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Fixity of Extended Tab Connection
3

Fixity of Extended Tab Connection

Fixity of Extended Tab Connection

(OP)
Hey everyone,

I need to cantilever a small length of wide flange about 4'. The moment is < 15 kip-ft (LRFD). I am hoping to avoid a field weld so I was thinking about using an extended "shear" tab but provide additional bolts and design the bolt group explicitly for the moment required (detail attached). I designed the plate at the first bolt line and the weld at the supporting girder for the moment required. If this were a critical member for the stability of my structure I would just extend the cap/base plates to form a paddle plate connection. However the cantilever is small and the implications of some slight rotations are minimal.

Every component is designed to carry the moment so it works on paper. Would you have reservations with using this detail?

Thanks!

RE: Fixity of Extended Tab Connection

I guess if you have justified everything it will be OK.

Still, it makes me a little nervous. I remember one of my college professors telling us you must "grab the flanges" to create a moment connection.

DaveAtkins

RE: Fixity of Extended Tab Connection

Is it possible to just shop weld the 4' section directly to the column or girder and ship and erect it?

RE: Fixity of Extended Tab Connection

(OP)
Thanks Dave. I understand the hesitation - and I agree it's always best practice to engage the flanges for moment. But considering the magnitude and the low utilization I feel reasonably confident - I just want to be sure I'm not overlooking some fatal flaw...

(edit) Jike - it is presumably possible. The job is galvanized and I'm not sure what the fabricators capabilities are for vat size. I'm trying to keep things broken down so I can avoid double dipping.

RE: Fixity of Extended Tab Connection

Your going to have increased deflection on the connection, bolt movement and plate yielding. You mention adding bolts but no mention of the extra forces on the shear plate shear plate. Your plate will also need to transfer tension and compression forces to the column. The moment and shear will need to be accounted from in the design.

RE: Fixity of Extended Tab Connection

(OP)
Sandman - that's correct. The bolt group is designed for the moment and shear. The net section of the plate is designed for the shear and moment and the welds back to the girder are designed for the same. Stability of the girder is provided by, at the very least, the reaction on the beam on the other side and the cap/base plate connections.

RE: Fixity of Extended Tab Connection

As long as all the forces (and load path) are accounted for....I don't see an issue. People use shear tabs all the time. All "shear" connections have a degree of eccentricity in them.

That being said....a few points:

1. If that shear tab really will generate 15 ft-k of moment.....I'd follow that closely. If it runs into that column, that's one thing....but if it tries to run out of that girder as a torque (at the ends) with clip angles (i.e. shear) connections, that's a whole other ball game.

2. Speaking of that column.....that (i.e. the fact it is interrupted like that) gives me more heartburn than anything else I see. Not sure what is above or below it....but I would check that closely. (Especially if that column is part of the lateral force resisting system.)

RE: Fixity of Extended Tab Connection

You've checked strength, but how closely have you looked at the stiffness? It won't be 100% accurate, but you could estimate the bending stiffness of the shear tab and ball park your cantilever deflections.

If you're going to use just the shear tab, I would make it as deep as possible while still providing clearances for bolting. That may mean your bolt spacing is no longer 3". Center the middle bolt and push the outer bolts as close as possible to the radius between the flange and web. Or add a 4th row of bolts.

Other options: You could weld a stub section of the cantilevered beam to the supporting beam in the shop, and provide a bolted end plate connection which would provide a true moment connection. Or you could extend the cap plate of the lower column and the baseplate of the upper column and bolt to the cantilevered beam flanges. Fit up on site wouldn't be too difficult because the cantilevered beam would be bolted to the shear plate and cap plate, then the column placed above.

RE: Fixity of Extended Tab Connection

I would not use that detail. It creates an apparent instability in the joint. The only member which can resist the moment is the beam on the opposite side of the girder which has a three bolt connection. At the very least, it should have a similar six bolt connection.

Better still, engage the flanges of both beams by extending the cap plate and base plate of the columns.

BA

RE: Fixity of Extended Tab Connection

I'm with Canpro: weld stubs on & do endplate connections. Reliable & easily quantifiable.

RE: Fixity of Extended Tab Connection

I've used a similar connection on some curvy buildings to cantilever infill beams out past girders and pick up slab edge angles and cladding. A design assist steel fabricator talked me into it on the basis that it works find and saves a LOT of money in some applications. I had rotational restraint out at the cantilever tips though. It would be nice if you did too.

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.

RE: Fixity of Extended Tab Connection

(OP)
Thanks everyone - I have rotational restraint at the end of the cantilever provided by a continuous edge beam. I definitely see the merit of the welded stub with an end plate and will keep that one in my pocket for larger load applications. For this though I feel reasonably confident the bolt group will work. I have increase the bolt spread and will try to locate the bolts as close to the girder as possible. I know I didn't provide a larger sketch but this is for a small residential deck detail and there is redundancy in the load path proved by some adjacent framing.

RE: Fixity of Extended Tab Connection

The girder beam will not provide rotational restraint, remember you are using the shear plate to transfer the loading. Your shear plate now takes compression/tension and must be provide sufficient stiffness to be stable or additional structural connections would be required to restrain the end of the beam at the column.

RE: Fixity of Extended Tab Connection

(OP)
Good point sandman. I do have flooring attached to the top flange but I wasn't going to consider it a diaphragm. That's probably the area I've overlooked - I'll take a closer look at the torsional capacity of the shear tab. If it's close I'll probably just change to the stub/end plate.

RE: Fixity of Extended Tab Connection

T Bat, what takes the moment from the cantilever beam? Not the girder, not the column above or the column below. If it's the beam on the opposite side of the girder, should it not have a moment resisting connection similar to that for the cantilever beam?

BA

RE: Fixity of Extended Tab Connection

(OP)
I'm either going to provide a similar bolted connection for the beam on the other side and/or see if I can't get the cap plate to resolve it. With the full depth shear tabs on both sides I basically have a stiffener to help distribute the moment out to the four bolts on the cap plate - not to mention help with the bolt tension from the reaction of the column above. Not an ideal situation sure - and for any significant loading I would look for another path but it seems reasonable to me. The moment is less than 15 kip-ft - the bolts are 3/4" A325 and the post below is a W8x31 with a 3/4" cap plate.

RE: Fixity of Extended Tab Connection

I have done this with the same depth shear tabs on both sides when the cantilever is small. For lightly loaded construction it seems like overkill to put in a traditional moment connection. We know the bolts offer significant moment resistance, why not use it?

RE: Fixity of Extended Tab Connection

Why not, indeed!

BA

RE: Fixity of Extended Tab Connection

I've done this once or twice. What bothers me is the rotational restraint at the joint itself. Cantilever stability is tricky to begin with without adding in an instability at the point of peak moment. If you're very sure you've got it heavily restrained elsewhere and have a bunch of extra capacity there are times when I'd use this.

In this case, though, you should probably be able to do this without a field weld. Make your column end plates longer left to right so that you can get some bolts into the beams. Then you have a continuous plate tying the flanges of the beams together. It could help with erection if you do it right, because you could use the plate as a seat to set the beams on. You have to make those joints slip critical, but the lowest level of surface prep isn't a big deal.

RE: Fixity of Extended Tab Connection

2
A couple of thoughts:
1. I'd definitely want to check the combined shear and bending on the shear plate, not just each one independently. I'd probably use a von Mises type stress combination just to make sure I the utilization was as low as I thought.
2. What type of bolted connection are you using? Are the bolts SC or just bearing? Are the holes standard holes or OVS/slotted. Bearing bolts in OVS/SSLT's would be a no-no, but make sure you specify STD holes if your typical notes/details allow for anything else. You can get a fair amount of rotation just from the slop in STD holes though so you may want/need to consider that possibility.
3. Some other thoughts: Why just use the standard 3" spacing the bolts instead of spreading out the bolts more? Deepening the connection provides lower stress and the top and bottom of the shear tab are closer to being laterally restrained.
4. As far as the rotational restraint goes, welding a "flange" onto top and bottom on one side of the tab to make it a "C" instead of just a plate would add a bunch of stiffness.
5. Even just artificially increasing the shear tab thickness helps a ton. And the cost difference for the extra plate thickness is practically negligible.
6. Even just extending the cap plate below to get even two bolts in each beam's bottom flange adds a bunch of comfort to me. The compression element of the connection is then a flange rather than just a vertical plate = lots more stability and stresses way lower.
7. At the end of this all... why? The cost of the engineering time you've put into this connection this way as opposed to the traditional way is way more expensive than the extra cost of the detail with extended cap/base plates. Unless there's some reason not to do that which complicates erection, why do we do this to ourselves?

RE: Fixity of Extended Tab Connection

(OP)
Again thanks for all the responses. After some changes to the design I have a larger moment here now - looks like shop welded stub with the end plates is the way I'm going.

RE: Fixity of Extended Tab Connection

On both sides of the girder, I hope.

BA

RE: Fixity of Extended Tab Connection

(OP)
Yes definitely. Need the continuity for sure now.

RE: Fixity of Extended Tab Connection

Sounds good.

BA

RE: Fixity of Extended Tab Connection

It's unstable laterally, so don't do it regardless of load magnitude.

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