0.6D+W Load Comb and 1.0 Factor of Safety

[vandede427]

Before I type, I'll go ahead and reference a previous thread that has been closed.

thread507-202887

http://www.eng-tips.com/viewthread.cfm?qid=202887#post

I think we all agree that the FS for sliding/overturning should be 1.0 with the 0.6D+W combo. But where in the IBC 2006 does it say this? I can't find it.

It is very specific about retaining walls FS (1.5), but it doesn't say if this only applies to the D+H combo. Surely you don't use the 0.6D+H combo. That's 1.5x1.5=2.25 FS.

 

[kslee1000]

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.

 

[WillisV]

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.

 

[vandede427]

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.

 

[JoshPlumSE]

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

 

[JoshPlumSE]

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.

 

[vandede427]

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

 

[SeanMD]

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.

 

[nutte]

The Metal Building Manufacturer's Association recommends you NOT use the collateral load when checking for wind uplift, as in 0.6D+W.

 

[vandede427]

thanks guys.