AISC unity check for legs

[Klikoos]

Hello everyone,

I am calculating legs for storage vessel and I came across the following formula;

Sc = (Sma/Sa) + (0.85*S)/((1-Sma/Spex)*Sb)

I know the following...
Sc = Unity check
Sma = axial compression
Sa = allowable axial compression
S = bending stress
Sb = allowable bending stress

Can someone explain to me why the bending stress gets reduced (since it's multiplied by 0.85) and also what Spex means. I read somewhere else that Spex is the combined allowed stress but in the example that I saw it seemed way too high to be that.

Cheers

Alex

 

[SteelPE]

That equation looks similar to the old AISC 9th edition unity check (I believe it is exactly the same with different variable names). If you have an old copy of the manual look in there.

My guess is the Spex is the Euler stress of member (in the AISC this is divided by a FOS).

The 0.85 is a modification factor placed on compression members subjected to joint translation (sidesway).

 

[Klikoos]

Thank you very much Steel PE,

I looked into the 9th edition of AISC and it turns out it's formula H1-1 in 3-9 with different symbols