For decades, the ASCE 7 design response spectrum was drawn from two anchor values: SDS, the short-period plateau, and SD1, the one-second spectral acceleration. Plateau, then a 1/T decay — the "two-period" spectrum every practicing engineer can sketch from memory. ASCE 7-22 added something new: the multi-period response spectrum (MPRS), a spectrum defined at 22 discrete periods, delivered per site through the USGS seismic hazard data service. The two-period spectrum did not disappear, and knowing when each one applies — and when the multi-period spectrum is telling you something real versus an artifact of sparse data — is now part of the job.
Why the change was made
The two-period shape is an approximation. On soft-soil sites and in regions where hazard is dominated by large, distant events, the true uniform-hazard spectrum can carry significant demand at periods the plateau-plus-decay shape understates. The multi-period spectrum removes the approximation: instead of collapsing the hazard into two parameters and reconstructing a shape, the code now hands you the shape itself. For sites where the old approximation was unconservative — notably soft-soil sites with longer-period content — that is a genuine correction.
What the code permits
The practical split in ASCE 7-22:
- Equivalent Lateral Force procedure (§12.8): the two-period parameters remain the working basis. The seismic response coefficient Cs is computed from SDS and SD1 exactly as engineers are used to, with the §12.8 upper- and lower-bound equations deciding which expression governs. (In 7-22 those anchor parameters are themselves derived from the multi-period spectrum rather than mapped directly, but the ELF math is unchanged in form.)
- Dynamic procedures — modal response spectrum analysis, response-history analysis — use the multi-period spectrum, which is the point of having the full shape: those methods integrate over all of it, not just two anchor points.
The Central & Eastern U.S. problem
In the Central and Eastern United States, the strong-motion record is thin. The ground-motion models behind the 7-22 hazard data compensate with conservatism at very short periods, and the result is visible in the delivered spectra: a pronounced spike in the 0.05–0.1 second range at many CEUS sites. That is precisely where stiff, short-period structures live — low-rise masonry and tilt-up buildings, braced one-story industrial structures, anchored equipment. Designs in that band jumped relative to ASCE 7-16 without a corresponding change in observed performance of those building types.
This is not a fringe complaint. The ASCE 7-28 committee chairs published guidance on exactly this issue in an SEI Update (Soules & Guglielmo, STRUCTURE, January 2025), acknowledging the spike as a data-sparsity artifact and laying out a rational path for designers while the 7-28 cycle works on a fix.
The recommended hybrid approach
The approach recommended in that SEI Update — and it is the authors' recommendation, not code text, so treat it as guidance to discuss with your AHJ:
1. For structures designed by ELF with a fundamental period T < 0.2 s, design using the two-period spectrum parameters — don't chase the multi-period spike with a method that never uses the full spectrum shape anyway.
2. For modal response spectrum analysis, run the multi-period spectrum, but scale results to the ELF base shear computed from the two-period parameters — consistent with the scaling discipline the code already imposes on MRSA.
The logic is sound on both ends: ELF was calibrated as a two-parameter method, and MRSA's code-mandated base-shear scaling already anchors dynamic results to the ELF value, so anchoring to the two-period ELF shear keeps short-period CEUS designs from inheriting the artifact while preserving the better-shaped spectrum's distribution of forces up the height.
A practical workflow
- Pull both spectra for the site from the USGS data service — the multi-period spectrum and the derived SDS/SD1 pair. Look at them together; a CEUS short-period spike is obvious on a plot.
- If ELF is permitted for the structure and T is short, proceed with §12.8 on SDS/SD1 as always — and record in the calc narrative which spectrum basis was used and why.
- If MRSA is required, use the multi-period spectrum and scale to the two-period ELF base shear, citing the SEI Update as the basis for the choice.
- Either way: document it. The reviewer question is coming, and "here is the plotted comparison and the published guidance" is a one-minute answer if you prepared it.
Where StructuralClaw fits
StructuralClaw's Seismic ELF module runs the §12.8 procedure from the SDS, SD1, R and Ie you provide — base shear, vertical distribution, story forces and drift — with each governing equation cited on its value, so the calc package records exactly which Cs bound controlled. The spectrum-selection judgment above stays where it belongs, with the engineer; what the software guarantees is that once you've chosen your parameters, every number downstream is traceable to its clause.
- ASCE/SEI 7-22, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, §12.8 (ELF) and the Chapter 11 seismic ground motion provisions.
- Soules & Guglielmo, "SEI Update: ASCE 7-22 Ground Motions — A Rational Approach for Structural Engineers," STRUCTURE, January 2025.