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Structural Performance of Cold-formed Lean Duplex Stainless
Steel Beams at Elevated Temperatures
Citation for published version:
Huang, Y & Young, B 2018, 'Structural Performance of Cold-formed Lean Duplex Stainless Steel Beams at
Elevated Temperatures', Thin-Walled Structures, vol. 129, pp. 20-27.
https://doi.org/10.1016/j.tws.2018.03.031
Digital Object Identifier (DOI):
10.1016/j.tws.2018.03.031
Link:
Link to publication record in Edinburgh Research Explorer
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Peer reviewed version
Published In:
Thin-Walled Structures
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Download date: 10. Aug. 2022
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Structural Performance of Cold-formed Lean Duplex Stainless
Steel Beams at Elevated Temperatures
Yuner Huang
1
* and Ben Young
2
1
Institute for Infrastructure and Environment, School of Engineering, University of Edinburgh, UK
2
Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China
ABSTRACT: The structural performance of cold-formed lean duplex stainless steel beams at
elevated temperatures ranging from 24 900 ºC was investigated in this study. A finite element
model was developed. The numerical analysis covered the specimens of square and rectangular
hollow sections. The material properties obtained from tensile coupon tests on lean duplex
stainless steel at elevated temperatures were used in the finite element model. A total of 125
numerical flexural strengths were obtained from the finite element analysis. The numerical
results were compared with the design values calculated by the existing design rules, including
the American Specification, Australian/New Zealand Standard, European Code, direct strength
method and continuous strength method. The suitability of these design rules for lean duplex
stainless steel beams at elevated temperatures was assessed using reliability analysis. It was
shown that the existing design rules are generally quite conservative in predicting the flexural
strengths at elevated temperatures, except that the modified direct strength method provides
accurate and reliable predictions. Therefore, it is recommended that the modified direct
strength method be used for cold-formed lean duplex stainless steel beams at elevated
temperatures.
Keywords: Beam; cold-formed; elevated temperatures; lean duplex; stainless steel; structural
design.
Corresponding author. Tel.: +44 (0) 131 650 5736; Fax: +44 (0) 131 650 6554.
E-mail address: Yuner.Huang@ed.ac.uk
2
1. 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
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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 proposed 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 24 900 ºC, 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.
2. 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 flexural members at elevated temperatures, except that
the materials properties at room temperature were replaced by the reduced material properties
obtained from tensile coupon tests at elevated temperatures [13]. The mechanical properties of
section 50×50×1.5 obtained from the tensile coupon tests at 24 ºC, 300 ºC, 500 ºC, 700 ºC and
900 ºC using the steady-state test method were used in the FEM. ABAQUS allows for a multi-
linear stress-strain curve to be used. Similar to the Huang and Young FEM [7], the first part of
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the curve represents the elastic part up to the proportional limit stress with the measured
ratio taken as 0.3. In the plastic analysis, the static stress-strain
curve obtained from tensile coupon tests was converted to true stress and logarithmic true
plastic strain curve, as described by Huang and Young [7]. The material properties adopted in
the FEM, including the modulus of elasticity, yield strength, and ultimate strength at high
temperatures ranging from 24 900 C, are summarized in Table 1. Similar to the FEM for
beams at room temperature, the local imperfection of t/10 was incorporated into the FEM,
where t is the thickness of the sections. The residual stresses in the sections were not included.
3. Parametric Study
A total of 125 cold-formed lean duplex stainless steel flexural members at elevated
temperatures, ranging from 24 900 ºC, 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 had one SHS of 300×300 (overall depth ×
overall width) as well as four RHS of 100×50, 50×100, 300×100 and 100×300. Five different
thicknesses were designed for each section, in order to cover a wide range of slenderness ratios,
from stocky to slender sections. The length of moment span between the two loading points
was equal to the length of shear spans between the loading points to the supports for the flexural
members. The lengths were designed carefully so that the section flexural capacity could be
reached without shear failure. The specimens with the same cross-sectional dimensions and
specimen lengths were investigated under five different temperatures in the finite element
analysis, including 24 ºC, 300 ºC, 500 ºC, 700 ºC and 900 ºC. The RHS specimens were
subjected to both major and minor axes bending. The specimens in the parametric study were
labelled such that the cross-section dimension, specimen length and the specimen temperature
could be identifid the cross-
sectional dimensions (D×B×t was the
specimen length in millimeters was the specimen
temperature in degrees Celsius. For example, the label 100×50×5L900T300 defined the
flexural member with cross-section (D×B×t) of 100×50×5 in millimeters, and the specimen
length of 900 mm as well as the specimen temperature of 300 ºC. The dimension of the overall
![Table 1: Material properties of the specimens at high temperatures [12]](/figures/table-1-material-properties-of-the-specimens-at-high-2x8tg5z5.webp)
