Expansion Anchors in Slotted holes 1

[kaffy]

Good Evening guys,

I am working on a project where an angle bracket will be attached to concrete with post installed expansion anchor.
Initially, I wanted to put a slot on the angle bracket for adjustment but as it will be a slip critical connection with slot now, I need to have a minimum pretension of 85kn (CISC) / 19kips(AISC) which comes out to be close to 200ft-lb torque.

As per the expansion anchor technical guide, Torque value is only 80 ft-lb. So what should I do?

Option 1: Forget about the slot and stick with hole
Option 2: Tighten the kwik bolt at 200ft-lb
Option 3: Find the clamping force at 80ft-lb and then friction force = coefficient of friction * clamping force. If the friction force is more than acting shear force, Just tighten it to 80 ft-lb and forget about AISC/CISC

Please share any related references

Thank You
Newbie

 

[rowingengineer]

We would typically add an over sized washer that was fixed after installation by either welding or screws.

 

[phamENG]

I'd just use a standard hole and not try to make it slip critical. If you try to take it 200ft-lbs you'll probably break it (or the concrete).

Nice thing about post installed anchors is that you can put your angle up where you want it, mark the holes, drill them, and be reasonably accurate (as long as you know how to use a drill, of course).

 

[kaffy]

oh I see but what about the pre tension?

As per the attached article, over torque could result in concrete splitting

https://www.structuremag.org/?p=11312

Do u mean that by addition of washer result in this being bearing joint instead of slip critical joint. If yes, what kind of screws do u use? Also as the washer thickness is pretty small so bearing joint will have really small bearing resistance. Is that right?

 

[kaffy]

@PhamEng Yeah I think I will have to add another hole(spaced properly on steel and making sure that minimum anchor hole to hole is being maintained) in case the anchor bolts are hitting the rebar in concrete.

 

[bones206]

I don't think slip-critical connections with steel/concrete faying surface is covered by any code provisions. I don't think it's something that could be reliably depended on, even if high initial anchor pretension was possible.

Some proprietary products out there solve the adjustability issue by introducing serrated surfaces into the assembly (Halfen, for example). If you have a lot of brackets to install you could look into using something along those lines.

 

[dik]

You are limited by the allowable torque on the fastener... limited by expansion or by adhesive.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[kaffy]

@bones206 Thank U very much.. I checked the product but unfortunately this is good only before pouring the concrete. Most of the time, When the project comes to me, concrete is already poured so the only option is post installed anchors

 

[kaffy]

@Dik yeah I thought so. I think I need to look into those rebar scanners.

 

[dik]

My preference is adhesive anchors, but I use both.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[kaffy]

@dik but there is a limit on max torque we can have with adhesive anchors also and that is pretty small so that doesn't solve the problem with slots. Having said that why do u prefer adhesive anchors?

 

[dik]

Slip critical is not possible. You have to go for a welded plate washer.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[bones206]

I checked the product but unfortunately this is good only before pouring the concrete. Most of the time, When the project comes to me, concrete is already poured so the only option is post installed anchors

You should be able to utilize these serrated products as a post-installed solution. See below:

This is just an example and there are probably many more products out there that are suitable. This is just the one I am most familiar with. If you look through their manual they also have custom brackets that you can order assembled, where you can specify your desired adjustability for field installation and they configure the bracket to suit.