Mr/Mot >= 1.5? 1

[msquared48]

In older UBC codes, there was a requirement (cannot put my finger on it right now) to have the Resisting moment exceed the Overturning moment by a factor of at least 1.5. This was used to size hold downs at the ends of plywood shear walls.

Now I cannot find a specific reference in the IBC or ASCE7 for this factor.

Has this 1.5 factor been effectively worked into the load factors of ASD and LRFD now, or has it been scrapped entirely?



Mike McCann, PE, SE (WA)

 

[jdgengineer]

Yes, I believe that is baked into the load combo now with 0.6D. 1/0.6 = 1.67 ~ 1.5.

 

[P1ENG]

jdg is correct except that the ASD load combination changed from 0.9D to 0.6D (at least for seismic combinations I believe), so it is actually 0.9/0.6=1.5.

I like it now, but for young engineers, it might have caused problems. I always wondered why I had to take a 40% reduction in dead load and always thought it was because engineers regularly "assumed" weights instead of adding up each component weight to get actual weights. I always thought, "My loads are right on, so why reduce them?". Also, seems like overkill since your connectors and materials also have built-in safety factors. I wonder if LRFD turns out better?

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

 

[RobertHale]

The last time an unqualified 1.5 overturning factor was in the code was 2006 IBC and it pertained only to retaining walls (1807). Now there is still a mention of the 1.5 overturning factor for retaining walls but it notes the use of nominal loads for use with the factor and not the load combinations.

Robert Hale, PE

 

[Nor Cal SE]

There is an intriguing aspect to this conversation. Noting RobertHale's post, which is consistent with what I know, you assume full D and not 0.6D when determining whether your retaining wall has 1.5 safety factor. Therefore, the bottom line safety factor for a retaining wall, whose overturning moment is constant and permanent, is about the same (or technically a bit less) than the required safety factor for infrequent seismic load condition.

Again, I believe everyone has stated the code requirements correctly, just curious whether anyone else has notice this, and if so, what your thoughts might be.

 

[JoshPlumSE]

Old requirement was:
0.9*DL + WL (or EL) = 1.5 overturning factor of safety. Total safety factor of 1.5/0.9 = 1.667

New requirement is now:
0.6*DL + WL (or EL) = 1.0.... Total safety factor = 1.0 / 0.6 = 1.667

So, it should lead to the exact same safety factor as before.

Personally, I might have have preferred keeping the load factors related strictly to load variability. And, using the 'resistance' as an additional safety factor or converting into a true resistance factor for the failure mode being considered. Seems intellectually cleaner to me.

 

[MotorCity]

The load combinations do not have anything to do with the factor of safety against any serviceability condition (overturning, sliding, etc.). They simply result in a force or condition that is compared to the resisting force, moment, etc. The load combinations cannot imply a factor of safety because they don't "know" what the capacity or resistance of the system is. In general, codes are silent on serviceability (such as wind or seismic drift). The capacity is calculated separately and is independent of the load combination used.