Friction and Shear Lug in Base Plates

[MrFurleyEIT]

I have got a question about making use of friction in the design of shear lug, or shear key, for base plates. AISC allows the use of friction, with a coeff. of 0.55 between steel and grout. However, I talked with my colleagues and most of them do not make use of friction to reduce their shear. Their reasoning is that the base plate may not be set right so friction is not developed, or that the anchor bolts are anchored in a way that does not develop friction fully. I would like to make use of friction to reduce my shear load. Who is correct? Please share your thoughts and why. Thanks in advance.

 

[Bagman2524]

I'd be very leary about taking credit for the friction. What happens when the concrete shrinks and pulls away from the steel?

 

[Atomic25]

I don't take it either, especially if the column is at the base of a brace that may be subjected to uplift, and hence no normal forces to give you friction.

I've just been out to too many sites where the column bases were buried in mud before they grouted them....and just makes me wary that nothing gets cleaned properly or installed correctly.

 

[UcfSE]

Along the lines of Atomic, what happens to the friction if the base plate has some residue or sand stuck to it? Do you want to bet the connection on that? Net uplift that often accompanies the shear load is another reason not to use friction.

 

[kelowna]

I have used the friction between the steel and the concrete before, it is not only allowed by AISC but also by other codes, however, this was a very controlled situation, correct grouting, cleaning of the plate before hand and post-tensioning of the bracket so that the load was actually there all the time.

In normal conditions I am with Atomic25 and UcfSE, do not use it. If you need shear resistance, have you considered a shear key?

 

[MrFurleyEIT]

Thanks for all your responses, guys. It really gave me a better insight on the issue of making use of friction on my shear lug design.

Oh BTW, kelowna, I am using a shear key (AISC calls it shear lug) for my base plate where they are significant lateral loads. I was just hoping that I could reduce my column base shear load by deducting that portion of it that the friction would take care of. From your responses so far, that might not be viable unless I am really strict and lay down some specific notes to make sure the grouting and base plate get done right.

 

[Lion06]

How do people typically design for shear transfer from column base into a footing? I can't imagine a shear lug/key goes into every lateral column in a building, especially if you use type II with wind connections and virtually every column is a lateral column.

 

[Gumpmaster]

Recess the base plate into a 6 or 8 in deep pocket in the footing. Non-shrink grout all around after column placement. The whole base plate acts as a shear lug then.

 

[UcfSE]

Str'l, use anchor rods until they don't work, then provide a shear key. It's a judgment call as to when anchor rods are too big to be practical.

 

[Lion06]

UcfSE-
do you typically weld the plate washers to the baseplate in the cases where you use the anchor rods for shear resistance to make sure you get a good load path for the shear through the anchor rods since the hole in the base plate are VERY oversized?

 

[UcfSE]

Yes.

 

[WillisV]

Use friction until it doesn't work, then use the anchor rods until they get ridiculous, then use an embed or post installed anchors attached to the baseplate, then use shear lug if all else fails.