Shear Connection Design 1

[strikengineer]

I was reviewing some general note sheets produced by another engineer, and came across this note regarding connections:
"Shear connections shall be designed for 1/2 of the allowable uniform loads table in Part 2 of the AISC (ASD)"

The note sheet was based on ASD 13th Edition, and when I asked the signing engineer what table he was referring, he referred me to a client specification that requested the note be used for the design basis. I still use my LRFD 2nd Edition, so I have a little trouble finding things in ASD 13th.

Does anyone know which table the note could be referring?

 

[JAE]

This is a method used over the years by a lot of engineers to provide a locked-in logical means of specifying connection capacities with minimal effort on the part of the engineer.

The ASD table, in the 13th edition, that would be applicable would be Table 3-6 (for wide flanges).

Note that this table has values for both LRFD and ASD and simply provides the maximum uniform load that a particular beam can support for various spans...all assuming that Lb = 0.

If the unbraced length is greater than 0, then the maximum load the beam can take is LESS that that given. So if a fabricator designs the connection for a beam at Lb=0, they are always going to have a connection that is at least as strong, if not stronger, than the beam.

 

[BAretired]

It is certainly more logical than some of the specs I have run into. In doing some work for a local steel fabricator, I found one consultant who demanded that all connections be designed to sustain the factored shear capacity of the beam. He was not prepared to back off even when shown that the calculated factored reaction was only a fraction of that.

BA

 

[nutte]

This an antiquated method, and it is despised by connection engineers. AISC even says not to do it this way. Their response (link below) says that doing this violates the Code of Standard Practice. I don't think I can bring myself to agree with that, but it should be avoided.

http://www.modernsteel.com/steelinterchange_details.php?id=237

Unfortunately, change will not be easy. "We've always done it that way..." Contractors say that and we get upset, but we're guilty of it, too.

 

[WillisV]

It is not that hard to write R=xxx on the drawings on each beam. If you are worried, increase the reaction some. It still will result in much more economical connections than using a portion or all of the uniform load capacity.

 

[abusementpark]

Quote from a frabricator: "An extra bolt here and there isn't driving up the cost of the steel."

 

[271828]

I agree with the folks who recommend to show the reactions. Sure, a bolt here or there isn't that big of a deal, but I honestly don't get the point to trying to save these few sec. I wrote reactions on the beams for my entire career and it's just not a big deal.

Here's another thought. I know of one fabricator that likes to use single angle connections. It's a lot easier to get these to work with reasonable reactions.

And another: Fabricators rightfully gripe about EORs who specify reactions using part of the max unif load because they sometimes end up with insane requirements. For example, say you have a W10x12, 5' long around a mech. opening. Good luck getting a reasonable connection to work for half the uniform load--75 kips. (Remember that at least the top flange will get coped in most cases like this.) I've used about a zillion beams like that, all with Ru = almost nothing written on them.

Sorry, but it's just plain old lazy to use this old method and needs to die.

 

[lrhg]

this modern steel article by a fabricator's engineer with a practical perspective on connections:

http://www.modernsteel.com/Uploads/Issues/July_2003/30719_connections.pdf

 

[hokie66]

I don't like the idea of writing design reactions on the end of each beam, just because it clutters up the drawing unnecessarily. And although I sympathize with providing shear capacity to account for future modifications, that can be achieved just by typical details. I specify the type connection and the number of bolts for each size beam in a table.

 

[271828]

hokie66, I think most people just write the reaction on the beam label, like W16x26 R=30k or something similar. I don't think this is too bad. I've seen R=whatever on both ends of beams and it is a bit much.

 

[JAE]

I do what hokie66 does - show actual connections for different beam depths or beam groups. Showing reactions is fine, but you still have to check the connections as the EOR if you use reactions on the drawings.

With spreadsheets, etc. that you can develop, it's really easy to verify standard AISC shear connections. We use single plate shear connections mostly - sometimes double angle.

 

[hokie66]

I should have clarified my comment in that in Australia the detailer doesn't design connections, the engineer does. So the design drawings have to show in some way how each member is to be connected, not the loads on the connections.

 

[engn555]

So just to clarify, does the OP's quoted rule mean that you would determine w(max) for your beam with an unbraced length of 0, via M=wL^2/8, and then design for V=wL/2? Regardless of the actual shear seen by the connection?

 

[Lion06]

We publish reaction tables that show the max reaction for each beam depth. That makes it much easier to check when the shops come in compared to have a different reaction on each beam.

 

[nutte]

Engn555: Yes.

27128's point about short beams at mechanical openings is the most obvious example of this requirement leading to ridiculous connection design. We ran into an EOR that absolutely refused to back off of it, and we had to haunch both ends of a W8x10 that was 3' long. That does cost more than a bolt or two. Unfortunately for the fabricator, it was his cost, not the owner's. Who's going to figure super-reinforced connections for beams at mechanical openings at bid time?

"Lazy" is the nicest word I can use to describe this.

 

[BAretired]

"Obstinate" is more accurate.

BA

 

[haynewp]

For composite beams the load at the end can be higher than the table value. I have seen drawings with this note but had end reactions on composite beams that exceeded the "1/2 of the allowable uniform loads table".

I have also seen drawings that have shown the reaction on the plan, and one number was covered up by a beam so it looked like something else (example; R=111kips was shown but looked like R=11kips because of cluttering). Obviously dangerous.