Section 3.4.7 Compression in Column, Axial, Gross Section

[pbardoff]

I am trying to check the effects of combined bending of an aluminum member that will be subjected to axial loads per Section 3.4.7 of the Aluminum Design Manual. My S1*< Lambda < S2*. The member I am checking is 14' long. Phi * FL (allowable compressive stress) is coming out as a negative number which to me is meaning that a beam this long cannot be subjected to axial loads? Something seems off. For this situation Bc = 39.37, Dc* = 13.12, Lambda = 3.08, and Phi = 0.95. I am unable to find any rules for what to do when Bc - Dc*(Lambda) < 0. Has anyone ever come across this situation before?

 

[JoshPlumSE]

I don't think that should be possible. One way to check to see if you are doing the calculation correctly is to assume Lambda = S1 and see if the two equations give you basically the same capacity. Stated another way, the code is designed such that the capacity does not experience a step function when Lambda is slightly less than or slightly greater than S1.

Then do the same calculation for Lambda = S2.