Basic nomenclature question

[cepe01]

What does or where did the subscript "a" come from for required strength in ASD design? As in Ma, Ra, Pa, etc.

Shouldn't it be called "r" instead or something so people don't confused it with "allowable"?

 

[thaidavid40]

Ever since I was in school back in 1976, I have used the "a" subscript to indicate "allowable".
Dave

Thaidavid

 

[Thoughtclaw]

Yup. Allowable.

 

[TehMightyEngineer]

Take a look at the start of section 2 of the AISC manual, this describes the nomenclature quite well.

Subscript n (R_n) is the nominal capacity. Essentially the capacity with no adjustment for LRFD or ASD.

Subscript a (R_a) is the required allowable capacity for ASD. R_a ≤ R_n/Ω.

Subscript u (R_u) is the required ultimate capacity for LRFD. R_u ≤ φR_n.

I can see your point, though. You would think subscript r would be used for "required". This is done somewhat, if you look at chapter H for combined forces in AISC 360 you see that R_r is the "required" strength for LRFD or ASD.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

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