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 →

You Didn’t Build That

At what point does a "customized version" of OpenSees become distinct from the main OpenSees? After tinkering with OpenSees, when does it become "your own"? In most cases, the answer to both of these questions is "never". Sure, you will learn a lot by implementing Concrete24, even if it is only an incremental improvement over … Continue reading You Didn’t Build That →

Slender Things

Using fiber sections and the corotational geometric transformation is an easy way to simulate combined material and geometric nonlinearity in column members. A previous post examined this approach for steel columns where residual stresses play an important role in the axial load capacity. In this post, I will show the corotational mesh approach for non-sway … Continue reading Slender Things →

OpenSeesBf

OpenSees started with Tcl and is now riding the wave of Python's popularity. But, it's not too soon to ask which language the next OpenSees interpreter will use. In my opinion, Julia is the front runner. However, I would like to put a bid in for Brainf*ck, an esoteric programming language shown to me a … Continue reading OpenSeesBf →

Interpolation of Ground Acceleration

There was a question on GitHub a few months ago about whether or not OpenSees uses linear interpolation when the analysis time step is smaller than the time step (digitization) of an input ground acceleration. This is a good question as I've used other software that does not interpolate and instead uses the acceleration of … Continue reading Interpolation of Ground Acceleration →

Don’t Think, Just Throw

The uncertainty of an OpenSees analysis often stops people in their tracks. What will happen if I use this input for that model? Will I be able to simulate this behavior? What if the analysis fails to converge? Don't think. Just throw. https://youtu.be/qa3EseH3Hgc Nothing bad is going to happen if you have an incorrect input … Continue reading Don’t Think, Just Throw →