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Is your feature request related to a problem? Please describe.
Defining a rigid diaphragm using multipoint constraints (MPC) like rigidDiaphragm, equalDof in openSees
Describe the solution you'd like
Possibility to model multipoint constraints.
As best I can tell this would take a significant rewiring of Pynite's stiffness matrix assembly and equation solver. It seems like a big effort. Maybe someday I'll get around to it, but in the meantime there is a workaround:
For each diaphragm level:
Create a diaphragm master node at that level (e.g., at the center of rigidity).
Connect all diaphragm nodes to the master node using very stiff members:
Very large EA → axial rigidity
Very large EI → bending rigidity
Very large GJ → torsion rigidity
DOFs then move together approximately as rigid.
Pros: Works today, easy
Cons: Adds artificial stiffness → keep stiffness large but not extreme to avoid ill-conditioning
Recommended approximate numbers (adjust to model scale):
Activity
JWock82 commentedon Dec 4, 2025
As best I can tell this would take a significant rewiring of Pynite's stiffness matrix assembly and equation solver. It seems like a big effort. Maybe someday I'll get around to it, but in the meantime there is a workaround:
For each diaphragm level:
Very large EA → axial rigidity
Very large EI → bending rigidity
Very large GJ → torsion rigidity
Pros: Works today, easy
Cons: Adds artificial stiffness → keep stiffness large but not extreme to avoid ill-conditioning
Recommended approximate numbers (adjust to model scale):
A = 1e6
Iy = Iz = J = 1e6
E = your_material_E
JBloss1517 commentedon Dec 5, 2025
@JWock82 approach is what Risa 3d does when you define a rigid diaphragm.
https://risa.com/risahelp/risa3d/Content/Diaphragms/Rigid-Diaphragms.htm