Space Frames - Structural Types.

Space frames are classified as single-, double- or multi-layered  structures, which  may be flat, resulting in grid structures, or may be curved in one or two directions, forming barrel vaults and dome structures. Grid structures can be further categorized into lattice and space grids in which the members may run  in two, three or four principal directions. In double-layer lattice grids the top and bottom grids are identical, with the top layer positioned directly over the bottom layer.Double-layer space grids are usually formed from pyramidal units with triangular or square bases resulting in either identical parallel top and bottom grids offset horizontally to  each other, or parallel top and bottom grids each with a different configuration  interconnected at the node points by inclined web members to form a regular stable structure.

Single-layer grids are primarily subject to flexural moments,whereas the members in double- and triple-layer grids are almost entirely subject to axial tensile or com- pressive forces. These characteristics of single-, double- and triple-layer grids  determine to a very large extent their structural performance. Single-layer grids, developing high flexural stresses, are suitable for clear spans up to 15m while double-layer grids have proved to be economical for clear spans in excess of 100m.

The main types of double-layer grids in common use are shown in Fig. 5.12.


Lattice and space grids
Fig. 5.12 Lattice and space grids

Skeletal space frames curved in one direction forming single- or double-layer barrel vaults also provide elegant structures capable of covering large clear spans.

Single-layer vaults are suitable for column-free spans of up to 40m, which may be substantially increased by incorporating selected areas of double-layer structure forming stiffening rings. Double-layer barrel vaults are normally capable of clear spans in excess of 120m. Figure 5.13 shows the main types of bracing used for single- layer barrel vaults.

Bracing of single-layer barrel vaults
Fig. 5.13 Bracing of single-layer barrel vaults

Dome structures present a particularly efficient and graceful way of providing cover to large areas. Single-layer steel domes have been constructed from tubular members with spans in excess of 50m while double-layer dome structures have been constructed with clear spans slightly greater than 200m.

Skeletal dome structures can be classified into several categories depending on the orientation and position of the principal members. The four most popular types usually constructed in steel are ribbed domes, Schwedler domes, three-way grid domes and parallel lamella domes.

Ribbed domes, as the name suggests, are formed from a number of identical rib members, which follow the meridian line of the dome and span from the foundations up to the top of the structure. The individual rib members may be of tubular lattice construction and are usually interconnected at the crown of the dome using a small diameter ring beam.

The Schwedler dome is also formed from a series of meridional ribs but, unlike the ribbed dome, these members are interconnected along their length by a series of horizontal rings. In order to resist unsymmetric loads the structure is braced by diagonal members positioned on the surface of the dome bisecting each trapezium formed by the meridional ribs and horizontal rings.

Three-way grid domes are formed from three principal sets of members arranged to form a triangular space lattice. This member topology is ideally suited to both single-layer and double-layer domes, and numerous beautiful large-span steel three- way domes have been constructed throughout the world.

The steel lamella dome is formed from a number of ‘lozenge’-shaped lamella units which are interconnected together to form a diamond or rhombus arrangement.The spectacular Houston Astrodome is an excellent example of this type of construction. This impressive steel double-layer dome was constructed from lamella  units 1.52m deep and has an outside diameter of 217m with an overall height of 63.4m. Figure 5.14 shows the four main dome configurations now in prominent use worldwide.

Dome configurations
Fig. 5.14 Dome configurations

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