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?

Update September 13, 2021: The issue described in this post has been resolved as of OpenSeesPy version 3.3.0.1. 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 … 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 →

Discarded Wrappers

In writing a previous post on material wrappers available in OpenSees, I knew there was a wrapper that never made it into cvs, svn, or GitHub. I thought it was no big deal and decided to let it go. Then, looking back at some previous work on SSI in developing this modeling challenge, I re-discovered … Continue reading Discarded Wrappers →

SSI Modeling Challenge

I know enough soil-structure interaction (SSI) to be dangerous. This means I'm no expert in the subject, only someone who is capable of describing an SSI modeling challenge. The subject of this challenge is the soil-bridge model shown below. The model is based on research by a former M.S. student and a couple colleagues in … Continue reading SSI Modeling Challenge →

There’s a Wrapper for That

Many publications describe software design patterns for reusing object-oriented software. The most widely read book on design patterns is so influential that it has its own Wikipedia page. In this book, the "Gang of Four" offers two guiding principles for software design patterns. The first principle is to program to an interface, not an implementation, … Continue reading There’s a Wrapper for That →

Integration Points with Negative Weight

A colleague in Eastchester once told me that faculty have three, maybe four, good ideas over their career. In other words, a faculty member could have over a hundred papers, but there's only three or four underlying concepts. Perhaps it was "two, maybe three", but you get the point. Playing with integration points and weights, … Continue reading Integration Points with Negative Weight →

Where Is Eastchester?

I often mention Eastchester in posts, referring to a conversation with "a colleague in Eastchester" or showing an example from "a class I teach in Eastchester". But where is Eastchester? You won't find it on any map of the US Pacific Northwest. Eastchester is the home of fictional Cascadia College in Bernard Malamud's novel A New Life, … Continue reading Where Is Eastchester? →

Results of a New Challenge

After proposing a modeling challenge for linear-elastic analysis of a strongback frame, I proposed a second challenge for linear-elastic analysis of the Ziemian frame. There were eight participants in this challenge, an increase from five for the first challenge. Due to the light gravity loads on the frame, whether or not the analysis included self-weight … Continue reading Results of a New Challenge →