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The structural responses of cold-formed high strength steel (HSS) square and rectangular hollow section (SHS and RHS) beams at elevated temperatures were examined in this study. Stress-strain relationships of cold-formed HSS at elevated temperatures were proposed and verified against material test results. The proposed stress-strain relationships were then employed in a finite element (FE) analysis to study the behaviour of cold-formed HSS SHS/RHS beams at elevated temperatures up to 1000 °C. The developed FE model was verified with available test results of cold-formed HSS SHS/RHS beams; upon verification, a total of 252 numerical flexural capacities were gained from FE analyses. The numerical results were used to investigate the suitability of existing cross-section slenderness limits to the HSS tubular sections at elevated temperatures. The applicability of current flexural design provisions in the Eurocode 3, AISC and AISI specifications to the investigated HSS tubular beams at elevated temperatures was also examined. Overall, it is shown that the codified provisions can provide quite conservative predictions; an improved design rule is proposed by modifying the direct strength method (DSM) in the AISI specification. Furthermore, reliability analyses were carried out to assess reliability levels of codified and modified provisions. It has been demonstrated that the modified DSM can produce accurate and reliable design and therefore is recommended to be used for cold-formed HSS SHS/RHS beams at elevated temperatures.

The structural response of cold-formed high strength steel (CFHSS) rectangular hollow sections (RHS) under localised bearing forces is investigated. Web crippling tests were undertaken on cold-formed steel RHS that had nominal yield strengths (0.2% proof stresses) of 700 and 900 MPa. The experiments were conducted under end and interior load cases. The experimental results were utilised in a numerical investigation to verify finite element (FE) models. Upon verification, an extensive parametric study was conducted. The ultimate capacities obtained experimentally and numerically were compared with nominal resistances computed by the current North American Specification, European Code and Australian Standard. Overall, the codified resistance provisions gave either unconservative or overly conservative predictions. Therefore, improved design rules for CFHSS RHS under localised bearing forces were proposed. The reliability of the codified as well as modified design provisions were appraised. It is demonstrated that the modified design rules proposed in this paper can provide reliable predictions for the CFHSS RHS under localised bearing forces.

The flexural buckling behaviour and residual strengths of stainless steel circular hollow section (CHS) columns after exposure to fire were studied, based on a thorough experimental and numerical modelling programme, and reported in this paper. The experimental programme was performed on three series of specimens, and each series contained five geometrically identical specimens, with one unheated and the other four heated to different levels of elevated temperatures (namely 300 °C, 600 °C, 800 °C and 1000 °C). The detailed heating, soaking and cooling processes, material testing and pin-ended column tests were described, with the derived key experimental results fully presented. The testing programme was supplemented by a numerical modelling programme, including a validation study where finite element models were developed and validated against the test results, and a parametric study where the validated finite element models were employed to derive further numerical results over an extended range of cross-section dimensions and member lengths. Due to the absence of existing design codes for stainless steel structures after exposure to fire, the codified design provisions for stainless steel CHS columns at ambient temperature, as established in the Europe, America and Australia/New Zealand, were assessed for their applicability to stainless steel CHS columns after exposure to fire, based on the obtained test and numerical data. The assessment results generally revealed that the design buckling curve, as adopted in the European code, and the tangent modulus method, as employed in the American specification, lead to unsafe and scattered design flexural buckling strengths for stainless steel CHS columns after exposure to fire, while the explicit approach, as used in the Australian/New Zealand standard, yields a high level of accuracy and consistency in predicting the post-fire flexural buckling strengths of stainless steel CHS columns.

The behaviour and resistances of cold-formed stainless steel rectangular hollow section (RHS) members undergoing combined bending and web crippling were studied based upon experimental and numerical investigations. A test program consists of 4 pure bending tests, 7 pure web crippling tests and 23 web crippling-bending interaction tests was conducted. The RHS specimens were cold-rolled from lean-duplex and ferritic stainless steel sheets. Numerical models were built and validated against the test results. Upon validation, parametric studies comprised of 312 finite element analyses were undertaken. The obtained test and numerical results were compared with nominal resistances predicted from the American, Australian/New Zealand and European design standards for stainless steel structures. Moreover, the provisions in the North American Specification for cold-formed steel members were also evaluated. The comparison results indicate that the codified design provisions are generally safe to use for design of cold-formed stainless steel RHS members undergoing combined bending and web crippling, among which the European provision yields overly-conservative predictions. The codified web crippling-bending interaction curves can be applied for designing the stainless steel RHS members undergoing combined bending and web crippling, whilst improved predictions could be achieved by employing recently proposed bending and web crippling design rules.

Design of cold-formed stainless steel tubular columns undergoing local-overall flexural interactive buckling is investigated in this paper. A numerical study has been carried out to study the interactive buckling behavior of square hollow section (SHS) and rectangular hollow section (RHS) columns of various stainless steel grades that are employed in structural applications. A finite element model was developed and verified against available experimental cold-formed stainless steel SHS/RHS column data. Extensive parametric studies, with cross-section aspect ratio, plate slenderness, overall slenderness and stainless steel grade selected as key variable parameters, were carried out thereafter to generate further numerical data featuring the tubular columns undergoing local-flexural interactive buckling. The obtained interactive buckling strengths were used to assess the design provisions in current European, American, Australian/New Zealand and Chinese design standards for stainless steel structures as well as available Direct Strength Method (DSM) design rules in the literature. It is shown that better predictions can be achieved when material nonlinearities of different material grades are accounted for. A modified DSM, which relies on simple hand calculations, is developed in this paper. It is demonstrated that the modified DSM is a suitable and efficient design alternative for cold-formed stainless steel SHS/RHS columns subjected to local-flexural interactive buckling.

This paper presents experimental and numerical investigations on cold-formed ferritic stainless steel (CFFSS) square and rectangular hollow sections undergoing web crippling. A total of 44 web crippling tests was carried out under the four codified web crippling load cases as per the American cold-formed stainless steel specification. Numerical model of each load case was built and verified with the web crippling test results. After the verification, a parametric study comprised of 154 finite element analyses was undertaken thereafter to gain further insight into the behaviour of the CFFSS sections undergoing web crippling. The current design provisions in the American, Australian/New Zealand and European stainless steel codes of practice were assessed. Moreover, the North American Specification (NAS) provisions for carbon and low-alloy steel sections were evaluated. Furthermore, design recommendations in the literature for stainless steel sections were examined. Improved design rules are proposed in this study for CFFSS square and rectangular hollow sections by modifying NAS and Direct Strength Method. Reliability analysis was also undertaken to assess the reliability levels of the existing and modified provisions.

The research work described in this paper was supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 17209614).

This paper presents an experimental investigation on residual mechanical properties of high strength steels (HSS) after exposure to fire. A test program was conducted to study the post-fire mechanical properties of cold-formed HSS. A total of 41 tensile coupon specimens was extracted from cold-formed tubular sections with nominal yield stresses of 700 and 900 MPa. The specimens were exposed to various elevated temperatures ranged from 200 to 1000 °C and then cooled down to ambient temperature before testing. Stress-strain curves were obtained and the mechanical properties of Young's modulus, yield stress (0.2% proof stress), ultimate strength, ultimate strain and fracture strain of the cold-formed high strength steel materials after exposure to elevated temperatures were derived. The post-fire retention factors that obtained from the tests were compared with existing predictive equations in the literature. The influence of heating rate on post-fire mechanical properties was also investigated. New predictive curves for the determination of residual mechanical properties of HSS after exposure to fire are proposed. It is demonstrated that the proposed predictive curves are suitable for both cold-formed and hot-rolled HSS with nominal yield stresses ranged from 690 to 960 MPa.

The behaviour and resistances of concrete-filled ferritic stainless steel tubular (CFFSST) joints were studied and presented in this paper based upon experimental and numerical investigations. A test program was firstly undertaken on a series of X- and T-joints with square and rectangular hollow section (SHS and RHS) chords. The stainless steel SHS/RHS tubes were cold-rolled from EN 1.4003 ferritic grade sheets, and the specimens were in-filled with either normal or high strength concrete. The experimental study including the detailed material properties, joint test setups, test procedures and results is reported. Numerical models were also developed and verified against the CFFSST joint test results, and followed by a parametric study generating further numerical results over a wider range of key joint parameters. The obtained experimental and numerical CFFSST joint capacities were compared with existing design recommendations as per the CIDECT Design Guide and design rules proposed by previous researchers. The comparison results indicate that the existing provisions are not capable of predicting the capacities of the investigated composite tubular joints in an accurate and reliable manner. Hence, modified design rules are put forward in this paper for the design of CFFSST joints.

The behaviour and design of cold-formed ferritic stainless steel tubular structural members subjected to concentrated bearing loads are presented in this paper. A total of 37 web crippling tests was conducted on cold-formed square and rectangular hollow sections of grade EN 1.4003 ferritic stainless steel. The tests were conducted under end loading and interior loading conditions, which closely simulated the support conditions of floor joist members seated on solid foundation. Finite element (FE) models were developed and validated against the experimental results. Upon validation of the FE models, a parametric study comprised 160 FE analyses was performed using the validated models. The web crippling strengths obtained from experimental and numerical investigations were compared with the nominal strengths calculated using the current American, Australian/New Zealand and European specifications for stainless steel structures. Furthermore, the North American Specification (NAS) and the Australian Standard AS4100 for carbon steel structures as well as the suggested design rules in the literature for stainless steel structures were also compared. Improved design rules are proposed for ferritic stainless steel tubular structural members subjected to concentrated bearing loads by means of modified NAS and Direct Strength Method. The reliability of the proposed design rules has been assessed through reliability analysis.

This paper presents an experimental investigation of cold-formed high strength steel tubular sections undergoing web crippling. The tests were conducted on square and rectangular hollow sections of high strength steel with nominal 0.2% proof stresses of 700 and 900 MPa. The measured web slenderness values of the tubular sections ranged from 8.3 to 35.8. Tensile and compression coupon tests were conducted to obtain the material properties of the test specimens. The web crippling tests were conducted under the four loading conditions as specified in the North American Specification and Australian/New Zealand Standard for cold-formed steel structures, namely, the End-One-Flange, Interior-One-Flange, End-Two-Flange and Interior-Two-Flange loading conditions. It should be noted that the web crippling design provisions in these two specifications were mainly developed based on sections with web slenderness value greater than 40 and 0.2% proof stress less than 500 MPa. The test strengths obtained from this study were compared with the nominal strengths calculated from the North American Specification, Australian/New Zealand Standard and European Code for cold-formed steel structures. Furthermore, the test strengths were also compared with the nominal strengths calculated from the Australian Standard AS4100. Reliability analysis was performed to assess the reliability of the design provisions in the aforementioned specifications. Generally, it is shown that the nominal strengths predicted by the codified web crippling design provisions are either unconservative or very conservative. Hence, the existing codified design provisions are not appropriate for the cold-formed high strength steel square and rectangular hollow sections undergoing web crippling.

This paper presents the material properties of cold-formed high strength steel at elevated temperatures. Material properties at elevated temperatures have a crucial role in fire resistance design of steel structures. The fire resistances of steel structures in the existing international standards are mainly based on experimental data of hot-rolled mild steel. However, investigation of high strength steel at elevated temperatures is limited. Therefore, a test program has been carried out to investigate the material properties of cold-formed high strength steel at elevated temperatures. The coupon specimens were extracted from cold-formed high strength steel square and rectangular hollow sections with nominal yield stresses of 700 and 900 MPa at ambient temperature. The coupon tests were carried out through both steady and transient state test methods for temperatures up to 1000 °C. Material properties including thermal elongation, elastic modulus, yield stress, ultimate strength, ultimate strain and fracture strain were obtained from the tests. The test results were compared with the design values in the European, American, Australian and British standards. The comparison results revealed the necessity of proposing specified design rules for material properties of cold-formed high strength steel at elevated temperatures. New design curves to determine the deterioration of material properties of cold-formed high strength steel at elevated temperatures are proposed. It is shown that the proposed design curves are suitable for high strength steel materials with nominal yield stresses ranged from 690 to 960 MPa at ambient temperature.

This paper presents experimental and numerical investigations of cold-formed high-strength steel (CFHSS) tubular structural members under combined bending and bearing. A test program that c...

Orthotropic Steel Decks (OSDs) are widely used in various steel bridge types and however, their fatigue performances remain a prominent issue. In this paper, experimental work was conducted to investigate the hot spot stresses at various details of rib-to-deck-to-floor beam (RDF) welded connection in OSDs reinforced by ultra-high-performance concrete (UHPC). Quadratic extrapolation approach was applied to determine the hot spot stresses at weld toes of different fatigue prone details according to IIW specifications. Parametric studies were performed using finite element analysis to examine the effect of structural key parameters on the hot spot stresses including UHPC layer thickness and UHPC elastic modulus. Experimental results indicated that longitudinal flexural increased the out-of-plane distortions of the U-rib walls, therefore the hot spot stresses at locations on the U-rib were remarkably higher than that around floor beam cut-out. It is observed that hot spot stresses at region A on the U-rib wall directly under the acting load were controlled by tensile stresses, while region B on the other side of the acting load had compression stresses. The application of 60 mm UHPC layer significantly reduced the hot spot stresses at critical location on the U-rib and around floor beam cut-out by 57.8% and 36.5%, respectively. Parametric study results demonstrated that increasing UHPC layer thickness significantly reduced the hot spot stresses at all considered locations, while increasing the elastic modulus of UHPC layer slightly influenced the hot spot stresses and its effect can be neglected.

Orthotropic steel decks (OSDs) are widely used in long span steel bridges owing to their outstanding structural characteristics and lightweight; however, the fatigue resistance of OSDs is still a prominent problem of such deck structures. Ultra-High-Performance Concrete (UHPC) has been recently employed in orthotropic steel decks to enhance their structural behavior and extend their fatigue life. In this paper, experimental work has been carried out to investigate the effect of employing UHPC overlay on the hot spot stresses at fatigue prone details of OSDs. Three full scale rib-to-deck-to-floor beam (RDF) welded connections were fabricated and tested. One centric wheel load was applied; so that, the effect of in-plane and out-of-plane distortions of floor beam web can be considered. In parallel with the experimental program, a finite element (FE) analysis was also performed. Upon validation of FE models against experimental results, a parametric study was carried out to study the effect of key structural parameters that primarily influence the hot spot stresses. Experimental results indicated that, the stresses around floor beam cut-out were slightly higher than the hot spot stresses at considered locations on U-rib due to in-plane and out-of-plane distortions of the floor beam web under centric loading. The application of 60 mm UHPC layer effectively reduced the hot spot stresses by 26–83% at the considered locations of the test specimens. Results of parametric study indicated that, increasing UHPC layer thickness significantly enhanced the stiffness of the bridge deck and therefore effectively reduced the hot spot stresses at considered spots. On the other hand, the effect of increasing UHPC elastic modulus slightly influenced the hot spot stresses at considered locations and its effect is deemed neglectable.

This paper presents an experimental investigation of concrete-filled high-strength steel tubular X joints. The high-strength steel tubes were cold formed into square and rectangular hollow ...