Software does exactly what you tell it to do. But problems arise when you don't know--or assume you know--what instructions you're giving. For example, many people like to use the 'fullGenLapack' eigenvalue solver in OpenSees because it will compute all N eigenpairs (eigenvalue with corresponding eigenvector) for a model whereas the default eigenvalue solver will … Continue reading More Than You or Eigen Chew

# Something, Something, Something Fiber

Two recent inquires asked why model response using frame elements with elastic fiber sections, i.e., sections with fibers whose stress-strain response is $latex \sigma=E\varepsilon$, differs from the response with elastic elements, i.e., defined by E, A, Iz, and Iy either with elasticBeamColumn elements or nonlinear elements with elastic sections. I've touched on this subject a … Continue reading Something, Something, Something Fiber

# OpenSees Scattergories

This is the blog's 200th post, which I will celebrate by not writing something technical. Instead, I will let you know that I organized the other 199 posts into the following Categories: Community - how to be a good OpenSees citizen; modeling challengesConstitutive Models - material, fiber, and section modelsElement Formulations - frame and solid … Continue reading OpenSees Scattergories

# Two Fibers Explain So Much

In a previous post, I asked how well we can capture the moment-curvature response of a rectangular section with EPP material using different integration methods with two fibers. For flexural response, two is the minimum number of fibers necessary to satisfy section equilibrium--one fiber for tension, the other fiber for compression. The previous post showed … Continue reading Two Fibers Explain So Much

# Much Ado About Damping

I do not remember why I was searching the internet for "damping" a couple weeks ago, but I came across this document on constructing a Rayleigh damping matrix, $latex {\bf C}=\alpha {\bf M}+\beta {\bf K}$, as a linear combination of mass and stiffness matrices. Instead of taking the usual approach of specifying damping ratios for … Continue reading Much Ado About Damping

# Major League

I refer to Bull Durham in more posts than in any other blog about OpenSees. I own up to that. After all, Bull Durham was an excellent movie--favorably-reviewed by critics and a box office success, grossing well more than its budget. The movie also won a few awards and was nominated for Best Original Screenplay … Continue reading Major League

# See the Convergence

Surely you have seen norms fly across the screen when running OpenSees with the print flag of the convergence test set to 1. The screen output slows down your analysis significantly, so you should only use print flag equal to 1 when you are trying to diagnose convergence issues. From a Jupyter Notebook. With OpenSees.exe, … Continue reading See the Convergence

# Two Fibers, Five Ways

I occasionally go down rabbit holes of numerical integration. These trips led me to Gauss-Radau integration, all the element integration options available in OpenSees, and, recently, Chebyshev integration. The latest rabbit hole, described in this post, deals with different ways to integrate section moment-curvature response using only two fibers. Consider a rectangular cross-section with EPP … Continue reading Two Fibers, Five Ways

# Verifying Will Never Be Easy

A previous post compared the natural periods computed by OpenSees for a relatively simple one-story, one-bay, elastic frame to published ETABS results. Many easy to make modeling choices (mass distribution, rigid joint offsets, relative stiffness, etc.) led to "incorrect" periods. The "correct" modeling choices gave periods from OpenSees that were very close to ETABS--close enough … Continue reading Verifying Will Never Be Easy

# The Stiffness Matrix Isn’t Everything

After several deliveries of graduate level courses in linear and nonlinear structural analysis, I have started to think that we over-emphasize the stiffness matrix in linear structural analysis. And this emphasis can lead to conceptual difficulties in nonlinear structural analysis. The steps to a linear analysis are presented as: Form the stiffness matrixForm the load … Continue reading The Stiffness Matrix Isn’t Everything