2005 AISC Spec - Beam with cover plates 1

[enginerding]

When finding the capacity of a beam with cover plates welded to the flanges, J is needed to determine Lr and Fcr when Lb > Lr.

An approximate equation for J is: J = [Σ](1/3 b t³) of each component of the cross section. For the flanges, is it common to consider the flange and cover plates as separate elements (thus having small t's) or is more typical to use the total built-up thickness for t? This will make quite a difference since J varies with the cube of t of the components.

 

[271828]

That's a tough one, especially because you probably have intermittent welds. I say separate, if you consider the plate in J at all.

 

[WillisV]

Agree with 271828. I would consider it, however the approach would be to add the J of the beam and the plate together, not to use a larger t. Adding them together is slightly conservative, but not by much.

For more information on a similar problem, see Ellifritt & Lue, AISC Engineering Journal, Q2, 1998. They look at calculated J and Cw for a beam with a channel on it. They proposed a simplified formula to account for the contributions of both together:

Jtot = Jbm+Jchannel+bf*twc*tfl*(twc+tfl) where twc is the web thickness of the channel and tfl is the flange thickness of the beam.

I would consider it reasonable to modify their approach to this problem which would yield:

Jtot = Jbm+Jcoverpl+bf*tcp*tfl*(tcp+tfl) where tcp is the thickness of the coverplate and tfl is the thickness of the adjacent flange.