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
Category: OpenSees
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
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
Not Just a Modeling Term
Master/slave is common terminology to describe relationships in many technical fields, e.g., between tables in databases or between devices in control systems. The terminology also appears in finite element analysis where the response of one node controls the response of another node through constraints. However, this terminology is based on archaic relationships within our society. … Continue reading Not Just a Modeling Term
Handle Your Constraints with Care
Manipulating the nodal equilibrium equations is necessary to enforce constraints between degrees of freedom (DOFs) at two or more nodes in a structural model. These multi-point constraints arise from assumptions of axial and flexural rigidity of frame elements, e.g., rigid diaphragms, and also between two nodes at the same location where some of the DOFs … Continue reading Handle Your Constraints with Care
Gimme All Your Damping, All Your Mass and Stiffness Too
Just because OpenSees is open source does not mean it is a fully transparent box. This is mostly because documentation has lagged behind development. So, pessimists would say the box is semi-opaque while optimists would characterize it as semi-transparent. But a few parts of OpenSees are definitely housed in an opaque box. Take, for instance, … Continue reading Gimme All Your Damping, All Your Mass and Stiffness Too
Meshing for Column Loads
For material nonlinear analysis of frame models, you can improve the computed response by using more displacement-based elements or more integration points in a force-based element. The material nonlinearity occurs inside the basic system, also known as the natural system or the kernel. To capture geometric nonlinearity due to large displacements, you have to go … Continue reading Meshing for Column Loads
