The static integrators in OpenSees, including displacement control, arc length, and minimum unbalanced displacement norm (MUDN), are based on an incremental-iterative framework. After an initial load increment, each integrator imposes a constraint on the change in load factor at subsequent equilibrium iterations within a pseudo-time step. Displacement control calculates the change in load factor necessary … Continue reading It’s Not Load Control
A Very Stable Challenge
With linear structural analysis, a number is a number. However, reality starts to creep in when physics dictates that you need to account for nonlinear effects. The three member structure shown below is a perfect example--Example1.1.tcl, one of the most OG OpenSees examples. Have you ever paid attention to the member sizes though? If so, … Continue reading A Very Stable Challenge
Fiber Section Centroids
If you define a fiber-discretized cross section in OpenSees using section coordinate systems A and B shown below, will you get the same response when you apply axial load and bending moments to the sections? The answer is yes. You will get the same response from sections A and B. The fiber sections compute the … Continue reading Fiber Section Centroids
Nonlinear Elements, Elastic Sections
Using nonlinear elements, particularly the forceBeamColumn element, with elastic sections is just as good as, if not better than, using the elasticBeamColumn element for many reasons. Not only do force-based elements with elastic sections make the transition to material nonlinearity easy, they also facilitate debugging your model. Another reason I like force-based elements is you … Continue reading Nonlinear Elements, Elastic Sections
Load Patterns and Time Series
In nonlinear structural analysis, loads add together, just not the load effects. So, the total mechanical load applied to a structural model can be expressed as the sum of time-varying load vectors. $latex {\bf P}(t)={\displaystyle \sum_{i=1}^N} \lambda_i(t){\bf P}_{ref,i}$ Each load vector is the product of a time-varying scalar function, $latex \lambda(t)$, and a non-time-varying reference … Continue reading Load Patterns and Time Series
OpenSees Blog Delurking Week
It's 2021 International Blog Delurking Week--the first full week of January. A lot has changed since last year's delurking week. Whether you lurk, running OpenSees in your cave, or actively comment here on the blog or on the OpenSees message board or Facebook group, please say "Hello" in the Comments section and let everyone know … Continue reading OpenSees Blog Delurking Week
Sensitivity Training
Sensitivity of structural response with respect to modeling parameters provides search directions for gradient-based algorithms in reliability analysis, optimization, and system identification. In addition to these applications, stand-alone sensitivity analysis gives useful information about the effect of parameters on the structural response. There are three methods to compute response sensitivity for nonlinear, path-dependent analysis of … Continue reading Sensitivity Training
Hysteretic Pinching Parameters
I receive a lot of questions about the pinching parameters for the HystereticMaterial in OpenSees. Despite the best of intentions, one-off responses to these queries often went unanswered. But now, with the blog, a one-off response has staying power. So, here goes. The HystereticMaterial dates back to the G3 days. Along with Steel01 and Concrete01, … Continue reading Hysteretic Pinching Parameters
Every Ending Is a New Beginning
Simulation of structural response to sequential hazards, e.g., fire following earthquake or tsunami following earthquake, is something OpenSees can handle. But suppose you want to look at different tsunami scenarios after a single earthquake. Tsunami loading occurs over a few seconds where the preceding earthquake lasted a minute or two. Do you want to repeat … Continue reading Every Ending Is a New Beginning
When a Deal Breaker Is Not a Deal Breaker
We often place too much emphasis on obtaining mathematically exact solutions for structural models. While it's important to obtain exact solutions, e.g., for element development and comparing softwares, it's not always necessary and definitely not always a deal breaker. The important thing is to know whether or not an exact solution is possible and the … Continue reading When a Deal Breaker Is Not a Deal Breaker
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