A strategy for modelling external user element in ANSYS: The Bouc-Wen and the skyhook case

The ANSYS environment allows for implementing user elements in Fortran subroutines, compiling a new executable file; however, this requires the user to: a) understand the interfacing routines provided with the ANSYS code; b) to correctly guess how the interfacing routines will be actually called by the ANSYS environment. This burden can be avoided by exploiting the APDL (ANSYS Parametric Design Language) command \SYS, which allows to call an external executable program from within the main ANSYS analysis procedure, or from within an APDL subroutine. The main idea is to pass quantities of interest (e.g. nodal displacements, velocities, etc.) to the external executable program via an intermediate data file written by standard APDL commands. The external executable replies with some computed quantities (e.g. nodal forces) in a second data file that will be read, processed and applied to the structure by APDL statements. It is worth underline that this implies a decoupling between the process of achieving a structural equilibrium configuration and that of the state determination of the external element. The usefulness and the limits inherently with this strategy will be shown with reference to the implementation of a new external non-linear element devoted to modelling passive (Bouc-Wen) and semi-active (Skyhook) control devices; these will be used in companion papers for the control of a bridge structure under seismic and wind loading. The external user element is finally evaluated by a comparison with a proven implementation inside the MATLAB environment, which explicitly considers the coupling between the structural state determination and that of the external element, with reference to seismic excitation.