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Soil included in .6D of load combination?

Soil included in .6D of load combination?

Soil included in .6D of load combination?

(OP)
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.

RE: Soil included in .6D of load combination?

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.

RE: Soil included in .6D of load combination?

(OP)
Thanks.

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

I appreciate your quick response StructuralEIT.

RE: Soil included in .6D of load combination?

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.

RE: Soil included in .6D of load combination?

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.

RE: Soil included in .6D of load combination?

(OP)
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?

RE: Soil included in .6D of load combination?

(OP)
Thanks sctiger07.   

RE: Soil included in .6D of load combination?

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

RE: Soil included in .6D of load combination?

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....

RE: Soil included in .6D of load combination?

"or" ...not "of"
sorry

RE: Soil included in .6D of load combination?

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.     

RE: Soil included in .6D of load combination?

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.

RE: Soil included in .6D of load combination?

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.

 

RE: Soil included in .6D of load combination?

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.  

 

RE: Soil included in .6D of load combination?

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.

RE: Soil included in .6D of load combination?

@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.

RE: Soil included in .6D of load combination?

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?  

RE: Soil included in .6D of load combination?

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.

RE: Soil included in .6D of load combination?

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.

 

RE: Soil included in .6D of load combination?

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.

RE: Soil included in .6D of load combination?

I would consider using 30 degrees or less measured from vertical for the soil weight. This is based on the internal angle of friction for the soil backfill.  Since it is fill, compaction would be necessary in order to use this angle or else just use the soil directly above the footing.

RE: Soil included in .6D of load combination?

From design point of view, I will not argue with not to use 45 degree soil. Most of the time I will probably Take the same design strategy as JAE's, just for play it safe. However, if strictly consider from academic point of view, if the soil internal angle is 30degree, then the soil cone which will behave as dead load to the footing will be at 60 degree from vertical because the footing uplift action to the soil makes the soil in passive rankin state.

RE: Soil included in .6D of load combination?

my thoughts, 0.6 for the dead load and soil cone with no factor of safty for overturning, 30 degrees for the soil angle (maybe 45 if I was desporate like a crack addict and had a good geotech dealer).  Allowances for footing "suction" is pretty wild and not designing anchor rods for the 0.6D + 1.0W is frightening.  

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