Eccentric loading of a channel

[penpe]

A C15x40 channel framed as a beam supports a load from an HSS4x4 tube column. The column load is axial and wind load lateral, parallel to channel length. The tube column is connected to the channel with bolts through the web, with the tube against the outer face (opposite the toes of the channel). The top of the channel is also loaded with equipment, whose loads are of a similar magnitude to the point load from the tube steel post, (but are distributed instead). Does this loading situation warrant any special analysis because the centroid of the post is 2" from the outer face of the channel? The channel is braced by other horizontal steel framing 2' away on one side and 4' away on the other.

I realize that the loads' magnitudes are always a factor, but in this case I'm not seeing the reason for any special concern, as it's pretty common (I think) to connect to a structural channel by bolting to the outside face.

I've analyzed this platform in RISA-3D, and the channel was found to be adequate, but I made no special effort to model this specific connection detail.

Without going into more specifics, does this strike you as a problem?

 

[BAretired]

The shear center of the channel is probably not far from the center of the HSS, so it does not sound like a problem.

 

[DaveAtkins]

Agree with BAretired. Remember, the shear center of a channel is actually on the side of the channel web where your tube is positioned.

DaveAtkins

 

[JAE]

A C15x40 shear center is 0.767" outside the web. For a 4x4 tube your theoretical eccentricity is 2" - 0.767" = 1.23".

There is inherent stiffness in the tube so it might be surmised that the channel doesn't "feel" the eccentricity and the tube simply sucks up the bending back into itself (i.e. the axial load sort of flows into the outer tube flange against the channel) and this is also due to the fact that the channel may have very little torsional stiffness anyway - but here is a bit of effect that will occur there.

It could be modeled to test how much effect it has.

 

[BAretired]

It would be prudent to laterally brace the channel at the point of contact with the HSS to prevent it from moving sideways. With that precaution, rotation of the channel is prevented by the stiffness of the HSS.