Modal and Stiffness Proportional Damping

OpenSees allows you to use both modal damping and stiffness proportional damping in a dynamic analysis. This combination of damping models is useful when you want to control damping in the low frequency modes and not let undamped high frequency response tarnish the analysis. Consider a simplified model of a 40 story building. The story … Continue reading Modal and Stiffness Proportional Damping

Only a few years ago I realized that you do not have to use natural frequencies--you know, the ones you obtain from an eigenvalue analysis--to compute Rayleigh damping coefficients. This may not be news to some of you--I am often a little slow on the uptake. But I actually read a couple papers (here and … Continue reading More Ado About Damping

A previous post showed that, when compared to a couple of brute force approaches, using the sdfResponse command is the most computationally efficient approach to generating an earthquake response spectrum. During an OpenSees Cafe, Dr. Silvia Mazzoni suggested taking a more intelligent approach by "batching" the brute force SDF analyses. Instead of analyzing one oscillator … Continue reading Multi-Threaded SDF Analysis

Another Way to Get Bad Eigenvalues

With daily posts during NaBloPoMo, LBUs are highly coveted. And I'm not afraid to partake in incremental blogging. Heck, LPUs and incremental publishing seem to be de rigueur. Anyway, with zero shame, here's an insidious variation of a recent post on how to get bad eigenvalues from your OpenSees model. If negative mass can lead … Continue reading Another Way to Get Bad Eigenvalues

One Way to Get Bad Eigenvalues

If one of the eigenvalues for your model is zero or negative, you likely made a modeling error. The error could be due to boundary conditions, element stiffness, or mass definition. Let me show you how easy it is to make an error and get bad eigenvalues due to an error in mass definition. Suppose … Continue reading One Way to Get Bad Eigenvalues

Quick (and Dirty) Modal Damping

Frank recently told me about "quick" modal damping and explained it as "adding the modal damping forces to the right-hand side but not adding the modal damping terms to the dynamic tangent". The rationale for "quick" modal damping is to reduce computational expense due to: Assembly of modal damping terms into the dynamic tangent must … Continue reading Quick (and Dirty) Modal Damping

Parametric Oscillator

I sometimes ask collaborators for post ideas. At the end of our meeting this week, Prof. Mark Denavit suggested modeling a parametric oscillator in OpenSees and referred me to the following video. https://youtu.be/MUJmKl7QfDU OK, if you didn't watch the video, the response of a parametric oscillator is modified by some frequency-dependent parameter of the system. … Continue reading Parametric Oscillator

Negative Feedback Loop

Although it has its proper uses, I'm not a fan of the linear algorithm. I'm even less of a fan of modal damping. However, it's totally reasonable to use these two analysis options together--and if you do, watch out! I'll walk you through a recent encounter with this lethal combination, experienced during a live presentation … Continue reading Negative Feedback Loop

Get the Accel Out

In OpenSees, a UniformExcitation pattern is functionally equivalent to a regular load pattern, fitting into the framework of a time-varying scalar load factor and constant reference load vector. The scalar load factor is the input ground acceleration, $latex \ddot{u}_g(t)$, while the reference load vector is $latex {\bf P}_{ref}=-{\bf m}{\boldsymbol \iota}$ where $latex {\bf m}$ is … Continue reading Get the Accel Out