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
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