This is the first paper fruit of the on-going collaboration with Federico Pisanò at TU Delft. Here we tackle the problem of formulating a constitutive model, derived from the SAniSand framework by Yannis Dafalias, which can replicate the *ratcheting* phenomenon on sands.

What is ratcheting?

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Read moreThe magnitude I gave came from the Tsunami threat record. pic.twitter.com/2h5dppVLLQ

— José A. Abell 🇻🇦🇨🇱 (@jose_abell) June 23, 2020

Stuck teaching during COVID-19 lockdowns in Chile has reminded me one crucial thing
about statics (the class I'm teaching this first semester in the southern hemisphere): **it can be very frustrating for students**. This frustriation comes from unmet expectations. They're just finishing their early maths and physics formation (Calculus, Linear Algebra, Differential Equations, physics etc. ), an ideal world where problems have nice answers such as \(\sqrt{2} \pi\) or no answer at all. Then, along comes statics, and the world seems upside down.

I wrote the *gmshtranslator* tool a while back during my …

Summary: **Finally!** First paper since PhD was accepted for publication …

The RC shear-wall building is modeled in OpenSees using non-linear fiber based beam column elements. Soil is modeled as a continuum using quad elements and linear stress-strain relationship. Soil shear wave-speed is varied in depth such as to obtain a \(V_{s30}\) consistent with a class B site according to chilean seismic code. Lysmer-Kulhemeyer dashpots are used along the soil boundary to model seismic radiation and earthquake wave-field input.

The performance of the building will be assessed for varying site fundamental periods. This is an aspect of SSI that is not covered by the chilean seismic code, and has been shown to be a problem in past earthquakes.

This is part of an on-going study with Prof. Carolina Magna from Adolfo Ibañez Unversity and her MS student Miguel Ángel Rodriguez from UDP.

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With my student, Pablo Ibañez, we implemented a new catenary cable element in OpenSees. This element is based on the work by Salehi et al[1].

The stiffness of this element is obtained using a flexibility formulation. Basically the shape of the cable is determined by the integral:

$$
\newcommand{\pare}[1]{\left( #1 \right)}
\newcommand{\brak}[1]{\left[ #1 \right]}
\newcommand{\brac}[1]{\left\lbrace #1 \right\rbrace}
\newcommand{\vect}[1]{\boldsymbol{#1}}
\newcommand{\uv}[1]{\hat{\boldsymbol{#1}}}
\newcommand{\ud}{\,\mathrm{d}}
\begin{align*}
\vect{x}(s) = \vect{x}_1 - \int_0^s \dfrac{\vect{w}s + \vect{f}}{\Vert \vect{w}s + \vect{f} \Vert^2}\pare{\dfrac{\Vert \vect{w}s + \vect{f} \Vert}{EA} + \pare{1 + \alpha \Delta T}} \ud s \\
\vect{w} = \brak{w_1,\, w_2,\, w_3}^T \qquad
\vect{f} = \brak{f_1,\, f_2,\, f_3}^T
\end{align*}
$$

Where \(\vect{x_1}\) is the position of the first node of the cable, \(\vect{w}\) is the weight vector in each direction, \(EA\) is the stiffness, \(\alpha \Delta T\) is the change in strain due to temperature and \(\vect{f}\) is the force vector at the start node. This equation is iterated (with the forces as variable) upon until the the shape of the cable matches the nodal postiions imposed by the finite element program (trial displacements). Then it is used to derive a stiffness matrix.

The element, as is, passes all our static verification tests. With the additional assumption of a lumped-mass matrix, we're currently working on a dynamic verification suite as well as some validation experiments.

The animation above was created using OpenSees to simulate the cable and Blender to render it.

Reference

[1] Salehi Ahmad Abad, M., Shooshtari, A., Esmaeili, V., & Naghavi Riabi, A. (2013). *Nonlinear analysis of cable structures under general loadings.* Finite Elements in Analysis and Design, 73, 11–19. https://doi.org/10.1016/j.finel.2013.05.002

One of the gripes a lot of people have with …

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\newcommand{\pare}[1]{\left( #1 \right)}
\newcommand{\brak}[1]{\left …

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This is a talk given at the IngeoKring 2016 Autumn …

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Read moreA nice extensive tutorial can be found here.

In a …

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