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

# Tag: Frame elements

# 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

# 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

# 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

# More Is Not Always Better

I sometimes run across simulations where frame member response is computed using displacement-based beam-column elements with more than two Gauss points per element. These elements require at least two Gauss points to ensure a complete solution and to capture the exact solution for a linear-elastic, prismatic member. While it is well known that you can … Continue reading More Is Not Always Better

# Better Late than Never

Frame finite elements abound in OpenSees, but something very useful has always been missing--a linear-elastic beam element with moment releases. Sure, you can define two nodes at the release location and use the equalDOF command; however, that can be error prone and you're at the mercy of the constraint handler. On the positive side, this … Continue reading Better Late than Never

# A Tale of Two Element Formulations

The question of whether to use the force-based or displacement-based formulation for material nonlinear frame analysis is one that comes up a lot. The answer depends on a few factors, mostly the material and the element length. To get a sense of the basic issues, I will compare the two element formulations with a numerical … Continue reading A Tale of Two Element Formulations

# Be More Specific than “Fiber Element”

I hear the term "fiber element" a lot, as in "We used fiber elements to model the response of this reinforced concrete moment frame". Usually, when people say something like this, they are referring to "force-based distributed plasticity frame elements with fiber-discretized cross sections". That is a mouthful. I can see the desire to shorten … Continue reading Be More Specific than “Fiber Element”

# Is Plastic Hinge Integration a Fruit or a Vegetable?

If distributed plasticity elements are fruits and concentrated plasticity elements are vegetables, then plastic hinge integration is something like a tomato. Based on papers I've read, manuscripts I've reviewed, presentations I've seen, etc., there's some confusion as to whether plastic hinge integration is a fruit or a vegetable. Here's my two cents. Concentrated plasticity elements … Continue reading Is Plastic Hinge Integration a Fruit or a Vegetable?