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🔌 Alpaca4d and OpenSees ​

Alpaca4d does not solve the model itself. It writes the model as an OpenSees input file, a Tcl script, runs the OpenSees program installed on your computer on it, and reads the results back from the file OpenSees records them in. Grasshopper is where the model is built and the results are read; OpenSees is where it is solved.

What happens when you press run ​

  1. The deck is written. Assemble has already made the components one model, and each piece knows the OpenSees command it stands for: element forceBeamColumn, fix, pattern Plain and the rest. Run Analysis writes them out as a .tcl deck: the model, then the recorder, then the analysis block from Analysis Settings, then wipe.
  2. OpenSees runs. Run Analysis starts the OpenSees executable set under Alpaca4d ▸ Settings ▸ Set OpenSees Executable…, with the deck as its argument, and waits for it to finish.
  3. The results are recorded. As OpenSees solves, its MPCO recorder writes the results of every converged step to a results file.
  4. The model comes back. Everything OpenSees printed becomes the log output. Run Analysis reads it for errors and warnings, and hands back the model with its results attached, or an empty output if the run failed, so the result components go quiet rather than show stale numbers.
  5. The results are read. The result components and Model View read the results file when they compute, not when the analysis runs.

The other analyses work the same way, with a deck of their own. Natural Vibration runs an eigen command, reads the eigenvalues from what OpenSees prints and the modal properties from a report file. Linear Buckling has OpenSees print the stiffness matrices and solves the eigenvalue problem in Alpaca4d itself, since OpenSees has no buckling analysis. Moment Curvature runs a section-analysis deck, which its log output shows in full.

The results file ​

The MPCO recorder, written by M. Petracca and G. Camata at ASDEA Software Technology, writes an HDF5 file that the result components read back, and that STKO opens too. Run Analysis records a set of results to suit the analysis type: a transient analysis adds velocities and accelerations to what a static one records. An MPCO Recorder on Assemble chooses the results instead: leaving out what you will not read makes a large analysis faster and its file smaller. Every converged step is recorded, so the steps the result components can read are the steps of Analysis Step.

Where the files go ​

The decks and the results files go into a folder called AlpacaResults, next to the Grasshopper file. Each name carries the first eight characters of the analysis component's InstanceGuid and the number of the run, such as recorder_<id>_0.mpco. Two analysis components in one document, or one component solving a tree of models, one per branch, each keep files of their own, and solving again overwrites the same files rather than adding new ones. A results file named on an MPCO Recorder goes beside the Grasshopper file itself.

Save the Grasshopper file first. Unsaved, Run Analysis writes its files to a folder in the system's temporary folder, and Natural Vibration refuses to run. Since the result components read the results file later, keep it where the analysis left it.

Why OpenSees is installed separately ​

OpenSees is open-source software from UC Berkeley, under the Regents' own licence. It is free to use, but selling software that includes OpenSees source code needs a licence from UC Berkeley. Alpaca4d is a commercial plug-in, and it does not include, bundle or resell OpenSees: your use of OpenSees runs under UC Berkeley's terms, which you accept with them when you download it (License). You download it yourself and point Alpaca4d at it once, as Setup describes; the path is kept from one session to the next.

It also leaves the choice of OpenSees to you:

  • The OpenSees from UC Berkeley solves on one core. One built with Mumps factorises on every core, which makes a large model 2 to 18 times quicker. The default System, Auto, uses Mumps when the OpenSees has it.
  • The Ten Node Tetrahedron needs an OpenSees built from master since 16 October 2026. Run Analysis says so on an older one, rather than solve.
  • Linear Buckling on OpenSees 3.8.0 is limited to 4000 equations; a newer build has no such limit.

The deck is yours ​

The .tcl deck is plain text in OpenSees's own documented format. You can read it to see exactly what was solved, keep it with the project, or run it with any OpenSees build, without Alpaca4d. Serialize writes the model part of the deck without running it, and Deserialise reads one back into a model. When a result looks wrong, reading the deck and the log is the quickest way to find out why.

How closely the answers match textbook and NAFEMS results is shown in the benchmarks.

See also ​