Getting the Number of Nodal DOFs

Do you ever get tired of typing a lot of zeros, e.g., to apply a 20 kip load in the X-direction at node 18 on a 3D frame model? ops.timeSeries('Linear',1) ops.pattern('Plain',1,1) ops.load(18,20,0,0,0,0,0) I know what you're thinking. Did I put five zeros after the 20 kip load or only four? Well, to tell you the … Continue reading Getting the Number of Nodal DOFs →

Long Term Column Loading

Practically all analyses of reinforced concrete columns in OpenSees assume the loading is short term--concrete as strong as it was at 28 days out of the hopper. Depending on what you're doing, not accounting for long term load effects, i.e., concrete creep and shrinkage, may not be a big deal. But the effects of creep … Continue reading Long Term Column Loading →

OpenSees Tom Swifties

According to its Wikipedia page, a Tom Swifty is "a phrase in which a quoted sentence is linked by a pun to the manner in which it [the quoted sentence] is attributed". For example, here is a Tow Swifty to which OpenSees users can relate: "I completed my first nonlinear frame analysis!", Tom beamed. This phrase uses "beamed" to … Continue reading OpenSees Tom Swifties →

The Maximal Broken Example

When I ask for a minimal working example, I often receive responses that are the opposite of minimal and working, i.e., maximal or broken, in some cases reaching new lows. The following situations can be categorized as MBEs (maximal broken examples), i.e., not a minimal working example. 1. An Example from Your Journal Article I … Continue reading The Maximal Broken Example →

Dangling Recorders

Because I don't perform very large or time-consuming OpenSees analyses, I use recorders rather infrequently, instead preferring commands like nodeDisp and eleResponse. An issue with node and element recorders is they can segmentation fault after the node or element to which they point is removed from a model--a common scenario for progressive collapse simulations. A … Continue reading Dangling Recorders →

No Reaction

As the old mantra goes, "you can displace without deforming, but you can't deform without displacing". Other than a post here and there on particle dynamics, the examples on this site deal with element deformation (or strain), which requires nodal displacement. But what about examples with elements that undergo only rigid body displacement with no … Continue reading No Reaction →

The Old Code OpenSees

You have no doubt read grammatically correct sentences whose meaning you interpreted incorrectly. Only after re-reading did you realize the author's intent. For example, We modeled the walls with cracks. Your initial interpretation of this sentence could have been equivalent to "We used cracks to model the walls", which makes no sense and was not … Continue reading The Old Code OpenSees →

Sequential Ground Motions

How to impose sequential ground motions while keeping the damaged state of the model from the end of one ground motion to start of the next is a question asked with moderate frequency (including today, which encouraged me to take this post from draft to reality). Below is a minimal working example where two ground … Continue reading Sequential Ground Motions →

Getting the Digits

In a recent post, I pointed out that the LobattoBeamIntegration class uses only ten, instead of 16, significant figures for the locations and weights of the integration points. Those six missing digits made for a perfectly fine demonstration of how to use the isclose function for verification. But, in the bigger picture, omitting six significant … Continue reading Getting the Digits →

Critical Buckling Loads via CBDI

Curvature-based displacement interpolation (CBDI) is a method of approximating the transverse deflection at each integration point, or section, of a force-based frame element in order to account for geometric nonlinearity within the basic system, i.e., "P-little-delta" effects. Neuenhofer and Filippou (1998) describe the complete CBDI formulation for force-based elements. But one piece of the formulation, … Continue reading Critical Buckling Loads via CBDI →