Building Structures

STRUCTURAL SYSTEMS 33
Figure 1.41 Basic aspects of tension systems.
may thus be supported by hanging as well as by columns,
piers, or walls.
There are many possibilities for the utilization of tension
elements in structures in addition to the simple draped or
vertically hung cables. Cables can be arranged in a circular
radiating pattern with an inner tension hub and an outer
compression ring similar to that in a bicycle wheel (see
Figure 1.42).
Cables can also be arranged in crisscrossing networks,
as draped systems, or as restraining elements for air-inflated
membrane surfaces. Membrane surfaces can be produced by
air inflation, by edge stretching, or by simple draping.
Tension elements can also be used in combinations with
compression elements, as they are in trussed structures. For a
spanning truss, the bottom chords and end diagonals actually
constitute a continuous string of tension elements, which
might actually be developed as such. Tension ties for rafters
and arches are another example of this type of system.
Surface Structures
The neatness of any categorization method for structural
systems eventually breaks down, since variations within one
system tend to produce different systems, and overlapping
between categories exists. Thus the rigidly connected posts
and beams become the rigid frame and the vault and dome
become surface structures. As a general category, surface
structures consist of any thin, extensive surfaces functioning
primarily by resolving only internal forces within their surfaces
(see Figure 1.43).
We have already discussed several surface structures. The
wall in resisting compression or in acting as a shear wall acts
like a surface structure. As with other rigid surface elements,
the wall can also resist force actions perpendicular to the wall
surface, developing bending and out-of-plane shear.
The purest surface structures are flexible tension surfaces,
since they are usually made of materials with no out-ofplane
resistance. Thus the canvas tent, the rubber balloon,
and the plastic bag are all limited in function to tension
resistance within the planes of their surfaces. The forms they
assume, then, must be completely ‘‘pure.’’ In fact, the pure
compression surface is sometimes derived by simulating it in
reverse with a tension surface. There are, however, other
structural elements within the surface structure category