Building code requirements apply to glazing systems, which comprise frame, attachments and the glazing infill. Window and door, skylight and curtain wall manufacturers, therefore, apply an integrated approach to designing their systems in order to pass missile impact and cycling tests, as well as required air, water and structural tests.

Laminated glass is a critical component of any impact resistant glazing system. It is manufactured by sandwiching "interlayers" (generally tough polymer sheet materials) between two pieces of glass and forming them into a composite by controlled processes combining heat and high pressure. The processing typically is done in large autoclaves capable of making finished, laminated glass suitable for buildings.

Its major benefit is that it remains intact even if broken, providing a weather barrier that reduces the likelihood of total collapse due to internal pressurization of the structure or widespread water damage. When adhesive plastic interlayers such as polyvinyl butyral (PVB) are laminated between glass, the plastic absorbs the energy of the impact, thus resisting penetration. This helps both hold the glazing system together even if the glass breaks and prevent injuries related to flying glass or exposed shards. When windows and doors are impact resistant, there is no need to board up window openings when a hurricane advisory is issued. Nor is there a need for the activation or mounting of shutters.

Depending on the location, size of the panels, and design pressures, the interlayer used in the laminate may be of different thicknesses or types. For example, a .060-in PVB interlayer is a typical thickness and type of interlayer for small missile glazed areas. Other stiffer, more structural, interlayers, in thicknesses of .090 and .100-in, provide better resistance to tearing when subjected to large missile impacts at higher design pressures. The strength and other properties of laminated glass can be tailored to meet specific needs.

Test Requirements and Standards

The Miami-Dade County test requirements currently define two distinct impact areas on a structure. Large missile impact resistance for glazing systems is required for the first 10 meters or 30 feet above the finished grade. Above the first 10 meters, small missile resistance is required for glazed openings to the top of the structure. The large missile impact test requires two impacts per specimen using a 9-pound 2-foot x 4-foot wood timber traveling at 50 feet per second. The small missile test requires two impacts to the glazing system, each with ten 2-gram steel balls traveling at 130 feet per second.

Once impacted, the test specimens are subjected to 9,000 pressure cycles, 4,500 positive pressure cycles and 4,500 negative pressure cycles. Failure is defined as a tear greater than 1/16-inch x 5-inches.

In 2002, the ASTM E1886 Test Method for Exterior Windows, Curtain Walls, Doors, and Impact Protective Systems Impacted by Missile(s) and Exposed to Cyclic Pressure Differentials was finalized. Similar to the Dade County protocol, the ASTM standard has both the large and small missile requirements. There are, however, some differences. In most areas, only one impact location is required per specimen. Two impact locations for each specimen are required in wind zones greater than 140 mph and within 1 mile of the coast. Failure is defined as an opening through which a 3-inch sphere can pass. No impact is required on the mullion outside of the 140 mph wind zone. Small missile impact resistance is required from 30 to 60 feet in height above grade.