5.1 Structural design of exterior windows, curtain walls, doors, and impact protective systems is typically based on positive and negative design pressure(s). Design pressures based on wind speeds with a mean recurrence interval (usually 25 to 100 years) that relates to desired levels of structural reliability and are appropriate for the type and importance of the building (1).5 The adequacy of the structural design is substantiated by other test methods such as Test Methods E330 and E1233 which discuss proof loads as added factors of safety. However, these test methods do not account for other factors such as impact from windborne debris followed by fluctuating pressures associated with a severe windstorm environment. As demonstrated by windstorm damage investigations, windborne debris is present in hurricanes and has caused a significant amount of damage to building envelopes (2-7). The actual in-service performance of fenestration assemblies and impact protective systems in areas prone to severe windstorms is dependent on many factors. Windstorm damage investigations have shown that the effects of windborne debris, followed by the effects of repeated or cyclic wind loading, were a major factor in building damage (2-7).
5.1.1 Many factors affect the actual loading on building surfaces during a severe windstorm, including varying wind direction, duration of the wind event, height above ground, building shape, terrain, surrounding structures, and other factors (1). The resistance of fenestration or impact protective systems assemblies to wind loading after impact depends upon product design, installation, load magnitude, duration, and repetition.
5.1.2 Windows, doors, and curtain walls are building envelope components often subject to damage in windstorms. The damage caused by windborne debris during windstorms goes beyond failure of building envelope components such as windows, doors, and curtain walls. Breaching of the envelope exposes a building's contents to the damaging effects of continued wind and rain (1, 4-7). A potentially more serious result is internal pressurization. When the windward wall of a building is breached, the internal pressure in the building increases, resulting in increased outward acting pressure on the other walls and the roof. The internal pressure coefficient (see ANSI/ASCE 7), which is one of several design parameters, can increase by a factor as high as four. This can increase the net outward acting pressure by a factor as high as two.
5.1.3 The commentary to ANSI/ASCE 7-93 discusses internal pressure coefficients and the increased value to be used in designing envelopes with “openings” as follows:
| “Openings” in Table 9 (Internal Pressure Coefficients for Buildings) means permanent or other openings that are likely to be breached during high winds. For example, if window glass is likely to be broken by missiles during a windstorm, this is considered to be an opening. However, if doors and windows and their supports are designed to resist specified loads and the glass is protected by a screen or barrier, they need not be considered openings. (109) <......ASTM E1886-13 history
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