The Material Characteristics of Fabrics - Tensile Fabric Structures: Design, Analysis, and Construction - page 34

Generally, the plain weave has the greatest angle deformation characteristics (the
allowance of perpendicular fibers to rotate into non perpendicular positions under
tension) of the two threads in building applications, with the panama next and the
stitch bonding process being the stiffest. However coatings play a major role in the
stiffness characteristics also and these will be discussed later in the coatings section.
A typical plain weave is shown in Figure 3-2. The weaving causes a different
behavior of the warp and fill yarns under load. The approximately straight warp
yarns simply stretch under load whereas the bent fill yarns flatten where they cross
the warp yarns. There is also a small amount of bending of the warp yarns by the
tension force of the fill. Understanding this characteristic helps in the tensioning of
frame structures where pulling on the warp yarns shorten or prestress the fill yarns.
These geometrical distortions cause an orthotropic behavior (one that has different
material properties or strengths in different orthogonal directions) of the fabric with
the application of load. To overcome this perceived drawback, some fabrics have the
yarns pre-stressed before the coating process by stretching them with a series of pins
on the edges of the rolls. This results in both yarns being crimped around each other
so the behavior of the fabric is more nearly equal in both directions. Whichever way
the fabric is made, generally the cross sectional area of the warp and fill fibers per
fixed length will not be the same. Most fabrics have greater strength in the warp
direction than in the fill direction, which also contributes to the orthotropic behavior
of the fabric. In addition the twist of the yarns and the tightness of this twist affects
the behavior. This is why it is essential in design of fabric structures to have biaxial
testing done on each roll of fabric to fully understand its behavior.
Figure 3-2
Plain weave cross section
If the structure is purely a shade or passive solar device and does not need weather
tightness, an open weave fabric with or without coating may be all that is needed.
However, if an enclosed weather tight structure is needed the fabric must be coated
with a material that accomplishes this function. The coating may be poured in liquid
form over the fabric and then allowed to harden or it may be spread with a knife and
forced through the fabric. Although the coated material takes on properties of the
two materials, the coating contributes little to the overall strength of the fabric.