MPC modelling

[dbpatankar]

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.

[JWock82]

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:

1. Create a diaphragm master node at that level (e.g., at the center of rigidity).
2. 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
3. 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):

A = 1e6
Iy = Iz = J = 1e6
E = your_material_E

[JBloss1517]

@JWock82 approach is what Risa 3d does when you define a rigid diaphragm.
https://risa.com/risahelp/risa3d/Content/Diaphragms/Rigid-Diaphragms.htm