Leaning Column

A “leaning column” approximates the global P-Delta effects of gravity loads supported by vertical load resisting systems that are connected to, but not explicitly included in models of, lateral load resisting systems. The leaning column approach can simplify the analysis of frame structures for lateral loads, as well as gravity loads with initial out-of-plumb imperfections. … Continue reading Leaning Column →

The Three-Act Verification

Although I would like to write more posts on OpenSees verification examples, I believe a post still needs context, or a story, to not become a dull trope. “I found this example, here’s the OpenSees model and some assertions, and look, the assertions pass” is not a compelling three-act structure of setup, confrontation (or conflict), and resolution. … Continue reading The Three-Act Verification →

Invertible Does Not Mean Stable

That you can invert a stiffness matrix does not tell you everything about the numerical stability of a structural model built in OpenSees or any other finite element software. In fact, no finite element software actually forms the inverse of the stiffness matrix, but let's go with the misleading terminology anyway. Consider the beam shown … Continue reading Invertible Does Not Mean Stable →

Not All at Once

I've been working on a project where we use OpenSees to analyze structural models for several load cases. We only care if the models can support the full load, not so much how they get to the full load. We apply all the load in one step with load control and Newton-Raphson. If that succeeds, … Continue reading Not All at Once →

A Very Stable Challenge

With linear structural analysis, a number is a number. However, reality starts to creep in when physics dictates that you need to account for nonlinear effects. The three member structure shown below is a perfect example--Example1.1.tcl, one of the most OG OpenSees examples. Have you ever paid attention to the member sizes though? If so, … Continue reading A Very Stable Challenge →