Full-Depth Bolted Connection and a Stiffened Seat

[FLSE]

All,

I have a situation with a beam design where I'm not able to obtain enough shear capacity from a full-depth bolted connection. I want add a stiffened seat to provide additional support strength as well as provide a margin of safety. We've used standard design methods for each connection from the SCM (14th Ed.).

I do have a concern about the shear engagement of the two connections. If the beam is first set on the seat, then the bolts may not be in full bearing. If the beam is first bolted with the seat either welded in the field or in the shop, then the seat may not be engaged without deformation of the bolt holes.

Another option would be to first install the beam by bolted connection and then, in the field, push the seat tight up against the bottom of the beam and weld it. This would allow the bolts to engage and the seat to be in full contact with the bottom flange of the beam.

Has anyone used this approach?

Comments? Opinions?

Thanks in advance.

 

[canwesteng]

I've added the seat in situ before as a means to beef up existing connections. You could add shims to the seat and then bolt from the beam flange to seat as well if you're concerned about getting bearing.

 

[KootK]

I do have a concern about the shear engagement of the two connections.

I share this concern. I consider it a permutation of the problem that we have when welds are used in combination with bolts in the same connection. Because welds tend to be much stiffer than bolts, at least initially, the welds and bolts tend to resist load sequentially rather than concurrently and the desired combined capacity is not obtained (welds fail before bolts fully engage).

If anything, it would probably be best to engage the bolts first and the welded seat later since, of the two shear resisting mechanisms, the bolted one will be the most ductile.

I'd recommend bolting your seated connection to the column so that both parts of your shear connection utilize bolts rather than mixing bolts and welds.

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.

 

[hokie66]

I wonder about the statement that you can't get enough capacity out of a full depth bolted connection. What type connection? What size bolts? Rather than provide a separate seat, can you just extend the connection beyond the depth of the beam so as to include more bolts?

 

[thaidavid40]

That must be some heavy reaction at the end of the beam to exceed the feasible load capacity of a full-depth connection using normal sized bolts (3/4", maybe?)! Have you the option to upsize to a beam having a thicker web, so that the shear capacity is increased? What size bolts are you using? You might want to look into going up to as much as 1-1/4 diameter bolts, if that would help.
Dave

Thaidavid

 

[FLSE]

canwesteng: That may be an option in lieu of installing the seat in the field.

kootk: AGREED ... "If anything, it would probably be best to engage the bolts first and the welded seat later since, of the two shear resisting mechanisms, the bolted one will be the most ductile."

hokie66: I'm not sure I follow. How would extend the connection beyond the beam? I can't extend it up, as there is no column. If I extend it down, we're back to a seat-type connection.

thaidavid40: Yes. It is a very large load. We're using 7/8" bolts. The size of the bolts is limited to the beam web thickness. The web thickness is limited, too.

Thanks for the comments!

 

[JAE]

What hokie66 is talking about is extending your beam web downward (coping your bottom flange off and extending the web down with added plate).

But I would also ask about what sort of shear connection / reaction do you have here (and beam size) that is requiring all this?

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[MotorCity]

It seems as though you are only concerned with shear at the reaction, but what about shear in the beam located away from the reaction. What does your shear diagram look like such that you are only concerned about shear at the beam reaction? The magnitude of shear on a shear diagram is usually constant, or decreases slightly in a uniform or parabolic fashion as you get further away from the reaction. My point is that your shear may also be an issue beyond the connection which would require a larger or reinforced beam.

 

[FLSE]

JAE: OK. This might work fine. I'll consider this. How would you handle the unbraced extension for buckling? Would you add stiffeners?

MotorCity: The shear for the beam works. The bolts are the failure mode for the connection.

Add'l Info:
Refer to Table 10-10a (pg 10-111)
Beam: W44x335
Shear Reaction: 230K
Connection Plate: 1/2"
Bolts: 12 - 7/8" dia.

 

[structSU10]

you could use two columns of bolts.

 

[JAE]

Yes - whatever stiffeners are needed but usually you don't want to leave a thin web end unstiffened if it has any sort of compression in it.

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[hokie66]

I was actually thinking of an end plate connection, and that plate could be extended down a fair way. No stiffeners required, as the column will stiffen it.

 

[JAE]

I thought of that too - just wasn't sure if you then begin developing a moment connection. Sort of like this one?

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[hokie66]

JAE,

Yes, except that one is a moment connection, while the OP only wants a shear connection. The stiffener and bottom flange extension can be deleted, and more bolts can be added between the beam flanges.

 

[KootK]

Some examples that I ran across this afternoon. Note:

1) I doubt the extensions were really necessary. Probably just EOR over specifying connection demands.

2) I too wonder about buckling checks that might apply to the extended web plates. I'm not familiar with an accepted method for evaluating such connections.

3) assuming that the web really is okay for shear, I favour using two columns of bolts as suggested previously.

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.

 

[haynewp]

Did you consider an all-welded double-angle connection?

 

[FLSE]

All,

I really appreciate all the responses!!

To me, it seems that the most simple and efficient connection is the Double Bolt Column. Second would be the Extended Web but that really isn't appropriate for this connection.