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Grid Loading

Grid loads are loads that are not applied directly to a node or element, but are located in space or, more precisely, on a grid surface which is located in space. Internally, the load is applied to the structure by considering the location of the load relative to adjacent structural elements.

Grid loads can be grid point loads, grid line loads or grid area loads. In all cases these loads are expanded to beam type elements.

The loads are not applied directly to the structure but by comparing the location of the loads relative to elements in the structure. The advantage of grid loads is that the loads can remain unchanged if the local definition of the structure changes, with the disadvantage that the loads are not tied to the structure.

The structure is considered as a collection of polygonal regions bounded by elements. These regions are referred to as panels. The panel associated with the loading is found and provided a number of checks are satisfied the load can be considered as a load on the structure. It is possible, though neither necessary nor desirable, to expand grid loading into an equivalent set of beam loads. Alternatively the grid loads are expanded as required by the solver.

Application of grid loading

The way in which the grid loads are applied to the structure depends on the span type associated with the grid surface.

Loads on a one-way spanning grid surface are assumed to span from one side of the panel to the other as is carried on a plank (or set of unconnected planks) spanning across the panel in the span direction. Thus a point load on a one-way spanning panel produces a pair of point loads on the panel edges.

Area loads on a two-way spanning grid surface are treated in a special way if the panel is triangular or rectangular and the simplified tributary areas option is set, otherwise two-way spanning panels are treated in the more general way described below. For simplified tributary areas the angle at each corner is bisected and the intersection of these lines is used to split the panel up in to three of four regions. For a rectangular panel this leads to the typical “back of an envelope” pattern. The load on each these regions is then assumed to be applied to the nearest edge.

Loads on two-way spanning panels are assumed to radiate in all directions from the point where the load is applied to result in a load which varies around the edge of the panel. Line loads are considered by dividing the line into a series of short segments each of which can be considered as an equivalent point load. Integrating the loading from each of these points gives the loading from the line load. Area loads are considered in a similar way but in this case a grid is superimposed on the loaded area. The cells of the grid are trimmed (if required) for panel and load boundaries and then the load is considered as set of equivalent point loads. As with the line loads the total load is the integral of all the point load contributions.

In all cases the load distributions are chosen to result in a set of loads which are in equilibrium with the defined grid loading.

Load cases and titles

The loading is stored by case number. However it is good practice to give load cases names. The load title gives access to the Load Case Titles wizard to simplify this procedure. The wizard is opened for the currently displayed load case.