What Is OpenSees? – A Comprehensive Introduction

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1. Introduction

Before beginning any modeling with specialized software, it is essential to understand its capabilities and applications. This article presents a detailed overview of OpenSees, an open‐source finite‐element framework widely used in civil and earthquake engineering.

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2. What Is OpenSees?

OpenSees (The Open System for Earthquake Engineering Simulation) is an open‐source finite‐element software developed at the University of California, Berkeley. It is designed for macro‐scale structural modeling and analysis, seismic simulations, soil and geotechnical modeling. Thanks to its high computational efficiency, OpenSees provides a versatile numerical platform for nonlinear analysis under seismic loading. While most civil‐engineering software integrates both design and analysis modules, OpenSees focuses exclusively on advanced structural analysis rather than on design workflows.

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3. Licensing

OpenSees is distributed freely under an open‐source license. Full details of its copyright and licensing terms are available at:
http://opensees.berkeley.edu/OpenSees/copyright.php

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4. Historical Development

The core of OpenSees originated in 1997 as part of a doctoral project at UC Berkeley, supported by the Pacific Earthquake Engineering Research Center (PEER). Under the direction of Gregory L. Fenves and developed by Frank McKenna, the software was initially named “G3” before being renamed OpenSees. Its object‐oriented architecture treats each variable as a feature of an ‘object,’ enabling users to add custom elements and material models to the source code. Contributions by researchers—such as Michael H. Scott and Minjie Zhu, who developed the Python interface—have continuously expanded the software’s capabilities. New elements or modules submitted by the community undergo review by the core developers, ensuring consistency and reliability.

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5. Software Architecture and Operation

OpenSees is implemented in C++ and leverages several Fortran numerical libraries for solving linear systems. Although these details are abstracted from the end user, extending OpenSees with new elements requires proficiency in C++. Users interact with the software via command scripts written in either Tcl or Python; each command is translated internally into C++ routines. While Tcl has a long history within the community—yielding a vast library of legacy scripts—Python’s richer ecosystem and modern syntax make it the recommended interface for new users. OpenSees remains predominantly an academic tool, with limited adoption in commercial engineering software.

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6. Macro vs. Micro Modeling

In finite‐element modeling, a micro‐scale approach subdivides a structural component (e.g., a beam or column) into thousands of small elements, each governed by detailed constitutive formulations. This yields high fidelity at the expense of computational cost. By contrast, a macro‐scale model represents the same component with a few “equivalent” elements, reducing the number of degrees of freedom and accelerating analysis. OpenSees supports both approaches: macro‐element formulations are prevalent in its structural analysis modules, while micro‐scale formulations are commonly employed in its geotechnical element libraries.

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7. Strengths of OpenSees

1. Open‐Source Flexibility
   – Researchers can rapidly integrate new materials, element formulations, and solution algorithms into the codebase—an often time‐consuming process in commercial packages.
2. Parametric Modeling
   – Through scripting, users can define structural models parametrically (e.g., specifying the number of bays or story heights) and automatically generate families of models for extensive parametric studies.
3. Extensive Nonlinear Libraries
   – OpenSees offers one of the most comprehensive sets of macro‐scale elements for structural nonlinear analysis, alongside robust micro‐scale geotechnical elements. Its modular solver architecture and diverse convergence algorithms help mitigate divergence issues common in nonlinear finite‐element methods.

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8. Limitations of OpenSees

1. Lack of Native Graphical Interface
   – OpenSees does not include a built‐in GUI for model setup or result visualization. Users typically employ external tools (e.g., MATLAB, Python libraries, Excel) or develop custom scripts for pre‐ and post‐processing.
2. Quality Control of User‐Contributed Models
   – While open source enables rapid innovation, it also requires careful validation of community‐submitted elements and materials to ensure reliable results.
3. Limited Educational Resources
   – As a research‐oriented tool, OpenSees lacks the extensive, readily accessible tutorials and support found with commercial software. To address this, the present website offers in‐depth conceptual explanations, derivations, and practical examples designed to accelerate the learning curve for new users.

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9. Comparison with Other Finite‐Element Software

Feature	OpenSees	Abaqus	ETABS / SAP2000
License	Free, Open Source	Commercial	Commercial
Macro‐element library (nonlinear)	Very extensive	Moderate	Relatively limited
Micro‐element library (nonlinear)	Robust for geotechnics	Very extensive	Relatively limited
Nonlinear solution algorithms	Very extensive	Moderate	Relatively limited
Script‐based modeling	Relatively straightforward	Moderately complex	Limited
Software customization	Unlimited	Restricted	Not available
Native graphical model creation	Not available	Strong GUI	Strong GUI
Native graphical result processing	Not available	Strong GUI	Strong GUI

OpenSees excels in macro‐scale structural analysis and large‐scale, complex modeling, whereas Abaqus offers superior micro‐scale detail and GUIs. ETABS and SAP2000 provide strong graphical environments but are less suited for highly nonlinear or dynamic research applications.

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10. Learning Resources for OpenSees

1. Official Command Reference
   – The Tcl and Python user guides on the OpenSees website provide concise command syntax and simple examples.
2. Theses and Peer‐Reviewed Articles
   – Many doctoral dissertations and journal articles include “numerical modeling” sections detailing OpenSees workflows; these can offer deeper insight but are often terse and dispersed.
3. Structured Online Courses
   – To overcome the limitations of the above resources, the courses offered on this website deliver comprehensive conceptual explanations, derivations of underlying equations, and extensive, varied examples—shortening the learning curve and enhancing practical proficiency.

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References

1. McKenna, F. (2011). OpenSees: A framework for earthquake engineering simulation. Computing in Science & Engineering, 13(4), 58–66.
2. OpenSees GitHub Repository: https://github.com/OpenSees/OpenSees
3. Python Software Foundation License / Python License 2.0.1
4. Tcl/Tk License Terms. Tcl Developer Xchange.
5. Creative Commons Attribution‐ShareAlike License 4.0
6. C++ Logo. Creative Commons Attribution‐ShareAlike License 4.0