The ASD load combinations that are being discussed (.6D + 1.0W and .6D + .7E) were updated 1998 edition of ASCE 7 to make them comparable to the LRFD load combinations (1.0D + 1.6W and.9D + 1.0E). (Note I dropped the lateral earth load H in all combinations since we are talking uplift only.) The .6 coefficient is not attributed to the uncertainty of the weight of the foundation or uncertainty of other dead loads used to resist the effect of wind or seismic. The majority of the uncertainty comes from the loads that the gravity forces are resisting. From the commentary, " ...the factor 0.6 on dead load is necessary for maintaining comparable reliability between strength design and allowable stress design."
Looking at the wind load combinations: 1/1.6 load factor = .625, which is approx 0.6.
The ASD Load combination using dead load and seismic .6D +.7E and LRFD load combination .9D + 1.0E are similar to wind combinations; noting that the additional .7 coefficient comes from the fact the "E" is calculated using strength design.
Looking at your comment below, I would suggest using the code required load combinations (either ASD or LRFD); but, seeing if you can justify using a larger dead load for you specific case. For instance, you might have soil above your foundation or your local foundation might be connected to the building slab or other foundation element that you can take a portion of that weight towards your dead load that you had not previously considered based on your comment "especially if it is attached to the rest of the foundation." In PEMB foundation design, some engineers consider a portion of the slab in their dead weight depending on their specific layout, connection details, and if they are able to justify that they have sufficient slab capacity (rebar and slab thickness) to engage a portion of the slab.
HTH, Donna Friis
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In California seismic uplift most always governs. That being said, I will
never reduce the footing dead load by 0.6 if I am designing the foundation
or if I know the exact dimensions of the existing footing.
I think that by reducing the footing dead load by 40% is an overly
conservative design, since the forces that are being designed for are a
statistical best guess. Also since the uplift forces for a shearwall or
moment frame are cyclic and only last for a very short duration I really
can't see the footing, flying up into the air, especially if it is attached
to the rest of the foundation.
This also brings up another issue that I have been thinking about a lot
recently.
As engineers I think that very often we loose sight of the intent as to why
we are consulted. Life safety first and foremost, but too many times I think
we design with the intent of a lawsuit that may occur if anything goes
wrong. Unfortunately this is the reality that there are too many greedy
as**^le lawyers as well as people that are all too willing to sue at the
drop of a hat.
Erik Gibbs
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