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!
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
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
RE: Fixity of Extended Tab Connection
(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
RE: Fixity of Extended Tab Connection
RE: Fixity of Extended Tab Connection
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
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
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
RE: Fixity of Extended Tab 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.
RE: Fixity of Extended Tab Connection
RE: Fixity of Extended Tab Connection
RE: Fixity of Extended Tab Connection
RE: Fixity of Extended Tab Connection
BA
RE: Fixity of Extended Tab Connection
RE: Fixity of Extended Tab Connection
RE: Fixity of Extended Tab Connection
BA
RE: Fixity of Extended Tab Connection
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
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
RE: Fixity of Extended Tab Connection
BA
RE: Fixity of Extended Tab Connection
RE: Fixity of Extended Tab Connection
BA
RE: Fixity of Extended Tab Connection