During the conceptual design stage of a project, the selection of materials and structural schemes are often governed by the requirement for solutions to be economically viable whilst equally providing a positive contribution towards the environment and society. High strength steels (HSS, defined here as materials with yield strength between 460 and 700 N/mm 2
in accordance with the Eurocode Part 1-12 ) have the potential to make a positive contribution towards these demands by reducing the material usage and hence weight of structural elements when employed in appropriate applications. Lighter structures lead to smaller foundations, reduced transportation costs and potentially reduced construction times and costs, as well as lower CO 2 emissions and energy use during construction.
One of the issues preventing more widespread use of HSS in structures is the lack of reliable information relating to the response of these materials during a fire. Although the Eurocode does include a section for HSS , the guidance for fire design is based on experiments on steels with yield strengths below 460 N/mm 2 .For HSS, there are limited data in the literature (e.g. [2, 3]) that present the effects of temperature on the mechanical properties in terms of reduction factors. Whilst the loss of strength and stiffness during a fire is inevitable, a recent review highlighted that the strength and stiffness of HSS at elevated temperature are directly related to the alloying elements and processing route employed . This implies that by choosing particular alloying elements and processing routes, possible metallurgical effects such as secondary (or precipitation) hardening could potentiallybe utilised to retard the loss of strength and stiffness of HSS during a fire, therefore buying valuable evacuation time. However, because limited metallurgical analysis was presented in the literature, the influence of strengthening mechanisms such as precipitation hardening on the performance of HSS at elevated temperature is not clear. In this context a primary aim of this work is to provide engineers and designers with essential and reliable information to support the safe design of fire resistant structures made from HSS. A further aim of the work is to develop a detailed understanding of the effects of steel alloying and processing routes on the structural response of HSS in fire as these are likely to have a strong influence on the degradation of mechanical properties.
A series of isothermal (steady-state) and anisothermal (transient state) tensile tests will be conducted for temperatures between 100-800°C in order to determine the elevated temperature data required for structural design under fire conditions. In parallel, a detailed metallurgical investigation will be carried out aiming to characterise the microstructural changes occurring with time and temperature. In particular, the influence of grain size and precipitates on the mechanical properties at elevated temperature will bestudied. A technique called electron backscatter diffraction (EBSD) will be used to characterise the grain size of the heat treated HSS samples whilst transmission electron microscopy (TEM) will be used to characterise precipitates and microstructure. Such information will support further material developments to optimise the ambient and elevated temperature properties by providing steel producers with preliminary guidance on the effects that chemical composition and processing routes have on the elevated temperature performance.
Additionally, the data from this project will be incorporated into ABAQUS models to evaluate the behaviour of HSS structural members during a fire in order to evaluate the suitability of existing design approaches.
EN 1993-1-12. 2007. “Eurocode 3 Design of Steel Structures - Part 1-12: Additional Rules for the Extension of EN 1993 up to Steel Grades S700,” CEN.
Chen, J., Young, B. and Uy, B. 2006. “Behaviour of High Strength Structural Steel at Elevated Temperatures,” J. Struct. Eng., 132(12):1948-1954.
Xiang, Q., Biljaard, F. and Kolstein, H. 2012. “Dependence of Mechanical Properties of High Strength Steel S690 on Elevated Temperatures,” Constr. Build. Mater., 30: 73-79.
Winful, D. A., Cashell, K. A., Barnes, A. M. and Pargeter, R. J. 2015. “High Strength Steel in Fire” Proceedings of CONFAB, the first International Conference on Structural Safety under Fire & Blast, Glasgow, Scotland. ASRANet Ltd, pp. 105-114.