Design of Anchor Bolts that is Tied With Hair Pin Tie 1

[pob11646]

I have got a question about the design of a group of four anchor bolts that are tied with hair pin ties. My hair pin ties are #7 bars and about 6'-6" long. They are in the shape of V with a "flattened bottom". The "flattened bottom" is pointed towards the outside of the slab and hooked around the two outermost anchor bolts to provide additional shear resistance to the anchor bolts when horizontal shear forces try to kick the columns out of the building.

My hair pin ties are anchored in the concrete, maybe about 2-1/2", or 3" from the top of the slab.

Now, I have got two uestions. First, when I design my anchor bolts, do I need to design for the horizontal shearing force again? I ask this because the hair pin ties were placed there to provide shear resistance at the anchor bolts? However, also consider that the hair pin ties can only provide shear resistance is out of the building, but not when the shear force is into the building.

Second, in the ACI 318-08 manual, Appendix D, one of the failure modes of anchor bolts in Figure RD.4.1 is the shearing off of the top of the anchor bolts due to shear. The caption reads "steel failure preceded by concrete spall". My second question is, will the hair pin tie prevent this kind of anchor bolt failure, considering that the hair pin tie is 2-1/2" to 3" below the top of slab.

Hope someone can shed light on this. Thank you so much.

 

[Lion06]

The only thing that the hairpin gets you out of checking in shear breakout. You still have to check the anchors in shear (steel), and pryout.

Additionally, if the hairpin is enclosing the anchor on the outside and fully developed to the inside, you are right that the hairpin is only helping for outward thrust. If you can show that it has adequate development on both sides of the potential failure surface then you can count on it in both directions. I would also say that for shear forces pointing inside the building, I am guessing that shear breakout won't be an issue because of the larger edge distance.

 

[kslee1000]

1. Yes. The base plate delivers horizontal load to anchor bolts. The the bolts passing the load to the hair pin through concrete bond.
2. See 1 above, the idea is similar.

 

[msquared48]

#7 Hairpins with only 6.5' legs - seems like the bar is large and the leg is too small to develop the tension into the slab.

Mike McCann
MMC Engineering

 

[BAretired]

I agree with Mike. Don't be stingy with development length in the slab.

BA

 

[pob11646]

Correct me if I'm wrong, but whenever I've designed anchor bolts, I have mainly designed for:
1. Breakout strength
2. Pullout strength
3. Side-face blowout strength, and
4. Pryout strength

I am going pretty much by Appendix D of ACI-318. In my particular case, I did not design for side-face blowout strength as I have provide hair pin ties to take care of that.

I have not specifically designed my anchor bolts to resist shear at the bottom of the column base. Whenever I have high shear forces at my column base, I would design with shear lugs.

Now, my question is do I specifically need to check my anchor bolts for shearing forces? Can you provide any references to state that this specifically required. And what is the best way to do that.

Also, what sort of pryout checks do I need to do. Thank you so much.

 

[Lion06]

The hairpin won't help with side face blowout. App. D explicitly states that you must check the anchor rods for shear in the steel. You just multiply the shear area times the futa and apply the required phi factor. The pryout check is also included in App. D.

Also, you should check
For shear: breakout, pryout, and shear in the steel.

For tension: breakout, pull-out, tension in the steel, and side face blowout.

Rebar does not benefit: pryout, pull-out, or side-face blowout.

 

[kslee1000]

The founction of the hair pin, or "tension tie", is simple to "connect" (that's all it does) the pedestal to the floor that enable the utilization of the floor weight to resist the lateral load applied on the pedestal through the column anchorage system. It has nothing to do with design of the anchors.

 

[kslee1000]

One good source to learn: "PCA Design Notes on ACI 318".

 

[BAretired]

StructuralEIT,

I am not familiar with the terms "pull-out" and "pry-out". Our code does not use that terminology.

I assume pull-out refers to an anchorage pulling out a cone shaped chunk of concrete. This type of failure is covered by our code.

Is pry-out different than pull-out? I don't understand the difference between the two.

BA

 

[Lion06]

kslee-
The hairpin DOES allow you to neglect the shear breakout. At least in ACI, it does. Well, it doesn't allow you to neglect it, you just use the capacity of the rebar instead of the shear breakout capacity.

BA-
Pullout is a bearing failure of the head of the anchorage against the concrete.
Pryout is harder to explain. If you have short stiff anchors they can rotate as a rigid body popping out a cone of concrete on the back side of the anchor (the side at which the shear force is approaching). The best way to visualize it is imagine you have a #9 bar embedded 2" with 4' sticking out of the concrete. Push on the #9 at 4' above the concrete and the cone that pops out on the back side is a result of pryout.

 

[BAretired]

StrEIT,

Got it. Thanks.

BA

 

[kslee1000]

EIT:

Now, my question is do I specifically need to check my anchor bolts for shearing forces

Above was the question I referring to, sorry didn't make it clear in my response. Also, I think you understand well, the primary function of hair pin is to "TIE" the concrete elements with preventing pry out as a side benefit only. The element directly responsible for preventing such failure is the stairrups.

 

[kslee1000]

Further note:

I don't know current code requirement. However, at all practical level, I have never considered hair pin and stirrups in anchorage design, since their's effectiveness is highly questionable under such circumstances (Pry out, break out).

 

[Lion06]

kslee-
Yes, you must check the actual anchors for shear.
Regardless of the primary function of the hairpin, it does help SIGNIFICANTLY with shear breakout. The stirrups could help with shear breakout, but may not, because they won't be developed back into the slab.
In either case, rebar does not help with the pryout failure mode at all.
I don't believe that the effectiveness of a hairpin is "highly questionable". It is now explicitly recognized in ACI 318-08 to be used in place of the shear breakout failure mode.

 

[kslee1000]

pob:

Attached is graphical description of anchorage failure modes (p. 1 & 2). I also made a hand sketch to help understanding (p.3).

Unless the code has changed again, the pryout would occur only the anchor embed length is too short, and bolts are placed relatively far apart.

Please note, bolt acting in a group behaves quite differently from the single bolt installation. However, the latter was the main base for the development of design criteria, which is quite conservative.

As the basic, anchorage design will involve the following cals:
1. Shear on anchor bolt.
2. Anchor pull out by tension.
3. Hair pin to transfer the load.
As a beginner, you may want to play with the rest cases (break out, pryout???) to have a feel when they will become critical. Ample edge distance and bolt spacing always help.
By the way, ignore the contribution from embed steel for above cals, but the case of pull out. For which case the transfer of tension to pedestal dowels is justified, but you will need to ensure there is adequate development length of dowel both above & below the failure cone.

 

[pob11646]

kslee1000,
Thank you so much for providing that post and taking the effort to provide that sketch. It is surely appreciated and helps a lot in the discussion. BTW, ACI 318-08 also presents the same drawings as "Failure modes for anchors" in Fig. RD.4.1 on Page 415.

BAretired,
ACI 318-08 provides for the pullout strength check in Section D.5.3.3 on Page 427. Pullout strength in tension of a single headed stud or headed bolt, Np, shall not exceed:

Np = 8 * Abrg * fc'

It is a function of only the bearing area (Abrg) of the acnhor bolt and the compressive strength of the concrete.

Thank you for the posts from everyone to help clarify this topic.

 

[Lion06]

kslee-

I think you may be confusing some failure modes. Rebar will NOT help with pullout. Also, you should ALWAYS check all of the failure modes. Pryout doesn't control often, but breakouts certainly can, and do.

 

[kslee1000]

pob:

Appreciation is well accepted, also thanks for the latest code information that confirms the original concept stand as it was.

I was only taking upon this opportunity to let people not familiar with ACI to know the issue under discussion, so potentially to hear more positive comments/feed-backs.

 

[kslee1000]

StrEIT:

Your energy could be better used to find more case study on real world failures that were attributed to the "break out", "pry out" phenomenon. Thus, you can affirm your understanding, and help others as well.