|Operating system||Linux, Windows and Unix-like|
OpenSees, the Open System for Earthquake Engineering Simulation, is an object-oriented, software framework created at the NSF-sponsored Pacific Earthquake Engineering (PEER) Center. It allows users to create finite element applications for simulating the response of structural and geotechnical systems subjected to earthquakes. This framework was developed by Frank McKenna and Gregory L. Fenves with significant contributions from Michael H. Scott, Terje Haukaas, Armen Der Kiureghian, Remo M. de Souza, Filip C. Filippou, Silvia Mazzoni, and Boris Jeremic. OpenSees is primarily written in C++ and uses several Fortran numerical libraries for linear equation solving.
The license permits use, reproduction, modification, and distribution by educational, research, and non-profit entities for noncommercial purposes only. Use, reproduction and modification by other entities is allowed for internal purposes only. The UC Regents hold the copyright for OpenSees. 
Users of OpenSees create applications by writing scripts in the Tcl programming language. The TclModelBuilder class in the OpenSees framework extends an instance of the Tcl interpreter with commands for finite element model building and analysis.
OpenSees developers access the source code using Apache Subversion. Although anyone may check-out the source code anonymously, only a handful of individuals have check-in access.
The proper acronym capitalization for the "Open System for Earthquake Engineering Simulation" is OpenSees, as opposed to OpenSEES. This reflects the same unconventional capitalization of Tcl.
Prior to taking on the name "OpenSees," the framework was simply called "G3" in reference to the name of the PEER research group tasked with simulation development. The doctoral thesis of Frank McKenna on parallel object-oriented structural analysis formed the basis for "G3."