Wall Footing Eccentricity

[design62713]

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.

 

[KootK]

Your footing was also tied into the ground slab.[highlight #FCE94F][/highlight]

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.

 

[civeng80]

Mike,

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

 

[civeng80]

hokkie

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

How would you approach it then?

 

[msquared48]

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

Mike McCann, PE, SE (WA)

 

[Leftwow]

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.

 

[DaveAtkins]

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

 

[DaveAtkins]

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

DaveAtkins

 

[KootK]

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.

 

[DaveAtkins]

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

 

[KootK]

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.

 

[civeng80]

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.

 

[bookowski]

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.