Shear Wall Footing Load Path

[msegerman]

Hi,

If I have a shear wall spread footing located in the center of my building, with a large overturning moment, should I design my footing for gravity loads only, or gravity loads + overturning/shear? Some have told me that the first floor slab takes lateral forces to the peripheral foundation walls. I don't want to make my shear wall footing larger/stronger to account for seismic if I don't have to.

What would happen if I neglected seismic forces and the basement wall theory is incorrect? Would the building actually topple over or could I say if the aspect ratio is low enough, this is not an issue?

Any input is appreciated.

Thanks,
Matt, EIT

 

[bookowski]

Simple answer = all loads.

You didn't provide much information - concrete, timber, steel?

Assume for a minute that your ground floor diaphragm was able to provide restraint via the connection to perimeter basement walls. This would provide a horizontal reaction point about which your wall is rotating, i.e. you would still get a reaction at the base of the wall (footing) - but it would be in the opposite direction. That assumes that everything is rigid - if you started accounting for some flexibility then the magnitude/direction of that reaction would depend on the stiffness of all the various elements. Either way it is not valid to just assume that the lateral force all gets sucked up and away to the perimeter and leave it at that.

 

[kylesito]

How stiff is your first floor...is it a concrete slab or is it untopped plywood? How about the shear wall...does it stop at the first floor or is it continuous to the top of the structure?

If your diaphragm is flexible and all other floors are flexible as well, it's probably conservative to assume the center wall takes the load in proportion to it's tributary mass. In this case, I would design the wall for the overturning moment.

If your base level diaphragm is stiff, and the wall does not continue to above floor levels, I wouldn't assume any shear load in this wall and only count for the perimeter walls to take all of the shear.

If your base level is stiff, the wall continues up to above floor levels, then I would check the bottom diaphragm for a concentrated load at it's center and verify it is strong/stiff enough to transfer the loads to the perimeter.

In any case, I would not make the assumption that the shear just magically goes to the perimeter walls. This is a pretty large assumption and would depend a lot on the specific case for this to work.

I would also use some engineering judgement though in your overturning calculations. Often times I have found problems with overturning in shear walls does not work but if I step back and really think "could this actually overturn" I will find that I can assume significantly more dead load that I can count on to resist overturning. Many times, these footings are controlled by the bearing of the soil and shear of the concrete more so than any realistic possibility for it to overturn.

PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi

 

[msegerman]

Thanks for the input. This question is not aimed towards a specific project, but yet a reoccurring situation I run into. Typically, most of my projects are 5-10 story concrete buildings with 8" slabs.

Also, what if I had a shear wall in the same plane as the peripheral foundation wall, would that change things?

 

[msquared48]

The slab may be able take the shear, but definitely not any overturning. If there is a net uplift, you have to counter it some way, either with extra weight, a larger footing, or grade beams.

As for a shear wall over a concrete foundation wall, that foundation wall becomes a shear wall, so the same overturning and shear forces have to be applied to it. Most of the time, due to the extent of the concrete foundation wall system, I just ignore it though.

I know. I'm bad.

Mike McCann
MMC Engineering

 

[msegerman]

also, bookowski, I have thought about your opinion before you posted it, and i tend to agree with you. However, I feel like if I'm trying to think of the wall behavior in the most realistic way, wouldn't the first floor slab put a force on the wall... and then would the slab on grade put another force on the wall at cellar level? (assuming 4-5 inch slab on grade 10' below first floor slab)

 

[bookowski]

Possibly - but like I said you didn't provide much info. If it's a slab on grade then you're not going to be transferring shear through the slab into the perimeter basement walls (I'm assuming there is a joint at the slab to wall). Maybe that opposing reaction would be taken by friction, but again isn't there a joint at the shear wall to slab interface? It really depends on your materials, relative stifnnesses, connection details etc. In general I wouldn't just wave it away though - you should at least be able to come up with a load path that accounts for all of the reactions (horiz & vert). You should be able to draw a little stick diagram of a cantilever and your various assumed reactions and roughly make sense of it.

 

[bookowski]

Check out this article in Structure - roughly what you are talking about. The gist of it is that it depends on a bunch of factors, so you need to evaluate on a case by case basis. I wouldn't assume it just dissolves away at the ground level unless you're dealing with something so negligibly small that it is obviously ok just by inspection.

http://www.structuremag.org/article.aspx?articleID=1476

 

[msegerman]

Thanks for all the advice.

Bookowski, very good article. Also, the article referenced in your article is very good as well: " A very good resource for an in depth discussion of the backstay effect and recommendations for modeling is Modeling and Acceptance Criteria for Seismic Design and Analysis of Tall Buildings, PEER/ATC 72-1, which is available as a free download from the PEER (Pacific Earthquake Engineering Institute) website."

 

[CURVEB]

For the loading, use the load combinations in the ASCE7 or IBC. Typically you do not have to design for full live load + full seismic (or wind). You also use a reduced dead load (0.9D for LRFD) when calculating your uplift force due to overturning.