Background

The UnxBoucWen2DLink class creates a uniaxial Bouc-Wen element with two-nodes in two-dimensions. The number of degree-of-freedom depend on the node. Currently node with 2 (two translation) and 3 (two translation and one rotation) dofs are supported. The main purpose of this element is to force uniaxial behavior along a certain direction. Figure provides a simple representation of this element where (1) and (2) represent the start and end nodes.

The UnxBoucWen2DLink mass matrix in global coordinates is \(\textbf{M}^\textrm{e} = 0\). On the other hand, the UnxBoucWen2DLink solves the differential equation:

\[ \dot{z}(t)=(1 - (\beta \, \textrm{sign}[z(t)\dot{u}(t)] + \gamma)\|z(t)\|^\eta )\dot{u}(t) \,, \]

and the force is computed as

\[F(z,u) = q_y\,z + \alpha\,K_0\,U \,, \]

where \(q_y\) is the yield force of hysteretic component.

The local to global transformation matrix is

\[ \textbf{T}_\textrm{e} = \begin{bmatrix} \hat{\textbf{N}} & 0 \\ 0 & \hat{\textbf{N}} \end{bmatrix} \,. \]

The stiffness matrices as well as the force vector in global coordinates are defined as:

\[ \textbf{K}^\textrm{e} = \textbf{T}_\textrm{e}^\top \, \mathcal{K}^\textrm{e} \, \textbf{T}_\textrm{e} \,, \; \textbf{F}^\textrm{e} = \textbf{T}_\textrm{e}^\top \, \mathcal{F}^\textrm{e} \,, \]

and \(\hat{\textbf{N}} = [\hat{\textrm{n}}_1, \hat{\textrm{n}}_2, \hat{\textrm{n}}_3]^\top\) is the unit matrix of local coordinate axes, and \(\hat{\textrm{n}}_\textrm{i}\) is the unit vector of the i-th local coordinate axis. Note that \(\hat{\textbf{N}} \in \mathbb{R}^{3 \times 3}\), and \(\hat{\textrm{n}}_\textrm{i} \in \mathbb{R}^3\). Note that \(\textbf{M}^\textrm{e}, \textbf{K}^\textrm{e} \in \mathbb{R}^{\textrm{N}_\textrm{dof}^\textrm{e} \times \textrm{N}_\textrm{dof}^\textrm{e}}\) with \(\textrm{N}_\textrm{dof}^\textrm{e} = 4, \text{ or } 6\) (solid or structural).

REFERENCE:

Bouc, R. (1971). "Mathematical model for hysteresis." Report to the Centre de Recherches Physiques, pp16-25, Marseille, France.
Wen, Y.-K. (1976). for random vibration of hysteretic systems." Journal of Engineering Mechanics Division, 102(EM2), 249-263.

Pre-Analysis

The python Pre-Analysis in the 01-Pre_Process/Method/Attach.py file provides with an interface to populate the Entities dictionary. This file contains several functions to populate specific fields. For example, to create a UnxBoucWen2DLink, using json format, use:

addElement(tag, name='UnxBoucWen2DLink', conn, attributes):
- tag : The identifier of this element, i.e., tag > -1
- name : Seismo-VLAB element class name
- conn : Connectivity array of this element
- attributes : Specific properties for the created element, for example
  - 'alpha' Post-yield ratio
  - 'eta' Yielding exponent (sharpness of hysteresis loop corners)
  - 'beta' First hysteretic shape parameter
  - 'gamma' Second hysteretic shape parameter
  - 'tol' Newton-Raphson Tolerance.
  - 'Fy' Yield force
  - 'k0' Initial stiffness of hysteretic component
  - 'dir' The direction of action of this link
Example

A UNXBOUCWEN2DLINK element can be defined using the python interface as follows:
SVL.addElement(tag=1, name='UnxBoucWen2DLink', conn=[1,2], attributes={'alpha': 0.1, 'eta': 1.0, 'beta': 0.5, 'gamma': 0.5, 'Fy': 250, 'k0': 250, 'dir': 1})

Application
Please refer to the A08-DY_2D_UniAxial_BoucWen_Link_Param1.py file located at 03-Validations/01-Debugging/ to see an example on how to define a UnxBoucWen2DLink element.

On the contrary, the 01-Pre_Process/Method/Remove.py file provides with an interface to depopulate the Entities dictionary. For example, to remove an already define Element, use:

delElement(tag):
- tag : The identifier of the element to be removed, i.e., tag > -1

Run-Analysis

The C++ Run-Analysis in the 02-Run_Process/04-Elements/02-Link/UnxBoucWen2DLink.cpp file provides the class implementation. A UnxBoucWen2DLink element is created using the built-in json parse-structure provided in the Driver.hpp. A UnxBoucWen2DLink is defined inside the "Elements" json field indicating its "Tag" as follows,

{
    "Elements": {
        "Tag": {
            "name" : "UNXBOUCWEN2DLINK",
            "conn" : [ ],
            "attributes": {
                "nmax": int,
                "Fy": double,
                "k0": double,
                "alpha": double,
                "eta": double,
                "beta": double,
                "gamma": double,
                "tol": double,
                "dim": double,
                "dir": int
            }
        }
    }
}

Variable	Description
`Tag`	Unique positive element identifier.
`conn`	The element connectivity node array.
`Fy`	The yield force.
`k0`	The initial stiffness.
`alpha`	The stiffness ratio \(\alpha \in [0,1]\).
`eta`	The hysteresis shape parameter.
`beta`	The hysteresis shape parameter.
`gamma`	The hysteresis shape parameter.
`nmax`	Number of maximum iteration Newton-Raphson.
`tol`	Tolerance to accept convergence of solution.
`dim`	The number of degree of freedom of the nodes.
`dir`	The direction of action in local coordinates (1, or 2).

Attention: The element acts only in one direction (local coordinates).; The element provides no-contribution to the mass matrix.; The element only provides contribution to the stiffness matrix.; The length of the link is not used.; Strain and Stress link responses are relative deformation and non-linear force and in local coordinates respectively.; Internal force responses are provided in global coordinates.

Example

A UNXBOUCWEN2DLINK element between nodes 1 and 2 with 2 dofs, acting on 1-direction in a 2D problem is constructed as:
{ "Elements": { "1": { "name" : "UNXBOUCWEN2DLINK", "conn" : [1,2], "attributes": { "nmax": 50, "Fy": 250.0, "k0": 250.0, "alpha": 0.1, "eta": 1.0, "beta": 0.5, "gamma": 0.5, "tol": 1.0E-08, "dim": 2, "dir": 0 } } } }