Eigenvalue analysis is often a necessary step before starting a dynamic response history analysis in OpenSees. You may want natural frequencies in order to compute Rayleigh damping coefficients, to apply modal damping, to compute modal properties, or to perform a response spectrum or modal superposition analysis. However, eigenvalue analysis can fail, indicating a problem with … Continue reading Why Your Eigenvalue Analysis Failed
Author: Michael H. Scott
I have been involved in the development, maintenance, and growth of OpenSees since its early days. I am interested in learning Python and improving my academic writing.
Partial-Span Uniform Loads
Although no one is going to use OpenSees to compute tributary loads for one-way floor systems, it’s still satisfying to analyze the resulting beam models. A common example is a beam that frames around one side of a floor opening. On the side of the beam with no opening, tributary area calculations produce a uniform … Continue reading Partial-Span Uniform Loads
Empty Spaces
Recently, a large OpenSees model was posted in an online forum with the poster asking why the analysis took longer than expected. Short answer: Not only did using a heavy-duty recorder that writes all node, element, and section data take up a big chunk of time, but using OpenSees's default linear equation solver, ProfileSPD, also … Continue reading Empty Spaces
Reverse Engineering the Equation Numberer
In OpenSees (and any other finite element software), equation numbering is a quiet, behind the scenes analysis option that users do not have to pay any mind. No matter how a user numbers the nodes in their model, e.g., for bookkeeping or from a mesh generator, the equation numberer will clean up any messes. But … Continue reading Reverse Engineering the Equation Numberer
2025 In Review
My annual review of blog stats and traffic might feel obligatory, but 2025 shows 2026 will likely ring in a new normal. Word Count The sum of all keys hit in 2025: 45 posts 26,253 words 583 words per post Compared to 2023 and 2024, the number of posts and words decreased, but the density increased. This slight … Continue reading 2025 In Review
Eigen Almost Hear You Sigh
ARPACK, the default eigenvalue solver in OpenSees, is very good at quickly finding a small number of eigenpairs (frequencies and mode shapes) for large models. Getting a few of a model’s lowest frequencies so you can check for rigid body modes and/or calculate Rayleigh damping coefficients is all most users care about. ARPACK gets the … Continue reading Eigen Almost Hear You Sigh
Modeling Is Always Nonlinear, but Not the Response
As a narrative device, an ellipsis omits events that are unimportant to a story and that an audience can easily understand and reconstruct if necessary. For example, a movie montage is a narrative ellipsis. In Bull Durham, we don't need to see every game in the Bulls' road winning streak. We just need to see … Continue reading Modeling Is Always Nonlinear, but Not the Response
Three Acts with the Two Node Link
This post follows three-act structure in verifying the flexural and shear response of the TwoNodeLinkSection element. Setup I’ve been working on the TwoNodeLinkSection element, which is like a TwoNodeLink, but uses a section object instead of multiple uniaxial materials. Basically the same spin the ZeroLengthSection puts on the ZeroLength element. Using a section object inside a two … Continue reading Three Acts with the Two Node Link
Combined Loadings
I talked with a graduate student from Mechanical Engineering the other day. The student is learning OpenSees and successfully analyzed a truss. No, not that truss from Example 1.1. After showing me the truss results, the student said something along the lines of “Deflections are fine and OpenSees does a good job, but I really … Continue reading Combined Loadings
Concrete23andMe
I’ve teased Concrete23 for some time now--so long that ChatGPT believes the model to be real. As you know though, Concrete23 was meant to be a riff on the material models in OpenSees that are clones of something else, but with one or two mutations. Like Concrete02IS’s addition of user-defined initial stiffness to Concrete02, or whatever insignificance ModIMKPinching02 … Continue reading Concrete23andMe
Portwood Digital 