sbeam — Simple Beam FEA

A Python finite element analysis tool for beam structures using NASTRAN BDF input format. Supports static analysis (SOL 101) and normal modes (SOL 103) via Euler-Bernoulli beam theory, plus static aeroelastics (SOL 144): steady vortex-lattice aerodynamics, flexible trim, stability derivatives, divergence, monitor-point section loads, and DLM-free quasi-steady transient maneuver loads (ZAERO-style MLOADS).

License: MIT (see LICENSE) — free to use, modify, and redistribute, including commercially.

Requirements

• Python 3.9+
• pip

Installation

Clone the repository and create a virtual environment:

git clone <repo-url>
cd sbeam
python3 -m venv .venv
source .venv/bin/activate        # Windows: .venv\Scripts\activate

Install the package and its dependencies in editable mode:

pip install -e .

The editable install registers sbeam into the active environment so it is importable from anywhere — including inside Streamlit.

Running the Viewer

streamlit run sbeam/viewer/app.py

The viewer opens in your browser. Upload a *.dat bulk data file to inspect model geometry, define case control, and launch analyses.

A sample model is provided at sample/simple_beam.dat — a 10 m steel cantilever beam (5 CBAR elements, SI units).

Running the Solver

python -m sbeam.main path/to/model.bdf

The solver reads the case control file (.bdf), which uses INCLUDE to reference the bulk data file (.dat). Results are written to a .f06 file in NASTRAN format.

Running Tests

pytest

Documentation

Full documentation lives in docs/, organised into four sections: code standard (10_standard/), theory & worked examples (20_theory/), future development (30_future/), and historic record (40_history/). Start at the documentation index.

Workflow

1. Define geometry in a *.dat bulk data file (GRID, CBAR, PBAR, MAT1, SPC, FORCE, etc.; for aeroelastic models add the AEROS/CAERO1/SPLINE2/TRIM card set)
2. Open the Streamlit viewer to inspect the model and run GPWG (mass/CG summary)
3. Define case control in the viewer UI and export a *.bdf file (SOL 144 trim/divergence/maneuver cases are authored directly in the BDF)
4. Run the solver against the exported *.bdf
5. Review results in the *.f06 output file or load them back into the viewer (SOL 144 adds trimmed flight-load / maneuver-load BDF exports and a monitor-loads CSV)

Supported BDF Cards

Category	Cards
Geometry	GRID, CORD2R
Elements	CBAR, CBUSH, PLOTEL, RBE2, RBE3, RBAR
Properties	PBAR, PBUSH
Material	MAT1
Mass	CONM2
Constraints	SPC, SPC1, SUPORT
Loads	FORCE, MOMENT, LOAD, GRAV
Eigenvalue	EIGRL
Aero geometry	AEROS, CAERO1, PAERO1, AEFACT, PSTRIP, STRIPK
Aero corrections	W2GJ, WKK, AECORR
Splining	SET1, SPLINE0, SPLINE1*, SPLINE2, ATTACH
Trim / divergence	AESTAT, AESURF, AELIST, TRIM, DIVERG, TRIMVAR, TRIMOBJ, TRIMCON
Monitor points	MONPNT1, MONPNT3, AECOMP
Transient maneuver	MLOADS, MLDTRIM, MLDTIME, MLDCOMD, MLDPRNT, TABLED1

* SPLINE1 is parsed but not yet implemented (use SPLINE2).

Full field-by-field definitions: docs/10_standard/02_card_reference.md.

Analysis Types

• SOL 101 — Linear statics: nodal displacements, SPC reactions, CBAR end forces/moments, stress recovery, CBUSH element forces
• SOL 103 — Normal modes: natural frequencies (Hz and rad/s), normalised mode shapes, modal mass fractions
• SOL 144 — Static aeroelastic trim (steady VLM): determined and over-determined trim, rigid + elastic-restrained stability derivatives, hinge-moment derivatives, divergence dynamic pressure (DIVERG), balanced maneuver loads with inertia relief, monitor-point integrated section loads, trimmed flight-load export
• SOL 144 + MLOADS — DLM-free quasi-steady transient maneuver loads (Phase G0, Level 1): open-loop commanded control histories integrated by Newmark-β from a trimmed initial condition, with time-history output and critical-sample load export

Known Limitations

Beam theory

• Euler-Bernoulli only — Timoshenko shear correction (PBAR K1/K2) is not applied. K1/K2 fields are parsed but silently ignored, which is the correct assumption for slender beams where shear deformation is negligible.
• Uniform cross-section only — tapered beams are not supported. PBARL (standard shape library) is not supported; cross-section properties must be entered directly via PBAR.

Coordinate systems

• CORD2R only — CORD2C, CORD2S, and CORD1R are not supported.
• GRAV requires CID=0 — a non-zero CID on a GRAV card raises a parse error.

Elements

• RBE3 simplified — uses same-DOF weighted averaging; rotation-to-translation lever-arm coupling across an offset is not applied. Use RBAR for kinematically exact rigid connections when grid offsets are present.
• CBUSH requires CID=0 — element offsets are not supported.

Loads

• PLOAD1 not supported — distributed beam loads are not implemented. A PLOAD1 card in the BDF is currently silently dropped with no user-visible warning. Do not use PLOAD1; a pre-solve validator warning is planned for a future release.

Solvers

• SOL 101, SOL 103, and SOL 144 only — frequency response (SOL 108 / SOL 111), transient response (SOL 109 / SOL 112), and buckling (SOL 105) are not yet implemented.

Aeroelastics

• Steady, subsonic VLM only — Mach ≥ 1 is rejected; no doublet-lattice (DLM) unsteady aerodynamics or flutter solution yet. Transient maneuver loads are quasi-steady (valid for reduced frequency k ≲ 0.1).
• Full-span models only — AEROS SYMXZ/SYMXY half-span symmetry is rejected; mirror the model to full span.
• Unrestrained (mean-axis) stability derivatives not yet computed — rigid and elastic-restrained columns are output; the trim solution itself is unaffected.

General

• No hard element count limit; memory and compute time are the practical constraints.
• Results are not certified for safety-critical or regulated structural design — see the Disclaimer below.

Author

Sean O'Meara — sean.c.omeara74@gmail.com

Bug reports and pull requests are welcome via the GitHub issue tracker.

Disclaimer

sbeam implements standard, well-published finite element methods (Euler-Bernoulli beam theory, consistent mass matrix, direct stiffness assembly) using established numerical libraries. It is intended as an educational and exploratory engineering tool.

Results are not certified for safety-critical or regulated structural design. The implementation uses simplified assumptions (uniform cross-sections, no shear deformation, global coordinate frame, dense direct solver). Verify any results against an established commercial solver and competent engineering judgement before relying on them for design decisions.

The software is provided "as is", without warranty of any kind. See the LICENSE file for the full disclaimer of liability.

License

Released under the MIT License. Copyright (c) 2026 Sean O'Meara.