Issues Related to the Design of Spatial Structures

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(4) Support Types and Placement

Spatial structures are usually supported by columns, walls, or footings using steel plates for their attachment. The size of the plate depends on whether the support point is pinned or sliding (which usually requires more area). Depending on the applied support reactions, the plate size can change. The uplift reactions from wind loads also determine the size and number of bolts required for the stability of spatial structures. At least three lateral supports are needed to keep spatial structure in equilibrium when subjected to lateral forces.

Roller supports through the use of sliding bearings can be provided. These systems use polytetra-fluorethylene (PTFE) between the spatial structure and its support system. If needed to restrict the support movements in a specific direction, guide plates are usually used.

A Roller Support for a Spatial Structure

The number, location and type of supports have a significant effect on member forces, deflection, and required sizes of the components (or the total structural weight) of spatial structures. In general, supports can be placed under any node of the spatial structure and may result in only a very small increase in the total costs if the column locations are changed for a building with an irregular plan shape. Obviously, columns should always be placed under a node to prevent local bending of the structural elements.

The best support locations depend on the configurations and architectural requirements of the spatial structure. If the column supports the top layer node, the adjacent bracing members are in tension and if supports the lower layer node they are in compression. These members are usually critically loaded and their failure may result in a progressive collapse of the entire structure. Therefore, if the structure is supported at the upper nodes it may be less susceptible to collapse since tension members do not buckle.

Placing columns along the edges of spatial structures results in a more efficient design than having them at the corners of the structure only. However, this will obviously increase the foundation costs. Generally, the member forces and maximum deflection of spatial structures are the smallest when supports are placed along all edges.

Placing columns at the center along the edges result in more efficient design (smaller deflection and member forces) compared to the corner- supported case. In this case most of the bottom layer members will be in compression and most of the top chords members in tension.

Placing supports at the extreme edges of the structure should be avoided since it results in large axial forces in the directly loaded members. Supports should be placed at least one module size in-board. However, in most cases, it is easy to cantilever spatial structures, which usually reduces the member forces and deflections. The optimum cantilever length is about 15% to 30% of the structure's back-span. However, if the cantilever becomes very long, the vertical deflections along the perimeter may become a controlling factor in the design. Cantilevers can provide circulation zones around the building perimeter, behind glazing, or provide a canopy for shading, etc.

The steel column supports can be in the form of straight columns, columns with pyramid capitals, columns with double pyramid capitals, and tree columns as shown below:

The columns with pyramid and double pyramid capitals and tree columns are very efficient and allow long-span structures with a minimum number of supports [ideal for exhibit halls, manufacturing plants, and wherever minimum number of supports (or maximum open space) are required]. They reduce the effective span between supports, the maximum vertical deflection, and member forces, which result in cost-saving - especially for spans over 100 ft.