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NBCC 2015 Seismic Foundation Provisions

NBCC 2015 Seismic Foundation Provisions

NBCC 2015 Seismic Foundation Provisions

(OP)
Hello, I'm having trouble deciphering the Seismic Foundation Provisions section of NBCC 2015 (or more specifically, NBC 2019(AE)). Section 4.1.8.16, see attached.

My understanding is you have to either capacity design the foundation, or design as elastic (with RdRo = 1.0). However, this clause appears to allow you to dip into overstrength factors of 1.3 or 1.5 (bearing). They increase the material resistances, instead of decreasing the loads, primarily because of overturning of spread/raft footings.

My questions are:
- I understand "all other resistances" to include the concrete/steel resistance, and I've been accounting for this by increasing Mr, Vr, etc. of the footings. Does this make sense? Increasing the resistance of the actual concrete/steel component (say 0.65*1.3 for concrete, or 0.85*1.3 for reinforcing steel) seems needlessly complicated.
- When checking footings, would you apply the 150% bearing resistance to the underside of the footing, to determine shears, moments, etc. in the raft? This seems like it would create design forces that are insanely large, compared to previous codes (when combined with RdRo = 1.0 and the increased spectrum values).

Thanks,

DG

RE: NBCC 2015 Seismic Foundation Provisions

Quote (DGetzlaf)

Hello, I'm having trouble deciphering the Seismic Foundation Provisions section of NBCC 2015 (or more specifically, NBC 2019(AE)). Section 4.1.8.16, see attached.

Yeah, this section of NBCC seems to change dramatically with each cycle and never quite have the same internal consistency as it's US code provision counterparts.

Quote (DGetzlaf)

I understand "all other resistances" to include the concrete/steel resistance, and I've been accounting for this by increasing Mr, Vr, etc. of the footings. Does this make sense? Increasing the resistance of the actual concrete/steel component (say 0.65*1.3 for concrete, or 0.85*1.3 for reinforcing steel) seems needlessly complicated.

I believe that the "other resistances" refers to other sources of foundation resistance to overturning and sliding resistance. Things like tension piles. I don't believe that this is meant to apply to internal force actions on your foundation elements.

Quote (DGetzlaf)

When checking footings, would you apply the 150% bearing resistance to the underside of the footing, to determine shears, moments, etc. in the raft? This seems like it would create design forces that are insanely large, compared to previous codes (when combined with RdRo = 1.0 and the increased spectrum values).

I believe that the intent is to apply the 1.5 factor OR the load associated with RdRo = 1.0, whichever is less. But not both concurrently.

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