Crane Beam Design (LRFD)

[DCBII]

I'm trying to find the nominal moment strength of a crane beam with a channel cap using LRFD. To find the moment strength I need the plastic section modulus, Z_x, which I can pull out of the section tables in the AISC Manual. Normally the channel cap is 36 ksi and the W section is 50 ksi, but in my case they are both 36 ksi (50 ksi steel is not available at this location). Do the values for Z_x in the table still apply, or are they a function of yield stresses?

Another question this brings to mind (as a sidenote):
If I do happen to have two different yield stresses, which one do I multiply Z_x by to obtain the plastic moment, M_p?

 

[HuckleberryFinn]

DCBIII:

The plastic section modulus is a function of the geometric properties of the section (not the material properties). In design, and specifically LRFD design, the plastic section modulus is used when the entire cross section is assumed to have yielded instead of only yielding at the extreme fibers (9th Ed. ASD and back utilized the regular section modulus Sx). It's worth noting that this is referencing only the yielding failure mode and not local buckling, LTB, WSSB or any of the other possible failure modes for your crane beam.

These sort of questions are commonly answered in undergraduate steel text books in the bending chapter. Perhaps you should ask your boss/supervisor for some guidance?

 

[HuckleberryFinn]

I tried to find an edit button for my original comment but couldn't so I will post my follow up remarks below:

Actually DCBIII what I just told you wasn't entirely accurate for your circumstances, my apologies, I was thinking of a typical doubly symmetric I-beam. The plastic section modulus is a function of the material properties for non-symmetric sections (i.e. your crane beam) and composite sections. The plastic section modulus is the sum of the tension/compression areas and their distance to the plastic neutral axis (the location where tension and compression forces for a yielded section are balanced).

 

[DCBII]

I don't know that that's entirely accurate either. Isn't the plastic section modulus a function of only geometry for sections made of just one material, regardless of symmetry? When different materials are involved, I think it's a function of both geometry and material properties. I could be wrong.

As another sidenote... my boss/supervisor wouldn't touch LRFD with a 10' pole.

 

[HuckleberryFinn]

DCBII:

You're correct; For a typical crane beam with differing steel types (50 KSI and 36 KSI) it would be a function of the material type, but since your beam and channel are both 36 KSI your plastic section modulus should be only a function of the geometric properties. Apologies for any confusion my comments may have generated.

So, coming back to your original question, since the AISC tables are setup for a 50 ksi beam and 36 ksi channel, you may need to recalculate the plastic section modulus by hand.

 

[ToadJones]

The problem you are going to have is not just checking yielding, that's the easy part. The design gets more difficult when designing using 360-05 or 360-10 and checking buckling from Sections F4 or F5.

http://www.eng-tips.com/viewthread.cfm?qid=329424