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 →

See You in December

This is my last post for November 2024. I will fail the NaBloPoMo one-post-a-day challenge. Or perhaps I will pass with 90%, or an A-, for 27 posts (including today) out of 30 days. Either way, there's nothing to prove. Time to focus on quality, not quantity. Thanksgiving is tomorrow in the United States. Or … Continue reading See You in December →

A Simple Base-Isolated Model

Although there are several base isolator elements available in OpenSees, I don't know much about base isolation. And when I don't know much about a topic, I look for a simple example to play with--usually something elastic that can be expanded later and that I can return to when I run into issues with nonlinear … Continue reading A Simple Base-Isolated Model →

Getting the Number of Nodal DOFs

Do you ever get tired of typing a lot of zeros, e.g., to apply a 20 kip load in the X-direction at node 18 on a 3D frame model? ops.timeSeries('Linear',1) ops.pattern('Plain',1,1) ops.load(18,20,0,0,0,0,0) I know what you're thinking. Did I put five zeros after the 20 kip load or only four? Well, to tell you the … Continue reading Getting the Number of Nodal DOFs →

Long Term Column Loading

Practically all analyses of reinforced concrete columns in OpenSees assume the loading is short term--concrete as strong as it was at 28 days out of the hopper. Depending on what you're doing, not accounting for long term load effects, i.e., concrete creep and shrinkage, may not be a big deal. But the effects of creep … Continue reading Long Term Column Loading →

OpenSees Tom Swifties

According to its Wikipedia page, a Tom Swifty is "a phrase in which a quoted sentence is linked by a pun to the manner in which it [the quoted sentence] is attributed". For example, here is a Tow Swifty to which OpenSees users can relate: "I completed my first nonlinear frame analysis!", Tom beamed. This phrase uses "beamed" to … Continue reading OpenSees Tom Swifties →

The Maximal Broken Example

When I ask for a minimal working example, I often receive responses that are the opposite of minimal and working, i.e., maximal or broken, in some cases reaching new lows. The following situations can be categorized as MBEs (maximal broken examples), i.e., not a minimal working example. 1. An Example from Your Journal Article I … Continue reading The Maximal Broken Example →

Dangling Recorders

Because I don't perform very large or time-consuming OpenSees analyses, I use recorders rather infrequently, instead preferring commands like nodeDisp and eleResponse. An issue with node and element recorders is they can segmentation fault after the node or element to which they point is removed from a model--a common scenario for progressive collapse simulations. A … Continue reading Dangling Recorders →

No Reaction

As the old mantra goes, "you can displace without deforming, but you can't deform without displacing". Other than a post here and there on particle dynamics, the examples on this site deal with element deformation (or strain), which requires nodal displacement. But what about examples with elements that undergo only rigid body displacement with no … Continue reading No Reaction →

The Old Code OpenSees

You have no doubt read grammatically correct sentences whose meaning you interpreted incorrectly. Only after re-reading did you realize the author's intent. For example, We modeled the walls with cracks. Your initial interpretation of this sentence could have been equivalent to "We used cracks to model the walls", which makes no sense and was not … Continue reading The Old Code OpenSees →