# Off the Hooke

To grasp nonlinear structural analysis, unlearn Hooke's Law, $latex \sigma = E\varepsilon$. And all derivative outcomes of Hooke's Law. Like $latex \kappa=M/EI$ and $latex {\bf K} {\bf U} = {\bf P}$. https://www.youtube.com/watch?v=z4jeREy7Pbc When steel yields, concrete cracks, etc., Hooke's Law no longer applies and holding on to it can lead to erroneous interpretations of nonlinear … Continue reading Off the Hooke

# Verifying Ain’t Easy

I've posted a few modeling challenges on frame analysis (strongback, Ziemian, and stability) and soil-structure interaction. However, I recently accepted a challenge from George Chamosfakidis to see if OpenSees can give the same periods and mode shapes reported in the ETABS verification example shown below. Published verification examples typically just show the "correct" result and … Continue reading Verifying Ain’t Easy

# Absolutely, It’s Relative

One of the most frequently asked OpenSees questions is whether node recorders record absolute or relative displacement (relative to the ground) when a model is subjected to a uniform excitation. There's several approaches to find the answer to this question. One solution is to apply a simple uniform excitation--like a constant ground acceleration--to an SDF … Continue reading Absolutely, It’s Relative

# You Gotta Keep ’em Aggregated

The SectionAggregator was one of my few useful OpenSees ideas. This class gives a flexible way to combine, or aggregate, modes of force-deformation in a single section model. The idea for SectionAggregator came from the Decorator software design pattern, the same pattern from which so many UniaxialMaterial wrappers were spawned (here and here). In fact, … Continue reading You Gotta Keep ’em Aggregated

# Minimal Working Example

When people post online or e-mail me about what could be a bug in OpenSees, I'll ask for a minimal working example (MWE), i.e., a simple script the demonstrates the problem. I don't want to deal with elaborate scripts--yours or mine. So, what does an MWE look like for OpenSees? Here's a non-exhaustive list of … Continue reading Minimal Working Example

# Tcl as a Front End for Python

I know I'm not the only one who enjoys converting between scripting languages or between structural analysis programs. I've had fun writing bespoke Tcl middleware between OpenSees and MATLAB, but now OpenSeesPy makes all of that obsolete. But, let's say you have an OpenSees Tcl script that you'd like to run in OpenSeesPy. There's a … Continue reading Tcl as a Front End for Python

# There’s Three, Actually

The displacement-based and force-based formulations garner a lot of comparisons for simulating nonlinear frame response. My Google Scholar alerts tell me so. And I even wrote a post comparing the two formulations. Doc Ock from Spider-Man: Into the Spider-Verse There is a third formulation--the mixed formulation. Alemdar and White compared three frame element formulations (displacement-based, … Continue reading There’s Three, Actually

# Random Bullets on Blogging

It's 2021. So, why blog when peak blogging was like 2007? Here are some random bullets on why blogging works for me. The more you write, the more ideas you generate. Usually it's ideas for more blog posts, but sometimes it's new ideas for research.Blog posts have a direct, conversational tone. Like I'm talking with … Continue reading Random Bullets on Blogging

# The Rayleigh Quotient

Eigenvalue analysis wasn't giving me what I wanted the other day. So, to make a long story short, I decided to try Rayleigh's method. I won't go through all the details of Rayleigh's method, but the basic idea is you can obtain a very good approximation of the fundamental frequency of a structural model by … Continue reading The Rayleigh Quotient