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Seismic questions

Seismic questions

Seismic questions

(OP)

In Part 4 of OBC dealing with the foundation design for seismic load (4.1.8.16.(1)): The clause says that the foundation should be designed to resist the lateral load capacity of the SFRS, except that when the foundations are allowed to rock, the design forces for the foundation need not exceed those determined in Sentence 4.1.8.7.(1) using an RdRo equal to 2.0.

Questions
1) Should the calculated base shear be magnified when designing foundation
2) If so, what force do we magnify to?
3) What is definition of rocking for a foundation?

RE: Seismic questions

Yeah, that's a problem clause. And from what I can tell, most designers either ignore it or are not aware of it. It can lead to some pretty beefy foundation designs. Other thoughts:

Quote (OP)

1) Should the calculated base shear be magnified when designing foundation

Yes, unless a correspondingly low RdRo was used for the rest of the building a well.

Quote (OP)

2) If so, what force do we magnify to?

a) If no rocking, magnify to the capacity of the system which is often enormous.
b) If rocking, magnify to RdRo = 2.0

Quote (OP)

3) What is definition of rocking for a foundation?

Typically, a rocking foundation is one that would stop absorbing seismic load prior to reaching a target seismic overturning demand. It would start literally overturning and, in the process, stop attracting subsequent seismic load. This is the only Canadian specific doc that I know of: Link

Here are some examples, according to KootK:

1) Shear wall on pad footing at grade-ish: rocking.
2) Shear wall on pile cap and piles that wouldn't easily rip out in tension: not-rocking.
3) Shear wall extending into subterranean levels: KootK says not rocking; other designers typically say rocking.

In material that I've read from other parts of the world, recommendations are made that one should actual determine at what load rocking would occur and design for that load with some safety margin. And that seems eminently reasonable to me. Complex, but reasonable. Not sure how we get away with less.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Seismic questions

(OP)
Thanks Kootk. You are an encyclopedia of knowldege. Do you teach engineering? We will study what you have sent. Much appreciated.

RE: Seismic questions

Quote (ajk1)

Do you teach engineering?

I wish. I moved back to Canada from the US in 2007 and, right out of the gate, screwed up royally with regards to this particular code clause. As such, it's etched into my memory pretty deep. On the plus side, if Calgary ever experiences an Armageddon-ish event of some kind, I know just what library elevator shaft I'll want to hunker down beside for shelter.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Seismic questions

Yeah, this is a completely terrible clause.

The problem is that it's completely impractical for conventional construction. On the other hand, it actually makes a lot of sense if you understand how the reduction factors and rest of the seismic code work. Holding the foundation to the capacity of the lateral system should be pretty fundamental.

The fact that there's nothing there that puts a hard force cap at RdRo=1 or maybe 1.3 on the design force is an oversight, though.

It also does a terrible job of defining what the 'foundation' actually includes. Sliding and geotechnical allowables are items that should really be specifically addressed and presumably wouldn't fall into this clause. They aren't actually excluded, though.

It's one of those clauses that I'm pretty sure exists in its current state because they don't have the manpower to come up with a better defined requirement, but there really has to be /something/ in the code because it's incredibly important. We have some really good seismic guys around the country, but there's a lot of things to cover in the code.

I think I saw some changes proposed in the draft NBCC 2015, but I don't remember the details and I don't think it solved much? I haven't got a copy of the published version yet, so if anyone does maybe they can fill us in.

RE: Seismic questions

Similar to Canadian LSD, huh?

Mike McCann, PE, SE (WA)


RE: Seismic questions

(OP)
Thanks TLHS for this further comment.
I am not sure I understand msquared comment though.

RE: Seismic questions

in lite of this proposed mission creep, does anyone have an example of actual fdn failure during past seismic events...

RE: Seismic questions

@SAIL3: in a way, you've spoken to the expressed logic behind the provision. Foundations are thought to often be very difficult to inspect after seismic events.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Seismic questions

Showa bridge 1964, failure of pile foundation.

Rocking is such a bad measure for designing of the foundation, removed from ASIC as a load limit. You can almost make the foundation anything you want with how the code defines rocking.

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