Alpaca4d

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⚙️ Setup ​

Alpaca4d uses OpenSees as its analysis engine. OpenSees is a free, open-source software developed at UC Berkeley — you need to download and install it separately, then tell Alpaca4d where to find it.

Step 1 — Download OpenSees ​

Go to the official OpenSees download page and grab the executable for your operating system:

👉 https://opensees.berkeley.edu/OpenSees/user/download.php

You can also reach this page directly from Grasshopper via Alpaca4d → Settings → Download OpenSees....

Save the executable somewhere permanent on your machine (e.g. C:\OpenSees\OpenSees.exe on Windows or /usr/local/bin/OpenSees on Mac/Linux).

An OpenSees built with Mumps solves large models on every core of the machine, and Alpaca4d uses it when it finds it: see Solving on several cores.

Note

The Ten Node Tetrahedron needs an OpenSees built from the master branch since 16 October 2026. OpenSees 3.8.0 and the releases before it do not have the element's fixes, and Run Analysis says so rather than solve with it. Every other element runs on 3.8.0.

Step 2 — Set the OpenSees Executable in Alpaca4d ​

Once OpenSees is downloaded:

  1. Open Rhino3D and launch Grasshopper.
  2. In the Grasshopper menu bar, click Alpaca4d → Settings → Set OpenSees Executable...
  3. A file browser opens — navigate to and select the OpenSees executable you downloaded.
  4. A confirmation message will appear: OpenSees path set to: ...

The path is saved automatically and persists across sessions — you only need to do this once.

Step 3 — Run an Analysis ​

With the executable set, any Alpaca4d analysis component (Run Analysis, Natural Vibration, Moment Curvature, etc.) will find and launch OpenSees automatically.

If the path is not configured, the component will show a red error:

OpenSees executable path is not configured. Use Alpaca4d → Settings → Set OpenSees Executable in the Grasshopper menu.

Resetting the Path ​

To clear a previously saved path, go to Alpaca4d → Settings → Clear OpenSees Path. You can then set a new one with "Set OpenSees Executable...".

Alpaca4d → Settings → Display... sets the colours Model View draws node ids, element ids, section names, loads, gravity, masses, result values and the elements' edges in. They are kept in the same settings file as the OpenSees path.

Solving on several cores ​

The OpenSees downloaded from UC Berkeley solves on one core. An OpenSees built with Mumps factorises on every core of the machine. On a large model that makes an analysis 2 to 18 times quicker, and it needs less memory. On a small one it makes no difference. Analysis Settings has the timings.

Alpaca4d works with either OpenSees, and nothing in a definition has to change. The default System, Auto, uses Mumps when the OpenSees has it, and BandSPD on one core when it does not. To use an OpenSees built with Mumps, point Alpaca4d at it like any other, with Alpaca4d → Settings → Set OpenSees Executable....

Every analysis then solves on all the cores. To use fewer, right-click Analysis Settings and pick a number under Cores for Mumps. Natural Vibration always uses every core, and so does Linear Buckling on an OpenSees newer than 3.8.0.