The 2016 edition of ASCE 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE 7-16), has three published supplements. Supplement 1 was published on December 11, 2018, and was adopted into the 2021 International Building Code. Supplements 2 and Supplement 3 have been recently published, and all three are available in the ASCE Library as free downloads.
Supplements 1 and 3 work together to address shortcomings in the 2016 standard and should be understood and used by engineers and building officials alike.
Technical Details – Research from USGS and the Applied Technology Council has shown that the use of only two response periods (0.2s and 1.0s) to define the Equivalent Lateral Force (and Modal Response Spectrum Analysis) design forces is reasonably accurate when the peak MCER response spectral acceleration occurs at or near 0.2s and peak MCER response spectral velocity occurs at or near 1.0s for the site of interest. However, the two-point spectrum is potentially non-conservative when the peak MCER response spectral velocity occurs at periods greater than 1.0s, particularly for structures on softer soil sites where large magnitude events dominate the seismic hazard.
As a result, for high seismic hazard locations located on Site Class D and E sites, ASCE 7-16 requires a site-specific ground motion hazard analysis be performed to determine the design response spectrum, including spectral acceleration parameters (SDS and SD1). ASCE 7-16 provides exceptions to requiring a ground motion hazard analysis but lacks sufficient direction to be clearly implemented. Supplement 1 was developed to clarify the requirement; however, the following two issues were uncovered subsequent to the approval of Supplement 1:
(1) For Site Class D sites, Exception 2 does not clearly identify the design spectrum that should be used for a modal response spectrum analysis. The intent of the exception is to use a spectrum that includes a 1.5 multiplier on SD1, but the language does not explicitly require this. This oversight can result in long period structures, including tanks, to be under-designed by a factor of up to 1.5. This oversight also leads to underestimating impulsive and convective forces in liquid storage tanks located on Site Class D sites covered by Chapter 15.
(2) For Site Class E sites, to properly utilize Exception 3, a value of Fa for locations where SS is greater than or equal to 1.0 is required. However, the language in the exception does not provide a value for Fa, nor does Table 11.4-1. This oversight leaves it up to the engineer to select a value without appropriate direction. This oversight also leads to underestimating impulsive and convective forces in liquid storage tanks located on Site Class E sites covered by Chapter 15.
Supplement 3 was developed to correct these issues. Also, Supplement 3 reorganizes the exceptions to simplify the requirements.
Supplements are developed by the ASCE 7 Standard Committee to address important issues in between cycles of development. Supplements 1 and 3 and all other Supplements and Errata to ASCE 7-16 and earlier editions are available at www.asce.org/seiasce7.
John Hooper, P.E., S.E., F.SEI, Dist.M.ASCE, is the chair and Emily Guglielmo, P.E., S.E., F.SEI, M.ASCE, is the vice-chair of the ASCE 7-22 Seismic Subcommittee.