Experimental and numerical studies on axially restrained cold-formed steel built-up box columns at elevated temperatures
Introduction
In current practice, cold-formed steel (CFS) built-up box-shape columns are commonly fabricated with C– and U-shape sections, which are connected by fasteners, such as self-drilling screws or seam welding. The built-up columns are generally utilized in the framing of CFS mid-rise buildings to carry concentrated vertical loads. Compared with single C-shape wall studs, the built-up box section has the advantage of large lateral and torsional stiffness, which leads to high load-bearing capacity. Numerous studies [[1], [2], [3], [4], [5], [6], [7], [8]] have been conducted on the behavior of single C– and U-shape CFS columns at elevated temperatures. However, axial or rotational restraint was not included in these studies. Columns in a building subjected to fire may be axially and rotationally restrained by adjoining unheated members, such as slabs and beams. In the case of local fire, the axial restraints will introduce additional internal force, which could result in completely different fire behaviors of columns.
Thus far, numerous studies have been conducted to investigate the fire resistance of restrained steel columns with hot-rolled or welded shapes. Literature review [9] summarized some fundamental research on the fire behavior of restrained steel columns, including fire tests and numerical simulation. Li et al. [[10], [11], [12]] conducted a series of experimental, numerical, and analytical studies on restrained steel columns at elevated temperatures. They found that the load ratio and axial restraining stiffness had significant effects on the fire resistance of columns. Correia and Rodrigues [13] demonstrated that the critical temperature of the restrained steel column might not decrease despite the increase in stiffness of the adjacent structural member. Correia et al. [14] subsequently conducted parametric analyses on restrained steel columns at elevated temperatures and proposed a simple approach for fire safety design.
However, the aforementioned studies were based on hot-rolled steel columns, whereas those on restrained CFS columns at elevated temperatures were limited. Craveiro et al. [[15], [16]] experimentally investigated the fire response of restrained CFS built-up columns with closed and open sections, including I, R, and 2R type sections, as shown in Fig. 1. The test results indicated that the axial restraint and applied load were the key parameters governing the fire resistance of the CFS columns. The investigation also noted that adopting 350 °C as the limit temperature for class 4 cross-sections as stipulated in EN1993-1-2 [17] was conservative in the absence of the critical temperature evaluation.
In Craveiro's tests [[15], [16]], global-local buckling was observed on the slender columns, where global buckling was the dominant buckling mode because the steel plate was relatively thick (2.5 mm). However, for a moderately slender built-up column, where local buckling becomes the dominant buckling mode, the fire-resistance behavior of the column may be different. Concerning cross-section type, the quadruple-limb built-up box cross-section was not investigated in their tests; this type of cross-section was commonly used in Chinese multi-story CFS buildings and recommended by Chinese specification JGJ 227–2011 [18]. The researcher at Sheffield University [19] recently conducted a load-bearing capacity study on this type of built-up column. Moreover, numerical simulations associated with the fire-resistance behavior of restrained CFS built-up columns are limited.
Experimental and numerical investigations on the fire-resistance behavior of the axially restrained CFS quadruple-limb built-up box column, where local buckling was the dominant buckling mode, were conducted to address the absence of published research in this field. The axial compression tests of the specimens at ambient temperature were also conducted as a benchmark. Failure mode, critical temperature, failure time, and internal force induced by restrained thermal expansion were recorded in the fire test, and the effect of axial restraint on the fire response of CFS built-up box columns was investigated. The finite element model of CFS built-up box columns was developed to reproduce the fire response, and the model was verified by the test results. Parametric analyses were also further conducted to investigate the influence of axial restraint, rotational restraint, and load ratio on the fire-resistance behavior of the column.
Section snippets
Specimen details
All CFS built-up box column specimens were manufactured with structural steel Q345 with a nominal yield strength of 345 MPa. Q345 is the most commonly used structural steel in Chinese steel structures. The thickness of the non-galvanized steel sheet used to manufacture C– and U-shape sections was 1.5 mm.
Fig. 2(a) and (b) illustrate the dimensions of CFS C– and U-shape sections, respectively. The built-up box sections were fabricated with two C-shape sections and two U-shape sections fastened
Furnace temperature
Fig. 10 shows a typical furnace temperature evolution curve compared with the designed temperature–time curve. The figure reveals that the furnace bottom temperature could not reach the design temperature due to the hot air rising in the furnace. Moreover, the middle zone temperatures of the furnace (TF3, TF4, and TF5 in Fig. 5(a)) were higher than the lower and upper zone temperatures (TF1, TF2, and TF6 in Fig. 5(a)) due to heat loss at both ends of the furnace. Thus, the temperature
Finite element model (FEM)
The commercial software ABAQUS (version 6.14) was utilized to simulate the restrained CFS built-up columns at elevated temperatures. Thermal and structural analyses were also conducted.
Concerning the necessity of conducting thermal analysis, Craveiro [16] indicated that the maximum temperature difference on the cross-section could reach 50 °C due to the change in the total overlapping thickness of the steel plate. Therefore, the thermal analyses were required in the numerical simulation to
Results of the load-bearing tests
At an initial stage, the axial displacement and stiffness of the test were relatively small due to the potential gaps between the specimens with the two endplates, as shown in Fig. 2(d). Fig. 24 reveals the comparison of load-axial displacement curves between the simulation and test results at ambient temperature at the load range higher than 100 kN, where the influence of gaps on axial displacement was eliminated. The simulation result showed a good agreement with the test results, and the
General information and definition
An extensive parametric analysis based on the developed numerical model was conducted to investigate the fire-resistance behavior of such restrained specimens. Concerning the label of the simulation models, the letter “S” denotes “specimen,” followed by a number representing the load ratio. The second and last digits respectively signify the axial restraining stiffness ratio (α) and the rotational restraining stiffness ratio (β). For example, the specimens “S-0.3-α0.2-β0.1” denotes the loading
Conclusion
A series of fire-resistance tests and numerical simulation were conducted to assess the response of restrained CFS built-up columns in fire. A total of six axially restrained columns were tested at elevated temperatures, and the local-global interaction buckling was found to be the buckling mode of the column at elevated temperatures. The specimens were sensitive to the longitudinally non-uniform temperature due to the local buckling, and the highest temperature along the height was recommended
Author statement
Jingjie Yang:Carrying out test, Writing- Original draft preparation.
Yu Shi: Visualization, Investigation.
Weiyong Wang:Conceptualization, Methodology, Supervision, Wring-Editing.
Lei Xu:Writing- Reviewing and Editing.
Hisham AL-azzani: Reviewing and Editing.
Declaration of Competing Interest
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgement
The authors wish to acknowledge the support of the Natural Science Foundation of China (51878096), the Fundamental Research Funds for the Central Universities (Grant No.: 2019CDQYTM027) and Chongqing University to the fourth author to the status of visiting professor. Any opinions, findings, and conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the views of the sponsors.
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