Eccentric Loading on Footing
Eccentric Loading on Footing
(OP)
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...
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...





RE: Eccentric Loading on Footing
DaveAtkins
RE: Eccentric Loading on Footing
RE: Eccentric Loading on Footing
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www.SlideRuleEra.net![[idea] idea](https://www.tipmaster.com/images/idea.gif)
RE: Eccentric Loading on Footing
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)??
RE: Eccentric Loading on Footing
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
RE: Eccentric Loading on Footing
Concerning footing geometry, the shape of the footing is not too important, the important factor is where the resultant of all loads is located.
www.SlideRuleEra.net![[idea] idea](https://www.tipmaster.com/images/idea.gif)
RE: Eccentric Loading on Footing
RE: Eccentric Loading on Footing
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
RE: Eccentric Loading on 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
RE: Eccentric Loading on Footing
Vulcan Hammer also provides a lot of good free material from their website.
RE: Eccentric Loading on Footing
RE: Eccentric Loading on Footing
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.
www.SlideRuleEra.net![[idea] idea](https://www.tipmaster.com/images/idea.gif)
RE: Eccentric Loading on Footing
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
RE: Eccentric Loading on Footing