Could Not Invert Element Flexibility

Due to inversion of section stiffness to flexibility, then inversion of element flexibility to stiffness, the force-based frame element implementation in OpenSees is finicky, to say the least. A common error message spawned from the force-based element is "could not invert flexibility..." You will see the same message many times, but, ultimately, you will get … Continue reading Could Not Invert Element Flexibility →

An Ode to Concrete23

You may know Trent Reznor as that Nine Inch Nails (NIN) guy who in 1994 released the controversial single "Closer" from the album The Downward Spiral. Alternative rock stations still play the radio edit of the song, where the verb in the chorus line is dubbed out. You may also know that Reznor had already … Continue reading An Ode to Concrete23 →

One and Only One

Two element formulations in OpenSees--forceBeamColumnCBDI and mixedBeamColumn--are capable of handling geometric nonlinearity within the basic system, i.e., P-little-delta effects. The CBDI formulation, described in Neuenhofer and Filippou (1998), approximates the transverse deflection using Lagrange polynomials fit through the curvature at each integration point. Due to the added computational expense and coding details, the forceBeamColumnCBDI element … Continue reading One and Only One →

Section Integration

If we can use procedures in Tcl or Python to lay out fiber patches and layers for standard section types, e.g., doubly-reinforced RC sections and wide flange steel sections, then why do we need the SectionIntegration class? You cannot say that it is not useful to have commands that define fibers in terms of basic … Continue reading Section Integration →

Life of OpenSees Pi

This is blog post #314, or $latex 100\pi$ to three sig figs. A more recognizable $latex \pi$-times-a-power-of-10 post than post #31 or post #3. And I'll have to maintain the current pace of post writing into my mid 80s in order to reach post #3141. In the G3 days of OpenSees, the inverse sine function … Continue reading Life of OpenSees Pi →

How to Mesh Shells

If you have created a mesh of planar (2D) elements in OpenSees, creating a mesh of shell elements (3D) is not that different. You can use either the block2D command or Minjie's line and mesh commands. However, you will quickly find that the block2D command--2D because the mesh is planar, not because the command is … Continue reading How to Mesh Shells →

Distributed Mass Beam Vibration

Most beam-column elements in OpenSees take mass density, $latex \rho$ (mass per unit length), as an input. The elements then return lumped mass $latex \rho L/2$ for the translational DOFs at the element ends. The elasticBeamColumn element can also return a consistent mass matrix with the -cMass input option. ops.element('elasticBeamColumn',tag,...,'-mass',rho,'-cMass') The dispBeamColumn and forceBeamColumn elements … Continue reading Distributed Mass Beam Vibration →

It’s OK to Be Negative

A common question is whether or not OpenSees allows negative tags for nodes, elements, materials, time series, patterns, etc. in a model. The obvious answer is "Don't think, just throw". But the polite answer is "Yes, you can define model objects with negative tags". Below is a minimal example where all tags are negative. import … Continue reading It’s OK to Be Negative →

Flipping Rotation Axes

Zero length rotational springs are a popular approach to model concentrated plasticity in frame members. Although distributed plasticity formulations are not perfect either, the zero length concentrated plasticity approach has a number of issues: implicit plastic hinge length (at best, it's a unitless 1) and having to define equalDOF constraints. There's also the outcomes of … Continue reading Flipping Rotation Axes →

Tuned Damper Models

An inerter is a passive vibration control device, where the force is proportional to relative acceleration, i.e., $latex F = b(\ddot{u}_2-\ddot{u}_1)$. The inertance, b, has units of mass. While working on inerter models in OpenSees, I found a paper by Lazar et al (2013) in which tuned inerter dampers (TID) were calibrated to give similar … Continue reading Tuned Damper Models →