what is this "hair pin" thing attached to anchor bolts? 1

[IJR]

I have come across friends suggesting someone use hairpins in conjuction with anchor bolts in anchoring steel frames to foundations.

I have no single idea what this technique and its rationale are

I wont mind a little ancyclopaedic hint.

Thanks,IJR

 

[prex]

I suppose you intend to weld the airpins at intervals onto the outer surface of the bolt.
In this case their goal would to raise the shear that may be transmitted from the bolt to the concrete and finally to make the bolt shorter.
However it isn't a practice familiar to me: the diameter of the bolt does not allow for good inspectable welds and the anchor must be of weldable material (this is normally the case).


prex
motori@xcalcs.com

http://www.xcalcs.com

Online tools for structural design

 

[Taro]

Hairpins are reinforcing bars bent into the shape of the letter "U". The bars are used to anchor inserts, anchors, or post-tensioning tendons into concrete to prevent the concrete from "blowing out" at a free edge. The name hairpin refers to the similarity in appearance and function to the pins used to hold one's hair in place.

 

[Ginger]

Be VERY careful if you are using rebar hairpins structurally. The bending process will work harden the steel. You may get microcracks in your bar which will reduce the steel bar's tensile/shear capacity. In reinforced concrete this is less of a problem as the bent bar is surrounded by concrete, however, in conditions described above there is the work hardening problem and also the likelyhood that you will be pulling on a single point at the top of the "U" bend which will induce other unknown stresses.

Regards Andy Machon

 

[Taro]

Hairpins are used structurally very commonly. The 180 degree bend diameter should conform to ACI 318 Section 7.2 to avoid cracking the bars. Also, the hairpins are not typically welded to the inserts or tendon, they simply enclose and confine them.

 

[MPHenry]

I have come across extensive use of "Hairpins" in the design of foundations for Pre-Engineered Buildings. The structural frames used are usually pin ended portal frames. The horizontal shear loads at the base of the portal frames are transfered to the floor slab of the building using a "hairpin" connection. The "hairpin" consists of a V-shaped length of rebar which is cast into the floor slab and around the anchor bolts of the portal frame columns. The result of this procedure allows one to design the portal frame foundation pad and pedestal for axial load only. All horizontal loading is carried by the hairpin connection. A considerable reduction in foundation size and cost is achieved.

 

[JAE]

Hairpins are most often seen and used in pre-engineered steel buildings where the exterior columns of the steel frame induce significant horizontal forces at the base of the column. The column base plate and anchor bolts transfer this force into the supporting concrete foundation.

However, many times the foundation pedestals and footings cannot be designed economically to resist this lateral force. If the footing is sunk deep for frost, or the base of the column is raised above the surrounding grade the high eccentricity would cause the footing to grow substantially. In these situations, engineers use the hairpin bars to transfer the lateral force back into the interior floor slab, which in this case should be reinforced.

Two problems with doing this: 1. The slab may be altered, cut, or slotted for drains in the future which could compromise the integrity of the system. 2. The slab-on-grade could settle, causing cracking and spalling around the interface between the foundation and the slab. You could wrap the rebar, in the slab adjacent to the foundation, with a foam insulation for about 12 inches of length. This would allow flex in the rebar if movement did occur.

 

[TowerEngineer]

There is more than one definition of the term 'hairpin' used in construction.
The reinforcing steel industry refers to supplemental cross ties as 'hairpins' which I think you are now familiar with based on the foregoing discussion.

There is another form of 'hairpin' used in my industry (guyed and self-supporting tower construction). The term commonly refers to a solid steel bar bent in the shape of a U-bolt and threaded just like a U-bolt. These tensioning devices are generally used for anchoring very large diameter guy wires. It is used for applying pre-stress to guy wires and forms part of the guy wire assembly. It attaches to the anchor steel by means of a shear pin. A good description of this type of 'hairpin' is available in the USSteel Wire Rope Catalog.

 

[rope]

Does nyone have a formula for the sizing of haripins for lateral transfer to slab?

 

[IJR]

rope

Your question interests me too. But new visitors to this thread may miss your question and may end up answering the question at the top. You understand THE TIME THING

Lets wait. If we dont get response, lets start a new thread so that everyone can see your question

Good luck

 

[prblmfxr]

Rope & IJR--

I had the same question a little while back. I got hairpin design information from Butler Manufacturing Co. (Butler Buildings) while we were planning on specifying their product for our project. I'm not sure if I'm at liberty to copy this information and give to you, so I will give you the reference that the information came from: "Foundation Design & Construction Manual", Butler Manufacturing Co., Computerized Structural Design, Inc. Hope this helps.

 

[IJR]

prblmfxr

Is Butler at

http://www.butlermfg.com?

 

[prblmfxr]

IJR- Yes, that is the correct website for Butler.

 

[gita]

IJR:
I hope this is not too late. The "Hairpins" are used to transfer the lateral forces into the slab to prevent the lateral thrust. The hairpins are nothing but the reinforcing bars that are bent at 30 degree with specified lengths and size to resist the horizontal loads. It is not always necessary to use hairpins sometimes the force can be taken care of by adding reinforing bars from one column base to another column base directly across from it.
Gita (Structural)

 

[evelrod]

Having worked for Butler Bldgs. about 40 years ago in Oklahoma, the type of slab construction you are talking about was common. However, here in California, where I have been since '66, footings require a great deal more than hairpins to take lateral loads into the main slab and welding is usually a no no on anchor bolts. Pre-fab buildings are available to anyone with the dough, but the slabs and erection must still meet Callifornia earthquake specs. To the point, hairpins are just re-bar and may on occasion be welded, but not usually to the bolts.

Rod

 

[Stumpy]

You size the hairpins as you would any other rebar in tension. Of course, they are at an angle to the applied force, so you have to resolve the forces to coincide with the orientation of the bar.

Then, you have to make sure the reinforcing within the slab is enough to carry the tension force to the opposite set of hairpins.

Personally, I don't use hairpins. I use bars hooked into the pier, embedded in a concrete strip below the slab, with ties at splices. To me, this is easier to rationalize and anyone digging in the future will notice they've encountered something worth thinking about (hopefully).

JPRiley

 

[mattshak33]

I am designing a foundation for a pre-engineered building and I wondering if anybody could point me to an example using a thickened floor slab as the footing.

 

[IJR]

Dont know exactly what you mean but isnt that going to be a mat foundation?

1-Your minimum depth is the frost line.
2-Your design depth is whatever comes from your analysis of mat foundation

3-If frost line governs then you can do some economy by keeping your slab thickness minimum and provide a thickened slab along the perimeter only.

4-You can do more economy by thickening the slab only under columns as required by analysis and cut down depth between columns. This is not difficult in practice and we would usually cast concrete twice, first for the thickened column areas and then in between columns.

My five cents worth boss

respects
ijr

 

[steve111]

yes on the hairpin thingy to transfer horizontal load from the pinned connected base of the rigid frame into the floor slab???, yes we have used to use this technique with many 'Pre-Engineered Buildings', always had trouble with that term, we could probably start a thread on that alone, "Pre-Engineered vs. Post-Engineered", actually we make a good living out of 'Post-Engineered' buildings, far better than 'Pre-Engineered'. But anyways there are about a million threads on this site with regards to slab on grade failures so draw your own conclusions. best to thicken the slab in the area of the hairpin, when you use them, and depending upon the magnitude of the lateral load, buttress the pilaster at the frame and design the required footing and pilaster (T-Beam) for the moment. Have also tied the columns side to side with renforcing ties that extend across the frame width but again in a thickened slab in that area. Metal Building Manufactures Association had a good book, but i threw it out so i can't reference it for u.