Cap Plate?

[SteelPE]

I have a client, a fabricator, who asked me my opinion about an issue they are having. I am not involved with the project at all other than the fact my client asked me to look at this picture.

The picture shows a round tube that is used to support a hanging running track inside of a building. The track is supported from the roof along the inside edge and by building columns along the outside edge. The building was completed about a year ago. The tubes were capped with a steel plate that was tack welded on the bottom of the hanger. As you can see, the tack welds are failing and the plates are starting to come loose. The client is wondering what is causing this issue.

I am wondering if the wall of the hanger is deforming slightly causing load to be attracted to the plate (which wasn’t designed or welded to resist such load). I am wondering if this is a valid analysis, or if something else is going happening?

 

[EngineeringEric]

The plate looks bowed out, could it have gotten water inside it and frozen?

 

[SteelPE]

The space is heated and covered, so the only time water was getting inside was during construction. If this were true, then it would have been corrected during construction.

 

[SAIL3]

yes, the wall of the tube is trying to deform @ the support and, as the OP mentioned, the only resistance is the cap pl...that's how I would design the conn using the cap pl to prevent ovaling, etc...

 

[JoshPlumSE]

Ye, but the plate should be more than just tack welded if it's going to prevent deformation of the CHS.

If the cap plate is genuinely structural and is there to prevent that deformation, then replace them with ones that are better connected. If the cap plate is purely architectural (which I suspect) then replacing it with something more flexible that will allow the deformation, but which still provides the desired aesthetic.

 

[dik]

Just a caution about offering an engineering opinion... even, if you aren't involved. [added] It appears that there is paint on the failure surface.

Dik

 

[MrHershey]

My guess is same to yours. You'll have a moment you need to resolve in the shear plate from the eccentricity between bolts and face of tube. Compression component of that is at the bottom of the shear plate and the tube doesn't run that far past the bottom. Tube wall thickness doesn't look large, so tube is probably trying to flatten out a bit and the cap plate won't compress so it pops the tack welds instead.

Assuming no other structural issues (wall thickness is sufficient for concentrated load from shear plate, overall tube section is okay for combined tension/moment from connection, etc.), would think you could probably unload the platform as best you can and weld cap plate back on with a sturdier weld.

 

[SAIL3]

depending on the loads ofcourse....this is an excellent example of why one should provide a cap pl in such a conn ,either that, or a ring on the tube @ top & bott of conn...

 

[SteelPE]

So, in digging into this a little more, I found out that the original detail called for a continuous weld around the plate. It is believed that of the large quantity of hangers that were fabricated, a few made it through QC with only a tack weld. These are the ones that are coming lose.

 

[Tomfh]

So, the cap is acting as a stiffner and they're popping off, like a pringles cap popping off if you squeeze the can?

 

[hokie66]

You've solved it. If the plate is going to keep the pipe round, it has to be well connected.

 

[SteelPE]

I guess the original intent wasn't for the plate to keep the pipe round, but it's presence just happens to keep the pipe round.

 

[KootK]

This feels like it could an Blodgett welding tip. Part Deux to this maybe: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.