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This paper reports on an investigation of the effects of fire on long span truss floor systems in a tall building environment. The effects of fire spread over multiple floors of a building are the focus of this research, especially where this may lead to progressive collapse. The results from an investigation of a two-dimensional model of a multistory office building analysis are presented. The model is representative of the type of construction used in the World Trade Center Towers 1 and 2. The local and global response of the model is described over the course of a concurrent fire on three floors reaching a peak compartment temperature of 800[degrees]C. The results of the analysis show that large displacements may occur in long span structural floor systems without failure, however, the interaction of the highly deflected floors with the exterior or perimeter columns can lead to structural collapse. Additional structural members, such as hat trusses, that allow redistribution of loads away from the exterior columns to the core columns have a significant beneficial impact on the robustness of the structure. CE Database subject headings: Buildings, high-rise; Fires; Framed structures; United Kingdom.