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
RBOC: Influence Lines
Searching for books recently, this title caught my eye. I do not plan to read the book, but I bet the message goes something like this: Instead of posting the latest notch on your CV and/or flashy results that no one can comprehend, post useful content in order to get leads and customers instead of … Continue reading RBOC: Influence Lines
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
You Get What You Give
In the early days of OpenSees, conventional wisdom was "You get what you pay for". Sure, the internet was around, but the only OpenSees resources were the wiki, the examples manuals, and the message board. Those resources are still around today. But there's a ton of other OpenSees resources available including packages, pre- and post-processors, … Continue reading You Get What You Give
Eigenvalues During an Analysis
How to compute the eigenvalues (natural periods) of a structural model during an analysis, as the stiffness changes due to yielding, unloading, reloading, large displacement, etc., is a common question. In general, periods elongate during yielding events, then shorten again upon unloading. The extent and duration of period change depends on the constitutive models and … Continue reading Eigenvalues During an Analysis
One Citation at a Time
Within the last day or two, I tripped 1000 citations for 2021 according to Google Scholar. Thank you, OpenSees! A colleague in Eastchester asked if the blog has helped my citation count. I don't think it has, except for reminding a couple people to cite OpenSees somehow instead of simply writing "We used OpenSees" somewhere … Continue reading One Citation at a Time
Section X
You can record section response in a frame element if you know the section number, or integration point, of the section whose response you would like to record. This is straightforward for distributed plasticity integration such as Lobatto or Legendre where section number 1 is at end I of the element and section number N … Continue reading Section X
Concrete02 with Control of the Initial Stiffness
Although they have the same compressive envelope, Concrete02 offers more features than Concrete01--tension strength, tension stiffening, and compressive unloading/reloading loops. However, one thing Concrete02 (and Concrete01) lacks is control over the initial stiffness, which is fixed at $latex E_c=2f'_c/\varepsilon_{c0}$. To overcome this limitation with Concrete02, Nasser Marafi created Concrete02IS (Concrete02 with control of the Initial … Continue reading Concrete02 with Control of the Initial Stiffness
Portwood Digital 