Short Pier 1

[WARose]

After a IFA/constructability review.....I've had my arm twisted into forming a short pier (about 6") above a mat for a column base plate. (I was going to form it with the mat.)

The anchor bolts will then present a bit of a problem......normally in a pier, I get the shear from the anchor bolts to the reinforcement via a strut and tie model and it goes into the support foundation as concrete shear. In this case....I don't have enough height above the mat to develop any reinforcing. (And ACI still requires full development for shear transfer. ) So I am trying to decide what to do.

2 options come to mind: running a long shear lug into the mat.....or possibly letting the anchor bolts transfer the shear across the plane of the mat/pier interface (with the pier able to handle any resulting flexural load and to keep the bolts from going into bending over that cantilever).

I've had to do something like this before.....and I believe I did the latter. I just wonder about the mechanics of the latter. It's like you would have to get the shear out of the bolts (to the re-bar).....and then get it back in (at the interface between the mat and the pier).

Thoughts?

 

[canwesteng]

Like option 1, don't like option 2. Unless the anchor bolts develop shear and flexure, making the pier redundant, the pier needs to take the shear. Don't think this can work simply like a wide flange beam where the web takes shear and the flanges take moment; shear goes from the base plate to the anchors, and either the anchors transmit shear to the pier or the anchors bend over the height of the pier.

 

[WARose]

Don't think this can work simply like a wide flange beam where the web takes shear and the flanges take moment....

That's the analogy I tried to talk myself into. Couldn't quite make it.

 

[KootK]

I like option #2. The anchor bolts themselves can provide the shear friction reinforcing as they can be anchored both above and below the concrete cold joint.

Load path = base plate --> bolts --> concrete --> ties --> concrete --> cold joint --> mat.

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.

 

[CANPRO]

Is it just one column? You could embed a short section of steel column in the mat and provide a cap plate to bolt the column. Similar to some tower crane anchor details.

 

[WARose]

Thanks for the feedback KootK. (I was hoping you'd weigh in on it.)

[blue](KootK)[/blue]

I like option #2. The anchor bolts themselves can provide the shear friction reinforcing as they can be anchored both above and below the concrete cold joint.

Load path = base plate --> bolts --> concrete --> ties --> concrete --> cold joint --> mat.

The question is: what do you want to call "anchorage" for the bolts [red]above[/red] the cold joint? Simply that they are bolted to the column? (With sufficient strength?) Or that they can transfer the load to the ties?

 

[Lomarandil]

I like CANPRO's idea. I don't know how cost effective it is comparably, but easy to construct.

----
The name is a long story -- just call me Lo.

 

[KootK]

Thanks for the feedback KootK. (I was hoping you'd weigh in on it.)

As though I could resist. I'll get back to the particulars but, for now, thought experiment! Let's put your example aside and go back to your normal case of a 4' deep pier or something akin. 4" down from the top of the pier, there's a horizontal plane that needs to transfer shear to keep the top 4" of the pier from being lopped off. And I contend that's shear friction keeping things together albeit the "monolithic construction" variety. Just a better friction value, and not even that much. So, in that case, what have you got providing clamping force to the joint?

1) Vertical bars partially developed above the cold joint.

2) Anchor bolts crossing the cold joint.

3) Any reliable compression coming in as axial compression or the compression part of a moment couple.

Now back to your goofball pier. You've got all the same stuff, verbatim! So why should you be any worse off? Whether you're fully developed per the inexplicable code clause or not, I say: what's good for the goose is good for the gander.

he question is: what do you want to call "anchorage" for the bolts above the cold joint? Simply that they are bolted to the column? (With sufficient strength?) Or that they can transfer the load to the ties?

That depends which aspect of "anchorage" you're referring to.

1) Shear friction. Anchorage is the connection to the column as you say. Upside down, it's basically one, great, kickass bearing plate. I'd get some slightly expansive grout in there for good measure.

2) Bolt tension. Anchorage is App D style capacity of the bolts being pullout out from the mat based on how far they're sunk into it. No place to fool around.

3) Bolt shear. Anchorage is the ability to engage the ties as you say.

I like CANPRO's idea.

I like it too and, for a certain scale of application, it would be my preferred simply because there's enough uncertainty in this that we're bothering to have this discussion. So what is this thing:

1) Diagrid connection at the base of Shanghai's next monster?

2) Work platform at the local power power station?

And what necessitates the 6" pier business? Durability? Sequencing issues? Could the pier not project up a development length? Could the column just be bolted to the mat and surrounded in concrete? If we're going to help, we deserve the opportunity to be catty and judgmental about your detailing choice. That's the meat in the help sandwich damn it.



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.

 

[WARose]

Thanks again for your help KootK.

[blue](KootK)[/blue]

And what necessitates the 6" pier business? Durability? Sequencing issues? Could the pier not project up a development length? Could the column just be bolted to the mat and surrounded in concrete? If we're going to help, we deserve the opportunity to be catty and judgmental about your detailing choice.

It was requested because the contractor (who gets the owner's ear more than I do) did not want to form it and the mat simultaneously. I can't have the base plate come down to the top of the mat because it is a fluid retaining area. (In the event of a spill. There is a curb around the perimeter.) I can't go up to get full development length because some steel is coming into a few columns low and I don't have time to re-work those details.

Basically this exercise boils down to keeping the detail as is.....but re-running the numbers with the new cold joint in mind. (And it checks out. Fortunately the shears aren't that high.....and I've got a ton of re-bar in the pedestal.)

So that's why I wasn't too interested in the embed idea. (Especially considering we are talking 18 bases.)

 

[KootK]

Got it -- thanks for the background info.

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.

 

[canwesteng]

I'm a little slow today so maybe kootk already covered this, but is sounds like you're married to the idea. What about counting the minimum dead load as the force between the pier and the mat for interface shear? Sounds like you have everything worked out anyway otherwise, if nothing else it's something to keep in your back pocket.

 

[WARose]

What about counting the minimum dead load as the force between the pier and the mat for interface shear?

That's the first thing I tried. Not enough dead load. (Especially in combination with wind uplifts.)

....but [it] sounds like you're married to the idea.

I explained why (in my last post). I'd rather re-run a calculation (that took a couple of hours) than change a bunch of details (that will take much longer).

 

[IFRs]

Extend vertical rebar above the cold joint into and above the short pier, form around the rebar, pour concrete?
Install the anchor bolts into the mat and extending up above the mat, use 4" of grout under the base plate?

 

[bearjew]

Could you not just reduce the capacity of your vertical bars by the ratio of (development provided/full development)? Why does ACI require the verticals to be fully developed for shear transfer here? Although you may not get much out of them considering your pier is only 6" in this case.

 

[WARose]

Why does ACI require the verticals to be fully developed for shear transfer here?

Good question. We've been asking that one for years.