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 →

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 →

Ways to Analyze This

A previous post challenged readers to analyze a simple frame model subjected to static loads. The model had material nonlinearity via tension-only diagonal cables and geometric nonlinearity via the P-Δ effects in the columns. The Challenge Despite these rather simple nonlinearities, analysis of the frame for the given vertical and lateral loads will fail using the default analysis options … Continue reading Ways to Analyze This →

Daisy Chains and Gaffes

When analyzing rigid bodies with multi-point constraints, one potential problem with the Transformation constraint handler is the sequencing of primary and secondary, or retained and constrained, nodes across multiple constraints. Using OpenSees to solve Problem 9.39 from J.C. Smith’s Structural Analysis is a perfect opportunity to show how defining constraints in series, i.e., daisy chaining the constraints, can knock … Continue reading Daisy Chains and Gaffes →