The 2024 International Building Code adopts ASCE/SEI 7-22, and Chapter 13 — the chapter that governs seismic design of nonstructural components — has been rewritten more thoroughly than in any edition since the equation was introduced. If you certify equipment, this is the change that matters.
What the old equation looked like
Through ASCE 7-16, the horizontal seismic design force on a nonstructural component was:
Three ideas were packed into it. The term ap was a blunt switch: 1.0 if the component was “rigid” (period at or below 0.06 s) and 2.5 if it was “flexible.” Rp was a single number meant to cover both the component’s ductility and its overstrength. And (1 + 2z/h) said that floor accelerations grow in a straight line from the ground to three times as much at the roof, regardless of what the building is or how it is framed.
All three were known to be coarse. The Applied Technology Council’s ATC-120 project spent several years on the problem and published its recommendations as NIST GCR 18-917-43, Recommendations for Improved Seismic Performance of Nonstructural Components (2018). ASCE 7-22 adopts that work almost directly.
What replaced it
Equation 13.3-1 in ASCE 7-22 reads:
with the same bounds as before — not more than 1.6 SDS Ip Wp, and not less than 0.3 SDS Ip Wp.
The useful thing about the new form is that it splits cleanly into two brackets. The first bracket is about the building: how much the structure amplifies ground motion at the component’s elevation. The second bracket is about the component: how much the equipment amplifies what its supports feel, and how much strength it has in reserve. Four factors carry that:
Hf — amplification with height (§13.3.1.1)
The successor to (1 + 2z/h). At or below the grade plane Hf = 1.0. Above grade it is a function of both the height ratio and the supporting structure’s approximate fundamental period Ta:
Where Ta is unknown, the standard permits the simpler Hf = 1 + 2.5(z/h). The tenth-power term is what makes this different: it is negligible everywhere except very near the roof, where it captures the higher-mode “whip” that instrumented buildings actually show and the old straight line missed.
Rμ — structure ductility reduction factor (§13.3.1.2)
A ductile building yields, and a building that yields does not deliver as much acceleration to the equipment sitting on its floors. Rμ is the credit for that, built from the supporting structure’s own R, Ω0 and Ie. It is 1.0 at or below the grade plane — nothing is yielding beneath you — and is floored at 1.3 above grade, which is also the default value when the seismic force-resisting system is unknown or is not one of the listed systems. This is the first time the standard has let the supporting structure’s ductility reduce component forces explicitly.
CAR — component resonance ductility factor (§13.3.1.3)
The ratio of peak component acceleration to peak floor acceleration — how much the equipment amplifies what its anchorage feels. This replaces ap, and it is where most of the added nuance lives. Instead of a rigid/flexible switch, components are judged on two questions: is resonance with the building likely, and how much ductility does the component and its attachments have? The values behind the tables in Chapter 13 come from the ATC-120 study:
| Ductility category | At or below grade plane | Above grade plane |
|---|---|---|
| Elastic (reference only) | 2.5 | 4.0 |
| Low | 2.0 | 2.8 |
| Moderate | 1.8 | 2.2 |
| High | 1.4 | 1.4 |
Component ductility categories, ASCE 7-22 Table C13.3-1. Components unlikely to be in resonance take CAR = 1.0.
Rpo — component strength factor (§13.3.1.4)
The component’s and its attachments’ inherent overstrength, tabulated in Tables 13.5-1 and 13.6-1. Under 7-16 this was tangled up inside Rp along with ductility; 7-22 pulls the two apart, so CAR answers “how much does it shake?” and Rpo answers “how much extra strength is already in it?” The separation also lets the standard tie anchorage overstrength, Ω0p, to the same categories.
What this means for shake table testing
Less than you might expect. ICC-ES AC156 — the acceptance criteria behind essentially every seismic certification by testing — builds its Required Response Spectrum from the building half of the equation only. The component bracket [CAR / Rpo] is deliberately set to 1.0 for testing, because the real component on the table supplies its own dynamic behavior and strength. So the change from 7-16 to 7-22 amounts to swapping one height term for another:
At grade level the two are identical: z/h = 0 gives (1 + 2·0) = 1.0, and Hf = Rμ = 1.0 gives 1.0. At the roof of a building whose dynamic properties are unknown — the usual case for a manufacturer certifying a product line — AC156 directs Hf = 3.5 and Rμ = 1.3, so the multiplier becomes 3.5/1.3 = 2.69 where it used to be 3.00. That is roughly a 10% reduction in ARIG-H and in AFLX-H.
In other words, the test demand went down slightly. A product qualified to the old spectrum still envelops the new one, and Ip continues to play no part in setting test levels — it governs the post-test functionality that must be demonstrated, not the shaking.
The Ip problem in certification by analysis
Certification by analysis is a different story, and there is a real error in the published standard.
Sections 13.2.3.3 (Item 3) and 13.6.2.1(b) permit nonactive components to be certified by analysis using a seismic demand based on [CAR / Rpo] = 2.5 — the tabulated values for elastic component behavior. The intent is plain: certify the component for a demand that keeps it essentially elastic in the design earthquake, independent of how important the component is.
But Ip is still sitting in Equation 13.3-1 as an explicit multiplier. Take a ground-mounted component — z/h = 0, so Hf = 1.0 and Rμ = 1.0 — with Ip = 1.5:
A 50% overstatement, and it scales with Ip. This did not happen under ASCE 7-16, where the equivalent provision required Rp/Ip = 1.0; because Ip appeared in the denominator there, it cancelled out entirely.
The components caught by this are exactly the ones you would not want to over-design by accident: fire pumps, medical equipment, and other designated seismic systems assigned Ip = 1.5 and mounted at or near grade. Pre Compliance has raised the issue with the ASCE 7 Seismic Subcommittee, proposing that the required ratio be written as [CAR / (Rpo/Ip)] = 2.5, which restores the cancellation and returns the certification-by-analysis demand to the intended elastic level at every value of Ip. For an Ip = 1.5 component that is a reduction of about a third.
Until that correction is published, it is worth checking any analysis-based certification of a high-importance, ground-mounted component to see whether the demand being used is the one the provision intended.
The practical takeaway
- Testing programs largely carry forward. The AC156 spectra moved by about 10% at z/h = 1 and not at all at grade, and in the conservative direction for existing reports.
- Analysis takes more inputs. Rμ needs the supporting structure’s R, Ω0 and Ie, and Hf needs its period. When they are unknown, the defaults (Rμ = 1.3, Hf = 3.5 at the roof) are what a product-line certification will use.
- Component classification matters more. CAR and Rpo are assigned per component type, and the spread between ductility categories is wide. Getting the category right is now a meaningful part of the design force.
- Watch Ip in analysis-based certification. See above.
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References
- ASCE/SEI 7-22, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, Chapter 13 and Commentary.
- NIST GCR 18-917-43, Recommendations for Improved Seismic Performance of Nonstructural Components, Applied Technology Council for the National Institute of Standards and Technology, 2018 (the ATC-120 project).
- ICC-ES AC156, Acceptance Criteria for Seismic Certification by Shake-Table Testing of Nonstructural Components, 2024 edition.
- 2024 International Building Code, Sections 1613 and 1705.14.2.
