The stiffness of gas given by the ideal gas law pV = nRT can be considered in the nonlinear dynamic analysis.
The calculation of gas is available for accelerograms and time diagrams for both the explicit analysis and the nonlinear implicit Newmark analysis. To determine the gas behavior correctly, at least two FE layers for gas solids should be defined.
The member type 'Dashpot' can be used for time history analyzes in RFEM/RSTAB with the add-on modules RF-/DYNAM Pro - Forced Vibrations and RF-/DYNAM Pro - Nonlinear Time History. This linear viscous damping element considers forces dependent on velocity.
With regard to viscoelasticity, the member type 'Dashpot' is similar to the Kelvin-Voigt model, which consists of the damping element and an elastic spring (both connected in parallel).
Due to the integration of RF‑/DYNAM Pro in RFEM or RSTAB, you can incorporate numeric and graphic results from RF‑/DYNAM Pro - Nonlinear Time History to the global printout report. Also, all RFEM and RSTAB options are available for a graphical visualization. The results of the time history analysis are displayed in a time history diagram.
The results are displayed as a function of time and the numerical values can be exported to MS Excel. Result combinations can be exported, either as a result of a single time step or the most unfavorable results of all time steps are filtered out.
Calculation in RFEM The nonlinear time history analysis is performed with the implicit Newmark analysis or the explicit analysis. Both are direct time integration methods. The implicit analysis requires small time steps to provide precise results. The explicit analysis determines the required time step automatically to provide the stability to the solution. The explicit analysis is suitable for the analysis of short excitations, such as impulse excitation, or an explosion.
Calculation in RSTAB The nonlinear time history analysis is performed with the explicit analysis. This is a direct time integration method and determines the required time step automatically in order to provide the solution stability.
RF-/DYNAM Pro - Nonlinear Time History is integrated in the structure of RF‑/DYNAM Pro - Forced Vibrations and extended by two nonlinear analysis methods (one nonlinear analysis in RSTAB).
Force-time diagrams can be entered as transient, periodic, or as a function of time. Dynamic load cases combine the time diagrams with the static load cases, which provides high flexibility. Furthermore, it is possible to define time steps for the calculation, structural damping, and export options in the dynamic load cases.
User-defined time diagrams as a function of time, in tabular form, or as harmonic loads
Combination of the time diagrams with RFEM/RSTAB load cases or combinations (enables definition of nodal, member, and surface loads, as well as free and generated loads varying over time)
Combination of several independent excitation functions
Nonlinear time history analysis with the implicit Newmark analysis (RFEM only) or the explicit analysis
Structural damping using Rayleigh damping coefficients or Lehr's damping
Direct import of initial deformations from a load case or combination (RFEM only)
Stiffness modifications as initial conditions; for example, axial force effect, deactivated members (RSTAB only)
Graphical display of results in a time history diagram
Export of results in user-defined time steps or as an envelope