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

# Category: Constitutive Models

# 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

# Something, Something, Something Fiber

Two recent inquires asked why model response using frame elements with elastic fiber sections, i.e., sections with fibers whose stress-strain response is $latex \sigma=E\varepsilon$, differs from the response with elastic elements, i.e., defined by E, A, Iz, and Iy either with elasticBeamColumn elements or nonlinear elements with elastic sections. I've touched on this subject a … Continue reading Something, Something, Something Fiber

# Two Fibers Explain So Much

In a previous post, I asked how well we can capture the moment-curvature response of a rectangular section with EPP material using different integration methods with two fibers. For flexural response, two is the minimum number of fibers necessary to satisfy section equilibrium--one fiber for tension, the other fiber for compression. The previous post showed … Continue reading Two Fibers Explain So Much

# Two Fibers, Five Ways

I occasionally go down rabbit holes of numerical integration. These trips led me to Gauss-Radau integration, all the element integration options available in OpenSees, and, recently, Chebyshev integration. The latest rabbit hole, described in this post, deals with different ways to integrate section moment-curvature response using only two fibers. Consider a rectangular cross-section with EPP … Continue reading Two Fibers, Five Ways

# Repeated Section Modes

If you use a section with linear-elastic response in the displacement-based, force-based, and mixed beam-column elements in OpenSees, you will get the same response from all three elements. TrueFalseIt depends The answer is it depends on the type of "section with elastic response" you use. Also, I wouldn't include "It depends" as a possible answer … Continue reading Repeated Section Modes

# Concrete02 with Control of the Initial Stiffness

Although they have the same compressive envelope, Concrete02 offers more features than Concrete01--tension strength, tension stiffening, and compressive unloading/reloading loops. However, one thing Concrete02 (and Concrete01) lacks is control over the initial stiffness, which is fixed at $latex E_c=2f'_c/\varepsilon_{c0}$. To overcome this limitation with Concrete02, Dr. Nasser Marafi created Concrete02IS (Concrete02 with control of the … Continue reading Concrete02 with Control of the Initial Stiffness

# Direct Moment-Curvature

That the force-based frame element and fiber sections are in an open relationship should come as no surprise. The displacement-based and mixed frame elements can use fiber sections and all three element formulations can use stress resultant sections. While this post used a coupled stress resultant plasticity model, you can also use the section aggregator … Continue reading Direct Moment-Curvature

# Making Sense Out of Concrete02

Those three extra parameters for Concrete02 are usually enough to make me stick with Concrete01. I struggle to come up with good values for the parameters $latex \lambda$, $latex f_t$, and $latex E_{ts}$ shown in the following image from the OpenSees Concrete02 wiki page. Image developed by Dr. Silvia Mazzoni The four compressive backbone parameters … Continue reading Making Sense Out of Concrete02