load comb and 1.0 factor of safety

huangyhg

0.6d+w load comb and 1.0 factor of safety
before i type, i'll go ahead and reference a previous thread that has been closed.
i have never used this combination to check stability, because i couldn't think of a logical case out of it. maybe others have better point of view, would like to hear it.
the ibc 2006 does not state directly "the 0.6d+h load combo is consider to meet the factor of safety required in section 1806.1" (though is should) - that is what the entire previous discussion and my response in the thread you referenced was meant to clarify.

i have some additional questions.
question 2
let's assume your designing a one-story building with a design roof dead load of 30psf (let's call this dmax) for example, but your actual self-weight is probably somewhere around 15 psf (let's call this dmin).  let's assume an applied wind of 20psf uplift.
if you're using something like a dmin, does the 0.9 to account for an over estimating on dead load need to be applied?  that would make the 0.6 factor become 1/1.5 = 0.667dmin + w.
which is correct?
(0.6 x 30 psf) – 20 psf = -2 psf
(0.6 x 15 psf) – 20 psf = -11 psf
(0.67 x 15 psf) – 20 psf = -10 psf
question 3
these load combo rules make more sense to me with a "normal" building where the dead load is equal to or greater than the wind load.  but do they hold up with a very light structure like a pre-engineered building where the dead load can be fractions of the wind load.
take for example, a pemb where the column dead load is 2 kips down and the column wind load is 11 kips up.
0.9d + w = (0.9 x 2) – 11 = -9.2 kips → 9.2 x 1.5fs = 13.8 kips resistance required → 92 cf concrete → 7' x 7' x 2' footing
0.6d + w = (0.6 x 2) – 11 = -9.8 kips → 9.8 x 1.0fs = 9.8 kips resistance required → 66 cf concrete → 6' x 6' x 2' footing
both of these answers are technically correct, but you can tell that they are obviously much different.  i would feel much better using 0.9d with 1.5 fs for pemb's than the 0.6d with 1.0 fs.

in response to your question #3.... i think the real comparsion should be this:
0.9 (dl + sw) > 1.5 * w
therefore, the required additional dead load is:
sw reqd > (11*1.5 - 0.9*2) / (0.9) = 16.3 kips
similarly, the 1.0 requriement would be:
0.6 (dl+sw) > 1.0 *w
sw required > (1.0*11-0.2*0.6) / 0.6 = 18.1 kips

correction to my last equation:
sw reqd > (11-2*0.6) / 0.6 = 16.3
therefore, it ends up being exactly the same as the old requirements.   
let me ask you this.
for pemb's, the worst case downward is usually
dead + collateral + (roof live or snow)
should collateral load be figured into the 0.6d+w combo?
is it (0.6 x self weight dead) + wind
or    (0.6 x self & collateral) + wind
i would not use 60% of the cl as part of the dl.  it is possible the cl will never be there.  i use 0.6*d, with d being only the loads i know will be there.
just went back and forth (repeatedly) with a pemb supplier about this, i couldnt get them to run the min column loads with 0.6*d + w.  they were including the cl in the dl case which in my case exceeded the normal dl.  and the cl will most likely never be there.
the metal building manufacturer's association recommends you not use the collateral load when checking for wind uplift, as in 0.6d+w.
thanks guys.