Advances in Structural Engineering

Experimental Study on Fire-Exposed Rectangular Concrete-Filled Steel Tubular (CFST) Columns Subjected to Bi-Axial Force and Bending by Shao-Fei Jiang, Zhao-Qi Wu and De-Sheng Niu Reprinted from Advances in Structural Engineering Volume 13 No. 4 2010 MULTI-SCIENCE PUBLISHING CO. LTD. 5 Wates Way, Brentwood, Essex CM15 9TB, United Kingdom

Experimental Study on Fire-Exposed Rectangular Concrete-Filled Steel Tubular (CFST) Columns Subjected to Bi-Axial Force and Bending Shao-Fei Jiang 1,*, Zhao-Qi Wu 1 and De-Sheng Niu 2 1 College of Civil Engineering, Fuzhou University, Fuzhou 350108, China 2 School of Civil Engineering, Shenyang Jianzhu University, Shenyang 110168, China (Received: 23 April 2009; Received revised form: 22 October 2009; Accepted: 19 November 2009) Abstract: This paper provides new test data and changing regularity of behavior of rectangular concrete-filled steel tubular columns that have been exposed to high temperature and subsequently are subjected to bi-axial force and bending. Fourteen specimens exposed to constant high temperatures were tested to investigate the residual behavior of them. The investigation emphasized the effect of experimental parameters, such as temperature, slenderness ratio, eccentric angle, height-to-width ratio and confining coefficient, on residual bearing capacity and ductility of members. It is found that the bearing capacity and ductility are mainly influenced by the testing parameters. After being heated to a high temperature lower than 600 C, the columns still have good carrying capacity. The ductility coefficients of all the specimens in the test are not less than 5.6, which indicates that CFST columns subjected to bi-axial compression and bending have good ductility even after high temperature exposure. The experimental results provide the foundation for assessing, repairing and strengthening the fire-damaged structures. Key words: concrete filled steel tubes, high temperature, bi-axial force and bending, carrying capacity, ductility. 1. INTRODUCTION In recent years, concrete-filled steel tubes are used widely in different areas of construction because they have many advantages in engineering practice (Zhong 1999; Uy 2001; Han 2007). One of the main advantages is a substantial increase in carrying capacity and ductility of the members. The steel tube provides confinement to the core concrete, and thus increasing its strength and ductility. Meanwhile, the core concrete reduces the possibility of local buckling of steel wall. In addition, compared with bare steel tubular columns, CFST columns are more fire-resistant. CFST columns also have much better endurance characteristics than conventional reinforced concrete columns under fire conditions as the steel tube prevents spalling of the concrete which remains better protected against fire. Another advantage is that no formwork is required for the concrete during construction. It is well known that fire is always one of the most serious potential risks to modern buildings. Due to the usage of the outside steel tubes, fire resistance is a key issue in the design of CFST columns. In the past, many studies have been performed on the fire resistance of CFST members (Klingsch 1985; Lie and Caron. 1988a, b; Lie and Irwin 1990; Lie and Chabot 1992; Lie et al. 1993; Lie and Stringer 1994; Lie 1994; Hass 1991; Wang 1991; Falke 1992; Kodur 1999; Kim et al. 2000; Han et al. 2001, 2002a, b, 2003a, b, c; Chen 2001). Most of the aforementioned studies have only considered the fire performance of CFST columns during the heating *Corresponding author. Email address: cejsf@fzu.edu.cn; Fax: +86-591-2286 5355; Tel: +86-591-2286 5379. Advances in Structural Engineering Vol. 13 No. 4 2010 551

Experimental Study on Fire-Exposed Rectangular Concrete-Filled Steel Tubular (CFST) Columns Subjected to Bi-Axial Force and Bending stage when both the fire and structural temperatures are increasing. Actually the post-fire performance of CFST may have to be considered when the fire damaged CFST structures are assessed and repaired. However, there is very little research on post-fire behavior of this kind of composite members (Han et al. 2002a, b, c; Chen 2001). The academic group led by Han has analyzed the postfire behavior of CFST members under axial force (Han et al. 2002a, b; Chen 2001), uni-axial force and bending (Han et al. 2002b, c). However, in some cases, CFST members are not always subjected to simple loads such as axial force, uni-axial force and bending. For instance, when used as certain supports at corners of buildings, the columns will be subjected to bi-axial force and bending. So, the behavior of fire-exposed CFST columns under complicated loads like bi-axial force and bending has to be considered. However, the work on the behaviors of fire-exposed CFST columns under complicated loading was not reported in past literature. From the above-mentioned studies, it is found that CFST columns subjected to complicated loading are common in practice and their behavior after fire is seldom in past investigations. Thus, it is indispensable to study the behavior of CFST columns subjected to complicated loading after fire, especially under bi-axial force and bending. The aim of this paper is to investigate the strength and ductility of post-fire rectangular CFST columns subjected to bi-axial force and bending. As part of this study, fourteen specimens were manufactured and tested. The specimens were first exposed to constant high temperatures. Then, the specimens were subjected to a bi-directional eccentrically load, consequently the bending moment and axial force increased in proportion. The effect of temperature, eccentric angle, confining coefficient, height-to-width ratio and slenderness ratio on the behavior of fireexposed rectangular CFST columns under this loading regime, was investigated in details and the test results were recorded. Conclusions and remarks were drawn finally. 2. EXPERIMENTAL PROGRAMS 2.1. Specimen Preparations Fourteen rectangular CFST columns were designed. Figure 1 and Table 1 show the details and dimensions of the specimens. Test parameters were the temperature exposed T, the slenderness ratio λ x (= 3L/ H, where L is the effective length of the specimen, which is the same as the physical length of the column with pinended supports; H is the depth of the rectangular section), the load eccentric angle θ, the height-to-width ratio of cross section H/B, and the confining coefficient ξ (= α f y /f ck, where α is the content of steel for CFST x t s o y θ Figure 1. Details and dimensions of a column specimen columns; α = A s /A c, A s is the area of the steel tube s cross section, A c is the inner concrete s area of the cross section; f y is the yield strength of the steel tube wall; f ck (= 0.67 f cu ) is the characteristic concrete strength; f cu is the characteristic 28-day concrete cubic strength). 2.1.1. Material properties Strips of the steel tubes were tested in tension in accordance with the Chinese standard GB/T228-2002. Three coupons were taken from each face of the steel tube. From these tests, the average yield strength ( f y ) and the ultimate strength ( f u ) of the steel Q235 were found to be 337.5 MPa and 409.8 MPa respectively. The average yield strength ( f y ) and the ultimate strength ( f u ) of the steel Q345 were 413.6 MPa and 555.8 MPa respectively. The modulus of elasticity for the steel Q235 and Q345 were 1.95 10 5 MPa and 2.10 10 5 MPa respectively. The concrete mix was designed for a cubic compressive strength ( f cu ) at 28 days of approximately 39.0 MPa. The modulus of elasticity (E c ) of concrete was measured in accordance with the Chinese standard GB/T50081-2002, and the average value was 2.744 10 4 MPa. For each concrete mix batch used, three 150 mm cubes were also cast and cured in the conditions similar to the related specimens. The mix proportions were as follows: Cement: 498 kg/m 3 Water: 204 kg/m 3 Sand: 612 kg/m 3 Coarse aggregate: 1185 kg/m 3 The average cubic strength at the time of test was 39.0 MPa. In all the concrete mixes, the fine aggregate used was silica-based sand; the coarse aggregate was P x 552 Advances in Structural Engineering Vol. 13 No. 4 2010

Shao-Fei Jiang, Zhao-Qi Wu and De-Sheng Niu Table 1. Details of specimens No. B/mm H /mm t s /mm L/mm f y /MPa f cu /MPa T / C θ/ H /B ξ 1 80 120 3.70 800 337.5 39 20 45 1.5 2.25 2 80 120 3.70 800 337.5 39 20 45 1.5 2.25 3 80 120 3.70 800 337.5 39 400 45 1.5 2.25 4 80 120 3.70 800 337.5 39 600 45 1.5 2.25 5 80 120 3.70 800 337.5 39 800 45 1.5 2.25 6 80 120 3.70 800 337.5 39 600 0 1.5 2.25 7 80 120 3.70 800 337.5 39 600 30 1.5 2.25 8 80 120 3.70 800 337.5 39 600 60 1.5 2.25 9 80 120 3.70 800 337.5 39 600 90 1.5 2.25 10 80 80 3.70 800 337.5 39 600 45 1.0 2.77 11 80 160 3.70 800 337.5 39 600 45 2.0 2.01 12 80 120 3.70 800 413.6 39 600 45 1.5 2.76 13 80 120 3.70 600 337.5 39 600 45 1.5 2.25 14 80 120 3.70 1200 337.5 39 600 45 1.5 2.25 carbonate stone from Shenyang City, North of China. 2.1.2. Specimens Steel tube consisted of four plates welded together with fillet welds. The weld was designed according to the Design Code for Steel Structures (GB50017-2002). The ends of the steel tube sections were cut and machined to the required length. The insides of the tubes were wirebrushed to remove any rust and loose debris present. The deposits of grease and oil, if any, were cleaned away. Each tube was firstly welded to a square steel bottom plate with 10 mm thickness. Two semi-circular holes with 20 mm in diameter, located at the junctions between the tube, the top plate and the bottom plate, were drilled in the section wall. They were act as vent holes for the water vapor pressure produced during the fire exposure. The specimens were placed upright to be air-dried until being heated. The concrete was filled in layers and was vibrated by a Φ50 poker vibrator. When the poured concrete reached 2/3 content of the tube, a thermocouple was inserted into the center of column to measure the temperature of the core concrete. During curing, a very slight amount of longitudinal shrinkage of 1.35 mm or so occurred at the top of the column. A high-strength epoxy was used to fill this longitudinal gap so that the concrete surface was flush with the steel tube at the top. The top end plate with the same dimension of the bottom plate was welded after the concrete was cast. 2.2. Fire Exposure Prior to loading test, the specimens were heated by exposed to heat in a furnace in Metalworking Laboratory, Northeastern University, China. Thermocouples were used to measure the temperatures of the hearth and the core concrete. During the test the free ends of the thermocouples were connected to the thermometer outside the furnace. The temperature of the hearth was controlled by the thermocouples furnished in the furnace. After the controller of the furnace was set at the desired temperature, the hearth began to be heated. During heating, the temperatures of the furnace and the core concrete were recorded at 5 minutes intervals. After the furnace reached the desired temperature, the controller automatically kept the temperature of the hearth unchanged. When the temperature of the core concrete reached that of the hearth, the heating stopped and the furnace was opened. The specimens were taken out of the furnace and cooled naturally to the ambient temperature. The ambient temperature at the start of the test was about 20 C. The desired heating temperature of each specimen was shown in Table 1. 2.3. Test Set-Up and Instrumentation The experiments were conducted with a 5,000 kn capacity universal testing machine. Columns under compression and bending were loaded eccentrically with a knife-edge at both ends of the columns. Six angles welded with the end-plate of the specimen [shown in Figure 2(a)] were used to prevent the knife hinge from moving to the endplates. An additional 900 600 12 mm steel plate was used to connect the knife hinge to the pressure machine. The plate was connected to the pressure machine with four Φ20 high strength bolts and was welded with six angles to prevent the knife hinge from moving to the additional plate. The photo of the knife hinge was shown in Figure 2(b). In order to study the failure modes and the behavior of the columns tested, a detailed instrumentation procedure was used. For each column, eight strain gauges were bonded to four faces of the specimen at the mid-height of the steel tube, and arranged at a 90 angle Advances in Structural Engineering Vol. 13 No. 4 2010 553

Experimental Study on Fire-Exposed Rectangular Concrete-Filled Steel Tubular (CFST) Columns Subjected to Bi-Axial Force and Bending (a) Angles to prevent knife hinge from moving (b) Photo of the knife hinge Figure 2. Connection details of a specimen with the pressure machine Pressure machine Knife hinge LDVT LDVT Strain gauge Strain gauge T T LDVT P y θ o x θ x y Pressure machine (a) Testing set-up (b) Location of the strain gauges and LDVT at the mid-height Figure 3. Diagram of testing set-up and instrumentation layout (a) Testing set-up (b) Layout of the strain gauges and LDVTs at the mid-height Figure 4. Photos of testing set-up and instrumentation layout 554 Advances in Structural Engineering Vol. 13 No. 4 2010

Shao-Fei Jiang, Zhao-Qi Wu and De-Sheng Niu in the longitudinal and transverse direction, as shown in Figure 3 and Figure 4. Besides the strain gauges, seven linear variable differential transducers (LVDTs) were installed to measure the deformation of the column. The longitudinal deformation of the column was measured by four vertically placed LVDTs. The other three horizontally placed LVDTs were used to measure the deflection about the major axis, minor axis and in the eccentrically direction of loading. 2.4. Loading Program The loads were applied in small increments. The strain and the deflection of specimens were recorded automatically with the data acquisition system, UCAM - 70A, after each load increment was finished. All specimens were loaded to be failure. In the elastic stage, the load increment was about 1/10 of the predicted ultimate capacity load. After the steel tube wall was yielded, the load increment was changed into 1/15 of the predicted ultimate capacity load. The duration of each load was 1 2 minutes. When the load force was near the carrying-capacity load of the specimen, the load was applied slowly. After reaching the ultimate capacity load, the load was reduced slowly until the specimen broke. 3. EXPERIMENTAL RESULTS 3.1. Test Observations and Failure Modes All of the columns were subjected to single curvature bending. No slipping at the knife-edge was observed until columns bowed after reaching the failure load. At the beginning of loading, the lateral and longitudinal deformations of the specimens were only a slight, and the steel tubes, as well as the core concrete were in the elastic stage. With increase of the load, the bending deformation of the specimens began to increase, but was still not obvious. Meanwhile, the rust on the surface of the hightemperature-exposed steel tubes broke off. When the load reached 60% 70% of the ultimate capacity load N u, the shear slippage lines (shown in Figure 5) began to appear on the wall of the steel tubes. When the load increased to 80% 90% of N u, the bending deformation of the specimen was very obvious, and the number of the shear slippage lines was still increasing. Meanwhile, the carbonized layer on the destroyed part (at the mid-height for most of specimens) of the column began to fall off and the crack came together. The steel tube walls in the compression zone at the mid-height of the column were considered as falling into the elasto-plastic stage according to the data of the strain gauges. The deformations on the horizontal and longitudinal direction of specimens were great, and they increased rapidly. The steel tube wall in tension was comparatively small and still in elastic stage. With increase of the load, the steel tube wall in compression came into the stain-strengthen stage. The wall in tension was still in elastic stage even when the ultimate load was applied. The welds did not break in the whole loading procedure. For all fourteen specimens, the failure position was concentrated at the mid-height of the column with an overall buckling failure mode. The steel tube walls in compression buckled at the mid-height where the moment reached its maximum value. The buckling steel tube wall were shown in Figure 6(a). In addition, it was found that the core concrete was crushed when the buckling steel tube wall was cut away after the tests finished. The crushed concrete (a) Buckling steel tube walls Shear slippage lines Figure 5. Shear slippage lines (b) Crushed core concrete inside of the buckling steel walls Figure 6. Typical failure modes of the specimens tested Advances in Structural Engineering Vol. 13 No. 4 2010 555

Experimental Study on Fire-Exposed Rectangular Concrete-Filled Steel Tubular (CFST) Columns Subjected to Bi-Axial Force and Bending 14 1 4 3 9 6 5 7 10 12 2 11 8 13 Figure 7. All specimens after tested was shown in Figure 6(b). Figure 7 shows all the specimens after tested. 3.2. Test Results 3.2.1. Carrying capacity loads The ultimate carrying capacity loads of all specimens tested were listed in Table 2. It is found that the rectangular CFST members after being exposed to high temperature still have relatively high carrying capacity. Where the specimens are exposed to the temperature lower than 400 C, and the reduction of the carrying capacity load is not evident. However, the carrying capacity loads of the specimens decrease obviously where the specimens are exposed to the temperature more than 600 C. Moreover, the eccentric angle, heightto-width ratio, confining coefficient and slenderness ratio affect the ultimate carrying capacity loads of the specimens. The details will be given in Section 4. 3.2.2. Ductility The ductility of specimens subjected to bi-axial force and bending is represented as µ = δ y / δ u, in which, µ is the ductility coefficient; δ y and δ u are the deformation of specimens at the state of yielding and failing. The deflection at the eccentrically direction of load was selected to be analyzed for the ductility and represented by δ m. When the carrying capacity load reduces to 85% of the ultimate carrying capacity, the specimen is considered as failed. The deformation of the specimens at the state of yielding and the corresponding ductility coefficients were defined by three methods as follows: (1) Method 1: Extended the elastic phase in the loading curve of force- deflection (N δm) to intersect it with the tangent line at the ultimate capacity load. The deformation at the intersecting point was considered as the yielding deformation (δ y) of the specimens. (2) Method 2: A loading force-strain relationship curve for the compression zone of specimens was obtained by using the data measured by the four vertically placed strain gauges at the mid-height of the steel tubes. The loading force at the yielding strain of compression zone was extracted from the loading curve of force-strain. The deformation of the specimens under the loading force extracted was obtained from N δ m relationship curve and was considered as the yielding deformation (δ y) of specimens. (3) Method 3: The loading force-axial deformation (N δaxi / L) relationship was also drawn according to the data measured by the vertically placed LVDT. The loading force at the yielding strain was extracted from the loading curve of force-strain (N δaxi / L). The deformation of the specimen under the extracted loading force was Table 2. Test results of all specimens Specimen Nu (kn) µ1 µ2 µ3 µ 556 1 2 3 4 5 6 7 8 9 10 11 12 13 14 680 9.0 11.4 7.3 9.2 660 10.8 11 8.5 10.1 620 14.0 5.6 7.0 8.9 590 15.0 14.2 12.2 13.8 380 10.4 9.9 14.2 11.5 582 8.2 15.1 9.7 11.0 606 17.0 11.7 10.3 13.0 573 17.8 9.1 6.5 11.1 530 11.8 14.4 6.0 10.7 371 8.1 10.5 9.3 730 8.4 8.5 9.7 8.9 652 11.5 15.1 6.6 11.1 620 14.3 16.0 12.0 14.1 500 14.2 8.6 9.7 10.8 Advances in Structural Engineering Vol. 13 No. 4 2010

Shao-Fei Jiang, Zhao-Qi Wu and De-Sheng Niu obtained from N δ m relationship curve and was considered as the yielding deformation (δ y ) of specimens. The ductility coefficients of the specimens predicted by the three aforementioned methods for the yielding deformation are summarized in Table 2. The subscripts 1, 2 and 3 are used to indicate method 1, method 2 and method 3 for the yielding deformation respectively; µ was the average ductility coefficient. It is found that in most cases, µ 1 and µ 2 are more than µ 3 ; all the ductility coefficients of the specimens predicted by the three aforementioned methods are larger than 5.0, the largest ductility coefficient specified by Code for Design of Steel- Concrete Composite Structures (DL/T 5085-1999), which indicates that rectangular CFST columns being exposed to high temperature still have good ductility. 4. DISCUSSION OF INFLUENCING FACTORS 4.1. Exposed Temperature Figure 8 shows a comparison between the loading forcedeflection (N δ m ) relationship curves of the specimens 2 5. The parameters of the specimens 2 5 are the same except for the exposed temperature. Specimen 2 was not exposed to high temperature, and the high temperatures which specimens 3, 4, as well as 5 were exposed to were 400 C, 600 C and 800 C, respectively. It is observed that the carrying capacity load of the column decreases with the increase of exposed high temperature. The carrying capacity load of Specimen 3, 4 and 5 was 7.46%, 11.94% and 43.28%, respectively, lower than that of Specimen 2. The reason for the reduction of carrying capacity load is that the core concrete was damnified due to the exposure of high temperature. The exposure to high temperature affects the deformation and ductility of the columns. The minimum displacement ductility coefficients of the specimens 2 5 were 8.5, 5.6, 12.2 and 9.9 respectively. All of them were more than the widely approbated ductility 3. The average ductility coefficients of specimens 2 5 were not less than 8.9. This indicates that the fire-exposed rectangular concrete-filled steel tubular columns still have good ductility. 4.2. Loaded Eccentric Angle Figure 9 shows the comparison between the loading force-deflection (N δ m ) relationship curves of Specimen 4 and specimens 6 9. The parameters of Specimen 4 and specimens 6 9 were the same except for the loaded eccentric angle. The loaded eccentric angles for the specimens 6, 7, 4, 8, 9 were 0, 30, 45, 60 and 90 respectively. It is observed that the carrying capacity loads of the specimens with varied loaded eccentric angles are different in Figure 9. The columns that have varied loaded eccentric angles bended about different axes of the cross section of the specimens. Accordingly, the section modulus of the cross section about the bending neutral axes and the carrying capacity load are different. The carrying capacity loads of the specimens 6, 7, 4, 8 and 9 are 582 kn, 606 kn, 590 kn, 573 kn and 530 kn, respectively. The minimum ductility coefficients of specimens 6, 7, 4, 8 and 9 were 8.2, 10.3, 12.2, 6.5 and 6.0, respectively. Namely, the ductility coefficients of the entire specimens exposed to 600 C were more than 6.0. This indicates the rectangular concrete-filled steel tubular columns after being exposed to high temperature still have good ductility. 4.3. Confining Coefficients 700 600 500 700 600 500 N (kn) 400 300 200 100 Specimen 2 (T = 20 C) Specimen 3 (T = 400 C) Specimen 4 (T = 600 C) Specimen 5 (T = 800 C) N (kn) 400 300 200 100 Specimen 6 ( θ = 0 ) Specimen 7 ( θ = 30 ) Specimen 4 ( θ = 45 ) Specimen 8 ( θ = 60 ) Specimen 9 ( θ = 90 ) 0 0 5 10 15 δ m (mm) 20 25 30 0 0 5 10 15 δ m (mm) 20 25 30 Figure 8. N δ m relationship curves of the specimens being exposed to different temperatures Figure 9. N δ m relationship curves of the specimens with different loaded eccentric angles Advances in Structural Engineering Vol. 13 No. 4 2010 557

Experimental Study on Fire-Exposed Rectangular Concrete-Filled Steel Tubular (CFST) Columns Subjected to Bi-Axial Force and Bending 700 800 600 500 600 N (kn) 400 300 200 100 Specimen 12 ( ξ = 2.76) Specimen 4 ( ξ = 2.25) N (kn) 400 200 Specimen 11 (H/B = 2.0) Specimen 4 (H/B = 1.5) Specimen 10 (H/B = 1.0) 0 0 5 10 15 δ m (mm) 20 25 30 0 0 5 10 15 δ m (mm) 20 25 Figure 10. N δ m relationship curves of the specimens with different confining coefficients Figure 11. N δ m relationship curves of the specimens with different H/B The experiment changed the confining coefficients of the specimens by using different steel materials. Specimen 4 and Specimen 12 used the steel Q235 and Q345, respectively. The confining coefficients ξ (= α f y /f ck ) of Specimen 4 and Specimen 12 were 2.25 and 2.76 respectively. Figure 10 shows the comparison between the N δ m of Specimen 4 and Specimen 12. It is obvious that the carrying capacity load of Specimen 12 is more than that of Specimen 4 due to the larger confining coefficient of Specimen 12. Although the specimens are exposed to 600 C, the steel tubes still confine the core concrete efficiently. On the contrary, the deformation-resistant capability is improved due to the larger confining effect, so the ductility coefficient of Specimen 12 is lower than that of Specimen 4. 4.4. Height-to-Width Ratio of Cross Section A comparison between the loading force-deflection (N δ m ) relationship curves of specimens 10, 4 and 11 is made as shown in Figure 11. The parameters of the three specimens were the same except for the height of cross section. Thereby, specimens 10, 4 and 11 had different height-to-width of cross section. It is found that the height-to-width of cross section affects the carrying capacity load of specimens remarkably (shown in Figure 11). The carrying capacity loads of the specimens 10, 4 and 11 were 371 kn, 590 kn and 730 kn respectively. One of the main reasons is that the cross sections of specimens become larger with the increase of height-to-width. The maximum ductility coefficients of specimens 10, 4 and 11 were 8.1, 12.2 and 8.4, respectively, all of which were not less than 8.0. 4.5. Slenderness Ratio N (kn) 700 600 500 400 300 200 100 Specimen 13 (λ = 17.32) Specimen 4 (λ = 23.09) Specimen 14 (λ = 34.64) 0 0 5 10 15 20 25 30 35 δ m (mm) Figure 12. N δ m relationship curves of the specimens with different slenderness ratios Figure 12 shows the N δ m relationship curves of the specimens 4, 13 and 14. The parameters of the three specimens were the same except for the length, i.e., the slenderness ratio λ. It is found from Figure 12 that the smaller the slenderness ratio λ, the more loads the specimen could carry, and the less the specimen deforms at the maximum carrying load. The reason is that the specimen with the larger λ produces higher second-order effects. The carrying capacity loads of specimens 13, 4 and 14 were 620 kn, 590 kn, 500 kn respectively. In addition, the minimum ductility coefficients of the three specimens were 12.0, 12.2 and 8.6, respectively. 5. SUMMARY AND CONCLUSIONS This paper provides new test data and changing 558 Advances in Structural Engineering Vol. 13 No. 4 2010

Shao-Fei Jiang, Zhao-Qi Wu and De-Sheng Niu regularity of behavior of rectangular concrete-filled steel tubular columns that had been exposed to constant high temperature and subsequently were subjected to bi-axial force and bending. The following conclusions can be drawn based on the experimental results of the study: (1) Rectangular concrete-filled steel tubular columns after being exposed to high temperature still have relatively high carrying capacity. When the specimens are exposed to the temperature less than 400 C, the reduction of their carrying capacity is no more than 7.5%. However, the carrying capacity load will reduce evidently when the specimens are exposed to the temperature more than 600 C. (2) Rectangular concrete-filled steel tubular columns after being exposed to high temperature still have good ductility. The ductility coefficients of all the specimens in the test are more than 5.0, the largest ductility coefficient specified by Code for Design of Steel-Concrete Composite Structures (DL/T 5085-1999). (3) The temperature to which the specimens are exposed, slenderness ratio, loaded eccentric angle, confining coefficient and height-to-width ratio are the main factors affecting residual behavior, i.e., carrying capacity and ductility, of fire-exposed rectangular CFST columns subjected to bi-axial eccentric compression. The higher temperature the specimens are exposed to, the less load the specimens can carry. ACKNOWLEDGMENTS The work described in this paper was supported in part by the Science Funds of National Construction Ministry (No.2000034 and No.2009-K2-40) and Fujian Province Excellent Youth Founds (No. 2009J06027), China. The writers would like to thank Prof. L.H. Han, Mr Q.H. Yu and Dr J.S. Huo. REFERENCES Chen, X.L. (2001). 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Experimental Study on Fire-Exposed Rectangular Concrete-Filled Steel Tubular (CFST) Columns Subjected to Bi-Axial Force and Bending Lie, T.T. and Denham and E.M.A. (1993). Factors Affecting the Fire Resistance of Circular Hollow Steel Beam-Columns Filled with Bar-Reinforced Concrete, NRCCNRC Internal Report, No. 651, Ottawa, Canada. Lie, T.T. and Stringer, D.C. (1994). Calculation of the fire resistance of steel hollow structural section beam-columns filled with plain concrete, Canada Journal of Civil Engineering, Vol. 21, No. 3, pp. 382 385. Lie, T.T. (1994). Fire resistance of circular steel beam-columns filled with bar-reinforced concrete, Journal of Structural Engineering, ASCE, Vol. 120, No. 5, pp. 1489 1509. Uy, B. (2001). Strength of short concrete filled high strength steel box columns, Journal of Constructional Steel Research, Vol. 57, No. 2, pp. 113 134. Wang, Y.C. (1991). The effects of structural continuity on the fire resistance of concrete filled columns in non-sway frames, Journal of Constructional Steel Research, Vol. 50, No. 2, pp. 177 197. Zhong, S.T. (1999). High-Rise Concrete-Filled Steel Tubular Structure, Hei Longjiang Science and Technology Press, Harbin, China. (in Chinese) 560 Advances in Structural Engineering Vol. 13 No. 4 2010