Major Changes to Design Wind Pressures Coming in 2018 Codes

Julie Ruth
April 29, 2017
COMMERCIAL, RETAIL, FABRICATION : CODES & STANDARDS

What happened: The 2018 International Building Code references the 2016 ASCE 7 for determining design loads for buildings.

Why it matters: This change will bring in significant changes in design wind pressures for fenestration products in some parts of the United States.

The 2018 International Building Code is complete. Perhaps the most significant change in the 2018 IBC for fenestration in most parts of the United States will be the update of ASCE 7, which prescribes design loads for buildings.

The 2018 IBC will refer to the 2016 edition of ASCE 7, while both the 2012 and 2015 IBC referenced the 2010 edition of ASCE 7. This will bring in significant changes in design wind pressures for fenestration products in some parts of the United States. In some areas of the country, the negative design wind pressure on skylights will increase significantly. In others, the design wind pressure on vertical glazing will decrease. And in some cases, there will be no changes to design wind pressure at all.

Vertical glazing

For most of the United States the design wind speed will be significantly reduced, due to the reference to the 2016 ASCE 7. Affected areas include parts of Maine, New Hampshire, Vermont, New York, Pennsylvania, Virginia, North Carolina, South Carolina, Georgia, Alabama, Mississippi, Louisiana, Oklahoma and Texas, and all remaining states to the north and west. In other words, the entire country will be affected, except the immediate coastal areas on the East and Gulf Coasts. (See Fig. 1 below).

Fig. 1 — Areas Affected by Design Wind Speed Changes for Vertical Glazing in ASCE 7-16


For most of the United States the design wind speed will be significantly reduced with the reference to the 2016 ASCE 7. The affected areas are indicated in red.

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 also be a slight reduction of 8 to 10 percent in design wind pressure for residences less than 30 feet in height in suburban areas. This reduction will be applied in addition to the reduction due to reduced design wind speed.

In the unaffected areas, 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 the 2016 edition of ASCE 7 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 the 2016 edition of ASCE 7 will be more than twice what it is under the 2010 edition.

Net effect

Although the extent of this change will vary from building to building in different parts of the country, as a general rule, it can be anticipated that the required design wind pressure rating for vertical glazing will be reduced, while 15it will be increased for skylights.

Table 1—Maximum Required Design Wind Pressure


The comparison between the maximum required design wind pressure in the 2010 ASCE 7 and 2016 ASCE 7, for a window and skylight in a two-story commercial building in Reno, Nevada.

The table above gives a comparison between the 2010 ASCE 7 and the 2016 ASCE 7 for a 3-foot by 5-foot window and a 3-foot by 3-foot skylight on a two-story commercial building in Reno, Nevada, as an example. Note that the increase from -16 pounds per square foot to -34.6 psf for skylights only occurs on a relatively small portion of the roof. The increase is only about 29 percent for the majority of the roof, while a 30 percent decrease occurs for the vertical glazing.

As noted above, while the net impact of this change for vertical glazing will be a simple reduction in required design wind pressure rating for most of the country, it will be much more complex for skylights.

Julie Ruth is a code consultant for the American Architectural Manufacturers Association. She can be contacted through AAMA at 847/303-5664 or via e-mail. Ruth is also owner of JRuth Code Consulting.