Channel Cap Crane beam

[Girtz]

Hi, I am new to this forum and to Crane Beam Design. I have used AISC 360-16 and Design Guide 7 (3rd edition) to go through the design process. It has been long, but educational. I have some questions when it comes to compact/non-compact/slenderness. I am creating a spreadsheet and checking it against other designs (spreadsheets and examples) I found online. For the most part everything is close. So to the meat of the question. I am looking at a W36x150 beam with a cap channel C15x33.9. This is just an arbitrary size for checking, but it applies to all members. I am checking compression flange local bucking (AISC F3.2).
It defines λ = b_fc/2t_fc.
b_fc = width of the flange in compression
t_fc = the thickness of the compression flange.
To me, this is a composite member and the top flange of the beam and the channel will be in compression. If this was just a wide flange beam then I would use b=b_f/2 as shown in case 10 in Table B4.1b. This seams to match how λ is defined in F3.2. However, since the channel is attached I would considered the composite section as a stiffened element. Case 18 of Table B4.1b would seem to apply (closest thing). This defines λ = b/t where b is b_f of the beam. There would be some addition resistance to flange local bucking from the rest of the channel, but I believe it is conservative to use just width of the beam (correct me if I am wrong ). However it seems wrong to use λ =b/t when F3.2 says λ = b_fc/2t_fc. I checked for errata just in case but there is none. The other question is what to use for t_fc? One would think that it would be the t_{beam flange} + t_{channel web} since the both will be in compression. I have seen some examples I found online that use just b_beam/t_{channel web}. I don't know I agree with it. it is only using a small portion of the area in compression for the width-to-ration check. It is causing wide flange beams that are compact by themselves to be considered non-compact when the channel cap is added.

1. What would you use for b_fc?
2. What would you use for t_fc?


Girtz

 

[GC_Hopi]

I believe case 18 is correct and you should use the channel properties. The text in B4.1 says if the width-to-thickness ratio of one or more compression elements... to me that reads that you should consider the channel as one compression element. I would tend to use b_fc/2t_fc. I think it has the 2 because it is considering the fact that the web will provide some restraint to buckling.