Thermo-mechanical analysis of steel columns using different constitutive laws

Application of advanced calculation models in fire analyses requires special attention by the designer to the modelling details and the type of finite element adopted for the structural model. Obviously the type of finite element (FE) used in the analyses, as well as the assumptions made in modelling of the thermo-mechanical properties, influence the analytical results. This benchmark has been developed using three different software codes, in order to demonstrate and evaluate the differences in the results and the influence of various parameters on the same results. The codes used for the analyses are Vulcan, SAFIR and Strand7. The first two are purpose- built software for structural fire analysis, and correctly model the constitutive law for steel at high temperatures, in the form given in EN1993-1-2. Strand7 is general-purpose FE software. It allows more possibilities than the others to simulate different structural loadings, such as seismic ones, and it allows more possibilities for the use of different finite element types. Nevertheless in elevated-temperature analysis Strand7 is subject to certain approximations in its modelling assumptions. In particular, it only allows a simplified constitutive law (bilinear elastic-plastic) for steel to be used at high temperatures, thus neglecting the elliptical branch between the proportionality limit and the yield stress. The use of this type of simplified constitutive law does not significantly affect the results in terms of time of collapse when the structures or substructures analysed are characterized by collapse due to the attainment of limiting tensile stress in the material. However, for analyses in which buckling phenomena cause the dominant collapse mode, this simplified constitutive law can significantly, and sometimes deeply, affect the results, both in terms of time to collapse and displacement behaviour. This is because the buckling phenomenon often happens when much of the material stress is in the elliptical branch of the constitutive law. This is particularly important as the load ratio increases.