Structural fire design considerations for high strength steel

Abstract

High strength steels (HSS) defined herein as material with a yield strength between 460 and 700 N/mm2 are increasingly being employed in structures as an alternative to conventional steel grades (i.e. steel grades with yield strengths below 460 N/mm2) due to economic and environmental benefits related to reductions in section thickness and hence reduced weight (and overall material usage) as well as potentially lower transport and fabrication costs. The European structural design guidelines (Eurocodes) are based on previous work on conventional steel grades, which currently limit the use of HSS. Hence there has been a surge in research activity with the aim to enhancing the current understanding relating to the behaviour of HSS structural members under various loading conditions and including their performance in fire scenarios. This thesis examines the behaviour of various HSS grades at elevated temperature for structural fire design purposes. Fundamental to the understanding and safe design of structures under fire conditions is a detailed understanding of material properties at elevated temperatures. To this end, the material response of various commercial HSS grades, including quenched and tempered (QT) and thermomechanically control processed (TMCP) steels, were determined from elevated temperature tensile tests under two conditions: isothermal (steady-state) and anisothermal (transient-state), at temperatures up to 800°C. The strength and stiffness were obtained and converted into reduction factors, defined as the elevated temperature property normalised by the property at room temperature in line with the current Eurocode approach. The results demonstrate the variability in strength retention at elevated temperatures; this is attributed to the material history particularly the alloying elements and highlight the danger in generalising the material properties of different types of HSS for structural fire design purposes. Heat treatments and microstructural studies were conducted on the two HSS grades which demonstrated the best strength retention properties at elevated temperatures to deduce possible metallurgical effects, which could account for the better strength retention. The study highlighted that a sufficient amount of elements including molybdenum and chromium, as well as microalloying elements, niobium, vanadium and titanium can cause strength enhancement between 500 and 700°C through secondary hardening. Additionally, with the scarcity of performance data on HSS columns at elevated temperature, a numerical model, which considered geometric imperfections and material nonlinearity, was developed in ABAQUS and validated using experimental data on HSS at room temperature and mild steel grades at elevated temperature. After the model was validated, parametric studies were conducted, incorporating material properties of QT and TMCP steels, in order to determine the influence of the material history on the buckling behaviour and assess the suitability of the buckling curves provided in Eurocode 3 Part 1-2 (2005). The results showed that the Eurocode generally provides conservative (i.e. safe) results for TMCP steel columns with respect to the buckling coefficients and safely predicts the buckling resistance, but a lower buckling curve may be needed for QT columns. In addition, because of the various alloying and production routes employed to produce HSS, variations in the stress-strain response was also observed, in turn, this influenced the buckling response and highlighted possible unconservativisms (i.e. unsafe) in the Eurocode design approach as a result of generalising the material response.