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Meshing a Grillage

Classically, grillage bays are 4-sided.  There are two common ways of constructing a grillage model.

  1. 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. 
  2. 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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