See the Convergence

Surely you have seen norms fly across the screen when running OpenSees with the print flag of the convergence test set to 1. The screen output slows down your analysis significantly, so you should only use print flag equal to 1 when you are trying to diagnose convergence issues. From a Jupyter Notebook. With OpenSees.exe, … Continue reading See the Convergence →

Two Paths You Can Go By

I am confident we can use OpenSees to solve every truss, beam, and frame problem from any statics or structural analysis textbook as well as every single degree-of-freedom and rigid shear frame problem from a structural dynamics textbook. We can also solve any reasonable problem from a finite element textbook. My confidence starts to wane … Continue reading Two Paths You Can Go By →

Finite Differences

A previous post showed how to compute response sensitivity by the DDM, or direct differentiation method. Comparisons with finite difference calculations verified that the DDM results were correct. In this post, I'll dig a little deeper into finite differences. The advantage of the finite difference method (FDM) is it will work for any model parameter--you … Continue reading Finite Differences →

Cable Analysis

Analyzing cables subject to transverse loads is straightforward in OpenSees. Use a mesh of corotational truss elements with elastic uniaxial material. Of course, you can use any uniaxial material you like. The only trick is you have to scramble the nodes up a little bit--if you try to analyze a perfectly straight cable, you'll get … Continue reading Cable Analysis →

Eigenvalues During an Analysis

How to compute the eigenvalues (natural periods) of a structural model during an analysis, as the stiffness changes due to yielding, unloading, reloading, large displacement, etc., is a common question. In general, periods elongate during yielding events, then shorten again upon unloading. The extent and duration of period change depends on the constitutive models and … Continue reading Eigenvalues During an Analysis →

Slender Things

Using fiber sections and the corotational geometric transformation is an easy way to simulate combined material and geometric nonlinearity in column members. A previous post examined this approach for steel columns where residual stresses play an important role in the axial load capacity. In this post, I will show the corotational mesh approach for non-sway … Continue reading Slender Things →

Not All at Once

I've been working on a project where we use OpenSees to analyze structural models for several load cases. We only care if the models can support the full load, not so much how they get to the full load. We apply all the load in one step with load control and Newton-Raphson. If that succeeds, … Continue reading Not All at Once →

Discretized Members Only

I wrote a DiscretizeMember procedure in Tcl many years ago--I don't know exactly when, definitely after the 1980s and definitely before moving to Eastchester. "Members Only." by The Semi-Frozen Trombone is licensed under CC BY 2.0 After carrying the function around for years, and probably spawning more variations than uniaxial Concrete models, the procedure went … Continue reading Discretized Members Only →

Right Under Your Nose

I long ago accepted that buckling analysis would never be implemented in OpenSees. Although there is a getGeometricTangentStiff() method in the Element interface, only PFEM fluid elements use it. Implementing this method for frame elements, assembling the geometric stiffness, and solving the generalized eigenvalue problem would require several updates to the innards of OpenSees. Then … Continue reading Right Under Your Nose →

Going Through Stages

Most structural earthquake engineering deals with staged analyses of built infrastructure in one way or another. In the most common scenario, we apply gravity loads to a structural model, get the model in equilibrium, then simulate the model response to earthquake loading. Analysis of structural systems and components during construction is also important. Temporary construction … Continue reading Going Through Stages →