# rblock tractions command

Syntax

- rblock tractions keyword ... <range>
Primary keywords:

contact-active constant direction-x extra install-gaps overburden ratio

Compute tractions at rblock-rblock and rblock-facet contacts consistent with a prescribed stress state. A constant stress tensor or gravitational stress may be specified. If gravity is specified and a uniform stress tensor is not specified, tractions including gravitation are computed; the direction of gravity must coincide with one of the global axes. The gravitational overburden is computed, accounting for non-uniform densities and non-uniform upper boundaries. Note that if the contact model supports a reference gap it is computed so that when the compressive force goes to 0 to sum of the contact gap and reference gap goes to 0 as well.

- contact-active b
Specify whether or not forces are set for inactive contacts. If true (the default), then forces are set for only active contacts. If false, forces are set for all contacts and inactive contacts may be activated.

- constant f1 f2 f3 f4 f5 f6 (only 3 components in 2D)
Set the components of a constant stress tensor. All components must be specified. In 3D, 6 parameters must be specified in the order stress-xx,stress-yy,stress-zz,stress-xy,stress-sxz,stress-syz. In 2D, 3 parameters must be specified in the order stress-xx,stress-yy,stress-xy.

- direction-x v (3D ONLY)
If two parameters are given for the ratio keyword, then this keyword can be used to specify the local \(x\)-direction the system should use. The \(y\)-direction is determined automatically using the \(x\)-direction and the direction of gravity to create an orthonormal system. The \(x'\)-direction may be adjusted to make it normal to the direction of gravity. By default the \(x\)-direction is (1,0,0). Cannot be given with the constant keyword.

- install-gaps b <f >
Install gaps as the stress is initialized (the default behavior). When stresses are initialized between rigid blocks, the amount they overlap plays an important role in how they behave after the stress has been relieved. For instance, if rigid blocks are just touching and a large compressive stress is specified, it may be possible for the blocks to no longer physically be in contact even though compressive stress remains. This can result in undesirable behavior if a shear failure occurs as some portion of the stress has been relieved. In that case, the contact resolution may determine that the contact gap is greater than zero, meaning that all contact forces are set to zero. This command adds a small rounding contribution to each contact consistent with the reference gap specified in the contact model. It is not necessary to call the

`rblock contact-resolution install-gaps`

command if this is active. The maximum gap is bounded by the f (defaults to 0.1) multiplied by the distance between rigid block centroids.

- overburden f
Specifies an overburden stress added to the calculation, assuming the vertical boundary of the model does not represent the surface. This value defaults to 0.0. Remember that compressive stresses are negative, so this value will typically be negative. Cannot be given with the constant keyword.

- ratio f <fy > (3D ONLY)
Determines the vertical to horizontal stress ratio. Both horizontal stresses are set to the vertical stresses times this value. If the optional second parameter fy is given, then a different ratio is used in the local x and y directions. Note that the direction-x keyword is used in this case to distinguish these local directions. Cannot be given with the constant keyword.

Usage Example

Compute tractions using an overburden as calculated by the inline FISH, with a horizontal stress ratio in local \(x\) and \(y\) determined by the `direction`

given, for both active and inactive contacts.

```
rblock tractions overburden [(topOfModel+12.0)*density*9.81] ...
ratio 3.0 1.6 direction-x 1.0 0 0 contact-active false
```

from “stress.dat” in Rigid Block Model of Tunnel Excavation

Compute tractions at contacts according to the constant stress tensor indicated.

```
rblock tractions constant [xx] [yy] [zz] 0 0 0
```

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