Shear transfer mechanisms in base plates. 2

[LuisUgarte]

Hello, everyone.

Just for general knowledge, I would like to know what's the most common practice in your area to transfer column shear loads trough base plates into piers,I know there are three main ways to achieve that and all of the three have something to be concern about.

1.-Shear transfer by friction.

The thing is what about if the grout has not been placed in the right way or even it hadn't been placed. The other situation is about cracks in grout due to forces, which means there wont be any contribution of it.

2.- Shear transfer by anchor rods.

I like this one but I'm concern about the mechanism to be used. As almost of you all know, AISC recommends the use of half of the anchor rods contributing to the resistance, doing that you can ignore the tension forces in anchor rods due to bending, and of course we should make some reductions. The other one is taking into account all of the anchor rods by using washer plates welded to the base plate but this time we have to include tension forces in calculations.

I think the second one could be more expensive and take more time. But the first one could have a displacement before it works, and I'm not sure how can that impact in the base plate behaviour.

3.- Shear key

I haven't used this one, but I've seen a lot of recommendations using this instead of anchor rods, is it difficult to place a base plate with shear key during erection?


What are your thoughts?


Thanks in advance, and I apologize if you see some mistakes but I'm not a native speaker of English.

Regards

 

[Lion06]

We use the second condition you described and provide welded plate washers with standard holes at all lateral columns.

 

[JoshPlumSE]

The welded washers negate the AISC suggestion that you design the shear only for the edge bolts, correct?

Is anyone out there actually designing anchor rods for bending? Maybe if you don't grout your base plate, I can see how it could become important. However, if you do have grout, then then considering bending stresses in the rod seems to be a bit over-kill to me.

 

[Lion06]

Josh-
That's right. If you use the welded plate washers, you can use all anchor rods for shear.

ACI 318 App. D Accounts for the small bending associated with a grout bed by using a reduction factor of 0.8 for shear if there is a grout bed. If there is no grout bed and the bolts are actually bending then I would account for bolt bending.

 

[LuisUgarte]

JoshPlum-

That's right. But even if the base plate is grouted, I think we should consider bending stresses in rods because we still have a free space within the plate thickness due to the oversized holes, and that stresses could be significant. I understand that the reduction factor suggested by ACI 318 App.D is only to consider the effects of cracking within the grout bed, and it's correct for the first design procedure I described. However, it's not enough when we're using welded washers, that's what I don't like about anchor rods.

Thank you for your comments.

Regards

 

[Lion06]

Luis-
What in ACI 318 leads you to believe that the 0.8 reduction is only applicable if you don't have the welded plate washers and is only to account for cracking in the grout and that you need to account for bolt bending in some other manner, despite there being nothing in the code about it? I recognize that we can't blindly follow the code without understanding it, but I don't see anything in the code that suggests you need to do anything more than apply the 0.8 factor on the shear capacities and I also see no rational basis for doing more. This is all assuming the grout beds are installed correctly and functioning properly.

 

[LuisUgarte]

Lion06

I think ACI 318 does not explain either, that the 0.8 reduction factor is to account for bending in rods, but that's what we understand. My idea is:

If we use STD holes and grout pads, the shear will be transferred through bearing within the base plate and grout thickness. But what happen if the grout cracks, as we have noted the rod will bend in that area, that's what we use a reduction factor. But if we have oversized holes, the shear will be transferred through bending within the base plate thickness and bearing against the grout, then we have two different mechanisms where rod's length could be either, the base plate thickness + 1/2 of the washer thickness or the grout bed thickness + the base plate thickness + 1/2 of the washer thickness. Of course that two mechanisms will be present at different times. It could be that the reduction factor suggested by ACI 318 accounts for these mechanisms, but if it was I think we would have some limitations in member's thicknesses.

Now if we follow the code without questions, what would happen if we had a base plate with oversized holes and welded washers, but no grout bed. Can we still use the same reduction factor? o simply ignore the bending stresses just because the code does not say anything?

I apprecciate your replies, and of course this is only my opinion.

Regards

 

[KootK]

Back to the original poll, I've used all three details depending on the loading situation. I use number two most often but switch to other methods for serious shear. On the handful of occasions that I've used the shear keys, I've had no issues with quality control or contractor griping. I always expect to have those problems but they never materialize. When using the shear keys, I always take care to detail the connection carefully ensuring room for concrete placement and vibration around the key form template.

For serious shear, I often place a studded cast in plate near the baseplate and use it to drag the shear out of the bracing gusset. This can work if you've got a grade beam or frost wall nearby.

This document may be of interest to you Louis, particularly around page 72 where they get into the rod bending issue: Link

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.

 

[LuisUgarte]

KootK-

Thank you for your post, I'll make sure to take a look at that paper.

Now, something that led me to this post was:

Weeks ago I was told of a project where the contractor could not place the base plates correctly because he didn't know how to place them when they have a shear key, that's because they're very unusual in my area. I've done some research about the field procedures, but I haven't had luck. Do you have something that can help me with this, I mean, some sketches or photos to understand the key issues in order to be able to use that type of base plate without field problems.


Thank you in advance.

Regards

 

[KootK]

If you have access to AISC design guide #1, that's a good place to start. Some ideas:

1) Detail the pocket dimensions to be cast explicitly.
2) Detail the arrangement in plan including the anchor rods, rebar, and pocket and ensure that you've got at least a 6" access hole for pouring and vibrating.
3) Consider normal tolerances in the placement of everything.
4) Consider providing vent/access holes in the base plate.
5) Consider using a thicker than normal grout bed so that flowable grout can be used rather than dry pack.
6) Make sure this stuff makes it conspicuously onto the tender drawings to avoid surprising the folks doing the costing.

As I mentioned, this kind of detail seems to be well within the range of the commercial contractors that I've worked with. The paper that I linked above has some good photos of cast in grout pockets.

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.

 

[LuisUgarte]

Thank you,

I've read AISC design guide 1, but it does not say much at all. I've found more useful your ideas.

Regards