Open System for Earthquake Engineering Simulation (OpenSees) is a finite element program developed by McKenna and Fenves (2001) at University of California, Berkeley. The software is not commercialized yet, and it is still an open-source program to date, it has mostly been used in academia by several research teams around the world. OpenSees employs object-oriented methodologies to maximize modularity and extensibility for implementing constitutive models, solution methods, and data processing and communication procedures (McKenna and Fenves, 2001).

The program consists of a wide range of inter-related libraries of material constitutive models, elements, solution algorithms, integrators, and equation solvers. These libraries are independent allowing great flexibility in combining the libraries to solve different types of engineering problems. This also allows earthquake engineering researchers to build upon each other’s contributions and developments. In OpenSees, the numerical model is introduced as a domain object, which is composed of all analysis components such as node, material models, elements, boundary conditions, loading patterns, and equation solvers. This domain is created by the user for each problem through an interface script using Tcl programming language.