All structural engineering students learn the direct assembly method, where you fix all degrees of freedom (DOFs) in a structural model, then impose a unit value of displacement at and in the direction of the $latex j^{th}$ DOF in order to get the $latex j^{th}$ column of the stiffness matrix from the fixed-end forces of … Continue reading Closing the Loop on Direct Assembly
Category: OpenSees
How to Run an OpenSees Container in the Cloud
Now that OpenSees is just another Python module, you can do some pretty cool things in the cloud (and on the ground). One of those cool cloud things is running an OpenSees analysis in a container on Amazon Web Services (AWS) or Microsoft Azure. A KH-lenticular or Kelvin-Helmholtz wave cloud (from this tweet). With either … Continue reading How to Run an OpenSees Container in the Cloud
Global Distributed Loads
Distributed loads on frame elements in OpenSees are defined with respect to the local element axes as opposed to global axes. This choice made the implementation easy, but it can give OpenSees users more shadow work, like bagging your own groceries or pumping your own gas. When global distributed loads act on inclined elements, e.g., … Continue reading Global Distributed Loads
Thoughts After a Year of Blogging
Here are some random thoughts from the blog's first year. Prior to my first post, which was insubstantial and lame, I came up with about 45 ideas for posts of actual substance and saved them in my Drafts folder. Some of the ideas were, and still are, junk and will never be published. But, even … Continue reading Thoughts After a Year of Blogging
SSI Modeling Challenge Results
After two modeling challenges (here and here) for the linear-elastic response of frame structures, I thought it would be a good idea to do a soil-structure interaction (SSI) modeling challenge. I feared the large uncertainty of geotechnical engineering relative to structural engineering would stifle participation, but I received four entries. Three participants submitted a single … Continue reading SSI Modeling Challenge Results
Most Solvers Can Be Marplots
Have you ever tried to replicate the familiar beam stiffness coefficients $latex 12EI/L^3$, $latex 6EI/L^2$, $latex 4EI/L$, and $latex 2EI/L$ (there's a poem about them here) by imposing unit displacements and rotations at fixed supports? It should be one of the first sanity checks you make when using or developing new structural analysis software. You … Continue reading Most Solvers Can Be Marplots
A Marathon, Not a Sprint
When tasked with developing an OpenSees model for simulating the nonlinear dynamic response of, let's say, a multi-story reinforced concrete frame, you may be tempted to go straight to force-based frame elements with fiber sections comprised of Concrete23 and Steel08 material models. This sprint to the finish line will undoubtedly lead to an analysis that … Continue reading A Marathon, Not a Sprint
Recorders Not Recording?
Python is one of the best things to happen with OpenSees. Unfortunately, the break from Tcl has not been squeaky clean. A very sticky transition point has been element recorders. When we wrote the internal setResponse functions to identify which element, section, material, or fiber response to record, we put C-style int argc, char **argv … Continue reading Recorders Not Recording?
A Vector in the x-z Plane
Three-dimensional frame elements require user input for the local element axes. Although the local $latex x$ axis points from node I to node J, there is no automatic way to define the local $latex y$ and $latex z$ axes, i.e., how the section axes line up with the element. In two-dimensions, this is not an … Continue reading A Vector in the x-z Plane
One Iteration of a Second Order Analysis
I was recently asked if one Newton iteration of a second order analysis will give the same results as a first order analysis. This is a good question, and the answer depends on what you're after. I will explain the answer using "Benchmark problem Case 2" from Chapter C of the AISC Steel Manual Commentary. … Continue reading One Iteration of a Second Order Analysis
