCs at different low temperatures. 21 axial large scale SCFST-SCs at different low temperatures of 20, -30, 60, and 80 C were firstly
tested subjected to axial compression. The test results offered detailed information on typical failure modes and axial load versus displacement behaviour. The influences of different parameters, e.g., low temperatures, wall-thickness of steel tube, and shape of the cross section on axial compressive behaviours of SCFST-SCs were analysed, discussed, and reported. Including the experimental studies, this paper also made efforts to develop analytical models to predict the ultimate compressive resistances of SCFSTSCs. Their accuracies were also validated by the reported 21 test results. With these reported experimental and analytical studies, some conclusions were finally drawn at the end of the paper. 2. Testing program 2.1. Details of square and rectangular CFST stub columns Fourteen square CFST (SCFST) and seven rectangular CFST (RCFST) stub columns have been prepared for this testing program. Each stub column measures 350 mm in height. The width for the square cross section of the SCFST stub column measures 120 mm. The length and width for the rectangular cross section in the RCFST stub columns are 150 mm and 100 mm, respectively. Fig. 2 illustrates the general geometric details of the SCFST or RCFST stub columns, and more details are given in Table 1. For the SCFST, the studied parameters in this testing program are low temperature level (T) and wall-thickness of the steel tube (t). Two types of wall thickness for square steel tube, i.e., 3.2 mm and 5.8 mm, were used in SCFSTs. Thus, these 14 SCFSTs can be categorized into two groups with seven specimens in each group. SCFSTs in each group were tested at varying low temperatures of 20, -30, 60, and 80 C, respectively that simulates the low temperature environment in cold regions. For RCFSTs, only 4.2 mm-thick rectangular tube was used, and they were also tested at varying low temperatures of 20, -30, 60, and 80 C, respectively. For these SCFSTs and RCFSTs, two identical specimens were tested at each low temperature level except one specimen was tested at 30 C. More details of geometric and testing parameters can be found in Table 1. 2.2. Materials Two types of materials were involved in this experimental study, i.e., normal weight concrete for core and mild steel for steel tube. Since these materials were exposed to low-temperature environment, their low-temperature mechanical properties need to be required. Steel coupons were firstly prepared through cutting from the square steel tubes, and their geometries are shown in Fig. 3. Followed tensile tests were performed in a cooling chamber that utilized liquid nitrogen gas (LNG) to cool down the coupons. Finally, the obtained elastic yield and ultimate strength as well as Young's modulus of the steel tubes at different low temperatures are given in Table 1. Including tensile tests, compressive tests on concrete cubes with side width of 100 mm were also performed at low temperatures. Finally, the obtained mechanical properties of normal weight concrete that include modulus of elasticity and compressive strengths at different low temperatures are given in Table 1. 2.3. Test setup and measurements Compressive tests on SCFST-SCs were performed in a 500-ton MTS testing machine with a cooling chamber as shown in Fig. 4. Following the specifications in Chinese code GB51081-2015 [35], all the SCFST-SCs were firstly put in a cold storage for 48 h and cooled down to the target temperature before testing. During this cooling process, the PT100 type of thermal sensors embedded in the specimens and attached to the external surface of steel tube assist
J.-B. Yan et al. / Journal of Constructional Steel Research 164 (2020) 105812
3
Fig. 1. CFST column in steel-concrete composite bridges.
Fig. 2. Geometric details of circular CFST stub columns.
measuring the temperatures in the SCFST-SCs. Fig. 4 shows the representative compression-test setup for the SCFST-SC at low temperatures. After the specimens were cooled down to the target low temperatures in the cold storage, the SCFST-SC specimen was then moved into the testing frame with a cooling chamber. After
that, LNG was injected to maintain the low temperature during the process of compression testing. All the SCFST-SCs were firstly installed to a bottom rigid support, and displacement-controlled loading was applied to their top surfaces. The shortening of SCFST-SCs was instrumented by four linear varying displacement transducers (LVDTs) that locates at four corner of the loading platen as shown in Fig. 4. Vertical and horizontal strains developed in the external surfaces of steel tube were measured by the linear strain gauges as shown in Fig. 2. The reaction forces of SCFST-SC during the testing were measured by a load cell between the loading platen and actuator as shown in Fig. 4. Finally, the shortening of column, reaction forces, and the vertical and horizontal strains during the testing process were recorded by a data logger.
3. Test results 3.1. Failure modes Fig. 5 depicts the typical failure modes occurred to 21 specimens. In this figure, three types of failure modes are observed that
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Table 1 Details and results of CFST columns subjected to axial compression at low temperatures. Item
D (mm)
B (mm)
R (mm)
T (mm)
D/t Ratio
L/B Ratio
T (ºC)
fcT (MPa)
EcT (GPa)
fyT (MPa)
fuT (MPa)
EsT (GPa)
As (mm2)
Ac (mm2)
x
St3T20-1 St3T20-2 St3T-30 St3T-60-1 St3T-60-2 St3T-80-1 St3T-80-2 St5T20-1 St5T20-2 St5T-30 St5T-60-1 St5T-60-2 St5T-80-1 St5T-80-2 Rt4T20-1 Rt4T20-2 Rt4T-30 Rt4T-60-1 Rt4T-60-2 Rt4T-80-1 Rt4T-80-2
119.7 119.7 119.7 119.7 119.7 119.7 119.7 119.4 119.4 119.4 119.4 119.4 119.4 119.4 149.7 149.7 149.7 149.7 149.7 149.7 149.7
120.2 120.2 120.2 120.2 120.2 120.2 120.2 120.4 120.4 120.4 120.4 120.4 120.4 120.4 100.2 100.2 100.2 100.2 100.2 100.2 100.2
2.99 2.99 2.99 2.99 2.99 2.99 2.99 3.02 3.02 3.02 3.02 3.02 3.02 3.02 3.01 3.01 3.01 3.01 3.01 3.01 3.01
3.1 3.1 3.1 3.1 3.1 3.1 3.1 4.8 4.8 4.8 4.8 4.8 4.8 4.8 4.2 4.2 4.2 4.2 4.2 4.2 4.2
38.8 38.8 38.8 38.8 38.8 38.8 38.8 25.1 25.1 25.1 25.1 25.1 25.1 25.1 23.9 23.9 23.9 23.9 23.9 23.9 23.9
2.9 2.9 2.9 2.9 2.9 2.9 2.9 2.9 2.9 2.9 2.9 2.9 2.9 2.9 3.5 3.5 3.5 3.5 3.5 3.5 3.5
20 20 30 60 60 80 80 20 20 30 60 60 80 80 20 20 30 60 60 80 80
34.0 34.0 37.4 48.8 48.8 59.2 59.2 34.0 34.0 37.4 48.8 48.8 59.2 59.2 34.0 34.0 37.4 48.8 48.8 59.2 59.2
27 27 29 33 33 36 36 27 27 29 33 33 36 36 27 27 29 33 33 36 36
337 337 355 369 369 379 379 347 347 366 380 380 391 391 377 377 402 420 420 434 434
408 408 420 428 428 434 434 443 443 457 467 467 475 475 467 467 496 517 517 534 534
200 200 208 215 215 229 229 210 210 209 226 226 231 231 204 204 206 217 217 227 227
1448 1448 1448 1448 1448 1448 1448 2210 2210 2210 2210 2210 2210 2210 2029 2029 2029 2029 2029 2029 2029
12929 12929 12929 12929 12929 12929 12929 12168 12168 12168 12168 12168 12168 12168 12971 12971 12971 12971 12971 12971 12971
1.11 1.11 1.07 0.85 0.85 0.72 0.72 1.85 1.85 1.78 1.41 1.41 1.20 1.20 1.73 1.73 1.68 1.35 1.35 1.15 1.15
Item
DI Ratio
Е ( m)
Ke kN/mm
Pu (kN)
Pu,ACI (kN)
Pu,ACI/Pu
Pu,AI (kN)
Pu,AI/Pu
Pu,EC4 (kN)
Pu,EC4/Pu
Pu,GB (kN)
Pu,GB/Pu
St3T20-1 St3T20-2 St3T-30 St3T-60-1 St3T-60-2 St3T-80-1 St3T-80-2 St6T20-1 St6T20-2 St6T-30 St6T-60-1 St6T-60-2 St6T-80-1 St6T-80-2 Rt4T20-1 Rt4T20-2 Rt4T-30 Rt4T-60-1 Rt4T-60-2 Rt4T-80-1 Rt4T-80-2 Mean Cov
1.75 1.99 1.39 1.64 1.81 1.54 1.62 e 7.01 8.63 2.85 3.98 2.00 2.11 3.02 2.99 2.51 1.68 1.38 1.70 1.56
3500 3677 3362 2721 2116 2971 2430 4993 4587 3189 3353 3149 3283 3361 3655 3522 3127 3654 3326 3514 3706
1840 1840 1922 2103 2103 2284 2284 2279 2279 2318 2569 2569 2716 2716 2198 2198 2259 2475 2475 2656 2656
1044 1000 1202 1299 1261 1423 1484 1304 1314 1416 1578 1620 1773 1773 1308 1299 1398 1585 1632 1732 1714
862 862 925 1071 1071 1200 1200 1118 1118 1195 1345 1345 1476 1476 1139 1139 1227 1390 1390 1534 1534
0.83 0.86 0.77 0.82 0.85 0.84 0.81 0.86 0.85 0.84 0.85 0.83 0.83 0.83 0.87 0.88 0.88 0.88 0.85 0.89 0.90 0.85 0.03
857 857 920 1064 1064 1192 1192 1113 1113 1189 1337 1337 1468 1468 1131 1131 1217 1379 1379 1521 1521
0.82 0.86 0.77 0.82 0.84 0.84 0.80 0.85 0.85 0.84 0.85 0.83 0.83 0.83 0.86 0.87 0.87 0.87 0.84 0.88 0.89 0.84 0.03
927 927 998 1165 1165 1315 1315 1180 1180 1263 1434 1434 1584 1584 1206 1206 1299 1485 1485 1649 1649
0.89 0.93 0.83 0.90 0.92 0.92 0.89 0.91 0.90 0.89 0.91 0.88 0.89 0.89 0.92 0.93 0.93 0.94 0.91 0.95 0.96 0.91 0.03
1013 1013 1095 1309 1309 1501 1501 1207 1207 1293 1503 1503 1695 1695 1249 1249 1347 1574 1574 1779 1779
0.97 1.01 0.91 1.01 1.04 1.05 1.01 0.93 0.92 0.91 0.95 0.93 0.96 0.96 0.95 0.96 0.96 0.99 0.96 1.03 1.04 0.97 0.04
D is side length of square tube; B is length of shorter side of rectangular tube; r is radius at corner of square tube; t denotes thickness of steel tube; T is low temperature; fcuT and fcT are compressive strengths of concrete cubes and cylinders at T, respectively; EcT denotes modulus of elasticity of concrete at T; fyT, fuT, and EsT denotes yield strength, ultimate strength, and elastic modulus of steel tube, respectively; εsF denotes fracture strain; Ac and As denote cross-sectional area of concrete and steel tube, respectively; Ke and Ka denote experimental and analytical initial stiffness of CFST column, respectively; εu denotes compressive strain at ultimate compressive resistance for circular CFST column; DI denotes ductility index for square CFST column and rectangular CFST column; Pu experimental ultimate compressive resistance of CFST column; Pu, ACI, Pu, AI, Pu, EC4, and Pu, GB denotes predicted ultimate compressive resistance by ACI, AISC, EC4, and GB50936, respectively. Notes: the height of the specimen is all 350 mm, and the slenderness ratio (L/B Ratio) is listed above.
include outward local buckling of the external steel tube, concrete crushing, and tensile fracture at the corner position of the steel tube. Outward buckling occurred to the external steel tube in all tested SCFST-SCs. Concrete crushing in the core was also observed in most of specimens, and Fig. 5(l) shows the typical concrete crushing mode of SCFST-SCs. Tensile fracture of external steel tube occurred to all specimens due to excessive expansion of the concrete core. In addition, this tensile fracture becomes more serious for SCFST-SCs tested at low temperatures. This is because that the excessive expansion of the concrete core resulted in concentrated stress at the corner of steel tube. Moreover, the mild steel tends to be more brittle at low temperatures [25].
3.2. Load-shortening and load-strain behaviours Fig. 6(a)~(c) depicts the average load-shortening (P-D) curves of
SCFST-SCs with 3.1 mm- and 4.8 mm-thick steel tube and RCFSTSCs at different low temperatures, respectively. Fig. 7(a)~(c) shows the load versus vertical axial compressive strain and hoop strain for SCFST-SCs with 3.1 mm- and 4.8 mm-thick steel tube and RCFST-SCs at different low temperatures, respectively. These figures show that there are three working stages for the SCFST-SCs and RCFST-SCs under axial compression at low temperatures that include elastic linear, elasto-plastic, and post-peak recession working stage. The first elastic stage starts at the beginning of loading and terminates at about 70e90% ultimate axial compressive resistance. As shown in Fig. 6(a)~(c), the limit of the first working stage tends to increase as the low temperature decreases from 20 to 80 C. After the elastic working stage, the SCFST-SCs and RCFST-SCs behave nonlinearly due to the nonlinearity of inside concrete core. However, the low temperature tends to shorten this working stage due to the reduced ductility of concrete
J.-B. Yan et al. / Journal of Constructional Steel Research 164 (2020) 105812
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3.3. Ultimate compressive resistance, initial stiffness, and ductility ratios The ultimate compressive resistance (Pu) can be obtained from the P-D curves of SCFST-SCs and RCFST-SCs. Since the P-D curves of SCFST-SCs and RCFST-SCs at different temperature levels behave linearly till to 70e90% their ultimate axial compressive resistances, the initial stiffness (Ke) is determined by the load at 45% peak resistance to its corresponding shortening ratio as specified in ACI318M-05 [36], i.e.,
Ke ¼ P45% =D45%
(1)
where, P45% and D45% denotes 45% ultimate compressive resistance and its corresponding shortening, respectively. The ductility ratio is defined as D85% to Du ratio as the following [28];
DI ¼ D85% =Du
Fig. 3. Details of steel coupons and corresponding tensile-test setup.
materials at low temperatures [29]. At the end of elasto-plastic working stage outward local buckling took place and the SCFSTSCs or RCFST-SCSs achieved their ultimate compressive resistances. Through the load-strain curves in Fig. 7, they show that the vertical buckling strains in the steel tube are close to their yielding strains, which implies elasto-plastic or plastic buckling occurred to SCFST-SCs or RCFST-SCSs. After the peak load, the SCFST-SCs or RCFST-SCs at low temperatures exhibit much sharper reductions in their compressive resistance during the recession stage than those at ambient temperatures. This is probably caused by the reduced ductility of both steel tube and concrete core [25].
(2)
where D85% is the shortening at 85% ultimate compressive resistance at recession stage; Du is the shortening at ultimate compressive resistance. However, this method is only applicable for the specimens exhibiting sharp drops after their peak resistances. Thus, all these determined Pu, Ke, and DI ratios are given in Table 1. 3.4. Discussions 3.4.1. Effect of low temperatures Fig. 8 shows the influences of low temperature (T) on strength, stiffness and ductility of SCFST-SCs and RCFST-SCs. Fig. 8(a) shows that the Pu value of the SCFST-SCs generally increased as the low temperature decreases from 20 C to 80 C. For SCFST-SCs with
Fig. 4. Setup for compression tests on CCFST-SC or RCFST-SC at low temperatures.
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J.-B. Yan et al. / Journal of Constructional Steel Research 164 (2020) 105812
Fig. 5. Failure mode of CCFST-SC at low temperatures (CC¼concrete crushing, OB¼Outward buckling, TF¼Tensile fracture at the welding seam).
3.1 mm-thick steel tube, the Pu value is averagely increased by 18%, 25% and 42% as the T value decreases from 20 to 30, 60 to 80 C, respectively; meanwhile, these increments in Pu value are 8%, 22%, and 35% for SCFST-SCs with 4.8 mm-thick steel tube. For RCFST-SCs with steel-tube wall-thickness of 4.2 mm, the Pu value was averagely increased by 7%, 23%, and 32% as the temperature decreases from 20 to 30, 60 to 80 C, respectively. These significant increases in ultimate compressive resistances of SCFSTSCs or RCFST-SCs are mainly caused by the improved strengths of both steel and concrete materials [27,31]. As reported in Table 1, the increments of compressive strength of the normal weight concrete are 10%, 44%, and 74% whilst the increments of yield strength for steel tube are 10%, 18%, and 24%, respectively. Fig. 8(b) shows the influences of low temperature on initial stiffness (Ke) of the CFST stub columns. It shows that the initial stiffness of both SCFST-SCs and RCFST-SCs generally is positive linear relationship with the decreasing temperature. As the low temperature decreases from 20 to 80 C, the Ke of SCFST-SCs with 3.1 mm- and 4.8 mm-thick steel tube and RCFST-SC with 4.2 mm-thick steel tube were averagely increased by 35%, 60%, and 22%, respectively. This is due to that the low temperatures lead to increased elastic modulus of both mild steel materials and concrete materials. Previous experimental studies on mild steel [27] and concrete materials [29,31,32] showed that the strengths and elastic modulus of mild steel and concrete all increased with the decreasing temperature. Fig. 8(c) shows that decreasing the temperature level from 20 C to 80 C generally
decreases the ductility of SCFST-SCs, but exhibits marginal influence on the RCFST-SCs. The DI ratios of SCFST-SC with 3.1 mm- and 4.8 mm-thick steel tube are averagely decreased by 25% and 30%, respectively. However, the DI ratio of RCFST-SC is only decreased by 13%, 3%, and 0% as T decreases from 20 to 30, 60, and 80 C, respectively. Thus, the influences of the low temperature on behaviours of the CFST stub columns subjected to compression need to be well considered in the developments of prediction equations. 3.4.2. Effect of wall thickness of steel tube Fig. 9 plots the influences of wall-thickness of steel tube, t, on strengths and ductility of SCFST-SCs. Fig. 9 shows that Pu, Ke, and DI ratio increase as the t value increases from 3.1 mm to 4.8 mm. Fig. 9(a) shows that as t increases from 3.1 mm to 4.8 mm that corresponds to 55% increments in cross sectional area, the Pu value is averagely increased by 28%, 18%, 25%, and 22% for SCFST-SCs tested at 20 to 30, 60, and 80 C, respectively; meanwhile, the Ke value is averagely increased by 16%, 16%, 5%, and 38% for SCFST-SCs tested at 20 to 30, 60, and 80 C, respectively; and the DI ratio is averagely increased by 275%, 521%, 98%, and 30% for SCFST-SCs tested at 20 to 30, 60, and 80 C, respectively. This can be explained by that increasing the t value in SCFST-SCs actually resulted in the increased steel content of cross section, reduced radius-to-thickness ratio (D/t), and improved hoop confining effect. All these improvements resulted in the improved strengths and
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shows that as the t value increases from 3.1 mm to 4.8 mm, the confinement factor of the SCFST-SC was averagely increased by 67%. These increased confinement factors partially improve the ductility and compressive strength and of concrete, which improved compressive behaviours of the SCFST-SCs.
3.4.3. Effect of cross sectional shape of the CFST stub columns Since the SCFST-SCs with 4.2 mm-thick steel tube were designed with the close cross sectional area with those RCFST-SCs, the effect of cross-sectional shape on their compressive behaviours can be investigated. As reflected in Fig. 6(b)~(c), the RCFST-SCs exhibit sharper drops in their P-D curves at recession stages, which iss due to more serious local buckling occurred to the RCFST-SCs (see Fig. 5). In order to eliminate the influences of the differences in the cross sectional area of the steel tube and concrete core, the generalized compressive resistance ratio was used as the following;
P0 ¼
Pu fyT As þ fcT Ac
(4)
where, Pu denotes ultimate compressive resistance of CFST column. Table 1 lists all the calculated P 0 values for both SCFST-SCs and RCFST-SCs. It shows that the P 0 value for SCFST-SCs at different low temperatures varies from 1.10 to 1.13, which are averagely 5% larger than those of RCFST-SCs varying from 1.04 to 1.09. These reduced P 0 values of RCFST-SCs are due to that with the equivalent cross sectional area the rectangular shape of steel tube increases the radius-to-thickness ratio (D/t ratio) of the side from 25 to 36, which compromises the compressive resistance of steel tube and further reduced the compressive resistance of SCFST-SCs.
4. Prediction equations on ultimate compressive resistance of SCFST-SCs and RCFST-SCs at low temperatures 4.1. Code provisions This section makes efforts to modify the equations in different codes to evaluate the ultimate compressive resistance of SCFST-SCs and RCFST-SCs. In this section, ACI318 code [36], AISC360-10 [37], Eurocode 4 [38], and Chinese code [39]. ACI318 [36] specifies the ultimate compressive resistance of SCFST-SCs and RCFST-SCs as the following;
Pu;ACI ¼ fyT As þ 0:85fcT Ac
(5)
AISC360-10 [37] defines the ultimate compressive resistance of SCFST-SCs and RCFST-SCs as follows;
Pu;AI ¼ Fig. 6. Load versus shortening curves of the SCFST-SCs and RCFST-SCs at different low temperatures.
ductility of the SCFST-SCs. The confinement factor (x) of the steel tube on CFST columns is specified as the following;
x ¼ fyT As
.
0:658P0 =Pcr P0 ; 0:877Pcr ;
P0 ¼ fyT As þ C2 fcT Ac
Pcr ¼
p2 KL
EIeff
P0 2:25Pcr P0 > 2:25Pcr
(6)
(7)
(8)
where, C2 is adopted as 0.85 for rectangular cross section; EIeff ¼ Es
ðfcT Ac Þ
(3)
All the calculated confinement factors are listed in Table 1. It
s Is þ C1Ec Ic; C1 ¼ 0:1 þ 2 AsAþA 0:3. c
According to Eurocode 4 [38], the ultimate compressive strength of SCFST-SCs and RCFST-SCs are determined as follows;
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Fig. 7. Load versus strain curves of the CCFST-SCs and RCFST-SCs at different low temperatures.
8 > < h fyT As þ fcT Ac 1 þ h tfyT a c DfcT Pu;EC4 ¼ > : fyT As þ fcT Ac
ha ¼ 0:25ð3 þ 2lÞ
(9)
sffiffiffiffiffiffiffiffiffiffiffi Ppl;Rk l¼ Pcr
(12)
(10) Ppl;Rk ¼ As fyT þ 0:85Ac fcT 2
hc ¼ 4:9 18:5l þ 17l
(11)
(13)
where, Pcr is the elastic critical normal force that can be determined by the effective flexural stiffness (EI)eff as the following;
J.-B. Yan et al. / Journal of Constructional Steel Research 164 (2020) 105812
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Fig. 8. Effect of low temperature on strength and ductility of the SCFST-SCs and RCFST-SCs.
4.2. Validations
ðEIÞeff ¼ EsT Is þ Ke EcT Ic
(14)
where, Ke equals to 0.6; Ic and Is denote the second moment of area for concrete core and steel tube, respectively; EsT and EcT denote elastic modulus of steel tube and concrete at low temperature level T, respectively. According to GB 50936-2014 [39], the compressive resistance of SCFST-SCs and RCFST-SCs at low temperatures can be determined as follows;
0 2 Pu;GB ¼ ðAs þ Ac Þ, 1:212 þ mq þ nq f cT
(15)
. m ¼ 0:131fyT 213 þ 0:723
(16)
. 0 n ¼ 0:070f cT 14:4 þ 0:026
(17)
q¼
fyT As 0
f cT Ac 0
(18)
where, f cT denotes compressive strength of concrete prisms at low temperature level T. For prediction purpose, the partial safety factors in Eqn. (5)~(18) are taken as units.
The ultimate compressive resistances of 14 SCFST-SCs and seven RCFST-SCs at different low temperatures estimated by different code equations are tabulated in Table 1, and Fig. 10 compares the prediction-to-test ratios by different modified code equations. These table and figure show that the average prediction-to-test ratios for ACI318, AIJ, and AISC are 0.85, 0.85, and 0.84 with the same Coefficient of Variation (Cov) of 0.03 for the reported 21 tests. These three codes offer the most conservative estimations on the ultimate compressive resistances of SCFST-SCs and RCFST-SCs at different low temperatures. Eurocode 4 equations offer the second most conservative predictions on Pu of SCFST-SCs and RCFST-SCs at different low temperatures. It averagely underestimates the Pu values by 9% with a Cov of 0.03 for the 21 compression tests on SCFST-SCs and RCFST-SCs. GB50936 offers the most accurate predictions on the ultimate compressive resistances of SCFST-SCs and RCFST-SCs. It averagely underestimates the ultimate compressive resistances of SCFST-SCs and RCFST-SCs by 3% with a Cov of 0.03 for the 21 test results. However, as reflected in Fig. 10, it offers seven unsafe predictions for the 21 tests. Thus, considering both accuracy and reliability of the predictions, the modified Eurocode 4 equations were highly recommended to predict the ultimate compressive resistances of the SCFST-SCs and RCFST-SCs at low temperatures. However, these validations were based limited test results, further validations are still required.
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Fig. 9. Effect of t on strength and ductility of the SCFST-SCs.
thickness of external steel tube, and shape of the cross section were discussed and analysed. Finally, including the reported test results, equations in different design codes were modified to estimate the ultimate compressive resistance of SCFST-SCs and RCFST-SCs at low temperatures. Based on these investigation, the conclusions are given as follows;
Fig. 10. Scatters of prediction-to-test ratios by different code equations.
5. Conclusions Firstly, this paper performed axial compressive tests on 14 SCFST-SCs and seven RCFST-SCs at different low temperatures of 20~-80 C. Then, the test results reported detailed information on the failure modes and axial compression force versus shortening/ strain behaviour. Thirdly, the influences of low temperature,
(1) At low temperatures, three types of typical failure modes occurred to the SCFST-SCs and RCFST-SCs, i.e., concrete crushing, outward local buckling of the external steel tube, and tensile fracture of the steel tubes at their corner positions. (2) Three working stages of SCFST-SCs and RCFST-SCs subjected to compression at low temperatures were observed that include elastic, elasto-plastic, and after peak recession stages. The low temperatures shortened the second working stage and results in sharper P-D curves at the recession stage. Elasto-plastic or plastic buckling occurred to the SCFST-SCs or RCFST-SCs when they achieved their ultimate compressive resistances. (3) Low temperatures significantly increase the ultimate compressive resistances of SCFST-SCs or RCFST-SCs. The increments of the ultimate compressive resistance of CFST stub columns due to the low temperatures decreases with the thickness of the steel tube. The low temperatures also significantly increase the initial stiffness, but reduces the ductility of SCFST-SCs or RCFST-SCs subjected compression.
J.-B. Yan et al. / Journal of Constructional Steel Research 164 (2020) 105812
(4) Increasing the thickness of steel tube of about 55% resulted in 28%, 18%, 25%, and 22% increases in ultimate compressive resistance of SCFST-SCs at 20 to 30, 60, and 80 C, respectively; meanwhile, the Ke value is averagely increased by 16%, 16%, 5%, and 38% for SCFST-SCs tested at 20 to 30, 60, and 80 C, respectively; and the DI ratio is averagely increased by 275%, 521%, 98%, and 30% for SCFSTSCs tested at 20 to 30, 60, and 80 C, respectively. The low temperature reduces the increments of the DI ratio receiving from the increased thickness of steel tube. (5) With the close cross sectional area, the square CFST-SCs exhibit slight larger compressive resistance at different low temperature compared with the rectangular CFST stub columns. (6) The modified ACI318 and AISC code equations offer the most conservative estimations among the four codes. Eurocode 4 offers the second most conservative predictions on ultimate compressive resistances of SCFST-SCs and RCFST-SCs at low temperatures. GB50936 offers the most accurate predictions on the ultimate compressive resistances of SCFST-SCs and RCFST-SCs, but offers the most unsafe predictions. Considering both accuracy and reliability of the predictions, modified Eurocode 4 equations were recommended to predict the ultimate compressive resistances of the SCFST-SCs and RCFST-SCs at low temperatures. However, these validations were based limited test results, further validations are still required.
[12] [13] [14] [15]
[16]
[17]
[18]
[19]
[20]
[21] [22] [23] [24] [25]
[26]
Acknowledgment The authors would like to acknowledge the research grant 51608358 received from National Natural Science Foundation of China and Peiyang Scholar Foundation (grant no. 2019XRX-0026) under Reserved Academic Program from Tianjin University for the works reported herein. The authors gratefully express their gratitude for the financial supports.
[27]
[28]
[29]
[30]
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