Initial Conditions

There are a couple ways to set initial conditions for nodal displacements and velocities in OpenSees. If you look at the end of this post, you'll see the hard way to do it: set initial displacements as single point (sp) constraints in a load pattern, do one analysis step, then remove the load pattern prior … Continue reading Initial Conditions →

Monte Carlo Simulation

The uncertainty associated with a finite element analysis is as important, if not more important, than the results of the analysis itself. Thanks to Terje Haukaas, OpenSees has several modules for finite element reliability analysis: FORM, FOSM, SORM, and several other methods to quantify uncertainty. Unfortunately, those methods have not yet made their way into … Continue reading Monte Carlo Simulation →

Means and Ends

OpenSees is often a means to an end, or an instrument, with the end being an advanced degree. There is nothing wrong with that! But OpenSees can also be an end in itself. You can learn to automate workflows, to think algorithmically, and to manage data structures--all in the context of earthquake engineering. That end … Continue reading Means and Ends →

Five Tweets from the Bot

The OpenSees Twitter bot is almost two years old. The bot has 356 followers, compared to 200 followers a year ago. Impressive OpenSees work continues to be tweeted. Here are five of the most recent tweets gleaned by @OpenSeesTweets. https://twitter.com/NHE_SimCenter/status/1460763867047501831 https://twitter.com/ccaprani/status/1459045946159812608 https://twitter.com/mikusscott/status/1451608792135651330 https://twitter.com/SilviaMazzoni_/status/1450195727783641088 https://twitter.com/6icees2021/status/1447127835261480960

Repeated Section Modes

If you use a section with linear-elastic response in the displacement-based, force-based, and mixed beam-column elements in OpenSees, you will get the same response from all three elements. True False It depends The answer is it depends on the type of "section with elastic response" you use. Also, I wouldn't include "It depends" as a … Continue reading Repeated Section Modes →

Trying to Get a Reaction

OpenSees does not compute reactions automatically because this can be a time consuming process--OpenSees assembles reactions over all nodes in a model, not just over the nodes that are constrained. When performing response history analysis, assembling reactions is likely not something you want or need to do at every time step. You probably just want … Continue reading Trying to Get a Reaction →

Inerters Everywhere

Vibration control devices based on relative acceleration, or inerters, are all the rage these days. So it's no surprise that inerter models are making their way into OpenSees. As far as I know, two inerter elements are available: InertiaTruss and Inerter. There has also been a third attempt at inerters, but via a material model. … Continue reading Inerters Everywhere →

OpenSeesaw

A moderately frequent OpenSees question is what happens to a structural model when you apply a rotational ground motion as a uniform excitation. Sure, rotational masses will receive effective earthquake forces, but what about dynamic DOFs that are orthogonal to the rotational ground excitation, i.e., with a "moment arm"? And if there is a moment … Continue reading OpenSeesaw →

Your DLL Might Be Ignored

I am not a fan of DLLs (dynamic-link library) for material and element models in OpenSees. There are many technical reasons for my dislike of DLLs--fragile code, multiple versions swirling around online, keeping up to date with the latest OpenSees executable, debugging difficulty, and cross-platform compatibility to name a few. So, if you've published your … Continue reading Your DLL Might Be Ignored →