PERFORMANCE-BASED FIRE DESIGN OF COLD-FORMED STEEL STRUCTURES MADE OF NEW HIGH-STRENGTH STEELS

The development of new structural materials is a key enabler of innovation in the building construction sector, but their adoption requires the ability to assess the safety of the design under various loading cases, including fire. The performance-based fire design approach provides a rigorous framework to assess the structural fire safety of structures, provided the appropriate data and methods are available to evaluate the response of the structures under fire conditions. This thesis describes a study on the behavior, modeling strategy, and design method of cold-formed steel (CFS) structures made of a new class of materials. The study is based on material testing, advanced fire modeling, and numerical and analytical methods, for enabling the assessment of the structural response in fire. First, experimental studies are carried out on the elevated temperature and post-fire material properties of high-strength dual-phase and martensitic steels. Tests performed under steady-state, transient, and residual heating conditions show proportionally larger reductions in strength at elevated temperature and after cooling down for the investigated high-strength steels than for conventional cold-formed steels. New models are proposed for the temperature-dependent properties of the materials for implementation in design methods and finite element codes. Second, various computational modeling strategies are evaluated to assess temperatures in structural members exposed to fires. Based on modeling of experimental tests measuring the temperatures in steel members exposed to localized fires, a modeling decision flowchart is proposed for supporting the computational fire-thermal modeling in performance-based fire design analyses. An application to open car parks illustrates the range of applicability of different models for capturing the effects of localized fires on steel frames. Third, numerical analyses are used to investigate the stability under fire of CFS members made of the dual-phase and martensitic steels. A parametric study using validated finite element models allows evaluating the local buckling strength of hollow section columns at ambient and elevated temperatures. A modification to the current analytical Direct Strength Method (DSM) is introduced for estimating the local buckling capacity of hollow section columns at elevated temperatures, valid for the different steel grades at ambient and elevated temperatures up to 700 °C. Finally, the developed material data and modeling methods are applied to conduct the performance-based structural fire design of cold-formed steel assemblies from a prototype metal building. The results show that this fire design approach enables explicitly verifying achievement of the performance objectives while allowing enhanced flexibility and efficiency in design. Overall, this thesis proposes a roadmap to address structural fire safety when adopting new materials for enabling more efficient, sustainable, and resilient designs