Strength reduction factor in ASD and LRFD? 1

[wildwolfyan]

Can we move the strength reduction factor from the capacity side to the load side? For example for LRFD, suppose a design criteria is 1.2D+1.6L > 0.9Mc, where Mc is the nominal moment capacity. Can we move 0.9 to the load side so that the design criteria becomes 1.33D+1.78L > Mc? I heard it is OK to do so for ASD, but not OK for LRFD. Is that true? Why? Thank you!

 

[frv]

It's just algebra. The inequality must be satisfied regardless of how you look at it. You could choose your own inequality and it wouldn't matter, provided the proportions are the same as the code.. 2.4D+3.2L >1.8Mc sure, why not?

 

[JLNJ]

The math is the same, but it's better to think of it with the .9 attached to the Mc. It gives you a better feel for the intent of the code.

 

[dcarr82775]

The load factors are constant, the Phi factors vary. For a frame you would have to carry multiple load cases to check the various limit states.

 

[JAE]

Agree with dcarr827775.
Phi for shear is different than for moment, axial, columns, etc.

 

[MikeDB]

If you are doing a linear analysis you should get the same results either way. However, once you start to include secondary moments (P-delta or non-linear) then it does make a difference because putting it on the load side increases deflection.

 

[rscassar]

It will yield the same answer.

 

[paddingtongreen]

Why do it? with the computational tools available it can't save much work, and as JLNJ says, you keep track of what you are actually doing, you are designing a structure. You need to be aware of the load paths and of the available strengths, engineering is not just making the equations work, it is understanding the story that the equations tell.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[JAE]

kikflip - it won't yield the same answer - as we stated above, there isn't just ONE [φ] to move over. For different limit states there are multiple [φ]'s. What are you going to move over to the load side...0.9? If you do, then your shear calculations will be incorrect.

 

[PMR06]

The original question was asking about moment only (Mc), not necessarily shear and axial.

I agree though, why mix it up? Even if the algebra works out for that one case, you're just going to confuse people. At some point, someone (maybe even you 10 years from now) will ask "where did 1.78 load factor come from??"

 

[wildwolfyan]

Thank you all for your advise!