Cantilever with a shear connection

[shaneelliss]

If the numbers work out, would you ever use a shear connection to support a cantilever beam? In other words, would you be comfortable using something like a single angle three bolt shear type connection to support the "fixed" end of a cantilever steel beam if it is lightly loaded and the calculations show that the force couple on the shear bolts is low enough to work out?

I wouldn't use this type of connection, but I am working with an engineer who wants to do it and I don't know if I am being too critical in telling him that it is a stupid idea. I always hesitate to tell another engineer that their ideas are bad, because deep down I wonder if maybe they are smarter than me and I am being closed minded to an adequate solution.

So what do you think? Pinned cantilever ok or not?

 

[mike20793]

I assume you're not referring to stub cantilevers and the cantilever has a backspan to it. Do you mean the cantilever beam is supported by a simple shear connection at the point where it cantilevers? I do this all the time; it's analogous to a roller support condition. The moment is internal and isn't applied to the connection.

 

[shaneelliss]

Picture worth a thousand words and all that:

 

[CBEngi]

If your relying on the top bolt to carry tension and the bottom bolt to carry the compression reaction is it still a simple shear connection? I wouldn't think so.

I wouldn't be as worried about the connection resisting the force in the vertical direction, but if that was a single angle (or even double) and the load had any lateral component I would be worried about the beam 'flopping' to one side, thus twisting and failing the connection. I think in a perfect world it would work, but in reality its risky.

 

[nutte]

I wouldn't do it. Though I have seen it done on one project by another engineer. This was for a very small member, and the load was probably measured in pounds, not kips.

 

[KootK]

It's a long story but I had a project that used a zillion of these things to support floor deck that was all wavy in plan. The fabricator talked me into it and backed it up with some solid calculations. My conclusions were as follows.

1) The loads have to be pretty small. The upper limit on the moment connections probably shouldn't exceed about 15 kN*m.
2) The top flange of the cantilever should be well braced for LTB.
3) The connection should cover at least 2/3 of the cantilever beam depth.

In your particular case, I would at least have the angle swapped out for a welded single plate shear tab. I feel that the angle would introduce too much connection rotational flexibility.

In general, this is not a great connection. I only did it because I had hundreds of small load cantilevers and the cost savings were significant. For a one off on a column, I'd just use an HSS and a bolted end plate connection.


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.

 

[Lomarandil]

Agreed, I'd be worried about the torsional and weak axis support of the beam (unless there are bracing elements not shown in the sketch).

Also, you'd want to be careful of deflections in some applications -- since your connection is closer to a typical "pin" connection, I'd imagine that even for light loads, you may develop some rotation at the supported end as the bolts develop into bearing. I suppose slip critical installation could rectify that.

 

[Jc67roch]

I agree with KootK - an endplate connection, checked for moment capacity would be much better. OR at the least I would delete the shear angles at mid-depth and use a top and bottom flange connection angles to the column - these would develop the moment strength of the cantilevered beam, could still be a cheap bolted connection, and just check that the angles and bolts could develop the necessary shear capacity (which they likely would). See attached B1 - without the shear tab at mid-depth.

 

[shaneelliss]

Thanks for the input. It seems that most agree that it isn't an ideal connection, but can work in some cases.

In this particular case, the beam and connection are already built and in place and when I noticed it I talked to the design engineer and told him I thought it was not a good connection for a cantilever. He at first agreed but then said he would see if he could get the numbers to work out, which they did. It is a short cantilever (2'), with small loads (<2 kips) and it is restrained for LTB. Doing my own check on the numbers, it looks like it will work by the math, I just don't like the idea of using a "pinned" type of connection on a theoretically "fixed" end.

To me a cantilever should have a rigid connection, and a rigid connection means fixed flanges. It is like KootK said, this type of connection has too much rotational flexibility for me to be comfortable with, even if the numbers say it will work.

 

[dcarr82775]

The bolts would have to be slip critical with the appropriate surface preparation. Without that the beam can rotate until the bolt goes into bearing. Maybe not a big deal depending on what connects to it.

 

[rb1957]

are there displacement criteria (as well as strength) on the beam ? maybe the simple shear clip is too flexible ?

another day in paradise, or is paradise one day closer ?

 

[TLHS]

I've done a similar style connection on channels on occassion for light cable tray loads, conduit or similar tiny items.