# Grid surfaces

A grid surface is required to in order to apply grid loads to the structure. The grid surface details how the grid load is transferred on to the structure.

## Definition

### Name

A label for the grid surface. The name is shown when grid surfaces have to be selected.

### Grid plane

The grid plane that the grid surface is defined to use.

### Element list

Elements included in the grid surface. In some situations only some of the elements are to be considered to be part of the grid surface (e.g. secondary bracing) so the element list can be used to specify these.

### Element type

Elements type of a grid surface specifies the type of elements that are to be considered as part of the grid surface. It can be 1D or 2D.

### Grid load tolerance

This defines a volume (above and below the actual grid plane) within which elements are considered as candidates for the grid plane. It is used in conjunction with the element list to determine whether an element lies in the grid surface. A floor may use a relatively small tolerance while a roof structure may use a much larger tolerance.

### Span

The span defines the way in which loading on the grid surface will be transferred from space to the surrounding elements.

**One-way**– the load is transferred in the specified span direction. The direction is the angle to the x-direction of the axis defining the grid plane.**Two-way**– an area load on a complete panel. The load is expanded according to the specified algorithm and distributed in a pattern resembling the back of an envelope. The possible expansion algorithms are**Legacy**– Legacy option: loads assumed to ripple out to edges of panel (does not work on re-entrant panels)**Plane**– Uses a plane approximation to distribute the load and adjusts for aspect ratio (load distribution depends on model refinement)**Smooth (plane)**– Uses a plane approximation to distribute the load; adjusts for aspect ratio and smooths loads along edges**Corner (plane)**– Uses a plane approximation to distribute the load; adjusts for aspect ratio and smooths loads along edges, adjusting for corner effects (recommended)**Simplified tributary areas**option. Give simple distribution on fully loaded triangular and quadrilateral panels as shown below

## Grid load expansion algorithms

The grid load expansion option determines which algorithm is used to expand the grid loads.

The legacy option treats loads as a series of point loads applied across the grid plane which *ripple* out to the elements bounding closed areas. This does not work for re-entrant panels, e.g. L shaped areas.

The plane approximation options treats the load on a panel as a single load applied at the centre of loads and a set of loads applied around the perimeter that give equilibrium. The loads are determined so that the aspect ratio of the panel results in a greater load intensity applied to the long edges that the short ones. The basic version of this gives a mesh-dependent load distribution and can lead to the mid-span load intensities being too large. The *smooth* variant takes the uniformly distributed loads on the individual elements and aggregates these along the edges resulting in a linear variation along the panel edges. The *corner* variant assumes that the loads on the ends of edges at internal corners should be reduced so an adjustment is made to taper the load intensity to zero at these corners.