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Due to the rapid development in steel manufacture, a new generation of structural cold‐formed advanced high strength steels have been developed. However, due to the variations in chemical composition, their material properties have not been fully understood, which includes the mechanical performance when exposed to fire. Therefore, a series of steady‐state elevated temperature tests were conducted on dual phase (DP) and high‐strength low alloy (HSLA) steels with different thicknesses and nominal yield strengths up to 700 MPa to investigate their performance in the range of 20°C to 700 °C. This paper presents detailed results and discussion of the experimental study. A two‐stage plus linear model based on Ramberg‐Osgood equation is proposed to depict the stress‐strain relationship for the materials at elevated temperature. The prediction of the new material model and common existing material models to represent the material properties of steel at elevated temperatures are compared to the test data and current code predictions. [ABSTRACT FROM AUTHOR]

The objective of this paper is to propose relationships for the reduction in mechanical properties of cold-formed steels at elevated temperature. Predicting the degradation of strength and modulus of cold-formed steels with temperature is critical to enable fire design of cold-formed steel members. Here, the properties of elastic modulus, 0.2% proof stress, 2% stress, and ultimate stress are studied for grades up to and including 550 MPa. Data are collected from the literature as well as from recent tests conducted by the authors at Johns Hopkins University. Steady-state and transient test results in the range of 20°C–1,000°C are analyzed. Retention factors are then proposed for the mechanical properties adopting a standardized format developed through committee work with the American Iron and Steel Institute. [ABSTRACT FROM AUTHOR]

As cold-formed steel profiles are increasingly used for load-bearing structural systems, their fire design requires specific attention due to their thin-walled nature and high slenderness. In some specific applications such as in storage hall floor systems, these structures are used without sheathing or thermal protection on the fire-exposed side. Yet, there is a lack of data from fire resistance tests on full-scale load-bearing thin-walled steel structures, especially with directly exposed steel. This article describes two standard fire resistance tests on full-scale light gauge steel frame floors made of cold-formed steel lipped channel girders and joists topped by chipboard panels. The experimental program was designed to investigate the fire resistance of the unprotected girders. A specificity of this program was that the girders were subjected to a low load level to probe the ability to achieve without passive fire protection a 30-min fire resistance rating typical for storage hall structures in the Czech Republic. The absence of protection resulted in differences in thermal gradients and bracing compared to common fire tests, as well as a very low degree of utilization leading to an expected failure temperature higher than 800 °C. The results showed that the two floors remained stable during the 30 min with limited deflections, but failed the deflection rate criteria after 24 and 22 min, respectively. Comparison is provided with the calculation methods from the Eurocodes. The presented results provide new data on the response of full-scale, unprotected cold-formed steel floor systems subjected to fire, which can be used to calibrate numerical models and fire design methods outside of the range currently covered by the codes. • Fire design of thin-walled steel structures requires specific attention. • Lack of data from fire resistance tests on full-scale thin-walled steel structures. • Some thin-walled structures are designed without the use of passive fire protection. • Calculation methods show good agreement with test data in temperature predictions. • The test results can be used to calibrate numerical models and fire design methods. [ABSTRACT FROM AUTHOR]

A numerical investigation is conducted on the local buckling behavior of cold-formed square hollow section (SHS) and rectangular hollow section (RHS) columns made of normal and advanced high-strength steels at elevated temperatures. Shell finite element models validated against experimental results are used in a parametric numerical study to evaluate the local buckling strength at ambient and elevated temperatures of SHS and RHS columns fabricated with G450, dual-phase, and martensitic steels. The modeling uses recent experimental data on the elevated temperature behavior of advanced high-strength steels. The numerical results are compared with the design strengths calculated by the Direct Strength Method in AISI S100. Results show that the current method in AISI S100 consistently captures the trend but overestimates the ultimate capacity of G450 cold-formed columns both at ambient and elevated temperatures by about 10% for the SHS columns and 6% for the RHS columns. The current Direct Strength Method also yields higher estimates of capacity than the finite element models for advanced high-strength steel grades. Therefore, a modification to the method for local buckling capacity is proposed for these sections. The modified Direct Strength Method proposed herein can be used to evaluate the local buckling strength of cold-formed SHS and RHS columns made of normal and advanced high-strength steels at temperatures up to 700 °C. [ABSTRACT FROM AUTHOR]

• Fracture experiments were performed on dual-phase steel at temperatures from 20 to 700 °C. • Five specimen shapes were used to generate stress triaxialities from 0 to 0.51. • Finite element analyses with a plasticity-damage model were calibrated on the experiments. • The dependency of the damage and fracture initiation strains with stress state and temperature was identified. • The calibrated material model can be used to simulate fracture of steel members in fire. Structural steels can be subjected to a range of stress and temperature conditions, and accurate characterization of their limit states is a requirement for structural design. Yet data on ductile fracture at elevated temperatures remains relatively scarce especially for high-strength steel grades. This paper describes an experimental and numerical investigation of the ductile fracture of a high-strength dual-phase steel under multiaxial stress states at temperatures up to 700 °C. Experiments were carried out on five geometries of specimens and at four temperatures to identify the temperature-dependent fracture locus across a range of stress triaxialities. Finite element analyses of the experiments provided the equivalent strain at fracture initiation. It was found that the fracture initiation strain varies markedly with stress triaxiality and increases with temperature. The effect of strain rate become significant at 500 °C. A temperature-dependent plasticity-damage model was calibrated to predict ductile fracture initiation in the dual-phase steel. The proposed model can be used in finite element analyses to support performance-based design of steel structures at elevated temperatures resulting from accidental events such as fire. [ABSTRACT FROM AUTHOR]

Advanced high-strength cold-formed steels (AHSS) have been developed with yield strengths up to 1250 MPa and ultimate strengths up to 1900 MPa. Accurate stress-strain modeling of these new steel alloys are required for eventual application of AHSS into the building construction industry, including the material behavior of AHSS subjected to elevated temperatures. A series of new constitutive material models are proposed based on the experimental AHSS stress-strain database at elevated temperatures (i.e., steady-state and transient-state test methods) and post-fire scenario (i.e., residual test method). The stress-strain curves from experiments on two families of AHSS, including dual phase steel with nominal yield strengths of 340 MPa and 700 MPa, and martensitic steel with nominal yield strengths of 1030 MPa and 1200 MPa, reported in a previous study are considered. Existing stress-strain models are investigated and fit to the AHSS database, in addition to the development of new material models. The required input parameters of the proposed stress-strain models are determined, and their corresponding predictive expressions are proposed. It is shown the stress-strain behaviors of AHSS could be accurately described by the proposed models. The data generated by this research addresses fire safety design and will be essential in supporting the adoption of these next-generation steels in future infrastructure. [Display omitted] • Advanced high-strength steels (AHSS) exhibit different stress-strain behaviors under different elevated temperature cases. • The complex AHSS stress-strain behaviors cannot be described by any single existing model, thus new models are developed. • The required parameters of the proposed stress-strain models are given by predictive equations in a unified form. • Detailed modeling examples for AHSS stress-strain behaviors under different elevated temperature scenarios are given. [ABSTRACT FROM AUTHOR]

The objective of this paper is to provide experimental results related to the elevated temperature performance of connections between cold-formed steel members and sheathing. Cold-formed steel building structures rely on sheathing for their mechanical benefits including bracing against member twist, global flexural and flexural-torsional buckling, and cross-section distortional buckling, as well as to supply lateral strength and energy dissipation in shear walls and diaphragms. Sheathing is also relied upon for non-structural benefits, including: fire, acoustic, and thermal performance. Predicting the degradation of the connection performance between cold-formed steel members and sheathing at elevated temperature is critical for any attempt to predict the structural performance of cold-formed steel buildings under fire demands. Steady-state connection tests were conducted under in-plane shear and pull-through at temperatures up to 400 °C for cold-formed steel members attached to gypsum board and oriented strand board. By combining the conducted tests with others in the literature retention factors for initial stiffness and ultimate strength of the connections are proposed. • Sheathing bracing is critical to the performance of cold-formed steel studs employed in buildings. • Performance of OSB and gypsum sheathing connections degrade precipitously with elevated temperature causing a loss of stud bracing. • Fire performance-based design of CFS structures is enabled through retention factors for cold-formed steel-to-sheathing connections. [ABSTRACT FROM AUTHOR]

Strap-braced, cold-formed steel framed walls are frequently used as the lateral force resisting system in cold-formed steel construction. While the behavior of these walls has been studied under lateral loading and (to a lesser extent) under fire conditions, there is a need to comprehend the influence of multi-hazard interactions, in particular the effect of fire pre-damage on the lateral load resistance of the walls. In this paper, a numerical model of a strap-braced cold-formed steel wall is developed to analyze the thermal and structural response when subjected sequentially to fire followed by shear deformation. The numerical model is validated against full-scale experiments. Coupon tensile tests are conducted to characterize the post-fire properties of the cold-formed steel that are used as inputs to the model. The results show that the numerical model can capture the post-fire response of the cold-formed steel walls including the wall strength, stiffness and ductile failure by yielding of the strap. The lateral behavior of the walls depends primarily on the maximum temperature reached in the cold-formed steel members and the resulting residual properties. Thermal analysis by the finite element method can be used to predict the maximum temperatures across a wall section under a variety of design-relevant fire scenarios, but the results are strongly affected by the quality of the data on thermal properties and by the loss of integrity of the gypsum sheathing. This study validates the numerical modeling strategy and suggests that the post-fire lateral capacity of the walls can be predicted from ambient temperature methods with use of the cold-formed steel residual mechanical properties. • Propose a numerical strategy to assess the post-fire lateral capacity of CFS strap-braced walls. • Conduct new material tests to obtain the post-fire retention factors for CFS. • Simulate four experimental tests to validate the proposed numerical strategy. • Capable to model the failure mode, stiffness and ultimate strength of fire-damaged CFS walls. [ABSTRACT FROM AUTHOR]

An experimental investigation was conducted on the post-fire mechanical properties of advanced high-strength cold-formed steel alloys. Tensile specimens made of two dual-phase steels and two martensitic steels with nominal yield stress from 340 MPa to 1200 MPa were tested after exposure to temperatures up to 700 °C. The stress-strain curves and mechanical properties were obtained from the tests and post-fire retention factors were derived. Comparison with available test data on hot-rolled and conventional cold-formed steels showed the influence of cold-working and steel grade on the post-fire mechanical properties. The permanent reduction in strength after exposure to elevated temperature is more severe and starts at lower temperatures for cold-formed steels compared with hot-rolled steels. Furthermore, cold-formed steels made of advanced high-strength grade exhibit proportionally larger permanent reductions in yield stress and ultimate stress than those made of conventional grade. A model for the post-fire properties of advanced high-strength cold-formed steels was proposed based on calibration of a simple three-coefficient equation developed through committee work with the American Iron and Steel Institute. • Study on post-fire properties of AHSS alloys with up to 1200 MPa yield stress. • Tests of ASTM E8 coupons after exposure to temperatures up to 700 °C. • Comparison with literature data of hot-rolled and cold-formed steels residual properties. • Obtained AHSS data show greater post-fire loss than published data and design codes. • New predictive models are proposed to determine the post-fire mechanical properties of AHSS. [ABSTRACT FROM AUTHOR]

Advanced High-Strength Steels (AHSS) have high potential to be adopted in future innovative infrastructure. However, the material properties of AHSS at elevated temperature, which are necessary for analysis and design in the fire situation, are not yet specified in design codes or literature. An experimental investigation is carried out on cold-formed steels with nominal yield stress ranging from 340 MPa to 1200 MPa. The tested materials include two types of advanced high-strength steels (dual-phase (DP) and martensitic (MS) steel) and two conventional steels (high-strength low-alloy (HSLA) and mild steel). 88 tensile specimens are tested in steady-state and transient-state conditions at temperatures up to 700 °C. Material properties are obtained from the tests including elastic modulus, 0.2% proof stress, yield stresses at strain levels of 0.5%, 1.5%, and 2%, ultimate stress, and elongation at fracture. The test results are compared with the design codes provisions in Eurocode 3 and AISC 360, and with data from existing literature. Comparisons show that cold-formed DP and MS steels exhibit relatively larger reduction in stiffness and strength than conventional cold-formed steels at elevated temperature, which warrant the development of specific provisions. It is also found that transient-state tests at 5 °C/min heating rate yield approximately similar yield stress retention factors as steady-state tests but yield lower values of elastic modulus retention factors. New predictive models are proposed to determine the mechanical properties at elevated temperature of the cold-formed advanced high-strength steels with nominal yield stress up to 1200 MPa. Unlabelled Image • Elevated temperature tests on cold-formed steels with yield stress from 340 MPa to 1200 MPa • Stress-strain curves and mechanical properties under steady-state and transient conditions • Higher grades CFS are proportionally more affected by temperature than lower grades • Current fire design code provisions are not applicable to CFS AHSS • Proposed predictive equations for the retention factors of CFS AHSS up to 700 °C [ABSTRACT FROM AUTHOR]

This paper assesses the stability and strength of sheathed cold-formed steel studs at elevated temperatures. Short and intermediate-length studs braced with gypsum, fire-rated gypsum, and oriented strand board were subjected to compressive axial load and temperatures ranging from 20 °C to 600 °C. A total of 40 tests were conducted in the steady-state regime, where the studs were first heated, then a compressive axial load was applied until failure occurred. Results show that the load-carrying capacity of the structural members decreases with increasing temperature, as the mechanical properties of the cold-formed steel reduce, and the bracing provided by the sheathing degrades. Local and distortional cross-section buckling failures are observed in the cold-formed steel member. The stabilizing effect and increase of load-carrying capacity attributed to the sheathing at ambient temperature is eventually lost at elevated temperature and the behavior of the sheathed studs becomes similar to the behavior of unsheathed members. Direct Strength Method design equations provided in the U.S. AISI S100 design specification are used with experimentally determined elevated temperature properties to predict the load-carrying capacity of the studs, then compared to experimental results to explore the feasibility of current design methods for performance-based fire design applications. • Degraded strength of sheathing-braced cold-formed steel studs is measured as temperature is increased. • OSB sheathing provides beneficial bracing for temperatures less than 350 °C while fire-rated and normal gypsum boards provide sheathing bracing for temperatures less than 600 °C. • The Direct Strength Method of AISI S100 can reliably be extended to predict the strength of uniformly heated bare and sheathed studs at elevated temperature. [ABSTRACT FROM AUTHOR]