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WINDOWS & DOORS
Hurricane-resistant skylights, windows & doors: choices, protection methods, construction, standards & codes. Here we provide a guide to hurricane, storm, and wind resistant windows and skylights, including citing storm resistance standards, building codes, and products.
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In this article series we discuss the selection and installation of windows and doors, following best construction and design practices for building lighting and ventilation, with attention to the impact on building heating and cooling costs, indoor air quality, and comfort of occupants.
Page top photo: hurricane resistant windows required in Boca Raton, FL and at left in Jupiter Florida.
We review the proper installation details for windows and doors, and we compare the durability of different window and door materials and types.
In response to the devastating impact of Hurricane Andrew in 1992, Florida enacted stringent codes to protect homes from severe storms.
Other coastal states have followed suit in recent years, and now similar provisions in the International Residential Code (IRC) apply to coastal areas from Texas to Maine.
Researchers attributed much of Andrew’s destruction to wind penetration into homes through broken doors and windows, leading to extensive water damage and, in many cases, roofs blown off and houses destroyed.
Our photo (left) shows storm-exposed beach-front hotel windows at Boca Raton, FL.
The keys to preventing these problems were strengthening roofs and protecting windows and doors from wind and wind-borne debris. To protect windows, the new code allows three options:
The trend in new home construction is toward impact-resistant windows, sometimes marketed as “storm resistant” or “hurricane-rated.”
Miami-Dade County enacted the most stringent standard and test protocols, subjecting windows (and storm shutters) to a test in which a 9-pound 2x4 is hurled into the glass at 50 feet per second, followed by 4,500 cycles of positive and negative wind loads equivalent to a 146-mph wind.
Miami-Dade also conducts AAMA/NWWDA testing for design pressure and water intrusion, but it conducts the water intrusion test after the structural test is completed rather than on a new window. Windows and doors that pass the Miami-Dade Product Control Standards are required throughout Miami-Dade County and most other coastal areas in Florida.
The International Residential Code (IRC) requires impact-resistant windows in all hurricane-prone regions along the Gulf and Atlantic coasts from Texas to Maine. Depending on the wind-speed zones established in the IRC, windows need to meet design pressures ranging from 30 to 80 psf, and they must meet impact-resistance standards under ASTM E1886 or E1996.
The design pressure required depends on both an area’s design wind speed, found on IRC maps, and the building’s exposure rating from A to D. Most buildings are rated Exposure B for “urban and suburban areas or wooded areas” or Exposure C for flat, open terrain with scattered obstructions of less than 30 feet. Waterfront buildings exposed to winds flowing over open water for at least a mile are rated Exposure D, the most severe.
Under pressure from both the building codes and insurance industry, most major window manufacturers have developed impact-resistant windows for residential applications that feature laminated glass along with heavier frames and hardware. The glass is similar to auto windshields with a plastic interlayer, but it is significantly heavier. Double-glazed units get a second layer of tempered glass either on the interior or exterior.
Vinyl-framed windows are heavily reinforced with aluminum, and all windows use metal mullion bars anchored to the framing between mulled units. Window-to framing attachment methods are also beefed up to comply with the new codes, and in some cases, metal clips are used to anchor the window to the frame.
Storm-resistant windows cost from two to four times as much as standard windows; but under pressure from code agencies and insurance companies, these windows will soon become standard fare in coastal construction and other storm-prone areas.
How well a window performs when subjected to heavy rains and high winds is indicated by its performance grade and design pressure. The design pressure is a structural rating only, while the performance grade also indicates that a window has met the water resistance and air infiltration standards for that grade (see Table 3-2 below).
[Click to enlarge any image, photo, or table]
The minimum recommended design pressure for residential doors and windows is 15 psf. A design pressure of 15 means a window has been tested to withstand sustained wind pressures of 22.5 psf, roughly equivalent to a 95-mph wind, applied to either side of the window, simulating both positive and negative wind pressures.
The test pressure is always 150% of the rated design pressure to provide a safety factor. To earn a performance grade of 15, a window must also pass a water pressure test of 2.86 psf, which simulates rainfall of 8 inches per hour with a wind speed of 34 mph. In coastal areas or other areas prone to heavy winds or hurricanes, higher grade windows are recommended and may be required by code.
-- Adapted and paraphrased, edited, and supplemented, with permission from Best Practices Guide to Residential Construction.
Reader question: what is the wind load [on windows or other structural components] at 200 mph
2/9.2014 lw said:
winds at 200 mph and what load ?
Reply: wind load calculation references
LW; several government PDFs as well as the Engineering Toolbox convert wind speeds to loading forces. For example a 50 mph wind would impose a load of 2.15 k N/M2. It's not linear so one needs to do the calculation.
There is a host of wind loading calculation approaches but the underlying concept is force x area or F = A x P.
"Uniform Buiding Code UBC 1997 Div. 3, Section 1620 Wind Design" offers this approach:
where UBC 1997: Division 3 - Wind Design.
Section 1620: " Design wind pressures for buildings and structures and elements therein shall be
determined for any height in accordance with the following formula:
P = Ce Cq qs Iw kzt (20-1)
The wind exposure type (given UBC in Section 16.16 & described just below) is used to determine the coefficient Ce according to Table 16-G.
Ce = combined height, exposure and gust factor coefficient (Table 16-G)
Cq = pressure coefficient (Table 16-H)
qs = wind stagnation pressure at 33 feet (Table 16-F)
Iw = importance factor (Table 16-K)
kzt = topographic factor
Note that this UBC 97 wind design code assumes (for the sake of low buildings perhaps) that the fastest wind speed is at 33 feet above the ground, not the actual peak sustained wind speed. That wind speed is given by a map included in the code. And the UBC Offers these Wind Exposure Levels:
EXPOSURE B has terrain with buildings, forest or surface irregularities, covering at least 20 percent of the ground level area extending 1 mile (1.61 km) or more from the site.
EXPOSURE C has terrain that is flat and generally open, extending 1/2 mile (.81km) or more from the site in any full quadrant.
EXPOSURE D represents the most severe exposure in areas with basic wind speeds of 80 miles per hour (mph) (129 km/h) or greater and has terrain that is flat and unobstructed facing large bodies of water over 1 mile (1.61km) or more in width relative to any quadrant of the building site. Exposure D extends inland from the shoreline 1/4 mile (.40km) or 10 times the building height, which ever is greater.
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