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Concepts and glossary

Terms used throughout PySees and these docs.

Model dimension: ndm and ndf

ndm is the number of spatial dimensions (2 or 3). ndf is the number of degrees of freedom per node: 3 for a 2D model, 6 for a 3D model. Both are chosen when you create the model and cannot be changed. Shells and 3D fiber beams need a 3D model.

Axes

  • 3D models are Z-up. Plan is X–Y and levels rise in Z. Gravity acts along −Z.
  • 2D models lie in the XY plane with Y up.
  • Beam local axes: local x runs from the first node to the second. In 2D, local y is x rotated 90° counter-clockwise. In 3D, the transformation's vecxz vector lies in the local x–z plane, then y = vecxz × x and z = x × y. Beam loads and the force diagrams use these axes.
  • Shells: nodes are listed counter-clockwise and the normal follows the right-hand rule. Stress resultants are per unit length in the shell's local axes.

Units

The model is stored in one consistent system of length, force and time units, chosen at creation. Display units are presentation only: values convert at the screen and are never stored converted. A model can also be unitless. See Units.

Analysis terms

  • Stage / case: one step of the analysis sequence, such as a static load ramp or an earthquake. Results are organised by case.
  • Pseudo-time: the load factor in a static analysis, which runs from 0 to 1 across the steps.
  • Load pattern claiming: each analysis block names the load patterns it applies. An empty list means every pattern no other stage claims.
  • Hold loads: keeps the loads from a stage constant for the stages that follow (OpenSees loadConst).
  • Reset vs. wipe: Reset restores the model to its initial state. Wipe analysis only affects the exported script and has no effect on a Carapace run.
  • Relative acceleration: in an earthquake analysis, results are relative to the moving ground, not absolute.
  • Recorder: a command that tells the analysis what to save (node displacements, reactions, element forces).

Files

  • Project: an annotated OpenSeesPy script that PySees can reopen completely.
  • Export: a script for running the model in OpenSees. See Files and interoperability.
Interface tourNodes and masses