Couple on a foundation - How to design for that??
Couple on a foundation - How to design for that??
(OP)
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.
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.





RE: Couple on a foundation - How to design for that??
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.
RE: Couple on a foundation - How to design for that??
RE: Couple on a foundation - How to design for that??
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!
RE: Couple on a foundation - How to design for that??
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.
RE: Couple on a foundation - How to design for that??
RE: Couple on a foundation - How to design for that??