OpenSees (the Open System for Earthquake Engineering Simulation) is an object-oriented software framework created during the National Science Foundation-sponsored era (1997-2007) of the Pacific Earthquake Engineering Research (PEER) Center. OpenSees 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.

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.”