If you have projects in the Central and Eastern United States (CEUS), you may be in for a surprise when you use the new seismic ground motion values of ASCE 7-22.  The purpose of this article is to provide guidance to the structural engineer to avoid excessively conservative designs.

Two response spectra options are incorporated in ASCE 7-22, the Multi-Period Design Response Spectrum and the Two-Period Design Response Spectrum.  Either spectrum is allowed for use with the Equivalent Lateral Force (ELF) procedure.  Only the Multi-Period Design Response Spectrum is allowed to be used with the other analysis methods found in ASCE 7-22. 

For project sites located in the CEUS, the Multi-Period Design Response Spectrum results in a spike in spectral response accelerations at periods between 0.05 and 0.1 seconds.  This spike is primarily due to limited data and the corresponding high uncertainty that form the basis of the ground motion models used in the CEUS.  In the past, the short-period spectral response acceleration parameter (SDS) used in the Two-Period Design Response Spectrum was determined at a period of 0.2 seconds.  This is because it was concluded that 0.2 second was reasonably representative of the shortest effective periods of buildings and structures that are designed using ASCE 7, considering the effects of soil compliance, foundation rocking, and other factors typically neglected in structural analysis.  Unfortunately, this period limit was not applied to the Multi-Period Design Response Spectrum when it was introduced in ASCE 7-22.  A comparison of the two design response spectra options is shown in Figure 1 for a site in Charleston, SC for the default site class.

 

Figure 1. Comparison of Design Response Spectra in Charleston, SC
(Default Site Class)

To avoid excessively conservative designs, we recommend the following options for project sites located in the CEUS:

  1.    When using the ELF procedure for structures with a fundamental period of less than 0.2 seconds, use the Two-Period Design Response Spectrum to obtain design seismic ground motion values.
  2. When using the modal response spectrum analysis method, which must be scaled to the ELF base shear, we recommend using the Multi-Period Design Response Spectrum to obtain design seismic ground motion values for use in the modal response spectrum analysis and then use the Two-Period Design Response Spectrum to compute the ELF base shear for the scale factor.

These options will provide both safe and economical designs and comply with the seismic design requirements of ASCE 7-22.

Authors: J. G. (Greg) Soules, Ph.D., S.E., P.E., P.Eng., F.SEI, F.ASCE
Chair, ASCE 7-28 Main Committee

Emily M. Guglielmo, S.E., P.E., F.SEI
Chair, ASCE 7-28 Seismic Subcommittee