Existing frame finite elements for modeling the response of structures in fire are limited to the stiffness method of analysis, in which the element formulations are based on an assumed displacement field. Such stiffness-based elements tend to experience loss of accuracy in highly nonlinear problems since the basic conditions of equilibrium are often violated. This problem can usually be remedied with the use of more elements along the length of a member, a solution that also leads to significant loss of efficiency. Recent efforts in the analysis of structures at ambient temperatures have demonstrated that flexibility-based and mixed formulation elements tend to offer greater accuracy and computational efficiency, since the element formulations strictly enforce equilibrium.
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