OpenSees — A Guide for Structural Engineers: Free Nonlinear and Earthquake Analysis
OpenSees (Open System for Earthquake Engineering Simulation) was developed at UC Berkeley within the PEER centre, and it is one of the central tools in earthquake engineering research. It is not a replacement for ETABS or SAP2000 in everyday design — it is a tool for checking what really happens to a structure once it leaves the linear range.
OpenSees
Pacific Earthquake Engineering Research Center, UC Berkeley
Nonlinear and dynamic structural analysis driven by scripts, from the earthquake engineering world.
Get it from the vendorWhat you can do in OpenSees
- Material and geometric nonlinear analysis — force-based and displacement-based elements, P-Delta and large displacements.
- Fibre sections — a reinforced concrete section divided into confined concrete, unconfined concrete and steel fibres, each with its own material model.
- Pushover — the capacity curve of a structure under increasing lateral load.
- Time history — running real earthquake records on the structure.
- Soil and interaction — soil models, soil-structure interaction and seismic isolators.
Tcl or Python
Originally, OpenSees models were written in Tcl. Today most new users work with OpenSeesPy — the same engine as a Python package installed with one command. The advantage: you can build the model with loops, process results with NumPy and plot in the same script.
Python
Python Software Foundation
The language behind most modern engineering computation, and its full ecosystem.
Get it from the vendorWhat's missing
No official graphical interface — you build the model in code. Commercial interfaces exist, but they are paid. No built-in code checks (Eurocode, ACI or national codes) — it is an analysis engine, not design software. Most importantly: choosing material models and nonlinear parameters needs deep understanding, otherwise the results are meaningless.
Who it suits
Graduate students and researchers, engineers working on performance-based design, seismic assessment of existing buildings, seismic isolation, or any case where linear analysis is not enough.
How to start
1. Install OpenSeesPy. 2. Start from a simple 2D frame example in the official documentation. 3. Compare a linear result with familiar software — before adding nonlinearity. 4. Add fibre sections and run a pushover. For visualising complex results, ParaView helps.
ParaView
Kitware / Sandia / Los Alamos
Large-scale simulation post-processing — slices, streamlines and animation.
Get it from the vendorAlternatives and companion tools
For simple linear frame analysis — Ftool and Frame3DD. For solid finite elements — PrePoMax with CalculiX. And for complex geotechnical-structural problems — Code_Aster.
Ftool
PUC-Rio, Tecgraf
2D frame and beam analysis — fast to enter, aimed at learning and order-of-magnitude checks.
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Frame3DD
Frame3DD Project
3D frame and truss solver driven by an input file — excellent for automation and repeat runs.
Get it from the vendorPrePoMax
University of Maribor
Full graphical front end for CalculiX — meshing, materials, loads and results in one window.
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CalculiX
CalculiX Project
Open-source FEA with syntax close to Abaqus. The solver behind the FreeCAD FEM workbench.
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Code_Aster
EDF
EDF industrial finite element solver — nonlinear, dynamic and thermo-mechanical, open source.
Get it from the vendorBottom line
OpenSees is the tool behind many papers in earthquake engineering — and whoever knows how to use it can assess a structure at a level ordinary design software does not reach.