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Wall Footing Eccentricity

Wall Footing Eccentricity

Wall Footing Eccentricity

(OP)
I am trying to find out which section in ACI-318 talks about L/6 eccentricity limit for designing wall footings? Thanks in advance for help.

RE: Wall Footing Eccentricity

It's not a code requirement, but rather a general recommendation that you keep you're eccentricity within the middle third (kern area) of the footing. This ensures that you have compressive soil stress under the entire width of the footing. The force resultant can lie outside of the middle third, but it may require special attention at the end of the footing where there is no compressive stress on the soil.

RE: Wall Footing Eccentricity

(OP)
Got it. Thanks mike20793

RE: Wall Footing Eccentricity

Is your situation a property line strip footing at the bottom of a basement wall? If so, common practice is to assume a uniform soil pressure beneath the footing and resolve the eccentricity via a moment connection to the basement wall. The predominant issue is the design and detailing of the joint.

I feel like the kern business gets too much air play. It often results in unreasonably sized foundations.

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: Wall Footing Eccentricity

KootK may be correct that the assumption of uniform soil pressure is common, but that doesn't make it right. Reinforcing the joint for the moment does not change the soil reaction.

RE: Wall Footing Eccentricity

The soil pressure doesn't need to be "right" with a bearing medium governed by either plastic behaviour or settlement concerns. Besides, it's dirt. Whether you go with a retangle, a triangle, a cycloid, or even FEM results, it's a wild ass guess at best. Might as well make it easy.

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: Wall Footing Eccentricity

Somehow, I knew that was coming.

RE: Wall Footing Eccentricity

(OP)
KootK, you are sketch is correct.

RE: Wall Footing Eccentricity

Quote (hokie66)

Somehow, I knew that was coming.

By now, you know full well that I'm incapable of self restraint in these situations. Can you share what your assumption regarding soil pressure would be in this situation? I really do want to know. In my experience, no reasonable footing size will work for basement property line footings unless a moment connection to the walls is employed. And, once the moment connection is thrown into the mix, I really have no idea what the "right" soil pressure distribution ought to be.

Quote (design62713)

KootK, you are sketch is correct.

In my opinion, crap detailing is a much bigger issue in these situations than is correct assumptions regarding soil pressure distribution. I see the detail shown below about once a month on average and it fuels a lot of my binge scotch consumption. In my opinion, the key features of a proper design include:

1) One piece rebar crossing the footing and turning up into the wall.
2) Depth and rebar bend diameter chosen such that the last strut coming into the rebar corner could conceivably pass a strut and tie check. Usually, the proportions of these things are such that sectional shear check are questionable.
3) Some dirt side wall reinforcement continued to the nearest floor diaphragm above. I don't like to rely on lateral earth pressure that may not exist to reduce cut-off lengths.


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: Wall Footing Eccentricity

KootK,
It is just simple statics, you know that. If the footing is the same width as the wall, the pressure is uniform. If the footing is wider and moment connected to the wall, the reaction centroid moves over, with the soil pressure becoming trapezoidal, then triangular.

RE: Wall Footing Eccentricity

Quote (hokie66)

It is just simple statics, you know that.

Hardly. Rather, it's extremely complex, pretty much unknowable, indeterminate statics. The soil stress distribution would depend on the interplay between:

1) The flexural stiffness of the footng.

2) The flexural stiffness of the wall.

3) The vertical stiffness of the soil below the footing.

4) The lateral stiffness of the soil behind the wall.

5) Whatever flexibility may result from the presence of drainage mat etc behind the wall.

Quote (hokie66)

If the footing is wider and moment connected to the wall, the reaction centroid moves over, with the soil pressure becoming trapezoidal, then triangular.

Sure. But what good is this if you never really have any idea where your soil stress "neutral axis" is located? Under the wall? 6" inside? Philadelphia?

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: Wall Footing Eccentricity

If the slab and footing are poured monolithically, you can include top bars that extend back into the slab to counter the eccentric loading and alter the soil pressure seen.

Mike McCann, PE, SE (WA)


RE: Wall Footing Eccentricity

Quote (mssquared48)

If the slab and footing are poured monolithically, you can include top bars that extend back into the slab to counter the eccentric loading and alter the soil pressure seen.

This is common but, frankly, also bothers me. It strikes as a gross stiffness incompatibility in most cases. People wind up relying on the flexural stiffness of a 5" slab on grade with rebar at mid-depth (d=2.5") to iron out eccentric soil stress beneath the footing. I'll be curious to here what other's think about it.

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: Wall Footing Eccentricity

KootK,
The unknowns you listed are typical of essentially all structural engineering problems. We use models and methods, which are never precise, and don't have to be. But I will use mine, you are free to use yours, but not on my job. The assumption of uniform pressure is anathema to my way of thinking.

RE: Wall Footing Eccentricity

Quote (hokie66)

The unknowns you listed are typical of essentially all structural engineering problems. We use models and methods, which are never precise, and don't have to be...The assumption of uniform pressure is anathema to my way of thinking.

You can't have it both ways hokie66. On the one hand, you tell me that my uniform distribution is "anathama" to you because the soil distribution is not "right". Then, on the other hand, you tell me that there's no justification for accuracy in knowing the actual soil stress. Which is it?

And you still haven't indicated what soil stress distribution you would use or how you would determine it. Simply stating that the reaction centroid "depends" isn't saying much. If it's just simple statics, do tell. The method that you described above is only simple if you choose not to rely on the moment connection with the wall. And that will generally make for an unreasonable / impossible situation for a footing like the one being considered here.

It's also worth noting that, even with concentric footings, the assumption of uniform soil stress is also fiction. My Teng foundation book contains a figure much like that shown below.



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: Wall Footing Eccentricity

I'll just say that I approach this in the same way I approach any cantilevered retaining wall analysis.

RE: Wall Footing Eccentricity

I designed a eccentric monolithic footing with similar reinforcing into an 8" slab supporting a two story tilt-up concrete wall on the edge of a down slope to a railway line over 25 years ago and it has shown no distress, inside or outside of the structure. Somehow, it works - perhaps by extending the soil bearing prism into the slab area. Kinda like an "L" shaped retaining wall reinforcing pattern to spread the load.

I know that I sound like the proverbial contractor, but I am not. Trust me. 2thumbsup

Mike McCann, PE, SE (WA)


RE: Wall Footing Eccentricity

Let's not kid ourselves: everything seems to work out in the wild. Even the shabby, ill-conceived detail that I posted above is working just fine. Geotechnical safety factors being what they are, it's entirely plausible that none of our "eccentric" strip footings really need to be any wider than the walls that they support. I will concede, of course, that an 8" slab on grade with a fighting chance of having legitimate top steel is a good deal better than the 5" SOG scenario that I described above.

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: Wall Footing Eccentricity

Would tend to agree with hokkie on this one.

Also what if the wall is precast ?

What about tieing the ground slab to the footing ?

RE: Wall Footing Eccentricity

My wall was precast.

Mike McCann, PE, SE (WA)


RE: Wall Footing Eccentricity

Your footing was also tied into the ground slab.

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: Wall Footing Eccentricity

Mike,

I thought your footing and slab was poured monolithically, so how could it be a precast wall ?

RE: Wall Footing Eccentricity

hokkie

Lets consider this not a basement wall, so no retaining pressure, just load from wall.

How would you approach it then?

RE: Wall Footing Eccentricity

It was a precast (cast on site) tilt-up concrete wall set on a monolithic slab/footing foundation.

Mike McCann, PE, SE (WA)


RE: Wall Footing Eccentricity

I found this argument interesting.

The usual configuration of wall footings in my line of work usually deals with two pedestals with a 2 separate rectangular bases. They would be supporting large vessels filled with some type of liquid. Mostly if it is designed in a high seismic area we do the analysis (which is quite misleading at times), but the lateral forces cause a cyclic load due to sloshing on the sides of the wall. Which in turn the connection between the base and pedestal wouldn't be as critical in this particular situation, but more critical in an overturning sense. I think simple mathematics would prove that an eccentricity caused by sporadic lateral forces due to nature would conclude that differential settlement could occur. Designing with assuming that distribution of bearing pressures are uniform would assume that during these natural disasters there will be no change in bearing distribution on the base of the footing. If the corner stress exceeds the bearing capacity shouldn't we expect the soil to fail? I am trying to understand your argument.

RE: Wall Footing Eccentricity

Assume you step on very soft soil. If you allow your foot to pivot at the ankle, the bearing pressure under your foot will be highest at your heel, and lowest at your toes. Your heels will sink into the ground. But if you hold your ankle rigid, the bearing pressure will be uniform under your foot, and your foot will sink uniformly into the ground.

DaveAtkins

RE: Wall Footing Eccentricity

This assumes you have something to hold onto with your hands, to prevent your entire body from rotating backwards.

DaveAtkins

RE: Wall Footing Eccentricity

Quote (Leftwow)

If the corner stress exceeds the bearing capacity shouldn't we expect the soil to fail? I am trying to understand your argument.

I'll assume that you're speaking to my arguments above.

Were it the case that over-stressed soil would start flying out the side of the footing and wreaking havoc the instant that allowable stresses were exceeded, I would ensure that they were never exceeded. However, for the vast majority of projects that I've worked on, the allowable soil stress was chosen not to preclude failure but, rather, to limit settlement. And that implies a material capable of additional deformation before a true strength limit state failure would occur

When geotechnical engineers give us the allowable soil bearing stress, it is often a value that was calculated assuming a uniform pressure beneath some size of foundation element. And it was a value calculated with a particular value of settlement in mind, say one inch. Now, if you have a footing with overturning where the the peak allowable stress reaches the max allowable bearing stress at only one point, will you see that one inch of settlement? You will not. That's why, in these cases, I'm comfortable using a uniform soil stress distribution that would produce one inch of settlement.

I encounter this most often in the design of shear wall footings for taller structures. I call up the geotechical engineer to ask if I can do this, they say yes, and I carry on. Were I stuck never exceeding the allowable soil stress beneath these footings, they would become quite enormous.

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: Wall Footing Eccentricity

Maybe this is what everyone is getting hung up on. To make KootK's method work, you must have a horizontal reaction at the bottom of the footing and an equal and opposite horizontal reaction somewhere above the footing.

DaveAtkins

RE: Wall Footing Eccentricity

Quote (DaveAtkins)

To make KootK's method work, you must have a horizontal reaction at the bottom of the footing and an equal and opposite horizontal reaction somewhere above the footing.

I've actually shown those in my sketch. I chose the SOG and shear in the wall higher up.

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: Wall Footing Eccentricity

Fair enough Mike.

Been thinking about this one and if the wall is not a basement wall i.e. not a retaining wall, just a vertical wall load I would agree with KootK assume uniform pressure on the footing, what other pressure distribution can be used ? Unless the soil is soft and its reduced stiffness has to be taken in account.

RE: Wall Footing Eccentricity

My $0.02 - my vote is with kootk. Assuming the detailing is there and there is continuity between the wall/footing it's reasonable to assume uniform pressure. If you the footing width was large then maybe it makes sense to start trying to account for the compatibility between the soil stiffness and rotation at the joint to have a more trapezoidal distribution, but most wall strip footings are not that wide.

Why assume a uniform pressure under a concentric footing? The footing is flexible so you could make a case for more pressure at the center and diminishing towards the perimeter but who does that? It seems to me to be a similar argument.

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