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RE: Section 1633.2.6 and 1612.4

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Mike O'Brien wrote:

<This doesn't seem right to change the Omega factor due to the 
<material of the drag.  If the lateral load resisting system 
<and R factor do not change, why would the Omega?

(MS) You are correct, my mistake.  Thanks for the correction.
The omega factor would NOT change if the LFRS remains the same.
Omega remains at 2.2, but the load duration factor of 1.33 can
still be combined with the allowable stress increase of 1.7 
(only when using 1612.4).

My previous post should be revised as follows:
_____________________

If the collector is WOOD (with steel braced frames) then omega 
REMAINS THE SAME AT 2.2 BUT the load duration factor of 1.33 
MAY be combined with the 1.7 allowable stress increase, so the 
above equations yield the following:

                           demand < capacity
             94 UBC:          (F) < 1.33(Cap)
             97 UBC:  2.2(1.4)(F) < 1.33(1.70)(Cap)

which is an EFFECTIVE increase in the required
capacity (97 UBC vs 94 UBC) of: (2.2)x(1.4)/(1.7) = 1.8
_____________________


Mike O'Brien wrote:

<If you multiply back through with the 1.4 factor, you have 
<reconverted your forces to ultimate levels.  Could you 
<explain further why you think that 1.4 factor is necessary 
<if you are designing to ASD?

(MS) The short answer is: that's what the code says.

The long answer follows: In the example I gave, the base 
shear at ultimate levels is V=0.196W, but the collector 
force was derived from V=0.196/1.4W=0.14W.  Saying V=0.14W
really is not correct, and is misleading.  Really what I 
should say is E/1.4=0.14W since E=rhoEh+Ev reduces to 
E=Eh=V when rho=1 and one is using ASD (see the 
definitions in 1630.1.1).  Am I sufficiently confusing?

Here's another way: My collector force F was based on 
E/1.4, not on E.  The definition of Em is omega times Eh,
but Eh=E as I said above, so with omega = 2.2, and 
F=E/1.4=Eh/1.4, the winner is....

Em=2.2(1.4)(F)

I also think this is correct because in this example,
the old 3Rw/8 would have been 3 (since Rw=8 for steel
ordinary braced frames), and the new value is 
2.2(1.4)=3.1 which is approx the same.  (Of course,
not all systems happen to match the previous code as
this one did, but we can talk more about that later.)

Thanks for your patience.

Mark Swingle, SE
Oakland, CA

Disclaimer:  These are my own opinions.  They are
subject to change due to the ravages of time and 
after being subjected to reasoned criticism.