2413 Basic Concepts

The space frame can be formed on either a flat or a curved surface. The earliest form of space frame structure is single-layer grid. By adding intermediate grids and including rigid connection to the joist and girder framing system, the single-layer grid is formed. The major characteristic of grid construction is the omnidirectional spread of the load as opposed to the linear transfer of the load in an ordinary framing system. Since such load transfer is mainly by bending, for larger spans the bending stiffness is increased most efficiently by changing to a double-layer system. The load transfer mechanism of a curved surface space frame is essentially different from the grid system that is primarily membrane-like action. The concept of space frame can be best explained by the following example.

It is necessary to design a roof structure for a square building. Figure 24.1a and b shows two different ways of roof framing. The roof system shown in Figure 24.1a is a complex roof composed of planar latticed trusses. Each truss resists the load acting on it independently and transfers the load to the columns on each end. To ensure the integrity of the roof system, usually purlins and bracings are used between trusses. In Figure 24.1b latticed trusses are laid orthogonally to form a system of space latticed grids that will resist the roof load through its integrated action as a whole and transfer the loads to the columns along the perimeters. Since the loads can be taken by the members in three dimensions, the corresponding forces in space latticed grids are usually less than that in planar trusses and hence the depth can be decreased in a space frame.

Purlin

Latticed truss

Purlin

Latticed truss

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