Shear Transfer at Column Baseplates

[JStruct82]

Hello all --

I have a braced frame building that I'm designing, and I have a question about the transfer of shear to the foundation. I know AISC discourages taking shear through the anchor bolts, since the baseplate has to displace before the anchor bolts become engaged. I've read in Modern Steel Construction that friction can be used to take shear, but my office doesn't adhere to that philosophy. I have literature and design examples for shear lugs as an effective means, but to me they seem cumbersome to construct. I was thinking of welding plate washers to the top of the baseplate, then taking the shear through the anchor bolts since they become engaged immediately, and checking bending of the rods using the thickness of the baseplate and grout bed as the moment arm. What are your thoughts? Also, is my office an anamoly for not considering frictional resistance? I guess I'm wondering about the design philosophies of others.

Incidentally, I am a rookie, and I apologize if the question seems rudimentary. And this is also my first post in the forum, so if it belongs somewhere else, maybe just humor me a little.

Thanks for your thoughts!

 

[JAE]

We never use friction.

One detail we've used includes a lug as you discuss. Difficult to get built right.

Another way is to set the baseplate down in a large sunken key and reinforce around the plate in the higher concrete. Again...rather expensive.

 

[WillisV]

There are several methods for accomplishing this, a few of which have been mentioned. The three discussed in the second edition of AISC's Design Guide 1 for Baseplates and Anchor Rods (which I highly recommend as a reference) are:

1. Shear lugs
2. Bearing (by embeding the column as JAE suggested).
3. Anchor rods

You are correct in that AISC does not generally recommend taking shear through the anchor rods, but for small shears (80kips or so max) it is possible to weld plate washers to the baseplate and check combined tension and bending in the anchor rods, though you will end up with some fairly large, most likely GR105 rods. The method for doing this is clearly spelled out in Design Guide 1 (2nd ed).

The method I use, which is not provided in the DG, is to weld L5x5 angles to each side of the baseplate in the field after the base plate has been placed and then anchor them with post installed anchors into the foundation. This should be done after final column plumbing and prior to grouting.

 

[jike]

The friction method only works if under all load conditions (where shear is present), there is always sufficient compression on the column to resist the shear.

We find that the shear lug is not the most friendly method and presents construction difficulties.

We use a cast-in template with headed studs (designed to take out the shear). This plate is at least 2" larger than the base plate on all sides. It also serves as the template to hold the anchor bolts in place.

Once the column with base plate is set, 3/8" side plates get field welded to both the side of the base plate and the cast in template. The side plates are oriented parallel with the shear. They transfer the shear from the base plate to the cast-in plate in the pier.

This method seems to work well.

 

[ironmon]

I agree with the responses. For small shear loads 50-60 kips use the anchor bolts. (remember to check for shear and tension combined) For larger load, shear lugs are widely used, and not a problem to construct. (check bolts for tension only)
I would not be afraid of using shear lugs if the loads required it.

In addition you may want to consider adding more bays of bracing, to reduce the shear loads on any one column bace.

Good luck.

 

[dik]

I generally use a lug with a good pocket accommodate it... alternatively and for smaller loads, I use a plate washer with a 1/16" clearance hole welded to the base plate. So far... both work.

Dik

 

[csd72]

Alternatively, tie the column into the ground slab, taking the shear out that way.

This is standard practice in Australia for industrial portal frames.

Dont know if this violates any codes in the US though!