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Point/ Patch Loads (Discrete loads)

  1. Discrete loads are assigned to POINT features.  These point features do not need to be part of the structure; the point feature simply represents the position 0,0,0 in the loading attribute definition. This is contrary to other "Structural loads" (concentrated, locally distributed, body force, etc.), which must be assigned to a feature that is fully part of the model and is meshed appropriately, such that the loads are applied directly to the underlying elements of the feature.

  2. Discrete loads may be applied to structures which lay in any plane.

  3. Modeller automatically translates the projected discrete loads into equivalent nodal loads (concentrated loads).   It carries this process out when it creates the LUSAS datafile (*.DAT) using the element shape functions, and using search areas.  

  4. The area of a model to which a discrete load is applied may be restricted using SEARCH AREAS (Attributes>Search Area).  For example, a search area may be used to ensure that a discrete load placed on top of a multi-storey building is applied to the roof only, by defining a search area attribute and assigning it to the features which make up the roof.  In this example, illustrated below, the building frame is modelled with beam elements and a search area is assigned to the lines at the top of the model.

  1. If no search area is specified when the loading assignment is made, Modeller will assume that all of the model may potentially receive the applied loadings.  This may cause ambiguity when applying the loading and may result in the load being applied to the wrong region of mesh.  If the multiple intersections of the load projected onto mesh are possible, then this is reported in the Text Output Window with reference to the load which will thus have no effect on the mesh.

Rules for search areas:

  • If the model consists of line features (as in a grillage), they must form closed boundaries (for surface-based models there is no such requirement). 

  • These boundaries will form "bays" which may have a maximum of 30 elements circumscribing the 3 or more "bay" sides.    It is usually unnecessary to have anywhere near 30 elements per bay, though this will depend on the shape of the bay and the accuracy (and mesh density) of the elements being used.  When using GRIL elements to model a grillage often no more than 4 elements would be required as these elements allow a quadratic variation of moment and linear variation of shear along their length.  This limit can be adjusted by manually setting the system variable called "SareaMaximumCellEdges", if necessary.

More on using discrete loads on a 2D frame/ beam model

The following GRILLAGE and FRAME examples indicate the requirements diagramatically.  

1. Grillage :-

2. Frame :-

This is not a closed boundary system.
This is a closed boundary.  A line is created and assigned with a "null" line mesh (shown dashed), with an element type set to "none" and 1 mesh division specified.  This encloses the SEARCH AREA.

More on using discrete loads on a 2D frame/ beam model

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