This study aimed to investigate the static performance of notched hexagonal concrete-filled steel tube (CFST) stub columns through axial loading. Notch length, notch location, and notch direction in 14 CFST stub columns were experimentally studied. Stress process, failure mechanism, and ultimate strength in the notched CFST columns were analyzed. Results show that notches in steel tubes can weaken the restraining effect of steel pipes on core concrete and induce a decrease in the ultimate strength of specimens. The failure mode of components is greatly affected by notch orientation. The notch is closed under axial compression in the horizontally notched specimen, and the slotting indicates outward buckling in the vertically notched specimen. Based on the test results, a method for calculating the ultimate strength of notched hexagonal CFST columns was established. This research encourages the extensive application of these structures in civil engineering.
Concrete-filled steel tube columns are extensively used in engineering due to their simple joint structures, convenient connections, and excellent flexural behavior. CFST columns have been widely explored worldwide, and considerable progress has been made [
However, some CFST problems remain unsolved. Some damages weaken the mechanical properties of CFST columns, thereby affecting the structural integrity and reducing the working life of the columns. (1) As with other metal structures, initial geometric and material defects exist in the external steel pipes of CFST members, and CFST columns are inevitably affected by corrosion and other external loads upon use [
These problems have been addressed in recent years. (1) Regarding the geometric and material defects of CFST columns, Nia et al. [
In China, hexagonal CFST columns have been applied in the Gao Yin Finance Building in Tianjin and the CITIC Tower in Beijing [
Adhibition of hexagonal CFST columns in practical engineering projects. (a) Gao Yin finance building. (b) CITIC tower.
The present work focused on the static performance of notched hexagonal CFST columns based on our team’s previous research [
Twelve notched hexagonal CFST columns and two intact CFST columns were included in the experimental study. Notch length, notch location, and notch direction were considered and investigated. All specimens had the same section size, and the material performance of the steel and concrete cube was tested through standard method before the experiment. Table
Geometric properties and characteristics of CFST columns.
No. | Orientation | Location |
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1 | HCFT1 | Horizontal | Sidewall | 100 × 10 | 100 × 4 × 600 | 270 | 37.5 | 1468 | 1483 |
2 | HCFT2 | Horizontal | Sidewall | 60 × 10 | 100 × 4 × 600 | 270 | 37.5 | 1502 | 1529 |
3 | HCFT3 | Horizontal | Sidewall | 30 × 10 | 100 × 4 × 600 | 270 | 37.5 | 1527 | 1557 |
4 | HCFT4 | Horizontal | Corner | 100 × 10 | 100 × 4 × 600 | 270 | 37.5 | 1488 | 1456 |
5 | HCFT5 | Horizontal | Corner | 60 × 10 | 100 × 4 × 600 | 270 | 37.5 | 1514 | 1502 |
6 | HCFT6 | Horizontal | Corner | 30 × 10 | 100 × 4 × 600 | 270 | 37.5 | 1534 | 1538 |
7 | HCFT7 | Vertical | Sidewall | 100 × 10 | 100 × 4 × 600 | 270 | 37.5 | 1519 | 1530 |
8 | HCFT8 | Vertical | Sidewall | 60 × 10 | 100 × 4 × 600 | 270 | 37.5 | 1530 | 1558 |
9 | HCFT9 | Vertical | Sidewall | 30 × 10 | 100 × 4 × 600 | 270 | 37.5 | 1539 | 1574 |
10 | HCFT10 | Vertical | Corner | 100 × 10 | 100 × 4 × 600 | 270 | 37.5 | 1527 | 1468 |
11 | HCFT11 | Vertical | Corner | 60 × 10 | 100 × 4 × 600 | 270 | 37.5 | 1536 | 1516 |
12 | HCFT12 | Vertical | Corner | 30 × 10 | 100 × 4 × 600 | 270 | 37.5 | 1542 | 1569 |
13 | HCFT13 | Unimpaired | — | — | 100 × 4 × 600 | 270 | 37.5 | 1555 | 1579 |
14 | HCFT14 | Unimpaired | — | — | 100 × 4 × 600 | 270 | 37.5 | 1555 | 1580 |
Diagram of notched hexagonal CFST specimens. (a) Horizontal slotting. (b) Vertical slotting. (c) Dimension diagram of a notched specimen.
The test adopted a 5000 kN press. Figure
Sketch for testing the apparatus of CFST stub column. (a) Test setup. (b) Loading device.
Figure
Failure modes for hexagonal CFST columns. (a) Horizontal notch in the sideway. (b) Notch in the corner. (c) Vertical notch in the sideway. (d) Vertical notch in the corner.
The entire steel pipe was cut by the end of the experiment. Figure
Core-concrete damage of typical specimens. (a) Horizontal slotting in the sideway. (b) Horizontal slotting in the corner. (c) Vertical slotting in the sideway. (d) Vertical slotting in the corner.
Figure
Load-displacement curve of hexagonal CFST columns. (a) HCFT1∼HCFT3. (b) HCFT4∼HCFT6. (c) HCFT1∼HCFT3. (d) HCFT4∼HCFT6.
The hexagonal CFST columns are in the elastic phase when their strength is 0.7 of the ultimate strength, and the load-displacement curve is closely linear.
The axial displacement grows nonlinearly as the axial load achieves the yield load. The linearly increasing trend of axial displacement decreases with increasing load, showing a slow increasing trend. Buckling deformation does not appear in the steel pipe.
The strength of CFST columns decreases when the load exceeds the limit strength. Moreover, deformation intensifies in the slotting of the specimens with increased axial deformation. Bearing capacity increases to some extent after the axial deformation of some specimens reaches approximately 0.025 H.
Figure
Figure
Influence of notch length on the ultimate strength of specimens. (a) Horizontal notch (HCFT1∼HCFT6). (b) Vertical notch (HCFT7∼HCFT12).
In the horizontally notched specimens, the carrying capacities of the specimens with a notch length of 30 mm are 2.1% and 5.3% larger than those of specimens with notch lengths of 60 and 100 mm, respectively. In the vertically notched specimens, the carrying capacities of specimens with a notch length of 30 mm are 2.2% and 4.8% larger than those of specimens with notch lengths of 60 and 100 mm, respectively.
Figure
Influence of notch location on the ultimate strength of specimens. (a) Horizontal notch (HCFT1∼HCFT6). (b) Vertical notch (HCFT7∼HCFT12).
Figure
Influence of notch orientation on the ultimate strength of specimens. (a) Sideway notch (HCFT1∼HCFT6). (b) Corner notch (HCFT7∼HCFT12).
Ding et al. [
According to equation (
The slotting length, slotting orientation, and slotting location considerably affect the ultimate strength. Therefore, the ultimate strength of notched CFST columns can be expressed as
Table
Confinement factor for all the notched specimens.
No. |
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HCFT1 | 2000 | 25980 | 270 | 27.4 | 1468 | 215 | 0.147 |
HCFT2 | 2160 | 25980 | 270 | 27.4 | 1502 | 206 | 0.137 |
HCFT3 | 2280 | 25980 | 270 | 27.4 | 1527 | 199 | 0.130 |
HCFT4 | 2000 | 25980 | 270 | 27.4 | 1488 | 235 | 0.158 |
HCFT5 | 2160 | 25980 | 270 | 27.4 | 1514 | 218 | 0.144 |
HCFT6 | 2280 | 25980 | 270 | 27.4 | 1534 | 205 | 0.134 |
HCFT7 | 2360 | 25980 | 270 | 27.4 | 1519 | 169 | 0.111 |
HCFT8 | 2360 | 25980 | 270 | 27.4 | 1530 | 180 | 0.118 |
HCFT9 | 2360 | 25980 | 270 | 27.4 | 1539 | 188 | 0.122 |
HCFT10 | 2360 | 25980 | 270 | 27.4 | 1527 | 177 | 0.116 |
HCFT11 | 2360 | 25980 | 270 | 27.4 | 1536 | 186 | 0.121 |
HCFT12 | 2360 | 25980 | 270 | 27.4 | 1542 | 192 | 0.125 |
HCFT13 | 2400 | 25980 | 270 | 27.4 | 1555 | 194 | 0.125 |
HCFT14 | 2400 | 25980 | 270 | 27.4 | 1555 | 194 | 0.125 |
The notched steel pipe cannot exert sufficient restraint effect on the core concrete. This structure also has poor mechanical properties. Thus, estimating the restraint effect becomes essential. The ultimate strength of CSFT is the total of the contribution of the core concrete and steel pipe and the composite effect between the core concrete and steel pipe. It can be expressed as
For notched columns, only a part of the steel pipe can carry the compression. Thus, the ultimate strength for a notched specimen can be defined as follows:
For horizontal slotting, the effective area of the steel pipe (
For vertical slotting, the effective area of the steel pipe (
Hence, a confinement factor defined in equation (
Table
Fourteen CFST stub columns were included in the experiments, and notch length, notch location, and notch direction were considered. The effects of the structure-failure pattern were further investigated, and load-displacement curves were obtained. Finally, a method for calculating the ultimate strength of notched hexagonal CFST columns was established. The main conclusions are as follows. Compared with undamaged CFST columns under compression, the specimens showed failure modes that differ according to the slotting orientation. For horizontally notched specimens, the notch is closed under axial loading. For vertically notched ones, the slotting shows outward buckling phenomenon. The ultimate strength of notched CFST columns is less than that of intact CFST because the notched steel pipe cannot provide sufficient restraint on the core concrete. For notched hexagonal CFST column, the ultimate strength is less than those of undamaged ones. The experimental study illustrates that slotting length, slotting orientation, and slotting location considerably affect the ultimate strength of notched CFST columns. A formula proposed by Ding et al. [
All data used to support the findings of this study are included within the article. There are not any restrictions on data access.
The authors declare that they have no conflicts of interest.
This research work was financially supported by the Hunan Education Department Foundation Funded Project, Grant nos. 18B438 and 14C0217, the Project funded by China Postdoctoral Science Foundation, Grant no. 2018M632990, the Natural Science Foundation of Hunan Province, China, Grant nos. 2018JJ3021, 2018JJ2525, and 2019JJ20029, the National Key Research Program of China, Grant no. 2017YFC0703404, and the National Natural Science Foundation of China, Grant no. 51308548.