Hairpin Reinforcement for Anchors in Concrete 1

[bones206]

I am working on a project with hundreds of embedment plates that are anchored using either nelson studs or deformed wire. I was tasked with designing some sort of reinforcement to avoid a concrete breakout failure when the embedment is near an opening or free edge. My design uses #5 or #6 hairpins that enclose each row of anchors and extend beyond the shear breakout cone into the concrete wall/slab by a distance of ld, measured from the furthest row of anchors from the free edge.

My concern is that there is not a whole lot of literature that I have seen that deals with this sort of anchor reinforcment scheme. ACI 318 touches on it, and I have seen a few experimental studies and research, but with mixed results.

I was wondering if anyone had any practical design or construction experience with this. I do not like the idea of designing something based on theory alone, and I've been told that running our own physical tests is not an option in this company. When I asked the senior engineers what their experience has been with reinforcing anchors near an edge, they simply said that they avoided placing embedments near a free edge in the first place. They all consider my design to be a sound one, but as a first-year green engineer, I'd feel a lot more comfortable if I heard someone say "Yes, I've done this before, and it works".

Any insights or advice will be very much appreciated

 

[UcfSE]

Look for metal building system details. Hairpins are common in that case. Also, google a book by Alexander Newman. You may get some information there.

 

[DaveAtkins]

Are you talking about hairpins in plan (in a slab-on-grade), as UcfSE is describing, or vertical hairpins in a pier?

DaveAtkins

 

[jike]

Maybe I am missing something here but why can't you assume the concrete cracks, has zero resistance, and then put hairpins in to take the full load? Sounds pretty simple and logical.

There are not code provisions that cover everything.

 

[bones206]

The hairpins are in the plane of the wall or slab, whichever the case may be. This is a multistory, industrial building with mostly concrete walls and floors.

For example, picture an embedment plate that is cast into a concrete wall flush to the surface. A steel member is welded to the plate and is transmitting a shear force into the concrete towards an opening or free edge. The embedment is too close to the edge to develop its full ductile capacity, and will fail by concrete breakout without reinforcement.

I have also been told not to take credit for any other structural reinforcement in the slabs/walls because the hairpin design must be generic to all situations.

UcfSE, thanks for the reference. Unfortunately, I think google is hiding the pages I want.

 

[bones206]

jike,

That's exactly what I did. Here are a couple of the constraints I've been wrestling with:

1) development length. Very sensitive to the spacing between the legs of adjacent hairpins. If you have anchors spaced at 6" oc and #5 hairpins centered on each row, you will have a clear spacing of 1.625". Since the code requires 1" min. clear spacing, it doesn't leave much tolerance for field workers.

2) vertical placement of hairpins. If you observe the code min. of 0.75" clear cover from the surface of the concrete, you can pretty much place the hairpin up against the bottom face of the plate, as close to the applied shear as possible. However, some of these plates are 1.5"-2" thick. That creates an eccentricity between the applied force, and the resisting hairpin force. I accounted for the eccentricity by amplifying the force that needed to be carried by the hairpin in order for the anchors to develop full capacity. Does that seem reasonable or overly conservative?

 

[jike]

bones206:

1) The loads you are resisting seem like they might be quite large. Apparently, you have a hairpin around each row of anchors 6" apart. Do you really need them around each anchor? The breakout that you are trying to prevent is of the whole embedment not individual anchors, so can you wrap the whole embedment with one hairpin?

2) Your approach for the eccentricity sounds OK.

 

[bones206]

Yes, the loads are quite large. The range of plate shear capacities is from ~20 kips to ~125 kips. So for most plates, I need to have the one hairpin per row in order to develop the full capacity. For the plates that don't require a hairpin on every row -- say a smaller plate with 4 studs on the corners that mathematically only needs one hairpin -- I design it with 2 hairpins for the sake of symmetry.

I think that approach will simplify the matter for construction as well. The idea is that I have to develop this "generic design" concept so that the client doesn't have to pay us to design each of these as a special case. I have tried to build in as many conservatisms as possible because after the generic detail is issued, it will be up to the contractor to interpret and construct these.

Thanks for the responses so far. I am very interested to hear if anyone has actually specified this type of detail, and any issues they had with design or construction.

 

[JD2]

Can't you use straight rebar welded to the backside of the embed plate? Prefab the plates all the same and then rotate the plates as required in place to resist the direction of your lateral force. Reference the PCI Design Handbook for design examples.

 

[sandman21]

Using rebar welded to the back of the plate kicks you out Appendix D. If you don’t have the current ACI code pick up 08 they cover reinforcement acting with anchor bolts. “Anchorage in Concrete Construction” by Rolf Eligehausen, Rainer Mallée, John F. Silva is the book when it comes to anchoring to concrete; I have personally not read it so going off of what others have done. As long as you follow the basic behavior of reinforced concrete you can create many different details that accomplish the task.

 

[bones206]

sandman,

I purchased that book about a month ago and it is a pretty good reference to have, but also too expensive for what it is.

I am confident in the hairpin design, but I am trying to gauge how conventional or unconventional this detail is. Will the contractor laugh when he sees it? It's disconcerting that not one structural engineer in my entire company has ever called out this detail

 

[OzEng80]

We commonly use rebar in cast-in-plates as described by JD2. The tags can easily be lengthened so that they extend beyond the breakout zone.

 

[RHTPE]

If you choose to go with rebar welded to your embed plates, please consider:
- Specifying ASTM A706 bars (for weldability).
- How the bars welded to the embed plates will interface/interfere with other reinforcing in the concrete element they are being placed in. Typically the form guy is setting embeds (usually a carpenter) after the rebar cage is tied. If you have embeds on both sides of the wall or column you will create a real problem for the guys building it.