cande-wrapper

Python wrapper for the CANDE (Culvert ANalysis and DEsign) finite element engine.

The CANDE-2025 Fortran source is compiled via f2py into a native Python extension module, allowing direct calls from Python without subprocess overhead or DLL wrappers.

Source · PyPI

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Installation

Install from PyPI (includes pre-built wheels for Windows and Linux):

pip install cande-wrapper

For development (requires gfortran):

pip install -e ".[dev]"

Build prerequisites (development only)

Dependency	Version	Purpose
Python	>= 3.10	Runtime
gfortran	any recent	Compiles the CANDE Fortran engine
NumPy	>= 1.26	f2py extension building and runtime
Meson	>= 1.1	Build system
Ninja	any recent	Build backend for Meson

Installing gfortran on Windows

Via MSYS2:

# In MSYS2 terminal
pacman -S mingw-w64-x86_64-gcc-fortran

# Add to PATH (PowerShell)
$env:PATH += ";C:\msys64\mingw64\bin"

Or via conda:

conda install -c conda-forge gfortran

Quick Start

from cande_wrapper import CandeEngine

engine = CandeEngine(work_dir="path/to/my/models")
result = engine.run("EX1")  # reads EX1.cid, writes EX1.out
print(result.output_text)

Vehicle definitions

Standard AASHTO design vehicles are available for live-load analysis:

from cande_wrapper import Vehicle

hs20 = Vehicle.hs20()       # AASHTO HS-20 (72 kip)
hs25 = Vehicle.hs25()       # AASHTO HS-25 (90 kip)
tandem = Vehicle.tandem()   # AASHTO design tandem (50 kip)
cooper = Vehicle.cooper_e80()  # Cooper E-80 railroad (160 kip)

Custom vehicles can be defined with arbitrary axle configurations:

from cande_wrapper import Vehicle, WheelFootprint, Axle

truck = Vehicle(
    name="Custom Truck",
    footprint=WheelFootprint(length=12.0, width=24.0, spacing=84.0),
    axles=[
        Axle(weight=12000, spacing=0),
        Axle(weight=40000, spacing=180),
        Axle(weight=40000, spacing=54),
    ],
)
print(f"{truck.name}: {truck.total_weight/1000:.0f} kip")

CANDE Input Files

CANDE reads .cid input files (CANDE Input Data). These are fixed-format text files with sections:

• A-1: Master control line (analysis/design mode, solution level, title)
• A-2: Pipe type and element counts
• B-*: Pipe material properties
• C-*: Mesh, node, and element definitions
• D-*: Soil properties and load steps
• STOP: End-of-problem marker

Multiple problems can be run back-to-back in a single .cid file, each terminated by STOP.

An example input file is included at tests/example_data/MGK-IO.cid.

Output Files

After running engine.run("prefix"), these files are created in the working directory:

File	Description
prefix.out	Main analysis output report
prefix.log	Engine log messages
prefix.ctc	Table of contents for the output
prefix_MeshGeom.xml	Mesh geometry (for visualization)
prefix_MeshResults.xml	Mesh results (for visualization)
prefix_BeamResults.xml	Beam element results
prefix_PLOT1.dat	Plot data file 1
prefix_PLOT2.dat	Plot data file 2

The CandeResult object returned by engine.run() gives convenient access to the main output files:

result = engine.run("EX1")
print(result.output_file)   # Path to EX1.out
print(result.log_file)      # Path to EX1.log
print(result.output_text)   # Full contents of EX1.out

Testing

pytest -v                    # unit tests (no build required)
pytest -m integration -v     # integration tests (requires build)
pytest -m "" -v              # all tests

Troubleshooting

gfortran: command not found

gfortran is not on your PATH. Verify with gfortran --version. On Windows, ensure the MSYS2/MinGW or conda bin directory is in your PATH.

Linker errors about missing symbols

The meson.build file lists Engine source files explicitly. If you see unresolved symbols like plasti_ or prhero_, the corresponding .f file needs to be added to the engine_sources list in meson.build.

ImportError: CANDE Fortran extension not found

The package was imported but the compiled extension (_cande.pyd / _cande.so) was not found. Rebuild with:

pip install -e . --no-build-isolation

Large memory usage

The CANDE engine statically allocates ~320 MB for the stiffness matrix (REAL*8 A(20000,2000) in system.fi). This is normal for FEA solvers and is mapped as virtual memory.