Building Design and Construction Handbook - Merritt - Ventech!

5.138 SECTION FIVE
5.17 AIR-STABILIZED STRUCTURES
A true membrane is able to withstand tension but is completely unable to resist
bending. Although it is highly efficient structurally, like a shell, a membrane must
be much thinner than a shell and therefore can be made of a very lightweight
material, such as fabric, with considerable reduction in dead load compared with
other types of construction. Such a thin material, however, would buckle if subjected
to compression. Consequently, a true membrane, when loaded, deflects and
assumes a shape that enables it to develop tensile stresses that resist the loads.
Membranes may be used for the roof of a building or as a complete exterior
enclosure. One way to utilize a membrane for these purposes is to hang it with
initial tension between appropriate supports. For example, a tent may be formed by
supporting fabric atop one or more tall posts and anchoring the outer edges of the
stretched fabric to the ground. As another example, a dish-shaped roof may be
constructed by stretching a membrane and anchoring it to the inner surface of a
ring girder. In both examples, loads induce only tensile stresses in the membrane.
The stresses may be computed from the laws of equilibrium, because a membrane
is statically determinate.
Another way to utilize a membrane as an enclosure or roof is to pretension the
membrane to enable it to carry compressive loads. For the purpose, forces may be
applied, and retained as long as needed, around the edges or over the surface of
the membrane to induce tensile stresses that are larger than the larger compressive
stresses that loads will impose. As a result, compression due to loads will only
reduce the prestress and the membrane will always be subjected only to tensile
stresses.
5.17.1 Pneumatic Construction
A common method of pretensioning a membrane enclosure is to pressurize the
interior with air. Sufficient pressure is applied to counteract dead loads, so that the
membrane actually floats in space. Slight additional pressurization is also used to
offset wind and other anticipated loads. Made of lightweight materials, a membrane
thus can span large distances economically. This type of construction, however, has
the disadvantage that energy is continuously required for operation of air compressors
to maintain interior air at a higher pressure than that outdoors.
Pressure differentials used in practice are not large. They often range between
0.02 and 0.04 psi (3 and 5 psf). Air must be continually supplied, because of
leakage. While there may be some leakage of air through the membrane, more
important sources of air loss are the entrances and exits to the structure. Air locks
and revolving doors, however, can reduce these losses.
An air-stabilized enclosure, in effect is a membrane bag held in place by small
pressure differentials applied by environmental energy. Such a structure is analogous
to a soap film. The shape of a bubble is determined by surface-tension forces.
The membrane is stressed equally in all directions at every point. Consequently,
the film forms shapes with minimum surface area, frequently spherical. Because of
the stress distribution, any shape that can be obtained with soap films is feasible
for an air-stabilized enclosure. Figure 5.105c shows a configuration formed by a
conglomeration of bubbles as an illustration of a shape that can be adopted for an
air-stabilized structure.
In practice, shapes of air-stabilized structures often resemble those used for thinshell
enclosures. For example, spherical domes (Fig. 5.105a) are frequently con-