FGIA Analysis September 2021September 7th, 2021 by Nathan Hobbs
Location, Location, Location: With ASCE/SEI 7, Updates Continue, but It Depends on the Address
By Kathy Krafka Harkema
For many, the go-to technical reference for maximum wind speeds for U.S. localities, and for determining fenestration design windloads, is ASCE/SEI 7, published by the American Society of Civil Engineers (ASCE). In building codes throughout the U.S., ASCE/SEI 7 is often the governing standard referenced for design loads on buildings and other structures. For example, the 2016 edition, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, is referenced in 2018 editions of the International Building Code (IBC) and International Residential Code (IRC). Public comments for the 2022 edition of ASCE/SEI 7 closed August 2, so a new edition already is in the works.
In the meantime, what is important to understand about the version currently in place? The 2016 edition of ASCE/SEI 7 represented some significant changes over the 2010 version when it comes to fenestration. In general, in ASCE/SEI 7-2016 the design wind pressure on vertical glazing decreases, while the negative design wind pressure on skylights increases significantly. In some cases, there is no change versus the 2010 edition.
In general, ASCE/SEI 7-16 results in minimal changes to the design pressures for vertical fenestration, including doors, windows and garage doors. Wind speed maps outside hurricane-prone regions have changed, with wind speeds in many areas lowering five to 15 mph in ASCE/SEI 7-16. Wind speed contours in New England shifted toward the Atlantic Ocean, reducing the wind-borne debris region.
Hurricane wind speeds generally remain unchanged from Texas to the Carolinas.
In the 2010 edition, Risk Category III and IV had the same mean recurrence interval (MRI) for wind loads, so one map was provided for both categories. In the 2016 edition, Risk Category IV buildings (like hospitals, police, and fire stations) now have a higher MRI which results in a basic wind speed increase of about five to 10 mph (depending on location) compared with Risk Category III buildings (for example, schools). The 2016 version includes separate maps for Risk Categories III and IV.
A new ground elevation coefficient, Ke, reduces velocity pressure for site elevations above 1,000 feet, to consider the lower air density at higher elevations. For example, Ke for a building 5,000 feet above sea level is 0.83. The net effect is a 17% reduction in design pressures in ASCE/SEI 7-16.
In general, ASCE/SEI 7-16 results in significant changes to design pressures for skylights and sloped glazing due to changes in roof pressure coefficients for low-rise buildings. Negative pressures increase in most areas for skylights.
In the 2016 version, a chapter on design for tsunami loads was added, and seismic ground motion maps changed. A new appendix on fire resistance, and major changes in the snow load chapter are included, too.
Some state and local jurisdictions may adopt wind speeds that differ from ASCE/SEI 7. That’s why it’s important to consult with local code officials to understand what requirements apply to a given location.
Kathy Krafka Harkema is U.S. Technical Operations Director for the Fenestration and Glazing Industry Alliance (FGIA).
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