Eccentric Loading on Footing

[ChrisKn]

I'm pretty well versed on the design of a spread footing when loading by a column is concentric, however, I'm a little confused on a method to handle eccentricities in the loading.

Specifically I'm looking for a quick design method to handle a footings supporting a columns along the edge of a building where the foundations are unable to protrude beyond the edge of teh building (this usually occurs in retail where a store of a strip center is directly adjacent to another store). So basically the edge of the foundations in the stores abut one another creating a "zero clearance foundation".

1. In this condition you are usually left with a column loading along the edge of a rectangular foundation. The question is how do you resolve the magnitude of the soil pressure (which varies linearally vs. uniformly for a concentric laoding)??

How is the footing sized and how do you resolve the max moments and shear in the footing so that reiforcing can be sized? The loading will also create an overturning moment??

Any help would greatly be appreciated.

Chris

design of the reiforcing steel resolve

Any help is appreciated...

 

[DaveAtkins]

In a situation as you describe, I doubt a spread footing will work. I would use a continuous grade beam (or even a 4' deep continuous foundation wall). The column loads will push down on the beam or wall, and you can conservatively assume the soil will push uniformly up on the beam or wall (or run the model on RISA, with springs for the soil).

DaveAtkins

 

[UcfSE]

You can look in texts by Braja Das for some good information about eccentric foundation loads. You would then need to use a little gray matter to apply it specifically to your footing. I've done this before too. You get very large footings but it is what it is.

 

[SlideRuleEra]

The time-tested "classical" solution to design a zero-clearance foundation is to use a either a combined or strap footing. Here is a reference that describes the basics (see paragraphs 9.77 and 9.78)

http://books.google.com/books?id=Kt...sig=rwx8FqDY8WIawzWsy1xAlIqIbxk#PSA9-PA112,M1

http://www.SlideRuleEra.net

 

[ChrisKn]

Thank you for your help so far.

I guess what i really have is a combined footing anyway because the continuous (zero clearance) footing for the walls of the building tie into the zero-clearance footing at the column. Similar footings on previous jobs have shown an approx 2' wide continuous (zero cleance) footing for the walls and a 12'long x 4' wide footing under the column loaded (zero clearance) footing. The continuous wall footing has the wall located along the edge of the 2' wide footing. The 4'x12' column footing has the column located at the edge of the 4' wide section and the center of the 12'long section. Hope this makes sense becuase it is a little bit hard to explain.

The column loading for my job is approximately 50 kips and the soil brg pressure is 3000psf.

From this configuration can i assume a relatively uniform loading on the 4'x12' footing? Can i also use a portion of the 2' wide wall footing takes some of the loading (therefore allowing me to use it in my required area calculation)??

 

[csd72]

I have designed fairly lightly loaded footings like this with a beam and a counterweight underneath the internal slab so it forms a kind of I shape in plan. For service loads you can even take the self weight of the ground slab. This is a last solution if nothing else works as it is quite expensive.

The strip footing can work if there is enough dead load in the footing to bring the eccentricity back from the edge. For final service loads you can take a portin of the grond slab over to oppose the eccentricity of the footing. You do, however, need to make this work under construction loads or require that the ground slab is poured before the roof/sheeting.

csd

 

[SlideRuleEra]

ChrisKN - To accurately assume that a footing exerts uniform loading on the soil, the footing has to be rigid. To be rigid, it has to be thick (relative to the horizontal dimensions). This ratio of the thickness to width creates a relatively large moment of inertia to minimize deflection under load. You have not mentioned footing thickness.

Concerning footing geometry, the shape of the footing is not too important, the important factor is where the resultant of all loads is located.

http://www.SlideRuleEra.net

 

[UcfSE]

I think you should order or borrow the text or some other source of information you prefer and work out the engineering instead of trying to make too many assumptions to make it easier.

 

[ChrisKn]

Thank you csd,slideruleera and ucfse. You have helped me clarify some of my thoughts on this situation.

CSD- Your points are well taken, however, i do not want to put restraints on the order of construction...Therefore I dont want to count on the interior slab to "bring the eccentricity back from the edge" of the footing. I also do not have a nearby column to use in a combined footing to bring the resultant loading to the centriod of the footing.

SlideRuleEra - What kind of deflection would you allow in the footing to still consider it rigid? Your approach seems like the most economical solution to my particular job, espicially becuase my loading is relatively light.

UcfSE - Do you have any suggestions on materials with good examples of eccentric loadings on footings? Its just a shame that my text-book from college doesnt give any examples of eccentric loading on footing (except the use of a combined footing which in my case wouldnt be easily accomplished.)

Thanks again.

Chris

 

[DaveAtkins]

What you have is NOT a combined footing in the classical sense, which puts two columns on one large footing.
You cannot assume uniform bearing pressure under the 4' X 12' footing, UNLESS you use a base plate at the bottom of your building column that can transfer bending moment up into the column. You see, the centroid of your column load will not line up (vertically) with the centroid of the uniform bearing pressure under the 4' X 12' footing. Due to this eccentricity, there will be a moment (P*e) that must be accounted for. What I have done on occasion is "throw" the moment back up into the building column--it can then be resolved as a force couple, with the forces occuring at the base plate and the roof of the building. Of course, the column must be able to resist the moment as well.

DaveAtkins

 

[UcfSE]

I would look in texts by Braja Das, or Coduto. Bowles may have what you want as well, though the later editions of Bowles tend more toward the software packaged with the text.

Vulcan Hammer also provides a lot of good free material from their website.

 

[jike]

You either have to counteract the eccentricity with a combined footing, strap, wall, slab, etc. or take the load into the isolated footing and provide stability by counterbalancing the eccentricity with the soil pressure.

 

[SlideRuleEra]

DaveAtkins is right, you don't have a combined footing - the rigidity of the footing that you do have is irrelevant.

Suggest that you take UcfSE's and jike's advice and do the engineering. Either create a true combined footing or learn how to deal with what you have.

http://www.SlideRuleEra.net

 

[ChrisKn]

Thank you all for your help. Looks like I will be obtaining some text on how to deal with the situation.

Dave - Yes I'm very aware that i do not have a combined footing, but thanks for the clarification. The point i was trying to get across is that the nearest second column that could be used to consruct a combined footing (two columns on one footing) is about 30 feet away. Therefore it wouldnt be cost feasable to construct a footing that large. I was looking to obtain different ways to counterbalance the column load...and i recieved that answers that i was looking for from JIKE...

I truely do appreciate all the help you have given me and i think its great that everyone is so willing to share their knowledge with others.

Chris

 

[miecz]

Why can't you use a continuous foundation wall/grade beam, as suggested by DaveAtkins first reply? With only 50 kips on 3000 psf soil, the required continuous footing width is so small, couldn't you make it concentric? Then you would design the wall to span center to center of the columns for the uniform net upward load.