A Nod to Backward Compatibility

I didn't want to do it, but I imagined an OpenSees user somewhere out there converting OpenSees Tcl scripts to OpenSeesPy--either manually line by line or using a converter script--and ending up with lines of code that look something like this: ops.section('Fiber',5) ops.patch(...) ops.layer(...) # tag I J secI lpI secJ lpJ E A I … Continue reading A Nod to Backward Compatibility

Shear Verse, Same as the First

In the same vein as a previous post, this post will show a basic comparison of material nonlinear displacement-based and force-based formulations with axial-flexure-shear interaction in frame elements. The timoshenkoBeamColumn element interpolates constant shear deformation along its length, along with constant axial deformation and linear curvature. Two-point Gauss-Legendre integration over the element is sufficient to … Continue reading Shear Verse, Same as the First

A Solution, Just Not The Solution

Force-based elements satisfy equilibrium in strong form, even with member loads. However, this does not mean force-based elements always get the exact solution. Consider a simple prismatic, linear-elastic beam with a point load at mid-span. Using a single force-based element with a single point load applied to the element using the eleLoad command. E = … Continue reading A Solution, Just Not The Solution

Plane Sections Do Remain Plane

Here's another conversation I've had with a concerned user (CU) of OpenSees, not necessarily the same CU that was worried about OpenSees crashing due to non-convergence: CU: "Do plane sections remain plane in the material nonlinear range of response?"PD: "Yes."CU: "What about with force-based frame elements?"PD: "Yes."CU: "How is that possible?"PD: "Because there's no other … Continue reading Plane Sections Do Remain Plane

Non-Prismatic Frame Elements

Long before the BeamIntegration abstraction, there was only Gauss-Lobatto integration for force-based elements, with a single section model copied to each integration point. This made it impossible to use a single element to simulate the response of an RC member with different reinforcing details along its length, or any member that was inherently non-prismatic. While … Continue reading Non-Prismatic Frame Elements

Failed to Get Compatible …

If you've used the force-based beam-column element in OpenSees, you've likely come across this warning involving element forces and deformations: I've encountered this warning many times and so have others. In fact, I lifted the above image from a recent post on the OpenSees Facebook group. I tried to come up with a MWE to … Continue reading Failed to Get Compatible …

Force-Based Beam-Column Integration Options

I like numerical integration because it allows you to do a lot of interesting things with force-based frame elements--so much more than simulating the response of reinforced concrete moment frames. Numerous numerical integration options are available in OpenSees, so in 2011 I wrote and uploaded to the OpenSees wiki a PDF summarizing those options. According … Continue reading Force-Based Beam-Column Integration Options

A Tale of Two Element Formulations

The question of whether to use the force-based or displacement-based formulation for material nonlinear frame analysis is one that comes up a lot. The answer depends on a few factors, mostly the material and the element length. To get a sense of the basic issues, I will compare the two element formulations with a numerical … Continue reading A Tale of Two Element Formulations