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.

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.

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.

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