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Resisting Column Base Shear with Friction

Resisting Column Base Shear with Friction

Resisting Column Base Shear with Friction

(OP)
I generally only use friction to resist base plate shear loads for lightly loaded columns in non-seismic areas. For serious loads, I'll find a more positive shear transfer mechanism such as shear lugs.

The sketch below represents an as-built condition for a building currently under construction. It relies on friction to resist shear loads and the numbers work. On the one hand, it is very heavily loaded. Were it my design, there would probably be a shear lug for that reason alone. On the other hand, the situation is such that the shear load only exists if the vertical load does too. It's 100% reliable in that regard.

What do others think of this? Good or no good?

As a secondary question, how do we feel about steel shim stacks when we're utilizing base plate friction? Clean steel on steel gives a coefficient of friction of 0.5-0.8. Greasy surface come in around 0.16. So says the internet anyway.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Resisting Column Base Shear with Friction

I would normally be OK with using friction and/or the anchor rods to transfer the shear, but the fact that the horizontal force is 359 kips makes me nervous.

DaveAtkins

RE: Resisting Column Base Shear with Friction

That's an angled member... Is the shear resistance necessary for general stability under gravity? If it is, I'd be terrified of just using friction. I don't have a reference or anything that will tell you not to do it. It just worries the hell out of me.

RE: Resisting Column Base Shear with Friction

(OP)
Thanks for the feedback gentlemen. The shear resistance is indeed necessary for general stability under gravity. Any concerns regarding the shim stack? Or just equivalent to more grout from a capacity perspective?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Resisting Column Base Shear with Friction

I thought friction was generally a no no.

What is the coefficient between steel and concrete? How can that be ensured? If the plate was toothed, or roughened, ok. Can the friction that you have assumed still apply if the plate "jumps" due to a seismic event.

I would add a steel shear key, embed the baseplate, or provide a crapton of bolts.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

RE: Resisting Column Base Shear with Friction

(OP)
The column has 4 - 1" diameter bolts, clearly not intended for shear resistance. The first time that I saw it, it had just the shim stacks and no grout. Even with most of the load yet to be applied, I was nervous standing next to it.

It is pretty common to use friction. Quality control on the mu value is an issue for sure. For this particular condition, ASCE 10-97 lets you use 0.55. In seismic zones, I believve that there is code provision precluding the use of friction in this kind of situation.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Resisting Column Base Shear with Friction

Is it ok to use shim stack with that amount of load? Seems you would be better off using grout as is will conform to irregular surfaces better.

RE: Resisting Column Base Shear with Friction

(OP)
Agreed. In my market, standard practice is to place one stack of shims centred under the base plate and then fill in the space around it with grout sometime before close out. It makes me nervous too, particularly as I came up in market where levelling plates were common. At my recommendation, the PM requested that the grouting be done ASAP, before the serious loading is applied.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Resisting Column Base Shear with Friction

I NEVER NEVER use friction for any significant load(greater than 2kips)...friction may be here today and gone tomorrow..

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