Overturning Check

[mkrei]

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.

 

[frv]

The combination you mentioned is primarily intended to check uplift and overturning. It seems to me that if your soil pressures are out of whack with this combination (I'm assuming you mean toward the ground), then it would be much more so with the D+.75L+.75W combination.

In any case, your soil should satisfy all combinations.

 

[Lion06]

The soil pressure gets so crazy because there is no DL and only moment. I've encountered the same thing. You have to satisfy it for soil brg. It sucks...... I know.

frv-
The load combo you mention is much better in some cases than 0.6DL + W for soil brg.

 

[JStephen]

Using the factored loads, you should not exceed the ultimate bearing. (If you do, your model is no longer valid, for one thing). You can exceed an allowable bearing which is based on service-level loads when using the factored loading.

 

[frv]

Risking embarrassment, I must admit..

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?

 

[mkrei]

Some of the issue is how to apply the lateral loads at the base of the wall. This footing is supporting a C-shaped shearwall around an elevator shaft. So I am considering how to add for the additional depth of the walls below the slab on grade. When I add the lateral load in and add a moment arm equal to the elevator pit depth plus 1/2 the footing thickness my numbers start to get out of whack real quick. Unfortunately this is a in place condition I am backchecking. Talking to the geotech he indicated the allowable bearing pressure for a short term load such as wind is much higher than the allowable bearing pressure for the permanent loading of dead plus live he indicated a factor of (2) could be used for a short term load such as wind since settlement is not a concern and soil shear is a bigger concern and the allowable bearing for soil shear is much higher than that required for a settlement problem.

I think I am ok with the higher bearing pressures as long as I dont have an overturning problem or a soil shear problem.

 

[Lion06]

frv-
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.

 

[mkrei]

Another question. I have very large thick footings below the shear wall. In keeping with the .6 dl do I need to reduce my concrete weight to 87 pcf. That really starts to whack things out. Also most of the dead load coming down on the footing is from the concrete shear walls above. Seems like a very strict penalty and really starts to wreak havoc with my bearing pressures.

 

[Lion06]

I had a very similar problem once and I thought you should use the 0.6 on th concrete self eweight, but a senior engineer here told me that if I was having that much of a problem with it that he was comfortable using 0.9 for the concrete self weight. I don't know if that is violating code or not, just telling you what my senior engineer said.

 

[frv]

Thanks, StrEIT!

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.

 

[miecz]

See thread507-202887. I wonder if the code committee intended for bearing pressures to be calculated by traditional methods with the 0.6D+W+H combination.

 

[ron9876]

This was discussed in detail before. It is crazy and you get crazy answers but everything has to work for all load cases. Wonder if geotech put in writing a larger allowable for short term loading. To me the craziest result of all is the footing sizes under interior columns on one story buildings. Building codes out of control in my opinion.

 

[DaveAtkins]

I have struggled with this issue in the past as well.

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

 

[msquared48]

You know, that was, and is, the beauty of WSD. There is (and was) no confusion with these (*&^%^&* load factors.

Mike McCann
MMC Engineering