Volume 36, Issue 8, August 2001
Coming Soon to Your Area?
The bevy of codes in existence makes it hard sometimes to keep them all straight. Among the many codes are the International Codes which are published and maintained by the International Code Council (ICC). This is a family of codes designed to complement each other in safeguarding the public’s health and safety. The International Codes have been well-received, therefore they are being adopted or considered for adoption by states and jurisdictions throughout the country. So, let’s take a closer look at these codes, which may soon be adopted in your area.
Wind Load Provisions
The wind provisions of the International Building Code (IBC) require all buildings and structures and parts thereof to be designed to withstand the minimum wind loads prescribed by the code. Wind loads on every building are required to be determined in accordance with ASCE 7-981# or in accordance with simplified procedures of the code (IBC §1609.6#).2 Wind pressure (psf) used for the design of exterior component and cladding (C&C) materials are required to be on the drawings. This information was not identified in previous codes and should assist the decision-making process for selecting the glass and windows on a project.
Wind-borne Debris and Hurricane-prone Regions
Both the IBC and ASCE 7-98 define wind-borne debris regions. Wind-borne debris regions are defined in the IBC as “areas within hurricane-prone regions within 1 mile (1.61 km) of the coastal mean high-water line where the basic wind speed is 110 miles (48.4 m/s) per hour or greater; or where the basic wind speed is 120 miles (52.8 m/s) per hour or greater; or Hawaii.” (IBC §1609.2)
Hurricane-prone regions of the continental United States include the Atlantic and the Gulf of Mexico coasts where the basic wind speed is greater than 90 mph.
Exposure categories are intended to take into account the effects of irregular terrain and other obstructions. Ex-posure categories range from A through D with Exposure D resulting in the largest wind forces. The most common exposure categories are Exposure B and C with the vast majority of buildings being located in Exposure B. Exposures B and C are defined by the IBC as follows.
“Exposure B: Urban and suburban areas, wooded areas or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger. Exposure B shall be assumed unless the site meets the definition of another type exposure. Exposure C: Open terrain with scattered obstructions, including surface undulations or other irregularities, having heights generally less than 30 feet (9,144 mm) extending more than 1,500 feet (457.2 m) from the building site in any quadrant. This exposure shall also apply to any building located within Exposure B type terrain where the building is directly adjacent to open areas of Exposure C type terrain in any quadrant for a distance of more than 600 feet (182.9 m). This category includes flat, open, country grasslands and shorelines in hurricane-prone regions.” (IBC §1609.4)
Exposure categories are an important factor in determining the design wind pressure of a structure. The basic wind speeds on both the IBC and ASCE maps are higher than on former maps. However, the higher basic wind speed does not necessarily result in an increased design wind pressure.
Formerly, when designing for Exposure B, the coefficients for Exposure C were applied in determining C&C loads (i.e., windows and doors). The current codes separate the coefficients and provide adjustments for the exposure category. The result is reduced design wind pressures for components and cladding in many areas. Conversely, design wind pressures increase along coastal areas and other areas of Exposure C, especially in the higher wind speed zones.
What it all Means
How does all of this affect the glass and window industry? In many cases it will result in reducing the wind load for windows. Perhaps this is best illustrated with an example.
Consider a 10-story project, 85 feet in height, on the Atlantic Coast. Window openings are 9.5 square feet. (DP = Design Pressure)
1. ASCE 7-98 outside end zone; Exposure C 130 mph basic
wind speed: DP = +48.6 and -48.6 psf.
2. ASCE 7-98 outside end zone; Exposure B 130 mph basic wind speed: DP = +37.5 and -37.5 psf.
3. ASCE 7-98 in end zone; Exposure C 130 mph basic wind speed:
DP = +48.6 and -89.1 psf.
4. ASCE 7-98 in end zone; Exposure B 130 mph basic wind speed:
DP = +37.5 and -68.7 psf.3#
As may be seen from the comparisons above, the exposure category has a tremendous impact on components and cladding loads. From the industry perspective this may mean increased use and demand for windows with a lower pressure rating, particularly in high-wind areas.
Opening Protection Options
When designing an enclosed building in wind-borne debris regions, exterior glazing, which receives positive pressure in the lower 60 feet of the building is required to be protected (IBC §1609.1.4). The code provides three options for opening protection:
• Impact-resistant glazing, which is tested and meets an approved impact-resistance standard or ASTM E 1996 with ASTM E 1886;
• Impact-resistant coverings, such as shutters or storm panels, which are tested and meet an approved impact-resistance standard or ASTM E 1996 with ASTM E 1886;
• Precut wood structural panels in basic wind speed zones of 130 mph or less within limits specified by the code.
In lieu of opening protection, buildings in wind-borne debris regions may be designed as partially enclosed and required to resist greater internal pressures.
The International Codes were developed by the three model code groups forming
the ICC. The codes are designed to complement each other and are being adopted
throughout the country increasingly. The IBC and ASCE 7-98, while increasing
basic wind speeds, do not necessarily increase design wind pressures, especially
in higher basic wind speed zones. The inclusion of coefficients components and
cladding loads in Exposure B may have a dramatic effect on the design
requirements for windows. Openings in wind-borne debris regions are required to
be protected or the building is required to be designed to resist the increased
internal pressures for partially enclosed buildings.
1. American Society of Civil Engineers, Minimum Design Loads for Buildings and Structures, ASCE 7-98, Published January 2000, ASCE 1801 Alexander Bell Drive, Reston, VA 20191-4400
2. Citations to the IBC are to Section Number from International Building Code, 2000 Edition, International Code Council 5203 Leesburg Pike, Suite 708, Falls Church, VA 22041-3401
3. Courtesy of Jim Stropoli, Product & Application Engineering Inc.
Joseph D. Belcher serves as president of JDB Code Services Inc. in Orlando, Fla. His column appears monthly.
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