Leaning Column

A “leaning column” approximates the global P-Delta effects of gravity loads supported by vertical load resisting systems that are connected to, but not explicitly included in models of, lateral load resisting systems. The leaning column approach can simplify the analysis of frame structures for lateral loads, as well as gravity loads with initial out-of-plumb imperfections. … Continue reading Leaning Column →

Why Your Eigenvalue Analysis Failed

Eigenvalue analysis is often a necessary step before starting a dynamic response history analysis in OpenSees. You may want natural frequencies in order to compute Rayleigh damping coefficients, to apply modal damping, to compute modal properties, or to perform a response spectrum or modal superposition analysis. However, eigenvalue analysis can fail, indicating a problem with … Continue reading Why Your Eigenvalue Analysis Failed →

Reverse Engineering the Equation Numberer

In OpenSees (and any other finite element software), equation numbering is a quiet, behind the scenes analysis option that users do not have to pay any mind. No matter how a user numbers the nodes in their model, e.g., for bookkeeping or from a mesh generator, the equation numberer will clean up any messes. But … Continue reading Reverse Engineering the Equation Numberer →

Eigen Almost Hear You Sigh

ARPACK, the default eigenvalue solver in OpenSees, is very good at quickly finding a small number of eigenpairs (frequencies and mode shapes) for large models. Getting a few of a model’s lowest frequencies so you can check for rigid body modes and/or calculate Rayleigh damping coefficients is all most users care about. ARPACK gets the … Continue reading Eigen Almost Hear You Sigh →

Modeling Is Always Nonlinear, but Not the Response

As a narrative device, an ellipsis omits events that are unimportant to a story and that an audience can easily understand and reconstruct if necessary. For example, a movie montage is a narrative ellipsis. In Bull Durham, we don't need to see every game in the Bulls' road winning streak. We just need to see … Continue reading Modeling Is Always Nonlinear, but Not the Response →

Three Acts with the Two Node Link

This post follows three-act structure in verifying the flexural and shear response of the TwoNodeLinkSection element. Setup I’ve been working on the TwoNodeLinkSection element, which is like a TwoNodeLink, but uses a section object instead of multiple uniaxial materials. Basically the same spin the ZeroLengthSection puts on the ZeroLength element. Using a section object inside a two … Continue reading Three Acts with the Two Node Link →