Bearing Pressure Under Eccentrically Loaded Footings
Bearing Pressure Under Eccentrically Loaded Footings
(OP)
Hi all,
I'm having a frustrating problem that I would think has an easy answer. I'm designing a wall footing for bearing pressure right now, and can only increase the footing dimension in one direction. Imagine my surprise when my calculations say adding a foot in one direction of a footing increases the maximum pressure. I don't really believe this can be true. So I attached a basic calculation showing 3 cases, all with the load inside or at the kern point, that show adding footing area is increasing my applied bearing pressure. Please tell me what I'm doing wrong here, unless the calculations are correct, in which case I would need some hard convincing that more footing means more pressure.
http://files.engineering.com/getfile.aspx?folder=6...
Thanks in advance,
labeattie
I'm having a frustrating problem that I would think has an easy answer. I'm designing a wall footing for bearing pressure right now, and can only increase the footing dimension in one direction. Imagine my surprise when my calculations say adding a foot in one direction of a footing increases the maximum pressure. I don't really believe this can be true. So I attached a basic calculation showing 3 cases, all with the load inside or at the kern point, that show adding footing area is increasing my applied bearing pressure. Please tell me what I'm doing wrong here, unless the calculations are correct, in which case I would need some hard convincing that more footing means more pressure.
http://files.engineering.com/getfile.aspx?folder=6...
Thanks in advance,
labeattie






RE: Bearing Pressure Under Eccentrically Loaded Footings
You can use a constant soil pressure that only extends partial length under the footing. This would give you the same soil bearing in all three cases.
You could use a more complex curved or multi-linear profile.... as long as the centroid of the pressure resistance coincides with the point of the applied load.
RE: Bearing Pressure Under Eccentrically Loaded Footings
It will help if you include some dead load over the top of the footing. The dead load of the soil and (assumed) concrete slab over the footing will help create a more balanced load - thus less eccentricity and less of a spike in the bearing pressure as you approach the outside edge.
RE: Bearing Pressure Under Eccentrically Loaded Footings
Stand in a canoe on a lake. The best place for you is dead center. You can't sit on the edge unless you have a similar load on the other side of the boat.
When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.
-R. Buckminster Fuller
RE: Bearing Pressure Under Eccentrically Loaded Footings
Tt also highlights the Rigid Footing + flexible soil assumptions that we're making for soil /footing interaction. Which means the reality may be slightly different than what we assumed.
RE: Bearing Pressure Under Eccentrically Loaded Footings
https://res.cloudinary.com/engineering-com/image/upload/v1438886105/tips/SBizHubC22-15080600430_vflrf4.pdf
td:dr; The total bearing reaction stays the same (no increase in stress), but the distribution changes. Any eccentricity is going to increase your maximum pressure. See manstroms post for the great boat analogy.
RE: Bearing Pressure Under Eccentrically Loaded Footings
RE: Bearing Pressure Under Eccentrically Loaded Footings
RE: Bearing Pressure Under Eccentrically Loaded Footings
RE: Bearing Pressure Under Eccentrically Loaded Footings
RE: Bearing Pressure Under Eccentrically Loaded Footings
As a matter of principle, I don't for a second believe that adding more footing actually makes the situation worse out there in the wild.
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: Bearing Pressure Under Eccentrically Loaded Footings
RE: Bearing Pressure Under Eccentrically Loaded Footings
The linearly varying soil stress certainly does imply tilt. But then, eccentric footings are a pretty regular occurrence and we're rarely bothered by the degree of tilt in those instances.
There's kind of a catch 22 at play with the supported structure. If the supported structure and its connection to the footing were stiff enough to be adversely affected by small footing rotations, that same supported structure would probably be stiff enough to help resist that footing rotation. A shear wall or a beefy fixed base moment frame might be stiff enough to offset some rotation whereas a simple gravity column probably would not.
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: Bearing Pressure Under Eccentrically Loaded Footings
BA
RE: Bearing Pressure Under Eccentrically Loaded Footings
I have never done this, but I have considered this for a small moment frame where the post had to be at the end of the footing. I'm curious to what others think this in application. I had to "find" a horizontal force at the anchors for it to work. However, it seems to fit the discussion that if you permit bearing pressures greater than allowable, you should find a stabilizing force elsewhere. Otherwise, you violate the sum of all moments equals zero.
RE: Bearing Pressure Under Eccentrically Loaded Footings
There are a couple ways to look at this one. If you look at adding 1' of length to any size footing with a 0.5' eccentricity, then the max bearing pressure will asymptotically approach the bearing pressure for the concentric case that is 1' smaller as the footing size increases and P is constant.
If you look at keeping the original size constant, and increasing the footing size in 1' increments, then it's all over the map, because at some point you'll be outside the kern and also the larger the original size, the closer you'll be to q average right off the bat.