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.

 

[BAretired]

Does ACI provide information on the tensile capacity of smooth bars with threads at top and a hook at the bottom?

We had a tornado in our area in 1987 and I examined quite a number of industrial buildings which had fully or partially collapsed. Many of the anchor bolts had pulled out of the concrete, leaving a perfectly clean hole the same size as the anchor rod. The steel had simply straightened out and 'snaked' out of the concrete.

It seems clear that a hook on a smooth anchor rod does not fully develop the bar in tension, yet the practice continues.

BA

 

[kslee1000]

BA:

I think manufactures such as HILTI would have the information you are looking for. Try "

http://www.hilti.com".

For hook, I believe it was "banned" by the code officials during 90s after intensive studies & testing. The reason was exactly you have observed - the hook tends to straighten up when subjected to substantial tension (pull out), thus causing failures.

 

[pob11646]

Per ACI 318-08 D.5.3.5 - The pullout strength in tension of a single hooked bolt, Np [shall not exceed]:

Np = 0.9 * f'c * eh * da, where

3 da <= eh <= 4.5 da ( <= means "less than, or equals to")

eh = distance from the inner surface of the shaft of a J- or L-bolt to the outer tip of the J- or L-bolt, Unit: inch

da = outside diameter of anchor or shaft diameter of headed stud, headed bolt, or hooked bolt Unit: inch

 

[marinaman]

This sounds like a project where a pre-engineered building is being placed upon a slab (foundation). I typically try to stay away from the use of hairpins into the slab. I have investigated many projects where this was done, and over time, this method greatly increased the cracking of the slab-on-grade.

If you are designing a foundation/slab for a pre-engineered building, I strongly suggest you read "Metal Building Systems" by Alexander Newman. You can buy it from ASCE. It goes thru the foundation design for a building such as this very nicely.

 

[pob11646]

marinaman:
You hit the nail right on the head!

Yeah, the hair pin ties are used for a foundation designed for a pre-engineered metal building (PEMB), or a metal building system (MBS), as it is called nowadays. Are PEMB's or MBS's where hair pin ties are mainly used? I have not really used them before in my other "more conventional" foundations?

Why is this so? Is it because of the restriction on braced bays in the interior of the columns, large spacing between columns that makes hair pin ties necessary? There are no doubt significant outward thrusts at the exterior columns, and the hair pin ties surely will help with the concrete breakout due to shear. Hope you can shed some more light into this.

 

[tolchijb]

The reason we shy away from hairpins is that you never know if someone is going to cut the slab to install some new piece of equipment or something. If they cut the hairpin, your design is no longer adequate.

 

[BAretired]

I have seen the tie placed immediately below the slab and encased in concrete. This gets away from the tensile stresses going into the slab, but it has the same disadvantage that tolchijb mentioned. The tie is essential to the building. To cut through it is not an option.

BA

 

[hokie66]

pob,

Hairpins in the slab are used for the same reason that PEMB are used: cheap and dirty solution. There have been a lot of discussions about this on this forum, and I have argued as marinaman has here, but now am tired of the discussion. People get what they pay for.