Alleviating de-stabilizing forces on wall footings 1

[StrEng007]

When designing your stem wall footing (full height CMU walls), is it OK to allow passive soil pressure to resist lateral loads on wall footings (thus eliminating the force vector that causes lateral overturning). I'm fully aware of how to determine Uplift FOS, OT FOS, and Sliding FOS. My question is whether or not it's acceptable to use passive pressure to alleviate some of the applied OT on the footing. As you can guess, my current procedure considers full lateral loads (Using Enercalc)

I know that passive pressure is typically used to resolve sliding. However, with my combination of uplift and lateral loads, both of these are de-stabilizing moments and when combined cause my footings to increase in width and thickness. It's become hard to justify these wider footing sizes when the competition is getting by with small footings.

My only assumption is that I have to alleviate some of the loads within the FBD. If you look in the prescriptive sections of the IRC, in my case I'm looking at Florida's adoption, 2020 FBC, Residential 7th edition (Section R403), you'll find ridiculous size footings being specified (such as a 12"wide by 6" thick footing). I'm wondering how they ever get these to work with combined de-stabilizing moments. I don't actually use these tables, I prefer to calculate my own footings.

Also, I'm aware that the residential codes tell you not to worry about uplift when using CMU walls. So if you're so inclined to follow this rule, my question about passive soil pressure still stands.

 

[XR250]

You call it a "stem wall" but also "full-height CMU"
Where I practice, a stem wall implies only partial height CMU or concrete.
Can you clarify?
If it is full-height, I don't see where the issue lies.

 

[HTURKAK]

I don't have the copy of 2020 FBC, Residential 7th edition , but apparently specified footing sizes for residential and the load bearing interior and perimeter wall footings shall be rigid connected .Pls post the plan of the subject residential structure to see the applicable footing layouts.

Your concern is valid for wall footings similar to fence etc. But still it is not reasonable to rely on passive thrust since the passive thrust will develop when the deformation reaches around 50 mm , literally means a wall having 2.0 m height , may have tip deflection around 200 mm .

The following figure shows the lateral displacement vs wall ht. ( from the book EARTH PRESSURE and EARTH-RETAINING STRUCTURES R.I. ClaytonRick I. WoodsAndrew J. BondJarbas Milititsky )

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[haynewp]

It doesn't seem worth it for a cantilevered wall. Whatever moment resistance you get will be very low for a case like you show with 16" of embedment and a corresponding small moment resisting arm. The wall has to deflect as noted above to engage the passive so now there will be some tilt at the top of the wall that will destabilize it before engaging the passive you're counting on.

It also doesn't make sense what you're doing with the fixed base if this is a full height wall supporting a roof. Should be a pinned base if there are shear walls all around.

 

[StrEng007]

Pls post the plan of the subject residential structure to see the applicable footing layouts.

I'm showing three figures below. Keep in mind this model is simplified for the point of this discussion. I've removed the interior slab so we can get to the point without splitting hairs.

The model considers a 30ft wide building, subject to dead, live, and wind loads. Top of wall is +8'-6" above grade and top of foundation is -1'-4" below grade. The wall footing is 16"wide x 12" thick. All the loads in Figure A have been calculated for you. Please note, this wall is continuous from roof to foundation. IT IS A SIMPLE SPAN WALL. I've shown the upper half of the wall as a hatched line because I couldn't be bothered to give y'all a textbook illustration. If you're hung-up about my use of the word "stemwall", please understand this cross-section is what we call "stem-wall" in south Florida" (2-courses below grade are solid grouted).

Considering the wind load applied to the wall, I've shown the lateral reactions at the roof diaphragm and the wall footing.

In Figure B, I've shown a FBD of the wall footing with all superimposed loads. This figure illustrates the resulting FOS against overturning, if the applied lateral load is to be resolved through the 'stabilizing' vs 'de-stabilizing' forces applied to the footing.

•The stabilizing forces consider: Roof DL, wall DL, weight of soil, weight of footing
•The destabilizing forces consider: Roof wind uplift, wall wind lateral
•The point of rotation (POR) is taken as the bottom right of the footing

In this scenario, a 16" wide footing does not pass. In the area I practice, this size footing is commonly utilized and the loads on this model are appropriate (lower half of Florida).

Now, as suggested in my OP, I was toying around with the idea that lateral loads could be resolved via passive earth pressure. To me, this is one way that my local peers are getting these sorts of scenarios to work. Let's say the lateral load is supported by passive pressure (I'm not saying this is correct, but hang in there with me for a minute), then there is no applied lateral load to the footing. There are two schools of thought here: either there is no applied OTM, or in the case of 'stabilizing' vs 'de-stabilizing', there is a FOS against overturn = 1.38.

 

[XR250]

I can't imagine worrying about something like this

 

[dold]

Ditto for what XR250 said. Even for much taller walls and higher wind pressures I've never considered this, and nobody at any of the firms I've worked for have as far as I know. I'd consider out of plane OTM perhaps if you have a wood wall bearing on top of a tall stemwall (i.e., hinge between foundation wall and wood wall).

But, in a theoretical sense... I know you've designed the wall as pin-pin for out of plane. Realistically you have wall dowels developed into the footing. I'd wager that that joint is "rigid" enough to resolve this OTM back into the wall.

 

[StrEng007]

I can't imagine worrying about something like this

That's great news... anticlimactic but good that I'm thinking too far into this scenario.

So to answer the question in my OP... this is a non-issue because of passive pressure? Or because this is too much of a non-issue to be considered?

You see how I'm trying to quantify all of this? I've been asked this question by a building reviewer before. I'm trying to develop a reasonable answer as to how the actual mechanics work.

I'd wager that that joint is "rigid" enough to resolve this OTM back into the wall

Wouldn't any fixity at the base be a sure way to impart OTM into the foundation due to lateral? This is what I'm trying to get away from. Or you're saying the wall will take the moment... even though the foundation supports the wall... *wires just crossed*

 

[StrEng007]

But still it is not reasonable to rely on passive thrust since the passive thrust will develop when the deformation reaches around 50 mm

So to engage the passive pressure, up to 2" of deflection is required adjacent to the bottom of wall/footing intersection? I don't see that happening.

I'm about ready to just scrap this whole exercise as I've not been able to find any solid discussion on this.

 

[kissymoose]

I agree, never seen it brought up nor be a problem before.
But now that it's here with some FBDs drawn...

My thoughts on why it's not been a problem:
• ye olde conservative load cases, especially with simultaneous wind lateral and uplift. The uplift per calcs negates wall DL, which likely isn't accurate with full lateral.
• dowels between the footing and wall likely don't distribute the lateral load as just a point load at the top of footing.
• once rotating, the resisting wall DL will be at the outside edge, adding wall width/2 to the moment arm.
• the floor takes the lateral load, not the footing.

Another note, the POR at the outside footing edge is unconservative, the soil is not that stiff.

 

[StrEng007]

kissymoose, thanks for the response. Seems like this really hasn't been brought up before. I've done plenty of work without this being an issue, however, there was one time I was asked this question by a building official and it really got me thinking. It likely has to do with the 175MPH Exposure D projects I'm working on but I wanted to find an answer.

A couple responses for you.

My thoughts on why it's not been a problem:
• ye olde conservative load cases, especially with simultaneous wind lateral and uplift. The uplift per calcs negates wall DL, which likely isn't accurate with full lateral.
If you're looking at FOS against uplift only (Figure B), the sum of dead loads exceeds the wind uplift. Keep in mind, all the reactions I'm giving have calculated wind at the ultimate state. So 0.6D+0.6W is the load case.

• dowels between the footing and wall likely don't distribute the lateral load as just a point load at the top of footing.
Right, this was mentioned before. If you're implying that a moment is developed, doesn't that further reinforce the fact that there is rotation at the footing? Also, whether or not moment is developed, isn't the shear load the same? A fixed end beam and a simple span beam develope the same shear reaction at their supports, no?

• once rotating, the resisting wall DL will be at the outside edge, adding wall width/2 to the moment arm.
The FOS against overturning that I gave for Figure A and Figure B, takes into full account all loads and moment arms.

• the floor takes the lateral load, not the footing.
This is true for windward loads that inadvertently push against an interior floating slab. However, leeward winds and side wall wind loads (negative winds) receive just as much pressure, especially under +GCpi internal pressure coefficient. Often times these walls with negative pressure don't have any resisting slabs as you mentioned.

 

[phamENG]

StrEng007 - I would say that yes, soil pressure is your saving grace here. But not passive. I'd look at it as an at-rest condition. Passive and active pressures require movement into and out of the soil mass, respectively, to develop. At rest assumes that there is no movement. While not precisely true, it's closer than the other two assumptions, and more conservative than assuming full development of passive resistance. Full development of passive pressures is usually measured as a rotation, but I'm not sure how applicable any of the testing that derived those would be for such a small wall. I doubt you'd need a full 2 inches of movement in a 24 inch tall stem wall, but I could be wrong.

 

[StrEng007]

Thanks pham, that was extremely helpful. Even with walls taken up to 10 or 12 feet, I suppose these conditions would still apply.

I'm baffled that I CANNOT find a discussion on this anywhere. I know I have a tendency to ask questions that are really difficult to prove. I've always been that way with my structural work.

 

[phamENG]

I know I have a tendency to ask questions that are really difficult to prove. I've always been that way with my structural work.

Welcome to the club. I suspect that's why most of the regulars are here. Deciding where to draw the line between "I must understand every detail of this" and "I must actually make money and not piss off all my clients/boss" is a daily struggle.

 

[StrEng007]

Welcome to the club...

This response alone is probably the most silver lined response I have ever got on this forum. A reminder that where I'm struggling to draw the line is something y'all also wrestle with. Thank you for this humanizing response. I've come to believe that the MVP's are somewhat untouchable.

I've been doing this for 15 years and I still manage to confuse myself sometimes.