Category: Constitutive Models

Concrete Zero

What is the depth of concrete uniaxial stress-strain models compared to the pinnacle that is Concrete23? No, it's not Concrete01--that's base camp for all the ConcreteXY that followed. The most basic uniaxial concrete model in OpenSees, "Concrete00" if you will, is elastic-no-tension, or ENTMaterial. As its name implies, the ENTMaterial response is elastic in compression … Continue reading Concrete Zero →

Minimal Creep and Shrinkage Example

In class, I tend to avoid talking about creep and shrinkage of concrete. I say "compression steel is good because long term deflections due to creep are bad", then move on to seemingly more interesting topics like how to find the neutral axis, bypassing shrinkage altogether. However, creep and shrinkage remain large mysteries to most … Continue reading Minimal Creep and Shrinkage Example →

A Simple Material Tester

A UniaxialMaterial tester was my first foray into Tk widgets back in the early 2000s. The tester has come along for the ride through all the OpenSees source code repositories. First cvs, then svn, and now GitHub. You could select various materials and drag the slider back and forth to see the stress-strain response history. … Continue reading A Simple Material Tester →

How to Find the Neutral Axis

There is no recorder option in OpenSees to get the location of the neutral axis in a fiber section. Instead, you have to post-process the section deformations. The strain at any point in a 2D fiber section is $latex \varepsilon=\varepsilon_a - y\kappa_z$ where $latex \varepsilon_a$ is the section axial deformation, $latex \kappa_z$ is the section … Continue reading How to Find the Neutral Axis →

P-M Interaction by the Book

Find any indeterminate beam, frame, or truss problem from a structural analysis textbook, and you can make OpenSees solve it. But sometimes, replicating the basics is not so easy. Take, for instance, an axial-moment (P-M) interaction diagram of reinforced concrete (RC) sections. The typical approach advocated with OpenSees is to use repeated moment-curvature analyses over … Continue reading P-M Interaction by the Book →

Plane Sections Do Remain Plane

Here's another conversation I've had with a concerned user (CU) of OpenSees, not necessarily the same CU that was worried about OpenSees crashing due to non-convergence: CU: "Do plane sections remain plane in the material nonlinear range of response?"PD: "Yes."CU: "What about with force-based frame elements?"PD: "Yes."CU: "How is that possible?"PD: "Because there's no other … Continue reading Plane Sections Do Remain Plane →

Hysteretic Damage Parameters

Because the C++ implementation is a straight translation of his FEDEAS subroutine written in FORTRAN, Prof. Filippou receives numerous inquires on the formulation of HystereticMaterial in OpenSees. According to the comments in the FORTRAN files, the bilinear backbone implementation (Hyster1.f) was finalized on November 24, 1994 followed by a trilinear backbone implementation (Hyster2.f) finalized on … Continue reading Hysteretic Damage Parameters →

How to Record Section Curvature

I've seen recently a few people compute curvature for an OpenSees fiber section by dividing the difference between top and bottom uniaxial fiber strains by the distance between the fibers. While there's nothing technically wrong with this approach, it's a lot of work and it's error prone. In addition, this approach implies that using a … Continue reading How to Record Section Curvature →