How big are seismic forces… really? (NBCC rule-of-thumb)

I’ve had a lot of conversations lately about how “big” seismic loads actually are compared to gravity loads under NBCC 2020 / NBC(AE) 2023 – especially outside the obvious high-seismic pockets.

So, I put together a rule-of-thumb view of base shear as a percentage of building weight (V/W) by Seismic Category (SC1–SC4) and ductility level for typical regular buildings (normal importance, NBCC equivalent static):

🔹 SC1 – Very low seismic

• Ductile systems: ~1–2% of W

• Moderately ductile: ~2–3% of W

• Conventional: ~3–5% of W👉 Wind and gravity almost always govern here.

  
  

🔹 SC2 – Low–moderate seismic

• Ductile: ~2–4% of W

• Moderately ductile: ~3–7% of W

• Conventional: ~5–13% of W👉 Seismic starts to matter but often competes with wind.

  
  

🔹 SC3 – Moderate–high seismic

• Ductile: ~3–8% of W

• Moderately ductile: ~6–15% of W

• Conventional: ~13–30% of W👉 Choice of SFRS and ductility level really shifts member sizes.

  
  

🔹 SC4 – High seismic

• Ductile: ~10–20% of W

• Moderately ductile: ~15–30% of W

• Conventional: ~30–50% of W (normal importance)

• Conventional + post-disaster: up to ~50–60% of W in extreme cases👉 In high seismic + low ductility, seismic can be half or more of gravity.

Of course, the code doesn’t give a fixed % – it gives:

and the actual value depends on spectrum, period, ductility/overstrength and importance. But I find these bands are a great sanity check when reviewing models or mentoring younger engineers:

• If your V/W is way outside these ranges, it’s a red flag to double-check:

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Curious how other engineers are seeing this in practice across Canada: 💬 In your region, does seismic typically govern over wind – and at what Seismic Category does that really start to shift?

#structuralengineering #seismicdesign #NBCC2020 #NBC2023 #earthquakeengineering #canada #buildingcodes