Overturning Check
Overturning Check
(OP)
I have a question about the .6DL+WL overturning check.
When I run the numbers I get a FS for overturning greater than 1 for this load with equates to the old 1.5 (1.667) d+wl. However my soil pressures get way out of whack with the .6dl +wl. Is the .6dl+wl purely an overturning check or does soil pressure need to be satisfied under this condition.
When I run the numbers I get a FS for overturning greater than 1 for this load with equates to the old 1.5 (1.667) d+wl. However my soil pressures get way out of whack with the .6dl +wl. Is the .6dl+wl purely an overturning check or does soil pressure need to be satisfied under this condition.






RE: Overturning Check
In any case, your soil should satisfy all combinations.
RE: Overturning Check
frv-
The load combo you mention is much better in some cases than 0.6DL + W for soil brg.
RE: Overturning Check
RE: Overturning Check
I don't get it.
How can 0.6DL + WL be worse for soil bearing than D + .75L + .75W?
On one hand you have only 60% of the DL plus the full load of the moment due to wind; on the other you have the FULL dead load PLUS 75% of the Design Live Load PLUS 75% of the moment due to wind.
I know you have less wind, but only 25% less..
Am I overlooking something?
RE: Overturning Check
I think I am ok with the higher bearing pressures as long as I dont have an overturning problem or a soil shear problem.
RE: Overturning Check
If you have that large axial load (1.0DL + 0.75LL) and the smaller wind moment, the effective eccentricity of the axial load will be much smaller (M/P : as P goes up, e goes down) and the vertical load is considered to act near the centroid of the wall. This gives a much more uniform bearing pressure (still trapezoidal most likely with full bearing). It will most likely have a higher average bearing pressure than the next load combo, but not a higher maximumn bearing pressure and here is why.
As that axial load gets smaller (0.6DL) and the wind moment increases (1.0W), the location of that axial load moves further away from the centroid of the wall (M/P again), and the bearing pressure is not even close to uniform anymore (and is likely triangular without full bearing). Now the max bearing pressure in the soil goes way up to accomodate the large eccentricity.
RE: Overturning Check
RE: Overturning Check
RE: Overturning Check
That makes perfect sense. I probably should have prefaced my post by saying that in my neck of the woods, we rarely use footings- almost exclusively piers; consequently, I don't have much experience with them.
RE: Overturning Check
RE: Overturning Check
RE: Overturning Check
The Code, I think, would require the weight of foundations to be multiplied by 0.6. However, this is one of those "engineering judgment" times, when I would agree to playing a little loose with the rules. In fact, before my State (Wisconsin) adopted the IBC, it had its own Code, which DID allow using the full weight of foundations and soil overburden for counteracting overturning.
DaveAtkins
RE: Overturning Check
Mike McCann
MMC Engineering