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> Advice
Meshing a Grillage
Classically, grillage bays are 4-sided. There are two
common ways of constructing a grillage model.
- Using a net of short line features (between points which
define every corner of each grillage bay). In such a
model, it must be ensured that there is only one point at each
location and a mesh definition which has one division only may
be assigned throughout the model. The
"grillage wizard" uses this approach.
This method is the preferred approach due to achieving mesh connectivity
without the need for equivalence attributes.
- Using long line features, stretching the length of a span
and the width of the bridge. In such a model, mesh
definitions should have divisions that will result in a node
at (or very close to) each intersection point. This will
usually entail using several mesh definitions for the
different longitudinal and transverse members.
Once the mesh attributes have been assigned to the lines,
ensure that coincident nodes on longitudinal and transverse
members are merged for proper "connectivity".
You can use the menu item:
File > Model properties > Meshing > Equivalence tolerance
(check) Automatic
This second method is more complicated and could cause connectivity
problems if mesh densities are changed.
In fact LUSAS can solve
grillages with bays of more than 4 sides but since grillage
elements (GRIL) allow a quadratic variation of moment and
linear variation of shear along their length, in the normal
way it is an unnecessary complication to have more than 4.
Should you wish to model columns, upstands or
downstands, it
will be necessary to select a 3D beam element. Further
information on selecting an element type is available via the
"Help" menu:
Help > Help Topics > Modeller
Reference Manual > Chapter 5: Model Attributes > Meshing
> Element
Selection
Related topics: Bridge loading,
Point/ patch
loading & search areas, Shear
near supports and Mesh refinement
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Modelling Issues
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