footing overturning calculations

huangyhg

footing overturning calculations
is the required factor of safety = 1.5 for overturning and sliding in addition to the load combinations of 0.6d + w and 0.6d +0.7e?  1/0.6 = 1.67 factor of safety against overturning, if i try to 0.6d + w overtuning fs > 1.5, wouldn't this be more of a fs > 3?
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the 0.6d + w combination has the "1.5" safety factor built in as you surmised.
you don't need to add another 1.5 factor to it.
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i understand the 0.6d + w equation and how it results in approximately the old 1.5 factor of safety, but the 0.6d + 0.7e as mentioned in the original post is still unclear to me.  
i read this as 0.6d, no live load, 0.7e where e is calculated only for dead load.  if the dead load is small compared to a fully loaded structure then i think this condition is generally not significant.
if my assumption is true then what do you do when the maximum uplift is from:  d + 0.75l + 0.75*0.7e
it seems to me maximum loads up and down in this case would not include a factor of safety for overturning.
regards,
-mike
mrmikee - i'm not sure what exactly you are asking.
where do you get the load combination:  d + .75l + .75*.7e??
is this considering that l and e are "transient" loads per ibc?  i don't know for sure if l is considered transient, but even if it is, the overturning "check" is found in the .6d + .7e combination only.
mrmikee,
ibc 2006 ads load combination eqn16-13 is d+h+f+0.75*(w or 0.7e)+0.75l+0.75*(lr or s or r).  i think in this condition, you will find that the overturning is mcuh less because of all of the additional gravity loads you have in this combination.  your overturning fs would be pretty high, much higher than 0.6d+w or 0.6d+0.7e which typically has the lowest overturning fs.
the 0.75 factor doesn't apply to the 0.7 e earthquake load, even when there are two or more transient loads (ibc 2003 1605.3.1.1).  
the 0.6d + w equation does not have a factor of safety included against overturning.  the 0.6 d + w equation is part of the allowable stress design set of equations (ibc 2003 1605.3.1).  the loads you get from the basic load combinations are service loads.  you then size your   
according to asce 7-05, the 0.75 factor does apply to the 0.7e.  i've heard this has been corrected in ibc 2006.
ithw, respectfully i disagree with you.
ibc (2000) does not specify any additional safety factor for overturning checks.  you simply follow the load combinations.
for seismic - see 1617.4.5.  this simply refers to overturning moments at each story based upon fi at each level.  you still use the various combinations, plugging in fi as qe in section 1617.1.1 along with the vertical seismic component of .2sdsd and you have e to use in the combinations.
for wind, see 1609.1.3 for overturning.  again, no mention of any required 1.5 safety factor.  simply use a slightly underestimated dead load and use within the combinations - specifically 0.6d + w.  for the alternative load combinations of 1605.3.2, they require you to use 0.67d instead of d within the load combinations.
if you have any specific code reference mandating an additional 1.5 safety factor, i'd love to be shown, but i'm not aware of any in the current codes.
the commentary for asce 7-02, section 2, states:  "load combinations (7) and (8) [these are the .6d+w combos] were new to the 1998 edition of asce 7.  they address the situation in which the effects of lateral or uplift forces counteract the effect of gravity loads.  this eliminates an inconsistency in the treatment of counteracting loads in allowable stress design and strength design, and emphasizes the importance of checking stability.
there used to be confusion between strength and asd because of the traditional 1.5 safety factor.  i.e. how do you utilized a 1.5 factor for ot with factored loads?  this was removed in the recent codes and instead, was included in the load combinations.
i know i originally posted this thread concerning spot footing overturning, but the factor of safety against overturning also applies to retaining walls.  ibc 2006 1806.1 states that "retaining walls shall be designed to ensure stability against overturning, sliding, excessive foundation pressure and water uplift.  retaining walls shall be designed for a safety factor of 1.5 against lateral sliding and overturning."  doesn't mention if this factor of safety is included in the load combinations or is in addition to them.  i agree with jae, seems pretty ultra conservative to apply the factor of safety of 1.5 after using the load combinations.
my load combination is a simplification of equation 6 under section 2.4.1 in asce 7-02 which i assume is the same as equation 16-10 in the ibc when l and e are considered transient loads and the 0.75 factor is used.  the asce equation also shows that the 0.75 factor is applied to 0.7e too.
my conclusion is that the overturning "checks" are only in the 0.6d equations only as jae said.  i design elevated bins where the ratio of live load to dead load is probably much higher than with typical building structures, and they are relatively tall when compared to plan view dimensions.  in higher seismic areas the maximum uplift at the column bases is when fully loaded.  
the reason for my concern is that i need to communicate to foundation engineers the loads at the base of my structures that are applied under the different load conditions, and i want to be sure that all assumptions including overturning are clear.  i think i need to change how i am currently doing this, but am not yet sure how.
this thread made me think about these issues again.
thanks,
-mike