Couple on a foundation - How to design for that??

[PostFrameSE]

I have a project (think hotel entry area with a roof supported by four legs and not tied to another roof) where I have a 22" wide steel frame exerting a couple on a foundation. The couple is 21ft-kips and I have a slight lateral load of 1.7kips. My D + S is pretty insignificant at less than 3kips. This is a pretty tall, spindly structure. I have sidewalks on all sides of my foundation so I'm thinking that I can ignore my 1.7kip load from a foundation sizing perpsective.

It's really messing with my mind as to how to design this foundation. I would like to just do a 4.5' x 4.5' x 5' reinforced concrete block to resist these forces. I have an allowable soil bearing pressure of 3,000psf. I'm struggling with where I should sum the moments about to see if my allowable lateral or vertical soil bearing pressures are exceeded............or is that not how I should be looking at this? Any guidance would be appreciated.

Thanks.

 

[JedClampett]

Any basic foundation design book should have examples of a foundation with a moment on it. The issue isn't usually just the bearing pressure, but trying to keep the eccentricity (the moment resolved into the downward force at an eccentricity) of the weight within the middle third of the footing. This prevents uplift (actually loss of bearing)at the edge.
My favorite book for this is "Foundation Engineering" by Peck, Hanson and Thornburn, but it's a standard analysis that any design book should have. I even have the pages bookmarked.

 

[Ron]

agree with JC. Set your couple to span the middle third of the footing and then design to prevent contact loss

 

[PostFrameSE]

Thanks to both of you for your response. I have that book and have reviewed Chapter 24........at least the first part of it.

I think I'm over-thinking this. Rather than envisioning that this foundation is floating in the soil and is just wanting to rotate I think I need to take my total moment produced by the couple (21ft-kips) divided by a gravity load of a 6' wide x 6' long x 4' thick mass of concrete plus my dead load (23.6kips) to come up with an eccentricity of .89ft. That of course is less than L/6 = 1ft so my compression load is affecting all of the soil under my footing, which I think in your words Ron "prevents contact loss."

I buy that. What if I don't have room for a 6' x 6' footing? What if I can only go 4.5' x 4.5' x 5' deep? At that point my eccentricity is at 1.22 and my L/6 is .75' so I'm outside of the middle third. Is that all bad? If my allowable soil bearing pressures are not exceeded does it matter?

Last question................I don't need to consider a FS for overturning do I? If I DID..............where would I consider my point of rotation since I just have a couple-induced moment rather than a lateral load-induced moment? Is there a difference? I'm struggling with this because if I chose the center of my footing on which to take my moments.......it works out. What if chose a bottom corner and had to factor in the weight of my footing and the moment both acting in the same direction? Then it doesn't work. What am I missing?

Thanks much!

 

[JedClampett]

First of all, it's always a good idea (and code mandated) idea to consider overturning and apply a 1.5 factor of safety. But in your case and most cases, it's not an issue. Sliding is harder to apply the F.S. to.
As far as your point, that if the e is greater than L/6, is it a big deal? No, it can be accommodated, but things start to go bad much quicker once you've passed that point. It's just not a good neighborhood to spend a lot of time in.

 

[hokie66]

For footings subject to overturning like yours, and others like sign footings, it is often impractical to keep the centroid of vertical loading within the kern. It is not necessary to do so, particularly for transient loading, as long as the footing is stable, the rotation is acceptable, and the soil is not overstressed. You should always have a factor of safety, and 1.5 is typical, but in your design don't mix factored and unfactored parameters.

 

[PostFrameSE]

Thanks hokie & Jed. I'm with you............but I'm still missing something. Consider a cantilevered retaining wall. I have my active forces (soil) acting on my wall laterally and I have my resisting forces (weight of wall and soil acting vertically above it) to compare. In that scenario, the obvious point of summing my moments and getting a FS is the toe of the retaining wall. I think what I'm missing when I compare that example with what I'm doing now is that for the cantilevered wall......pinned at the toe......there are no soil forces acting upwards to contribute to the summing of moments for a FS calculation. Theoretically I should be able to sum the moments about any point and get the same answer right?.............which means that I would need to consider the soil forces acting upward on the foundation that is resisting both the weight of the foundation and the overturning moment if I happened to choose say the opposite end of my footing in my cantilevered wall example. I think I'm talking myself into the answer. If this makes sense........I would take my soil bearing capacity of 3,000psf and come up with an allowable triangular soil pressure acting 2/3 away from the corner to which I can then compare the weight of the foundation and the acting moment both working in the same direction. Phew!!!!!! I'm not a big soils guy, obviously, and so I appreciate your patience. I THINK I have it and my calcs seem to be working.