This work proposes a novel methodology to determine the design fire of modern buildings that are outside the range of applicability of nominal methods. Structural engineers face the problem of how to characterize the fire environment to be used in the design stage of buildings. This question is addressed here in the context of complex high-rise buildings where the resulting fire environments pose unique features that are not necessarily accounted for in traditional design fires. A comprehensive analysis of the fire environment, with the objective of maximizing the challenge to the structural elements is proposed to understand the particular fire dynamics of the building. The results must be expressed in simple terms without loss of generality in order to be of valuable engineering use. The methodology combines computational fluid dynamics and engineering simplifications such as using steady-state temperatures and dividing the fire environment into near and far fields. Both, fully distributed fires and travelling fires are investigated. The methodology is applied to a real building to illustrate the importance of this analysis and the comparison with the results from the standards.