Numerical modelling of steel members subjected to severe thermal loads

This paper addresses the structural behaviour of cold-formed steel members subjected to axial compression and severe heating. Analytical approaches used in retrospective numerical simulations of fire resistance tests are discussed. The developed model reflects the structural failure modes observed in the experiments and gives appropriate consideration to thermally-induced stresses and deformations. The behaviour of loaded steel studs is shown to be sensitive to structural boundary conditions and non-uniform heating. The numerical techniques are described in detail and demonstrated in lateral deflection simulations. The model produces structural failure time (fire resistance) predictions that are consistent with experimental data. INTRODUCTION In structural terms, the accidental situation of fully developed building fire can be analysed as an ultimate limit state event for the affected structural elements and systems. In some instances, such events may cause significant structural damage because severe heating leads to the deterioration of the strength and stiffness properties of structural materials. It is equally important to recognise that thermally-induced deformations can cause significant changes in the configuration of heated structures leading to unusual stress re-distributions and unexpected failure modes. Numerical simulations help in tracing the complex interaction of thermal and structural phenomena in construction assemblies subjected to fire attack. Computational techniques are becoming increasingly important as research tools in structural fire protection because they provide engineers with a better interpretation of experimental results. These tools can also help to produce an ‘educated guess’ of expected fire resistance in situations where results of standard fire tests are not available or not applicable (e.g. when the size of the structure precludes testing or when non-standard fire exposure must be considered). In this context, a computer program STUD has been developed recently at the National Research Council of Canada (NRC), in collaboration with the Canadian Steel Construction Council (CSCC), with the purpose to investigate the behaviour of cold-formed steel studs in loadbearing lightweight steel-framed (LSF) walls exposed to fire. This paper describes the analytical approaches and algorithms, used in the program, and presents the results of numerical simulations compared with experimental data. A survey of the literature pertaining to the fire resistance of loadbearing LSF walls is presented elsewhere . These walls, usually lined with gypsum board, are frequently used in industrial and commercial buildings, up to five storeys high. In North America, loadbearing LSF wall assemblies are increasingly being used in residential construction and multi-unit housing projects. In response to the growing demand for the proper assessment of the performance of these assemblies in building fires, several standard 3,4 fire resistance tests were recently conducted at NRC. These tests were well instrumented to provide detailed experimental data required for the development of the numerical model. A brief description of the tests and the observed patterns in structural behaviour is presented in the following section. FIRE RESISTANCE TESTS The wall assemblies tested (designated W1 through W6) were 3048 mm high by 3658 mm long. Each assembly consisted of a single row of galvanized cold-formed steel studs, protected with two layers of 12.7-mm thick fire-resistant gypsum board (Type X Firecode C) on each side. All steel studs had a C-shaped cross-section, nominally, 92.1 mm deep by 41.3 mm wide, with 12.5-mm flange stiffening lips and base metal thickness of 0.912 mm (control measurements showed an average thickness under-run of 0.01 mm). The minimum specified steel yield strength was Fy = 228 MPa. Each stud had four web perforations, 38 mm wide, spaced 610 mm o.c. along the stud. Figure 1 shows a typical steel frame fabrication layout, and Figure 2 schematically illustrates the crosssectional details, of the wall specimens. A short summary of the variable parameters and test results is provided in Table 1. The purpose of this test series was to investigate the effects of stud spacing, resilient channels and insulation type on the fire resistance of loadbearing LSF walls. An exhaustive description of wall construction details and the test setup is presented elsewhere . Figure 1. Typical Steel Frame Fabrication Layout for Wall Specimens. Figure 2. Cross-Sectional Details of Wall Specimens. 8 Studs @ 406 o.c.