Some items look like equipment but are governed by ASCE 7-22 Chapter 15 — non-building structures — instead of Chapter 13. The distinction changes the design force calculation, the response modification factors, and the deliverables. Engineers regularly misclassify silos, tanks, racks, and pipe bridges as "components" and end up with under-designed anchorage and a rejected submittal.

Chapter 13 vs Chapter 15 — the legal definition

  • Chapter 13 (nonstructural): equipment supported by another building structure. The building structure's lateral system delivers the seismic motion to the component.
  • Chapter 15 (non-building structure): a structure that is not a building but is itself the lateral system. It stands on its own foundation and resists seismic motion as a primary structure, even if it is housed inside a building.

Common Chapter 15 items

  • Above-grade liquid-containing tanks (water, fuel, chemical).
  • Silos and bins.
  • Industrial stacks and chimneys.
  • Storage racks and pallet rack systems (per Chapter 15.5.3).
  • Piping support pipe bridges between buildings.
  • Telecommunications towers.
  • Cooling towers and condensers when they sit on their own foundation, not on a building roof.

Chapter 15 design-force calculation

Chapter 15 splits the world into two categories:

  • 15.4 Non-building structures similar to buildings: design per Chapter 12 with R, Ω0, Cd from Table 15.4-1. Use the equivalent lateral force or modal procedure.
  • 15.5 Non-building structures not similar to buildings: special rules per Section 15.5 (tanks, stacks, racks, etc.) — usually with prescribed period formulas and lower R values.

Worked example — 10,000-gallon water tank on its own concrete pad

  • Classify as ASCE 7-22 §15.7 (tanks and vessels supported on grade).
  • Use the impulsive + convective sloshing decomposition per §15.7.6.
  • Compute the impulsive base shear from the impulsive period and impulsive mass.
  • Add the convective component (much smaller, longer period).
  • Anchor the tank for the combined base moment using the §15.7 anchorage rules.

That is a totally different calc from Fp = 0.4 · SDS · Ip · Wp · (Hf/Rμ) · (CAR/Rpo) — which would underestimate the impulsive base shear of a tank by an order of magnitude.

Worked example — pallet racks in a Costco

  • Per §15.5.3, storage racks > 8 ft tall are non-building structures with R from Table 15.4-1 (R = 4.0 for steel storage racks).
  • Design the rack frames for V = Cs · W with Cs from §12.8 using the appropriate R.
  • Anchor the rack to the slab per ACI 318-19 Chapter 17 with Ω0p on concrete-controlled limit states.
  • NOT a Chapter 13 component — the rack stands on its own; the building only provides the floor.

Items that look like Chapter 13 but are not

  • Roof-mounted cooling tower with its own structural frame and direct anchorage to the roof: Chapter 15 if it has its own resistance system; Chapter 13 if it is just an "equipment package" hung off the roof framing.
  • Floor-mounted process tank in an industrial facility: usually Chapter 15.7.
  • Pallet racks: always Chapter 15 once they exceed the threshold.
  • Free-standing antenna mast on the roof: Chapter 15 if > ~30 ft tall and not laterally tied to building; Chapter 13 if guyed and short.

The R, Ω0, Cd table difference

Chapter 15 uses Table 15.4-1, not Table 12.2-1. The values for non-building structures similar to buildings are similar to the building counterparts but often slightly more conservative. For non-building structures NOT similar to buildings, R values are notably lower (often 2.0 to 3.0) reflecting the reduced ductility of these systems.

Anchorage under Chapter 15

  • Anchorage is still designed per ACI 318-19 Chapter 17.
  • The seismic over-strength factor Ω0 from Table 15.4-1 is applied to anchor forces (instead of Ω0p from Chapter 13).
  • The §17.10.6 0.75 reduction on concrete-controlled limit states still applies.
  • For tanks, the §15.7 prescriptive anchorage rules govern — including the 1.5 anchor force amplification for hold-down rods.

Common mistakes

  • Designing a 12-ft-tall storage rack as a Chapter 13 component with CAR = 1.0 (rigid). Underestimates demand.
  • Using the Fp equation for a roof-top water tank instead of the §15.7 impulsive/convective decomposition.
  • Applying Ω0p from Chapter 13 to a Chapter 15 anchor (use the Ω0 from Table 15.4-1).
  • Forgetting that Chapter 15 structures need their own load combinations — the 0.5 wind/seismic carry-over rules from Chapter 12 apply.

Decision flowchart

  1. Does the item have its own lateral resistance system? If yes → Chapter 15.
  2. Is the item a tank, silo, stack, or rack named in §15.5–15.7? If yes → Chapter 15 with the section-specific rules.
  3. Is the item supported by the building structure and not self-resisting? → Chapter 13.
  4. When in doubt, ask the SEOR — and document the classification on the cover sheet.

How PANACHE ENGINEERING handles Chapter 15

We design tanks, silos, racks, and stacks per Chapter 15 with full impulsive/convective tank analysis where needed. For project-specific guidance, see our seismic anchor calculations page or contact our non-building-structure engineers.