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).⁶ 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 ASCE/SEI 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 ASCE/SEI 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)

Thus, there are two options in designing buildings for windstorms with windborne debris: buildings designed with “openings” (partially enclosed buildings) to withstand the higher pressures noted in the commentary to ASCE/SEI 7-93 and, alternatively, building envelope components designed so they are not likely to be breached in a windstorm when impacted by windborne debris. The latter approach reduces the likelihood of exposing the building contents to the weather.

5.2 In this test method, a test specimen is first subjected to specified missile impact(s) followed by the application of a specified number of cycles of positive and negative static pressure differential (8). The assembly must satisfy the pass/fail criteria established by the specifying authority, which may allow damage such as deformation, deflection, or glass breakage.

5.3 The windborne debris generated during a severe windstorm varies greatly, depending upon windspeed, height above the ground, terrain, surrounding structures, and other sources of debris (4). Typical debris in hurricanes consists of missiles including, but not limited to, roof gravel, roof tiles, signage, portions of damaged structures, framing lumber, roofing materials, and sheet metal (4, 7, 9). Median impact velocities for missiles affecting residential structures considered in Ref (7) ranged from 9 m/s (30 fps) to 30 m/s (100 fps). The missiles and their associated velocity ranges used in this test method are selected to reasonably represent typical debris produced by windstorms.

5.4 To determine design wind loads, averaged wind speeds are translated into air pressure differences. Superimposed on the averaged winds are gusts whose aggregation, for short periods of time (ranging from fractions of seconds to a few seconds) may move at considerably higher speeds than the averaged winds. Wind pressures related to building design, wind intensity versus duration, frequency of occurrence, and other factors are considered.

5.4.1 Wind speeds are typically selected for particular geographic locations and probabilities of occurrence from wind speed maps such as those prepared by the National Weather Service, from appropriate wind load documents such as ASCE/SEI 7 or from building codes enforced in a particular geographic region.

5.4.2 Equivalent static pressure differences are calculated using the selected wind speeds (1).

5.5 Cyclic pressure effects on fenestration assemblies after impact by windborne debris are significant (6-8, 10-12). It is appropriate to test the strength of the assembly for a time duration representative of sustained winds and gusts in a windstorm. Gust wind loads are of relatively short duration. Other test methods, such as Test Methods E330 and E1233, do not model gust loadings. They are not to be specified for the purpose of testing the adequacy of the assembly to remain unbreached in a windstorm environment following impact by windborne debris.

5.6 Further information on the subjects covered in Section 5 is available in Refs (1-12).

1.1 This test method covers the performance of exterior windows, curtain walls, doors, and impact protective systems impacted by missile(s) and subsequently subjected to cyclic static pressure differentials. A missile propulsion device, an air pressure system, and a test chamber are used to model some conditions which may be representative of windborne debris and pressures in a windstorm environment. This test method is applicable to the design of entire fenestration or impact protection systems assemblies and their installation. The performance determined by this test method relates to the ability of elements of the building envelope to remain unbreached during a windstorm.

1.2 The specifying authority shall define the representative conditions (see 10.1).

1.3 The values stated in SI units are to be regarded as the standard. Values given in parentheses are for information only. Certain values contained in reference documents cited herein may be stated in inch-pound units and must be converted by the user.

1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Specific hazard statements are given in Section 7.