A rational, analytical approach to a fire engineering design of load bearing steel structures is described. The method of design is directly based on the natural compartment fire concept and on strictly defined functional requirements and performance criteria. The method is permitted to be generally applied in Sweden, as one alternative, since more than ten years. For facilitating the practical application, a comprehensive design basis has been worked out in the form of temperature during the relevant compartment fire and the corresponding design load bearing capacity of the fire exposed structure. The design basis is presented in a manual which has been given type approval for practical use by the National Swedish Board of Physical Planning and Building. The design procedure has recently been further developed on order to arrive at a design method in regard to fire exposure which is in principal agreement with modern loading and safety philosophy for the non-fire state.