Storey stability of unbraced steel frames subjected to non-uniform elevated temperature distribution

• A model of column stability at non-uniform elevated temperature is proposed.
• Lateral stiffness of column at non-uniform elevated temperature is derived.
• A procedure to evaluate frame buckling strength at elevated temperature is presented.
• Results obtained from the proposed method are verified by finite element analysis.

Current studies on stability of steel frames are primarily based on the assumption that steel columns are subjected to uniform elevated temperature. However, the temperature distribution of real fire in a building may be non-uniform due to thermal buoyancy effect. In this study, the elastic stability of unbraced steel frames subjected to a non-uniform elevated temperature distribution along the longitudinal direction of the column is investigated based on the concept of storey based buckling and a two zone fire model. First, to simulate a steel column exposed to non-uniform elevated temperature, an analytical model is proposed to examine the effects of axial loading, non-uniform elevated temperature distribution, and thermal boundary restraints on the lateral stiffness of steel columns in unbraced frames. The lateral stiffness equation of the column model is derived based on Euler–Bernoulli beam theory. Then, the procedure to evaluate the stability capacity of unbraced steel frames subjected to non-uniform elevated temperature distribution is concluded. Numerical examples are presented to demonstrate the evaluation procedure of the proposed method and investigate the frame stability subjected to different scenarios of frame members exposed to the non-uniform elevated temperature distribution. The validity of the proposed method is verified by the numerical analysis with the use of finite element analysis.