Soil included in .6D of load combination?

[PostFrameSE]

I've got some columns with some pretty significant uplift on them. I've got some building dead load acting on the column of course. When I look at the load combination .6D + W, do I need to look at my soil cone that I'll be picking up as being reduced 40% from the calculated load or does the .6D just apply to the building?

Then............what type of FS is generally used against uplift?

Thanks.

 

[Lion06]

I use the 0.6 factor on the soil to, but then you just need to satisfy the load combinations and there is no additional factor of safety required.

 

[PostFrameSE]

Thanks.

One doesn't constitute a pattern Is that what most people do then do you think?

I appreciate your quick response StructuralEIT.

 

[leeStruct]

Soil weight can be included in resisting uplift, I remember one of the text book? or code book recommand to use 45degree line to calculate soil volume which can be used for dead load weight.

 

[sctiger07]

I use the 0.6 factor on the soil and the weight of the footing. I also agree with StructuralEIT that there is no additional safety factor required.

 

[PostFrameSE]

Whoa! 45 degrees seems AWFULLY generous!! The question more-so is........do you take .6 x the soil weight or nor?...........and the......do you put a FS on it or just do as StructuralEIT?

 

[PostFrameSE]

Thanks sctiger07.

 

[ToadJones]

Im confused here?
You designing the column or simply checking uplift on the footing?

 

[ToadJones]

I guess I would use a 1.5 or 2.0 FOS on uplift, and use the full dead weight of soil that you can count on being permanently in place.
Of course 60% of 1.67...six of one, half dozen....

 

[ToadJones]

"or" ...not "of"
sorry

 

[mijowe]

It has been posted on here several times, but when the load combination went from 0.9 dead to 0.6 dead the overturning ratio of 1.5 went away. 0.9 / 1.5 = 0.6.

The dead load has always been reduce because we tend to be conservative with dead loads, and in combinations that involve uplift, it is less conservative. ASCE provides specifically load combinations with H (Lateral Earth Pressure, but the weight of the soil would fall under the dead load, and should be multiplied by 0.6.

I would get a recommendation from a geotech before i used a 45 degree angle for the soil cone, with out a recommendation i would us only soil directly over the foundation.

 

[leeStruct]

PostFrameSE,

45degree is not my invention its printed black and white in a book. I will try to find that book and show you later. Of course this soil weight need multiply 0.6 because it is part of dead load.

 

[JAE]

I usually just use the soil directly over the footing. If I get desperate I might use a 30 degree. 45 degree might be closer to reality but I'd rather play it safe.

 

[asixth]

Call me "Mr. Conservative" but I only include structural elements in the resistance to uplift.

Despite the fact that I do not use the same code which specifies 0.6*D+W (ASCE by the sounds of it), 0.6*D would be a fair reduction for soil.

1. The unit weight of soil is variable (as is concrete and steel to a lesser degree).
2. The height of the soil is not fixed. For example, the contractor may not get the levels of the footings correct which results in you having less soil then assumed in the engineering design.

I will not comment on the angle of soil resisting uplift as I do not include it in design.

 

[ash060]

The thing that has always bothered me about the 0.6D for foundation design is the waste of concrete for the foundation.

For example you have a steel column with a net uplift of 10 kips on it. You have to have the foundation weigh about 17 kips (10k/0.6), but when you design the anchor rods into the foundation you are designing them for the 10k uplift not the 17 kip weight of the foundation. So your failure is built in at the anchor rods unless you design them to carry the actual weight of the foundation. I just don't see the point. For the same reason I don't put top steel in a footing for uplift because enough load will never get transfered to the foundation to uplift the footing because the anchor rods are not designed to pick it up.

Sorry for the ramble, but I would use the soil above the footing.

 

[slickdeals]

@ASH60,
You do bring up a valid point, one which I have tried to argue with code officials. I had to increase the thickness of a topping slab considerably to not violate the 0.6D rule.

I know the weight of the topping slab +/-. Why would I have to increase the topping slab to prevent uplift using a 0.6D and not say 0.95D or 1.0D.

 

[mijowe]

ash060, why are you not desinging the anchor bolts for the same load combination? The 0.6D is part of a load combination, and every structural element, anchor bolts foundation etc., should be checked agains all combinations right?

 

[ash060]

Yes when you design the anchor rods the combo you are using is 0.6D + W, but the footing does not add any uplift to the column, so the weight of the footing has nothing to do with the anchor rod design.

If you were using another combo the column would not be in uplift and the anchor rods would not have any tension.

 

[JAE]

With uplift there is the consideration that it is a non-redundant "failure" - i.e. if the overturning is exceeded the thing fails with no secondary effects there to stop it. Thus the 0.6D factor.

With anchor rods, you have the yield zone of the rods still there once you yield, thus the degree of safety may not be looked as as critical.

 

[ron9876]

0.6D is the same as using a safety fator of 1.67=1.0/0.6 as opposed to the former 1.5 safety factor.

I definitely use 45 degrees of soil above the footing plus some slab on grade. If anyone is not using this you must not be in a location with large wind loads.

What always has bothered me about this approach is that the design of the anchor bolts already has a material safety factor included. Using the 1.67 safety factor means the design of the anchor bolts, or similar elements like piles, has safety factor x safety factor. Makes no sense.

Has anyone ever seen an allowance for the suction that must surely occur if you actually tried to pull a footing out of the ground.