# Step 7: fortran-implementation

## Read First

- `/AGENTS.md`
- `/docs/ABAQUS_SUBROUTINE_AGENT_DESIGN.md`
- `/docs/ARCHITECTURE.md`
- `/docs/ADR.md`
- `/.codex/skills/abaqus-fortran-tdd/SKILL.md`
- `/docs/requirements/uel-3d-euler-beam.md`
- `/docs/research/uel-3d-euler-beam-research.md`
- `/docs/formulations/uel-3d-euler-beam.md`
- `/docs/numerical-reviews/uel-3d-euler-beam.md`
- `/docs/io-definitions/uel-3d-euler-beam.md`
- `/docs/reference-models/uel-3d-euler-beam.md`
- `/docs/build-test-reports/uel-3d-euler-beam-red.md`
- `/tests/fortran/manifest.json`

## Task

Implement the minimum Fortran production code needed to make the approved no-Abaqus tests pass.

Use the production source layout approved in `/docs/reference-models/uel-3d-euler-beam.md`. If the layout is missing, prefer:

- `/src/fortran/kernels/uel_3d_euler_beam_kernel.f90` for testable calculation logic.
- `/src/fortran/abaqus/UEL_3D_EULER_BEAM.for` for the thin Abaqus `UEL` wrapper.

Implementation scope:

- Build the 3D Euler-Bernoulli local stiffness matrix.
- Build and validate the local-to-global transformation.
- Compute static internal force/residual contribution according to the approved Abaqus `UEL` sign convention.
- Fill `AMATRX` and `RHS` for the supported `LFLAGS` subset.
- Validate element length, orientation, and material/section properties.
- Keep `SVARS` and `ENERGY` behavior consistent with the approved interface contract.
- Keep Abaqus ABI wrapper thin; testable logic belongs in the kernel or no-Abaqus driver layer.

Follow RED -> GREEN -> VERIFY. Do not change the approved tests unless the upstream contract is objectively wrong; in that case mark the step `blocked` and explain which upstream document must change.

After GREEN evidence, create `/docs/build-test-reports/uel-3d-euler-beam-green.md` with commands, compiler mode, and summarized results.

## Tests To Write First

- Tests were written in step 6 and must already exist.
- Add only narrowly scoped additional no-Abaqus tests if implementation uncovers an untested requirement. If added, run RED for the new test before code changes.

## Acceptance Criteria

```bash
python -m unittest discover -s scripts -p "test_*.py"
python scripts/validate_fortran.py
python scripts/validate_reference_artifacts.py
python scripts/validate_workspace.py
```

The step is complete only when the no-Abaqus Fortran validation passes, or the step is marked `blocked` with concrete environment evidence such as missing Intel Fortran compiler.

## Validation Notes

- Do not create or modify external reference CSVs in this implementation step.
- Update `/phases/uel-3d-euler-beam/index.json` step 7 with `completed`, `error`, or `blocked` and a concrete summary or reason.

## Forbidden

- Do not add JavaScript/TypeScript/npm fallback.
- Do not run Abaqus analyses from this project.
- Do not generate reference CSVs.
- Do not bypass or weaken TDD guard behavior.
- Do not break existing tests.