Keywords: OpenSees, stability, structures, fire, dynamic.

The response of structures under different fire scenarios is a complex phenomenon, and in order to represent it efficiently, software is required that can analyse the inherent nonlinear behaviour subject to elevated temperatures arising from second order effects of large deflections, progressive softening of materials with temperature rise and the effect of thermal expansion. This study examines the stability of steel structures subject to fire loading using OpenSees with a dynamic approach that can trace efficiently the nonlinear equilibrium path of a structure.

The OpenSees framework is being extended at the University of Edinburgh by adding classes that include capabilities of performing the analysis of structures in fire including both heat transfer and stress analysis. This work is still on going and it enables the analyst to perform in a single numerical tool a state of the art nonlinear analysis of structures under different situations (fire or earthquake) or multi-hazard (fire after earthquake). This extension currently includes truss and beam column elements capable of handling large displacements arising from uniform or non-uniform thermal loads and nonlinear uniaxial materials of which the temperature dependent material properties vary.

The various path following methods for modelling the response of structures in fire are discussed. The nonlinear static methods of load control, displacement control and arc length can be efficient for some cases but have limitations. A numerical procedure involving dynamic analysis is described. The numerical scheme selected for the dynamic analysis is an implicit solution with a Newmark integrator. Moreover, the lumped mass matrix method is preferred as a result of to its computational simplicity and the small storage requirements. It is shown through numerical examples that this approach can efficiently model structures subject to thermal loading experiencing sudden buckling phenomena and can follow the response of the load-deflection path of a structure even when limit points are present. A simply supported beam, a two member truss and twenty-four member shallow dome are examined. The results are compared against experimental tests or numerical analyses performed by other researchers.

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