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Design parameter Cb in ASD 9th edition

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Hello,

I have a question regarding Cb factor.

The question is that "If I design a structure by AISC ASD 9th edition and use defalut cb 1, is it overdesign?"

In case of latest AISC latest code, Cb is calculated as following,

Cb = 12.5 max / (2.5 Mmax + 3Ma + 4Mb + 3Mc)

and in case of ma=mb=mc=max, cb=1

On the other hand, in AISC ASD code (9th edition ASD)

Cb is calculated as following

Cb = 1.75 + 1.05 (M1/M2) + 0.3(M1/M2)^2

and in this case, even if M1=M2, Cb value is over than 1

But In staad design parameter, is shows "Cb default is 1.0"

I feel latest version of default cb=1 seems to be reasonable, (or maybe it can be little conservative value)

but ASD 9th edition case, it feels not. It seems like way over design parameters.

I am not sure I have a right view.

Please give me some advice regarding this issue.

If I design a structure by ASD 9th edition and use defalut cb 1, is it overdesign?


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