Nastran#

.bdf .nas .fem .dat 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 resolves by content (GRID / CEND / BEGIN BULK / SOL)

Read the full Nastran specification ↗

Reading#

import polyxios as px

mesh = px.read("model.bdf")
mesh.vertices          # (n, 3)
mesh.element_types     # one type code per element

Writing#

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

Format-specific options#

Option	Default	Effect
`field_format`	`"free"`	`"free"` writes comma-separated cards; `"large"` writes `GRID*` cards in large (16-column) field form. Element cards stay free field either way.

Quirks worth knowing#

All three field widths are read; writing emits free field, with large-field GRID cards on request (field_format="large").
.dat is shared with Tecplot, LS-DYNA and plain ASCII tables, so it is resolved by looking inside the file: a $ comment banner is stepped over and a GRID card, CEND, BEGIN BULK, NASTRAN or a SOL naming a solution number or name lands the deck here - a numeric table whose first row happens to read SOL 1.0 2.0 does not. px.read("model.dat", fmt=".bdf") still forces the issue, and writing to .dat needs fmt=".bdf" because an output file has no content to inspect.
Free-field coordinates are written with the shortest text that round-trips the double exactly, so polyxios reads them back bit for bit; a magnitude below one whose short spelling needs an exponent goes out in fixed point instead. A solver keeps only the first eight characters of a free-field entry, so a longer value is read rounded there, with a warning on write. field_format="large" fits each coordinate in sixteen columns, using the implicit-exponent shorthand the format allows (1.234-10 for 1.234E-10) when it saves digits; a value that still does not fit is rounded on the way out, with a warning.
Property ids land in element_attrs["pid"] and each distinct one becomes an element tag pid_<id>, so the deck’s grouping is preserved.
A card name does not say the element’s order, so the shape comes from the grid points the card carries: CTETRA with ten grid points is a quadratic_tetra, with four a tetra. Counting stops at the first blank field, because Nastran lets a mid-side grid point be omitted and that card is a linear element.
The axisymmetric cards read as their planar counterparts: CTRAX3, CTRAX6, CTRIAX6, CQUADX, CQUADX4 and CQUADX8, alongside CSHEAR.
One-dimensional cards (CBAR, CBEAM, CROD, CONROD, CTUBE, CBUSH, CGAP, CBEND, CVISC) read as lines rather than being skipped; CONROD names a material where the others name a property, so its grid points sit one field earlier.
A shell’s ZOFFS becomes element_attrs["zoffs"] when any card carries one, and is written back on the shell cards that have a field for it: CTRIA3, CTRIAR, CQUAD4, CQUADR, CTRIA6 and CQUAD8.
Nastran and VTK number the corner grid points alike, but not every mid-side one. CPENTA runs the prism’s vertical edges before its top ring and CHEXA does the same with the brick’s, where VTK runs the verticals last; CTRIAX6 interleaves corners and mid-side nodes rather than listing the corners first. All three are permuted on the way in; CPENTA and CHEXA are permuted back on the way out, while a quadratic triangle is always written as CTRIA6, whose order already agrees with VTK.
CTRIAX6 names a material in the field where the shells name a property, so its second field is not read as a property id. A CBUSH grounding its second end names one grid point and no element, and is skipped with a warning of its own rather than being counted among the cards this codec has no shape for.