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This section includes the original examples. The objective of this primer is to provide new users of OpenSees (Open System for Earthquake Engineering Simulation) familiar structural engineering examples as a convenient method for learning how to use the software. OpenSees is an object-oriented framework for building models of structural and geotechnical systems, performing nonlinear analysis with the model, and processing the response results. The goal for OpenSees is to support a wide range of simulation applications in earthquake engineering. The details, however, on how OpenSees accomplishes this goal are not particularly important for new users, who are primarily interested in how to solve problems.
This primer examines a few typical examples. Most users will conduct a simulation with a scripting language that has been extended to incorporate the features of OpenSees. As new features are developed, such as material models, elements, solution methods, etc., the scripting language can be extended to include them. The scripting language is named Tcl/Tk, and it has many features for dealing with variables, expressions, loops, data structures, input/output, that are useful for doing a simulation. Some of the basic features of Tcl will be illustrated in the examples.
Although users do not need to understand the object-oriented principles in the OpenSees framework, some terminology helps in the description of the examples. We talk about commands creating objects, which may be a specific material, element, analysis procedure, etc.
To conduct a simulation, the user creates objects for three main purposes:
Modeling: The user first creates a ModelBuilder object which defines the type of model, and commands available for building the model. With a ModelBuilder defined, the user then creates the Element, Node, LoadPattern and Constraint objects that define the model. In this primer, the use of a basic ModelBuilder will be demonstrated.
Analysis: After defined the model, the next step is to create the Analysis object for analyzing the model. This may be a simple static linear analysis or a transient non-linear analysis. In OpenSees, an Analysis object is composed of several component objects, which define how the analysis is performed. The component objects consist of the following: {SolutionAlgorithm}, {Integrator}, { ConstraintHandler}, { DOF\_Numberer}, { SystemOfEqn}, { Solver}, and { AnalysisModel}. This approach provides a great deal of flexibility in how an analysis is conducted.
Output Specification: Once the model and analysis have been defined, the user has the option of specifying what is to be monitored during the analysis. This, for example, could be the displacement history at a node or internal state of an element in a transient analysis or the entire state of the model at each step in the solution procedure. Several Recorder objects are created to store what the user wants to examine.
In the examples, Tcl scripts are used to create a model, analysis, and output specification. The examples are (1) simple truss structure, (2) reinforced concrete portal frame, (3) two-story multi-bay reinforced concrete frame, and (4) a three-dimensional frame. The examples are not meant to be completely realistic, but they are representative of typical structures. The analyses performed on these models consist of simple static analysis, pushover analysis and transient analysis. An example of moment-curvature analysis is also performed on a reinforced concrete section.
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