Making a Point Load

The midterm exam in my graduate course on matrix methods of structural analysis was based on a linear-elastic, geometrically linear model of what is essentially Lee's frame. I asked the students to analyze the model using two elements and treat the point load as a member load instead of defining an additional node and applying … Continue reading Making a Point Load →

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 →

Handling a Doubt

After a post on the "constraint matrix not identity" warning message, a couple people followed up (one by post comment and the other by direct communication) that they didn't realize the Plain constraint handler works with equalDOF constraints. The Plain constraint handler has not always been able to handle equalDOF constraints--I vaguely recall Frank modifying … Continue reading Handling a Doubt →

Constraint Matrix Not Identity

The equalDOF, rigidLink, and rigidDiaphragm commands constrain one or more nodes to move in direct proportion to the motion of another node. In the case of the equalDOF command, the constraint is of the form us = up, where us is the displacement of the secondary node and up is the displacement of the primary … Continue reading Constraint Matrix Not Identity →

Do It Your Self-Weight

Most solid elements in OpenSees don't do body forces very well, if at all. Among elements that have body forces, the implementation and execution are inconsistent. With surface loads, the story is even more convoluted. However, OpenSees is pretty good at dynamics, so all solid elements handle mass density. Yes, the implementations are inconsistent because … Continue reading Do It Your Self-Weight →

Eigenvalues of the Stiffness Matrix

Students are exposed to eigenvalues and eigenvectors a few times through their structural engineering education. After the math department's obligatory treatment to sophomores with definitions, characteristic polynomials, and toy matrices, two to three years pass before students see eigenanalysis again as first year graduate students: Structural dynamics - find natural modes of vibration of a … Continue reading Eigenvalues of the Stiffness Matrix →

How to Use Block2D

Although the mesh commands outlined in previous posts (here and here) are more powerful, the block2D and block3D commands remain useful, especially if you want to use quadrilateral or brick elements. Ed C++ Love based the OpenSees block commands on the meshing functionality in FEAP. So, if you are familiar with FEAP, these commands should … Continue reading How to Use Block2D →

Rigid Diaphragm for 2D Models

The rigidDiaphragm command allows you to constrain the motion of multiple secondary nodes to the motion of a primary node, e.g., when simulating rigid floor diaphragms in 3D structural models. But what if you want to enforce rigid diaphragm constraints for a 2D model, e.g., as shown below? Prior to PR #764, you would have … Continue reading Rigid Diaphragm for 2D Models →

Element Self-Weight

A question posted the other day in the OpenSees Facebook group asked how to add self-weight to elements. I gave the easy I-can-answer-this-in-under-two-seconds answer of "use the eleLoad command". It turns out the complete answer is not so simple as it depends on the type of element to which you want to apply self-weight. The … Continue reading Element Self-Weight →

OpenSees Spy

A previous post on the 'fullGenLapack' eigenvalue solver led me down a rabbit hole of sorting out all the equation solvers (linear and eigen) and equation numberers available in OpenSees. I have yet to emerge from the rabbit hole, but I wanted to show how to create "spy" matrices from OpenSees models. A spy matrix … Continue reading OpenSees Spy →