Based on the fire behavior of steel structures subjected to local fire

the collapse mechanism of buildings initiated by the localized fire can be classified as: interior column buckling

exterior column buckling and interactive failure.Corresponding sub-structure analysis models

taking the effect of loading and constraint into account

are introduced to analyze the performance of steel structures under fire.The constraint boundary conditions such as the neighboring beams

columns and the surrounding floors are considered.Finite element models are developed to simulate the fire response of the substructure.Results obtained by proposed models are compared with those of the monolithic steel structure.It is shown that the sub-structure analysis models are feasible to simulate and assess the performance of multi-story steel structures under fire.