Load Analysis - Tensile Fabric Structures: Design, Analysis, and Construction - page 99

4. The regions of the roof surface most likely to accumulate drifted and sliding
snow are generally in the roof's drainage path. Consequently, the snow mass
that accumulates will increase with water saturation; the source of which can
be the total up-slope roof area tributary to the location in question.
5. Membrane surface deformations and drainage characteristics of the roof
surface are load distribution specific. Large deformations of the tensile
membrane are common under snow loads.
6.4 General Design Parameters
As in all structural systems, the designer must consider a variety of possible failure
mechanisms. The tensile membrane structural system and its components must be
designed to withstand each of the failure possibilities. The membranes used in tensile
membrane structures resist loads by tension. Intermediate cables may be used to help
define the form and to reduce the effective span of the membrane. Surface cables are
stiffer than the membrane. Consequently, when the surface is loaded, the membrane
will deflect between the cables. This reduces the effective membrane span. A shorter
span deflects to a smaller radius of curvature for a given allowable strength or strain.
As noted in Section 5.4, the load carrying capacity of a membrane is inversely
proportional to its radius.
Membranes are typically prestressed to a uniform biaxial level. Under applied load,
the curvature and stress typically increase in one principal direction and decrease in
the other until there is no tension and the fabric is slack. It is generally undesirable to
lose tension over any significant area under any load condition. Once the membrane
loses tension, it may bag or wrinkle. A change in load which could suddenly
detension/tension or snap the membrane can be very destructive. Consequently, the
prestress level is generally set high enough to remove any looseness on the pattern
form and prevent slack fabric under applied load. As the prestress is increased,
fabrication tolerances and patterning become more critical, more effort in installation
is required and the strength reserve under maximum tension load is decreased. The
level of prestress is dependent on the strength and stiffness of the fabric as well as the
expected range of applied loads.
Deviating from installing membranes with a uniform prestress can be used to force a
difference in the two surface radii of curvature. For example, increasing the
longitudinal prestress relative to the hoop prestress in a cone like surface will tend to
make the sides flatter. Most structural fabrics are bidirectional materials.
Consequently it is possible to develop different prestresses in the warp and fill
directions. However due to the typically low shear strength of most fabrics;
developing different prestresses on a biased axis is usually not feasible. To the extent
that the principal axes of curvature on the surface of a fabric structure are not parallel,
patterning a material with parallel warp and fill fibers for an unequal prestress
becomes particularly challenging. Uneven prestress tends to encourage wrinkling or
loss of prestress perpendicular to the direction with the highest prestress.