Because sparse solvers like UmfPack and SparseGeneral are the fastest for analyzing large models in OpenSees, it is reasonable to assume that these solvers must also be the best for small models, even the smallest possible model–an SDF system. But what if you’re doing millions of SDF analyses, e.g., generating nonlinear response spectra over a database of ground motions, … Continue reading Best Solver for an SDF Analysis
All Possible Sets of Possible Redundants
Although the force method has some severe limitations, it still has many elegant mathematical aspects. One of those aspects is the selection of redundants, for which there are a few approaches that are not ad hoc. The approach I teach in Eastchester is to form the equilibrium matrix for a frame or truss model, then reduce that … Continue reading All Possible Sets of Possible Redundants
More Than One Way to Heat Up a Truss
OpenSees can solve every reasonable problem from any structural analysis textbook. But I nearly had a meltdown over a simple truss problem because truss elements in OpenSees do not directly support thermal loading. Consider Example 6.17, shown below, from J.C. Smith’s Structural Analysis. All members of the truss have E=29000 ksi, A=10 inch2, and α=6.5e-6 (coefficient of thermal expansion, … Continue reading More Than One Way to Heat Up a Truss
Inside the Equation Numbers
In a previous post, I showed the effect of equation numbering on matrix bandwidth for a large model. And in another previous post, I showed how to determine the sequence of nodes visited by the equation numberers in OpenSees. This post is the result of those posts going on a road trip to shed more … Continue reading Inside the Equation Numbers
Stop Cargo Culting BandGeneral and Plain Numberer
In the early days of OpenSees, perhaps in its G3 infancy, an example Tcl script used BandGeneral for the system and Plain for the numberer. numberer Plain system BandGeneral Who created the script, or why they chose those analysis options, is unimportant now–it could have been me for all I know. But this pair of … Continue reading Stop Cargo Culting BandGeneral and Plain Numberer
Remote Control
An OpenSees question posted online mentioned numerical issues when attempting to load a structural model via a “remote node” connected to the main structure by a rigid link. I quipped, “Why don’t you use an equivalent force-couple?” That response did not garner a reply, but the original question brought up a common issue with the … Continue reading Remote Control
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
Partial-Span Uniform Loads
Although no one is going to use OpenSees to compute tributary loads for one-way floor systems, it’s still satisfying to analyze the resulting beam models. A common example is a beam that frames around one side of a floor opening. On the side of the beam with no opening, tributary area calculations produce a uniform … Continue reading Partial-Span Uniform Loads
Empty Spaces
Recently, a large OpenSees model was posted in an online forum with the poster asking why the analysis took longer than expected. Short answer: Not only did using a heavy-duty recorder that writes all node, element, and section data take up a big chunk of time, but using OpenSees's default linear equation solver, ProfileSPD, also … Continue reading Empty Spaces
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