Eccentricity or not to Eccentricity? (on strip footing)

[EngStuff]

When is it okay to ignore eccentric loads on strip footings with stem walls. I am told at the office to include overturning, but it seems way too conservative and unnecessary.

Here is a scenario, I have a footing with a 13" wide stem wall. See pic below. Hypothetically, if that stem wall is deeper then 9" there should be no eccentric load on the actual strip footing. Thus the strip will see a force on the center, and the bearing force is evenly distributed underneath the strip. No need to check overturning(ignoring wind load and other horizontal loads)

Now, this is an industrial building with a very large load on the masonry wall above the stem wall. The brick goes only 8 feet high. I can see if there was no stem wall, and the masonry and brick go all the way down to the strip footing, there will definitely be an overturning force. but not in this case.

What do you guys think?

 

[KootK]

Hypothetically, if that stem wall is deeper then 9" there should be no eccentric load on the actual strip footing.

I feel that logic to be a serious violation of statics. Your assumptions regarding load spread to not change the center of action of the original force. If you draw an FBD, this should be self evident.

As to when I would ignore the eccentricy: never. I may not go into great detail but I would give it one of the following considerations at minimum:

1) Is the load comfortably within the kern?

2) Is there a restraining element at the top, or sides, of the stem walls that can rectify that eccentricity?

 

[EngStuff]

If I drew a FBD, my eccentric force will lean very heavily to the masonry side. Say we did that and got an eccentric point force at the top of the stem wall. However, considering that the loads spread 30-45 degrees on each side of that top point. Shouldn't there be a certain depth at the bottom of the stem wall that the load force is distributed throughout the bottom width of the stem? whether it be 9" deep or 30" deep or greater? then the strip itself sees a centric point load(not including the stem wall weight)

Or when creating the FBD do we just take that load at the top of footing and drop it down to the bottom of the stem without including effects of distribution?

Thanks!

 

[Celt83]

because the cmu/brick are eccentric the stress distribution through the stem is not uniform.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[Celt83]

the 1:1 load distribution only applies if the load can arch in both directions:

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[Enable]

With these kinds of things I find it fun to do some wonky hypotheticals / desk examples if I can.

Lets take a look at the picture below. What do you envision happening at point A?

Now lets make that bottom grade (instead of a teeter totter) and ask the same question.

But before you answer first lets see what happens when you press down on the edge of a card on your desk.

What do you think now?

 

[KootK]

Shouldn't there be a certain depth at the bottom of the stem wall that the load force is distributed throughout the bottom width of the stem? whether it be 9" deep or 30" deep or greater? then the strip itself sees a centric point load(not including the stem wall weight)

No sir. Internal stress distributions, whatever they may be, should not affect the overall FBD of an element assumed to be rigid.

Or when creating the FBD do we just take that load at the top of footing and drop it down to the bottom of the stem without including effects of distribution?

That one, all day long.

 

[Ron247]

I am not sure from your picture what loads you are working with. Is there a wall on top of the masonry block that also adds load to the footing? Like Koot, I do not see why 9" tall would mean anything to overturning. As others have stated, compute your loads, apply the centers of the loads where they occur and do statics.

The most common construction I see in residential almost always has overturning. The put an 8" block in the center of an 18" to 24" footing. Then they put the outside face of brick about 5" from the outside face of block. The brick can be 8' tall or 38' tall, same location. One house was 2 stories with a 12:12 roof. Since the center of block is at the center of footing, the center of the wall is on the same side of overturning as the brick. Kern = 4", brick center is about 7" and the wall center is 1.25" from footing center. Most calcs I have done tend to put the outside face of block at the center of footing or close to it.

 

[MIStructE_IRE]

Why not centre the footing under the load bearing inner leaf?

I agree with all above. You can’t simply keep your head in the sand and hope the eccentricity goes away.

 

[dik]

I'd center the footing on the load centroid... or close to it... as long as it's within the kern, if you have ample capacity should be OK, but loading may cause some sort of differential movement.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Tomfh]

Hypothetically, if that stem wall is deeper then 9" there should be no eccentric load on the actual strip footing. Thus the strip will see a force on the center, and the bearing force is evenly distributed underneath the strip. No need to check overturning(ignoring wind load and other horizontal loads)

It doesn't work that way. As KootK says, that violates statics.

The footing will apply an uneven bearing stress if the load on the footing is eccentric. It has to for equilibrium to occur.

I generally check peak bearing stress, or check that the wall (or slab) can rectify the eccentricity.

 

[EngStuff]

Thanks all,

I will center the footing at the load centroid. Much cleaner.