What is a Rocking Shear Wall Foundation (NBCC)?

[KootK]

Canada rocks! Well, at least our foundations do.

The Canadian building code contains a provision (4.1.8.16(1)) stipulating that foundations supporting shear walls must be designed for the forces corresponding to the flexural capacity of those shear walls. This provision applies in both high seismic and low seismic regions alike (my jurisdiction is low seismic). Moreover, the level of force to be transferred through the foundation is not capped at an effective R value of one, corresponding to elastic design.

This provision results shockingly large foundations, particularly in low seismic areas. An exception is made for foundations that are allowed to rock however. If a foundation is allowed to rock, it may be designed for a force level corresponding to an R value of 2.0 which is a large improvement in most instances. Unfortunately, the Canadian code offers little guidance with regard to what constitutes a foundation system that is "allowed to rock". Is it based solely on the ability of the foundation to lift off? Do special provisions need to be made to accommodate the displacements associated with rocking (my research thus far would suggest not)?

So, how can you tell if a shear wall foundation is "allowed to rock"? Here are some specific examples and my thoughts on whether or not they qualify:

1) Shear wall on pile cap and piles. I'm going to say that this system is NOT allowed to rock. Piles in tension would restrain the rocking substantially, even if not designed for tie-down forces.

2) Shear wall on standard shallow footing without basement levels. This does sound like a rocking foundation to me. Should one include the effect of soil overburden in restraining rocking etc?

3) Shear wall on standard shallow footing with basement levels. I suspect that this system does NOT qualify as a rocking foundation. If the shear wall extends into the basement levels, the relatively rigid diaphragms at those basement levels ought to restrain (shear reversal forces) the shear wall from rocking.

Thoughts? I'd love to get my hands on some papers discussing this issue if anyone knows of any. Right now I just have a paper written in 2003 by a fellow by the name of Anderson.

Thanks,

KootK

 

[msquared48]

What is a rocking shearwall foundation. Well, I would say unstable.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[KootK]

Here's a link to a paper discussing the concept: Link

Rocking shear walls are referred to in the NZ building code as well as in some others I believe. Paulay and Preiestly discuss it in their book as well: [URL unfurl="true"]http://www.amazon.com/Seismic-Reinforced-Concrete-Masonry-Buildings/dp/0471549150[/url]

Basically, the interface between your footing and the ground becomes your -- not so plastic -- plastic hinge.

 

[PicoStruc]

Use CSA A23.3-04 UPDATED Seismic Chapter (CH 21.11) to obtain foundation seismic forces that depend of capacity, rocking and level of ductility of the system.

USE THE UPDATED CHAPTER available on CSA Website.

 

[MiketheEngineer]

We all ROCK!!!!!!

 

[KootK]

@ PicoStruc,

Thanks for your response. I'm working from the 2010 version of A23.3-04. It contains the three updates that I'm aware of. Are there further updates?

I reviewed chapter 21 again. The only reference to rocking foundations is listed below. There they refer to "anchored foundations" which, I assume, refers to foundations that do not rock. These provisions do not technically apply to conventional construction in low seismic regions but one would think that if the RdRo cap of 1.0 is good enough for high seismic, it would also be good enough for low.

Still, I'm left with trying to figure out whether or not a shear wall locked into place by basement diaphragms qualifies as anchored. My gut feel is that it does.

KootK

21.11.1.2
The factored resistance of the foundation system and the supports of frames or walls shall be sufficient to
develop the nominal moment capacity of the frames or walls and the corresponding shears. Where the
factored moment resistance of any wall or frame exceeds the required factored moment, the following
shall apply:
(a) the factored resistance of unanchored footings supporting those walls or frames need not exceed the
maximum factored load effects determined with loads calculated using RdRo equal to 2.0; and
(b) where frames or walls are supported by anchored footings or elements other than foundations, the
factored resistance of those elements need not exceed the maximum factored load effects
determined with loads calculated using RdRo equal to 1.0.

 

[PicoStruc]

@KootK

First, the chapter 21.11 don't apply to conventional construction.

second, what you listed was the old code article. It's Not the updated one from August 2009.

 

[KootK]

I got my hands on the update PicoStruc. Thanks for directing me to that.

Clause 21.8.1 of the conventional construction section sends you to 21.11.1.2. It seems that the upper bound moment to be considered would be the minimum of sentence a, d, or e for conventional construction.

I'm still unclear as to exactly what qualifies as an un-anchored (rocking) footing however. It seems to me that a shear wall projecting below grade, into basement levels, would be considered to be anchored. Is that your understanding as well?