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.
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
Download nowINTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS Come Join Us!Are you an
Engineering professional? Join Eng-Tips Forums!
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail. Posting GuidelinesJobs |
Wall Footing Eccentricity
|
RE: Wall Footing Eccentricity
RE: Wall Footing Eccentricity
RE: Wall Footing Eccentricity
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
RE: Wall Footing Eccentricity
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
RE: Wall Footing Eccentricity
RE: Wall Footing Eccentricity
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.
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
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
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.
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
Mike McCann, PE, SE (WA)
RE: Wall Footing Eccentricity
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
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
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
RE: Wall Footing Eccentricity
I know that I sound like the proverbial contractor, but I am not. Trust me.
Mike McCann, PE, SE (WA)
RE: Wall Footing Eccentricity
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
Also what if the wall is precast ?
What about tieing the ground slab to the footing ?
RE: Wall Footing Eccentricity
Mike McCann, PE, SE (WA)
RE: Wall Footing Eccentricity
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 thought your footing and slab was poured monolithically, so how could it be a precast wall ?
RE: Wall Footing Eccentricity
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
Mike McCann, PE, SE (WA)
RE: Wall Footing Eccentricity
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
DaveAtkins
RE: Wall Footing Eccentricity
DaveAtkins
RE: Wall Footing Eccentricity
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
DaveAtkins
RE: Wall Footing Eccentricity
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
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
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.