0.6D overturning uplift foundation metal building continuation

[bdruehl]

thread507-352021

so, ya.. kinda clear as mud as far the application i need clarity on (unless ofcourse im reading it wrong again)....

when you are doing footing designs for METAL Building frames with no slab to count for contributory weight, it is a BIG DEAL to have to reduce your required concrete footing weight to .6D to counteract the uplift .9D would be acceptable, but how bout a similar note as ASCE 12.13.14 regarding an additional 25% overturning (uplift) reduction for seismic, but for wind instead ... is there one for wind? That would make the footings more manageable. Despite my love of conservativism, I have a hard time justifying the need for say 250 yards in a footing as opposed to 160 because of the 0.6D we are all used to.

 

[TLHS]

You definitely need to use that. If you're using the 0.6 factor, you're in ASD. The point of that combination is to give you a factor of safety against overturning. It's the same as applying a global factor of safety of 1.5 against overturning with a ten percent reduction in weight to act as a fudge factor.

The additional weight you're trying to avoid is the whole intended point of the factor you're talking about.

It's easy to justify the necessary concrete, since it's required to provide safety against the building flipping over in an extreme event.

 

[bdruehl]

ALSO... anybody employ the skin friction of the footing against the soil at .25 of dead load to help out with uplift? specifically, the values in the cbc for soils are desscribing allowable friction as a percentage of DEAD LOAD, which, by the nature of the load UPLIFT, probably should not be counted since there is no dead load (the footing is being lifted out of the soil...)... gigantic footings it is.

 

[bdruehl]

yes, TLHS, that is true, that is for ASD... in a way, I am getting confused by the old LFRD vs ASD and the fact that anchorage bolts design now have to use LFRD ultimate loads (right?!), so, all things being equal the .9D in LFRD should end up being equivolently as big of a footing as the ASD .6D. clear as concrete (mud) YES THATS RIGHT, DIG A 6X6X6 FOOTING AND FILL IT WITH REBAR!!!! and stop calling me! =)

 

[bdruehl]

wait, TLHS, are you implying (as maybe the previous thread did) that after your load combo wHEN USING .9D in LFRD, you need to also add in a 1.5 factor of safety for "overturning" .... that actually is part of my initial question -- 250 yds vd 160 yards. the previous post i referenced talked about how that is most likely an overly conservative way of doing it, since there is very little dead load question (as to how much weight you have)

 

[TLHS]

You don't need an additional factor of safety against overturning when using either ASD or LRFD. In ASD it's implicit in the 0.6 dead load factor, in LRFD it's implicit in the load factor for wind.

I can't help you with the anchor bolt issue. I'm not really all that conversant in current ASD practices in the US.

I wouldn't suggest using skin friction of your foundation except in some very controlled situations. If you want help from the soil against uplift, why don't you put a spread footing at the bottom?

 

[TLHS]

And just to be clear, the 0.6 factor in ASD has very little to do with uncertainty in the weight of the system. It's ensuring there's a factor of safety against overturning in the required code checks.

In ASD, all your material strengths are factored down. Your allowable resistance is some fraction of the ultimate resistance. This is how you get your factor of safety. Just think of the 0.6 factor as the allowable uplift resistance per pound of dead load. It's the same thing.

 

[wannabeSE]

Using the seismic reduction for overturning with wind loads may not be conservative. FEMA 750 states:

"C12.13.4 Reduction of Foundation Overturning. Since the vertical distribution of forces prescribed for use with the equivalent lateral force procedure is intended to envelope story shears, overturning moments are exaggerated. (See Section C12.13.3.) Such moments will be lower where multiple modes respond, so a 25 percent reduction is permitted for design (strength and stability) of the foundation using this procedure. This reduction is not permitted for inverted pendulum or cantilevered column type structures, which typically have a single mode of response.

Since the modal response spectrum analysis procedure more accurately reflects the actual distribution of shears and overturning moments, the permitted reduction is only 10 percent."