Nastran#

.bdf .nas .fem read + write eager

Summary of the specification#

A Nastran bulk data file is a deck of fixed-column cards. Each card starts with a name in the first field and continues across eight further fields. Three field widths coexist: small field (8 columns), large field (16 columns, signalled by a * on the card name), and free field (comma-separated). GRID cards define points; CTRIA3, CQUAD4, CTETRA, CHEXA and friends define elements by referencing grid ids and a property id, and continuation lines carry any fields that overflow the first card.

Specification at a glance#

field widths	small (8 col), large (16 col, name*), free (comma-separated)
points	GRID id cp x1 x2 x3
elements	CTRIA3, CQUAD4, CTETRA, CHEXA, CPENTA, CBAR …
grouping	property id (PID) on each element card
continuation	/ * continuation markers on overflowing cards
other suffixes	.dat is read via fmt=”.bdf”

Read the full Nastran specification ↗

Reading#

import polyxios as px

mesh = px.read("model.bdf")
mesh.vertices          # (n, 3)
mesh.element_types     # element groups found in the file

Writing#

px.write(mesh, "out.bdf")

Format-specific options:

Option	Default	Effect
`large_field_grid`	`False`	Emit GRID cards in large (16-column) field form for full float precision.

Quirks worth knowing#

All three field widths are read; writing emits free field, with large-field GRID cards on request.
.dat decks are recognised when the codec is named explicitly: px.read("model.dat", fmt=".bdf").
Property ids become element tags, so the deck’s grouping is preserved.