A grid can be comprised of two or more sets of parallel members intersecting each other at an angle and loaded perpendicular to their planes. If all the members of a grid are in the same plane, it is called a single-layer grid.
There are different types of single-layer grid used as the main structural systems for roofs and floors. These interconnected patterns consist of two-way, three-way or four-way configurations, with the most common pattern being rectangular grid in which the intersecting elements are perpendicular to each other. The diagonal grid, in which the members are oriented in an oblique manner to the supporting structure along the edge, has great rigidity (or stiffness), i.e., substantial reduction in deflection.
Rectangular
Diagonal
Two-Way Grids
Triangular
Triangular/Hexagonal
Hexagonal
Three-Way Grids
Several Types of Single-Layer Grids
Four-Way Grids
The three-way single-layer grids are generally used for larger span or when the structure is subjected to large concentrated loads. This system has a more even stress distribution that the two-way grids. The four-way grids are made of a combination of rectangular and diagonal grids. These systems are not commonly used. For up to almost 30 ft span single-layer grids are efficient, however, for larger spans double-layer grids are more appropriate, which can be used for spans up to about 300 ft. Since grids are very rigid structures, they require members that are shallower than planar systems. The span to depth ratio for single layer rectangular grids is about 30 and for diagonal grids is about 40.
Double layer grids (DLGs) are extensions of single-layer grid systems, which consist of two sets of parallel beams connected to each other at right or oblique angles and loaded by forces perpendicular to the plane of the grid.
Even though DLGs are customarily made of parallel planes, it is possible to create free-forms by modifying the shape of the top and bottom layers independently.
There are generally two main types of DLGs:
Direct (Truss) Grids: In this type of system the members of the top and bottom layers are in the same plane resulting in a series of intersecting planar trusses.
Space Grids: These are made of a series of polyhedral units with triangular, square, pentagonal or hexagonal bases.
Square-on-Square
Diagonal-on-Diagonal
Truss Grids
Square-on-Square Offset
Square-on-Diagonal Offset
Diagonal-on-Square Offset
Triangle-on-Triangle Offset
Space Grids
DLGs can be used for floors or roofs of multi-story buildings (though a majority of systems are used for roofs). The roof system in most cases consists of purlins supported by joints, however, in some cases the top layer directly supports the roof. In addition, DLGs have been used for vertical or inclined walls.
To see some of the most common configurations of flat DLGs click here. In fact, one can create an infinite number of configurations by placing members between different nodes of a DLG.
Robert Le Ricolais, a French engineer, designed numerous three-way grids. The three-way grids, made of interconnected tetrahedra and octahedra, are referred to as “Le Ricolais space grids” which are made of identical members interconnected with identical connectors.
Double layer grid configuration can also be considered to have been generated by the close packing of platonic solids such as tetrahedra, octahedra, etc., placed next to each other and connected at joints (sometimes with additional members). To see the assembly of some typical configurations from platonic solids click here.
