Allowable Bending Stress, Unbraced Length > Lu

[weron4u]

In section F1 of "AISC Manual of Steel Construction (ASD) - 9th edition," there are three different ways to calculate Fb. Two of them depend on the slenderness ratio, and the other can be used for any slenderness ratio.

When comparing the one that can be used for any slenderness ratio to the other two, I get completely different values in most cases when the unbraced length is over 8 ft.

Can someone tell me when to use which method of calculating Fb, and why there are three methods of calculation when one of them can supposedly be used in any condition?

 

[JAE]

weron4u
Here I am again....

Are you talking about F1.1, F1.2, and F1.3? If so, each of these are defined for specific cases -

F1.1 - For compact sections only....this is where the flange/thickness and web/thickness meets the "compact" criteria in Table B5.1 AND ... your unbraced length is less than Lc which is defined in F1.1

F1.2 - For non-compact section

F1.3 - For either compact or non-compact BUT the unbraced length is greater than Lc.

In F1.3 there are also THREE equations for Fb - two of these are for specific conditions of l/rT...so you use the appropriate one. The other is another check on lateral buckling. You get the appropriate Fb from the appropriate Equation F1-6 or F1-7. Then you get another Fb from Equation F1-8. You use the larger of the two Fb's.

 

[weron4u]

Thank you again JAE. Yes I was talking about F1.3, equations F1-6, F1-7, F1-8. That did answer my question. I needed to know to use the larger of the two values. Thanks again.