Mr/Mot >= 1.5?
Mr/Mot >= 1.5?
(OP)
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?
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)






RE: Mr/Mot >= 1.5?
RE: Mr/Mot >= 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
RE: Mr/Mot >= 1.5?
Robert Hale, PE
RE: Mr/Mot >= 1.5?
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.
RE: Mr/Mot >= 1.5?
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.
RE: Mr/Mot >= 1.5?