Change is in the Air

Changes to Design Wind Pressures in the 2018 IRC
Julie Ruth
May 27, 2017
COLUMN : Code Arena | Codes & Standards

The content of the 2018 International Building Code and 2018 International Residential Code has been finalized. We can now start to prepare for the potential impact of these new provisions in the marketplace.

Perhaps the most significant change in the 2018 I-codes for fenestration products will be the update of American Society of Civil Engineers 7. The 2018 IRC will refer to the 2016 edition of ASCE 7 for engineered design but the prescriptive provisions will continue to be based upon ASCE 7-10. This will be inconsistent with the 2018 IBC, which will rely entirely upon the 2016 edition of ASCE 7.

ASCE 7-16 brings significant changes in design wind pressures for some fenestration products in much of the U.S. In most locations, the negative design wind pressure on skylights will increase significantly. In some locations, the negative design wind pressure on vertical glazing will decrease.

Vertical glazing

For most of the U.S. (except for the Gulf and Atlantic coastal areas), the design wind speed will be significantly reduced. This will result in lower design wind pressures for vertical glazing in those areas. In some cases, this reduction will be as much as 30 percent.

There will be an additional, slight reduction in design wind pressure for residences less than 30 feet in height in suburban areas. In the remainder of the U.S., the design wind speed and resultant design wind pressure either remains the same or is only slightly reduced.

Skylights

Although the same reduction in design wind speeds that applies to vertical glazing will also apply to skylights, the net effect of the update to ASCE 7-16 on skylight design wind pressures will be more difficult to determine. This is because other factors that go into the equation to determine design wind pressure have also changed significantly.

The greatest will be an increase in design wind pressure for skylights that are within 4 feet of the roof ridge or eave. In some cases, the design wind pressure for skylights under ASCE 7-16 will be two to two-and-a-half times what it is under ASCE 7-10.

The increase in negative design wind pressure under ASCE 7-16 applies to all components of the roof. The main reason the prescriptive provisions of the 2018 IRC will not be updated to ASCE 7-16 is due to the roofing industry’s opposition to this increase.

Net effect

Most single-family homes and townhouses are built using the prescriptive provisions of the IRC. In those instances, there will be no change to the design wind pressures for vertical glazing and skylights from the 2015 IRC to the 2018 IRC.

The 2018 IRC will require the use of engineered design rather than prescriptive design for specific applications. These will include unconventional structural systems and the Gulf, Mid-Atlantic and Alaskan coastal areas. Conventional structural systems include light wood, balloon framing and concrete.

The table below gives a comparison for a 3-foot by 5-foot window and a 3-foot by 3-foot skylight on a two-story residence in Savannah, Georgia, as an example.

Note that in Savannah the builder could choose whether to build to the prescriptive requirements of the IRC, which are based upon ASCE 7-10, or use engineered design in accordance with ASCE 7-16. If the home were located a little further north in Charleston, South Carolina, and the 2018 IRC is adopted there, engineered design would be required and the higher pressures of ASCE 7-16 would apply.

The potential ramifications of this change on your product will vary greatly, depending upon the type of product you provide and where you are providing it.

Code Arena is brought to you by the America Architectural Manufacturers Association. Julie Ruth may be reached through AAMA at 847/303-5664 or via e-mail at julruth@aol.com.