movement. The defacto industry standard at the glass-
polycarbonate interface is 0.050 inch (1.2 mm).
Aspect ratios are an important consideration and must
be kept as close to one as possible. Avoid designing glazing
"ribbons" because they tend to create unbalanced stresses
relative to the long and short sides of a glazing which can pull
the laminate apart.
a) Symmetrical Cross-Section. The glazing industry
uses the term "symmetrical" for two different types of cross-
sections. The first type, also known as a balanced cross-
section, consists of a polycarbonate ply or polycarbonate
laminate (known as the core) sandwiched between two plies of
strengthened glass. This cross-section is primarily used to
protect against forced entry attacks. The second type is a truly
symmetrically cross-section with all materials (types and
thickness) uniformly distributed around a neutral axis.
Balanced cross-sections can also be used for protection
against ballistic attacks. The glass ply on the protected side
of a ballistic threat cross-section is a very thin piece of
strengthened glass. This arrangement, when used to protect
against ballistic attacks, is known as a "lo-spall" product.
b) Asymmetrical Cross-Section. Asymmetrical cross-
sections usually consist of an annealed, heat, or chemically
strengthened glass laminate on the threat side laminated to a
polycarbonate ply or laminate on the protected side. A
polycarbonate ply or laminate on the protected side provides
protection against spall and is known as a spall shield. This
product is primarily used to protect against ballistic attacks.
The primary benefit of this cross-section is its fragment
resistance. This arrangement is known as a "no spall" product.
TTG is not recommended for multiple shot ballistic
resistance because of its extensive break pattern. Semi-tempered
glass can be used to provide a satisfactory, but thicker,
outboard lite for blast and ballistic resistance. Because
ballistic penetration is an empirical science, field tests are
required to determine the actual level of ballistic resistance.
Glass-Clad Ionomer. Glass-clad ionomer cross-sections
are produced only in a symmetrical arrangement. They provide
excellent forced entry resistance, especially in cross-sections
incorporating the thicker ionomer cores. These cross-sections
perform well because the ionomer material is softer than