The influence of approximations in the nonlinear analysis of steel frames with semi-rigid joints at elevated temperatures

The analysis of building frames under thermal loading is an important and essential task in structural engineering. Linear analysis is generally inadequate for large temperature changes, and nonlinear analysis is normally required to correctly model the behaviour of the structure. Various nonlinear models are used in practice, and they usually involve approximations of the nonlinear effects. In this case, often it is not clear what effects the approximations have on the analysis. In this paper, we discuss a general approach for the nonlinear analysis of two-dimensional steel frames with semi-rigid joints under thermal loading. In particular, we consider the role of approximations in the governing equations of the structure and the semi-rigid joints, and the changes in material properties at high temperatures. The material and joint models use empirical formulations and code-based rules. Several examples are given that demonstrate the effects of the modelling assumptions on the results.