How to Bend Beams in 3D

Most structural frame models are analyzed in two dimensions (2D), for a variety of legitimate reasons. But sometimes, you have to go to three dimensions (3D). And the most confusing thing about making that jump in OpenSees is the geometric transformation and its "vector in the x-z plane". Instead of going into the details of … Continue reading How to Bend Beams in 3D →

Shutting Off the Containment Unit

If you've used OpenSees--even if you're a geotech--you've used the force-based element. When Remo implemented the force-based element, it was the only material nonlinear frame element available in OpenSees (G3 at the time); thus, the original name nonlinearBeamColumn. Only after a standard displacement-based frame element (dispBeamColumn) was added did we change the name from nonlinearBeamColumn … Continue reading Shutting Off the Containment Unit →

Eccentrically Loaded Bolt Groups

Tables 7-6 through 7-13 of the AISC Steel Manual contain values for C, the effective number of bolts that resist shear in eccentrically loaded bolt groups. For example, in a bolt group with three vertical rows of 4 bolts spaced s=3 inch with srow=3 inch row spacing and a load at $latex \theta$=30 degrees from … Continue reading Eccentrically Loaded Bolt Groups →

I Missed Again

When I come up with "great" exam questions, only a handful of students get the correct answer while the remaining students misunderstand the question. Same thing happens on social media. Prior to Twitter's full-on death spiral, I tweeted a quiz on the MVLEM element, a formulation available in OpenSees for modeling shear walls. Only 5 … Continue reading I Missed Again →

Geometric Transformation

OpenSees offers three types of transformations between the basic system and global system for frame (beam-column) elements: Linear - small displacement assumptions for compatibility and equilibrium PDelta - small displacement assumption for compatibility with the $latex P-\Delta$ term included in equilibrium Corotational - large displacement assumption for compatibility and equilibrium Use the geomTransf command to … Continue reading Geometric Transformation →

Simple Loads on a Cantilever

"What are the columns of output for a section force / section deformation element recorder?" is a frequently asked OpenSees question. The correct answer is "It depends on the type of element and section model." But a much better answer is "Apply simple loads to a cantilever and figure it out." Consider the cantilever and … Continue reading Simple Loads on a Cantilever →

Element Self-Weight

A question posted the other day in the OpenSees Facebook group asked how to add self-weight to elements. I gave the easy I-can-answer-this-in-under-two-seconds answer of "use the eleLoad command". It turns out the complete answer is not so simple as it depends on the type of element to which you want to apply self-weight. The … Continue reading Element Self-Weight →

Force-Based Element Moment Release

The common approach to adding a moment release at the end of a force-based frame element is to define an extra node then use the equalDOF command to constrain all but the released rotational DOFs. Although this approach is cumbersome, it is fine as long as the element lies within a global plane (X-Y, Y-Z, … Continue reading Force-Based Element Moment Release →

The Basic Natural Kernel in a Corotating Frame of Reference

Frame elements in OpenSees are formulated in a basic, or natural, system that removes rigid body displacement modes from the element displacement field, leaving only the deformational modes. The basic forces that correspond to the deformational modes depend on the element force-deformation relationship while the remaining local forces are found from rigid body equilibrium. The … Continue reading The Basic Natural Kernel in a Corotating Frame of Reference →

Unrolling the Four Node Quad

The FourNodeQuad was one of the first, if not the first, solid elements in OpenSees. Despite the element's mediocre implementation, the coding style was copied by others into subsequent solid element implementations. Fortunately, before that copying happened, I revised the implementation to be more computationally efficient--and I was kind enough to leave the original author's … Continue reading Unrolling the Four Node Quad →