Form Determination - Tensile Fabric Structures: Design, Analysis, and Construction - page 56

Chapter 5
Form Determination
5.1 Basic Design Principles
Designers attracted to fabric structures are often intrigued with the wide range of
forms which can be built. Although the range of possible forms is extensive these are
not "free-form" structures. They must rigorously conform to the physical principals
which govern their behavior as limited by the characteristics of the materials from
which they are built.
5.1.1 Tensile Behavior
Exploration and exploitation of the nature of tensile behavior is the basis for the
design of fabric structures. The use of structural systems and materials which resist
loads in tension govern their design. Ropes and cables are the simplest tensile
elements. Structural forms may be generated by arranging cables between fixed
boundaries. Compression struts or bending components may be incorporated into the
web of tension elements. Membranes are generally made by combining or weaving
numerous linear tension components. Membranes structurally act as tension surfaces.
Loads applied to membranes must be resolved in their surface, as they typically have
negligible compressive, flexural or shear strength. Cables may be used to establish
membrane boundaries or "reinforce" a membrane by dividing the surface into
manageable portions.
Supporting elements such as masts, arches and perimeter beams typically have
significant compressive, flexural and shear strengths. The tension membrane surfaces
are usually visible in the completed construction. Tensile structure systems typically
use different materials and forms to resist various types of forces. Their dramatic
imagery is often highlighted by the juxtaposition of the tensile, compression and
bending components.
5.1.2 Geometric Classification
Surfaces are generally classified by their curvature. Cylinders and cones are singly-
curved surfaces. These forms have curvature in only one direction and can be made
from a flat surface. They are considered developable. Dome and saddle shapes are
doubly curved and as such are not developable. Methods of approximating these
surfaces are discussed in Section 9.1.1, Patterning - Lay Down from 3D to 2D.
When the principal curvatures at a point are on the same side of the surface, as in a
dome, the surface is synclastic. Figure 1-4, radome, and Figure 1-5, Osaka Pavilion,
are examples of synclastic surfaces. When the principal curvatures at a point are on
opposite sides of the surface, as in a saddle shape, the surface is anticlastic. Figure 1-
6, German Pavilion, is an example of an anticlastic surface.
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