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STRUCTURAL GLASS FACADES - USC School of Architecture

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There are then two distinct classes <strong>of</strong> structure systems used in structural glass facades;<br />

closed and open systems. The primary attribute that differentiates them as a function <strong>of</strong> this<br />

classification is the requirement for prestress, which must be determined as a function <strong>of</strong> the<br />

design process and must be realized on site during the installation <strong>of</strong> the structure.<br />

6.1.1.2 Spanning Behavior<br />

Unidirectional Spanning: Systems spanning in one primary direction.<br />

Two dimensional (flat) trusses can span in only one direction, and systems built <strong>of</strong> such<br />

trusses are referred to as 1-way systems with respect to spanning. Morphologically flat<br />

trusses <strong>of</strong> any kind are only capable <strong>of</strong> unidirectional spanning. Strongbacks, fin-supported,<br />

and cable-hung structures are also only capable <strong>of</strong> unidirectional spanning.<br />

Multidirectional Spanning: Systems spanning in two or more primary directions.<br />

Additional spanning directions increase the efficiency <strong>of</strong> a structure, allowing for a more<br />

uniform stress distribution. Most common are 2-way systems. Orthogonal grid space frames<br />

and cable nets are examples <strong>of</strong> 2-way spanning systems. Triangular grid space frames and<br />

cable nets displaying 3-way spanning behavior are also conceivable. More complex<br />

geometries as can be developed with grid shell structures are capable <strong>of</strong> complex, highly<br />

efficient multidirectional spanning behavior along multiple load paths.<br />

Multidirectional spanning is not simply a matter <strong>of</strong> utilizing a 2-way system. A square grid<br />

octet truss space frame, rectangular in plan will at some point, as the plan length increases<br />

relative to the plan width, span only in the short dimension, behaving as a 1-way system with<br />

no benefit in efficiency from the other potential spanning direction. A square plan will span<br />

most efficiently, evenly distributing stresses along both spanning paths.<br />

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