Two previous posts showed how to use Minjie's meshing functions to create line meshes for beam-column elements and 2D meshes for solid elements. This post will complete the trilogy by showing how to make a 3D mesh for solid elements. The bar shown below is the same model used in the post on 2D meshing. … Continue reading Three-Dimensional Meshing
Category: OpenSees
Line Mesh
The DiscretizeMember function, which dates back many years, was recently superseded by the line mesh command, written by Minjie. In addition to creating boundaries for solid meshes, as shown in this post, you can use line meshes to discretize a frame member (2D or 3D) into beam-column elements--just pass the optional element type and arguments … Continue reading Line Mesh
Two-Dimensional Meshing
Although the material and element models are there, OpenSees is not well known for solid finite element analysis. Creating a good mesh is key to solid FEA and there aren't many meshing tools implemented in OpenSees. The OG block2D and block3D commands work fine, but you have to manually join or tie adjacent meshes with … Continue reading Two-Dimensional Meshing
Cable Analysis
Analyzing cables subject to transverse loads is straightforward in OpenSees. Use a mesh of corotational truss elements with elastic uniaxial material. Of course, you can use any uniaxial material you like. The only trick is you have to scramble the nodes up a little bit--if you try to analyze a perfectly straight cable, you'll get … Continue reading Cable Analysis
Repeated Section Modes
If you use a section with linear-elastic response in the displacement-based, force-based, and mixed beam-column elements in OpenSees, you will get the same response from all three elements. TrueFalseIt depends The answer is it depends on the type of "section with elastic response" you use. Also, I wouldn't include "It depends" as a possible answer … Continue reading Repeated Section Modes
Trying to Get a Reaction
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
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
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
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
Portwood Digital 