Full length articleStructural performance of cold-formed lean duplex stainless steel beams at elevated temperatures
Introduction
A relatively new type of cold-formed lean duplex stainless steel is becoming an attractive choice as a construction material. Lean duplex stainless steel is characterized by a low nickel content of around 1.5%. Thus, lean duplex stainless steel has economic advantages over the other types of stainless steel. In addition, it is regarded as a high strength material with the nominal yield strength (0.2% proof stress) of 450 MPa [1]. However, there has been limited research on the structural performance and design of lean duplex stainless steel members, especially at elevated temperatures. Therefore, research on the lean duplex stainless steel material and structural members is required.
Lean duplex stainless steel is a relative new construction material. The previous research on lean duplex stainless steel focused mainly on the material properties and design of structural members at room temperature. Huang and Young [2], as well as Theofanous and Gardner [3], conducted tensile coupon tests and stub column tests to investigate the mechanical and section properties of cold-formed lean duplex stainless steel rectangular and square hollow sections. Experimental and numerical investigations were carried out on cold-formed lean duplex stainless steel columns [3], [4], [5], [6], and the test and numerical data were compared with the predicted column strengths calculated by the existing design rules. It was shown that the existing design rules, including design rules in the European Code, explicit approach in the Australian/New Zealand Standard and the direct strength method, are quite conservative for the lean duplex stainless steel. The implicit approach for column design in the American Specification and Australian/New Zealand Standard provides accurate predictions, but the iterative calculation procedure is tedious. Therefore, modified design rules have been proposed for better prediction of lean duplex stainless steel structural strengths. Some research has also been conducted for cold-formed lean duplex stainless steel beams [7], [8], [9], [10]. This research indicated that the existing European Code and direct strength method are quite conservative for lean duplex stainless steel flexural members, while the continuous strength method provides a better prediction. The European Code and direct strength method were found to be suitable for the shear design of lean duplex stainless steel rectangular hollow beams. The existing design rules in the European Code and the Australian/New Zealand Standard are generally quite conservative for lean duplex stainless steel beam-column members [11], [12]. The mechanical properties of cold-formed lean duplex stainless steel at elevated temperatures have been investigated in previous research [13], [14]. Huang and Young [13] conducted tensile coupon tests on lean duplex stainless steel in both steady and transient states. The existing design rules for predicting the reduced material properties at elevated temperatures were assessed for lean duplex stainless steel. A modified design rule was proposed for lean duplex stainless steel material properties at elevated temperatures. Gardner et al. [14] summarized the results of tests on material properties of various stainless steel alloys at elevated temperatures, including the lean duplex stainless steel material reported by Outokumpu [15]. Reduction factors of strength and stiffness for lean duplex stainless steel were obtained according to the available data.
A search of the literature revealed a lack of research on cold-formed lean duplex stainless steel beams at elevated temperatures. Therefore, the objective of this study was to investigate the structural performance of cold-formed lean duplex stainless steel beams at elevated temperatures, ranging from 24 to 900 °C, using finite element analysis. The reduced mechanical properties at elevated temperatures were used in the FEM. A total number of 125 numerical flexural strengths were compared with the design values calculated from the existing design rules. The applicability of the existing design rules for the lean duplex stainless steel beams was assessed using reliability analysis. According to the comparison, recommendations for designing cold-formed lean duplex stainless steel flexural members at elevated temperatures are proposed based on this study.
Section snippets
Finite element model
The finite element model (FEM) for cold-formed lean duplex stainless steel flexural members was developed by Huang and Young [7] using the program ABAQUS version 6.11 [16]. The FEM has been verified with the test results of four-point bending tests at room temperature. The moment-curvature curves and the failure modes predicted by the FEM have been found to agree well with the test results. In this study, the FEM developed by Huang and Young [7] was used for the finite element analysis of
Parametric study
A total of 125 cold-formed lean duplex stainless steel flexural members at elevated temperatures, ranging from 24 to 900 °C, were investigated in the parametric study. The finite element model (FEM) in the parametric study was identical to the FEM developed by Huang and Young [7], except that the mechanical properties obtained from the tensile coupon tests at elevated temperatures were used. The parametric study included square hollow sections (SHS) and rectangular hollow sections (RHS), which
Design rules & comparison with beam strengths
The existing and modified design rules for cold-formed lean duplex stainless steel flexural members at elevated temperatures were assessed by comparing the design values with the 125 FEA flexural strengths (MFEA,T), as summarized in Table 3. The unfactored design strengths (nominal strength) were calculated using (1) American Specification (ASCE) [17] (Myielding,T, Minelastic,T), (2) Australian/New Zealand Standard (AS/NZS) [18] (Myielding,T, Minelastic,T), (3) modified ASCE and AS/NZS
Conclusions
The study reported here investigated the structural performance of lean duplex stainless steel flexural members at elevated temperatures. The flexural strengths obtained from the finite element analysis were compared with the design strengths calculated by the existing design rules. The design rules in ASCE [17] and AS/NZS [18] were found to provide quite conservative predictions for the cold-formed lean duplex stainless steel flexural members at elevated temperatures. Modifications to the
Acknowledgements
The research work described in this paper was supported by a grant from The University of Hong Kong under the seed funding program for basic research, and the Start-up Fund for Dr. Yuner Huang, sponsored by University of Edinburgh.
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