Postdata File Format¶

The .mpco.postdata file is a plain-text companion to the .mpco result file. It is generated automatically by parsing the OpenSees TCL model files and contains geometry metadata that the HDF5 file does not store.

Generation¶

# Automatic (generated on first run if .tcl exists)
ops-post my_model

# The parser follows all source commands in the TCL:
# my_model.tcl -> source nodes.tcl -> source elements.tcl -> ...

The .postdata file is regenerated when the .tcl file is newer.

File structure¶

#elem_id q.w q.x q.y q.z
*LOCAL_AXES
1 0.0 0.7071 0.0 -0.7071
2 0.0 0.7071 0.0 -0.7071
...
#profile_id n_vertices\n  y z (per vertex)
*BEAM_PROFILE
-1 4
-0.0285 -0.0285
0.0285 -0.0285
0.0285 0.0285
-0.0285 0.0285
...
#elem_id profile_id
*BEAM_PROFILE_ASSIGNMENT
1 -1
2 -2
...
#elem_id elem_type section_name
*ELEMENT_INFO
1 ElasticTimoshenkoBeam
239 ASDShellQ4 LayeredShell_10
...

Sections¶

*LOCAL_AXES¶

One line per element. Quaternion (scalar-first: qw, qx, qy, qz) defining the element's local coordinate frame.

elem_id  qw  qx  qy  qz

How local axes are computed from TCL:

Element family	Source
Shell/quad/tri with `-local vx vy vz`	Local-x direction + element normal
Beam with `geomTransf ... vecxzX vecxzY vecxzZ`	Element axis + vecxz plane vector
Zero-length with `-orient x1 x2 x3 y1 y2 y3`	Explicit local-x and local-y
Other	Computed from node positions (fallback)

*BEAM_PROFILE¶

One block per beam cross-section profile. Each block starts with the profile ID and vertex count, followed by 2D vertices (y, z in local coordinates).

profile_id  n_vertices
y0  z0
y1  z1
...

How profiles are derived from TCL:

For beams with inline section properties (ElasticTimoshenkoBeam, elasticBeamColumn), the cross-section dimensions are computed from the area and moment of inertia assuming a rectangular section:

h = sqrt(12 * Iz / A)
b = A / h

For beams referencing a section Elastic, the same formula is applied using the section's A and Iz values.

Profile IDs are negative (e.g. -1, -2) for inline beam properties to avoid collision with TCL section IDs.

*BEAM_PROFILE_ASSIGNMENT¶

Maps each beam element to its cross-section profile.

elem_id  profile_id

*ELEMENT_INFO¶

One line per element with its type and section name.

elem_id  elem_type  section_name

The section name is derived from the TCL section definition (e.g. LayeredShell_10 for section LayeredShell 10 ...).

Relationship to .mpco¶

The .mpco file stores:

Node coordinates and element connectivity
Section assignments and fiber data
All result data (displacements, stresses, strains)

The .postdata file adds:

Local coordinate frame orientations (needed for shell normal direction and beam cross-section orientation)
Beam cross-section profiles (needed for 3D beam visualization)
Element metadata linking elements to their TCL definitions

Together they provide everything needed for visualization.