OpenSees will never win any awards for documentation. Although many features, e.g., Concrete23, are not documented because no one uses them, a few useful items lack documentation. For instance, did you know that you can apply trapezoidal member loads to beam elements? You can do so with additional arguments to the eleLoad command with the … Continue reading Trapezoidal Beam Loads
Category: Element Formulations
Release the Plastic Hinge
How close to a true moment release can you make the plastic hinge at one end of an OpenSees beamWithHinges element? It's a question I've thought about before, and it came up again recently. A simple fixed-fixed beam can start to answer the question. Imposing a unit rotation at one end will produce moment reactions … Continue reading Release the Plastic Hinge
Heavy as a Chebyshev
Numerical integration, or quadrature, is essential for material nonlinear finite element formulations. Gauss, Gauss-Lobatto, or a plastic hinge approach is all you need for frame elements. And for fiber sections, midpoint integration gets the job done. Besides some highly specialized cases, there's no need to use other types of numerical integration in nonlinear structural analysis. … Continue reading Heavy as a Chebyshev
Did You Pass Your Patch Test?
The patch test is one of the standard "sanity checks" of finite element implementations. To pass a patch test, an irregular mesh of elements must be able to reproduce a constant stress field. Interior nodes of the mesh should also displace according to a linear displacement field. OpenSees is not known so much for its … Continue reading Did You Pass Your Patch Test?
Wind in Warped Sails
The Island of Unfinished OpenSees Business grows every year. I own a few acres on that island and I'm sure you do too. Occasionally, items make their way off the island and get shipped to the mainland, i.e., the OpenSees GitHub repository. Take, for example, the warping frame elements developed at the University of Sydney … Continue reading Wind in Warped Sails
Plastic Rotation
Plastic rotations are a common damage measure for frame members and frequently define limit states of structural performance under seismic loading. The calculation of plastic rotations for frame elements in OpenSees is based on a decomposition of deformations into elastic and plastic components. The elastic deformations are obtained by elastic unloading of the basic forces, … Continue reading Plastic Rotation
Nonlinear Elements, Elastic Sections
Using nonlinear elements, particularly the forceBeamColumn element, with elastic sections is just as good as, if not better than, using the elasticBeamColumn element for many reasons. Not only do force-based elements with elastic sections make the transition to material nonlinearity easy, they also facilitate debugging your model. Another reason I like force-based elements is you … Continue reading Nonlinear Elements, Elastic Sections
When a Deal Breaker Is Not a Deal Breaker
We often place too much emphasis on obtaining mathematically exact solutions for structural models. While it's important to obtain exact solutions, e.g., for element development and comparing softwares, it's not always necessary and definitely not always a deal breaker. The important thing is to know whether or not an exact solution is possible and the … Continue reading When a Deal Breaker Is Not a Deal Breaker
Rigid Joint Offsets
The geometric coordinate transformation objects handle rigid joint offsets for frame elements in OpenSees. This is nice because the code for the transformations of displacements and forces is not duplicated in the element state determinations. There are three things to keep in mind when using rigid joint offsets. First, the offsets are global with respect … Continue reading Rigid Joint Offsets
How to Bend Beams
How to Boil Water was one of the first cooking shows on the Food Network. Emeril Lagasse was the original host and the show was geared toward people with little to no cooking experience. Everyone starts as a beginner. I was no exception when learning OpenSees. But when I'm asked where's the best place to … Continue reading How to Bend Beams
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