Using Hairpins to Take "Kickout" force

[jmaddox]

I am planning on using full length #5 bars as hairpins to take the kickout force from the dead/collateral load in a premanufactured steel building. The idea is that the force will be transmitted to the slab and thus reduces the req'd size of my ftg. It is my belief that since I am using the slab to transmit load from the structure to the slab, the slab design is governed by ACI 318 (22.1.1.2). Because of this I am limited to the maximum reinforcement spacing (18") and minimum reinforcing ratios set by 7.12.2. (temp/shrinkage reinf.)
The reason that I am asking this is becasue the contractor wants to use #4's @ 24"oc in a 6" slab which violates both spacing and temp./shrinkage req's. Does anyone happen to know spacing/min ratio exceptions for this condition?

Thank you in advance for any thoughts on this issue.

 

[hokie66]

I think the reason that ACI318 does not address this is that ground supported slabs are not considered to be structural, and are not considered appropriate for loading by direct tension forces in this manner. Hairpins in ground slabs are a creature of the PEMB industry. Most of us who design our own steel structures use other means of resisting the thrust, e.g. tension ties under the slab, or moment resisting footings. Using a slab on ground for resisting tension relies on slip resistance between the slab and subgrade, and on continuity of the tension reinforcement. In industrial environments, this can rarely be depended on, as future modifications may result in compromise of the load path.

I am certain you will get other opinions in defence of the use of hairpins.

 

[haynewp]

For thrust loads over about 10 kips I start looking at using a tie-beam from one side of the building to the other instead of hairpins. I believe the tension tie requires mechanical splices per ACI. I would not consider the rest of the adjacent slab to be taking the tension and needing special reinforcing over what the slab would normally require, only the designed tie-beam. Hairpins are used a lot but they are not very desirable in my opinion for a couple of reasons.

 

[jmaddox]

Thank you for the responses guys. I know that hairpins are looked down on and are susceptible to the owner cutting holes in the slab and possibly cutting a hairpin. However, more to my point was does the slab need to be designed per aci 318? Reinf. spacing reqs, ect.

 

[Gumpmaster]

I would say yes, if you're using it for the overall structural stability. It's no longer just a slab on grade but a structural slab.

I agree with haynewp. If it's greater than 10 kips go for a tie beam. Either way, look at your overall elongation. If you have 50 kips over 100 feet, your tie reinforcing may stretch a couple of inches if based only on strength.

 

[Archie264]

>>> Hairpins are used a lot but they are not very desirable in my opinion for a couple of reasons<<<

haynewp, care to elaborate on that? I'm not disagreeing, I'm just curious what those (other) reasons are.

jmaddox, it seems to me that if the concrete is being used as a tension element, or a part thereof, that ACI318 would have to become the governing code and the element designed accordingly. In other words, it now becomes a "structural" element (for lack of a better description) and is no longer just a slab on grade. That's just my take on it. On the other hand you might be able to resolve the thrust with a continuous rod that wraps around bolts of opposing columns, thus removing the concrete from the equation and returning the slab to it's origninal status of being a slab on grade.

 

[haynewp]

WWR is often pulled up at the same time people are standing on it to get it into the "correct" elevation. The slab is cut at a control joint not usually far away which may cut the WWR if not placed correctly. This affects the thrust transfer ability in the slab. WWR is supposed to be lapped at joints but correct joint laps are hard to maintain when people are standing on the WWR and pulling it up at the same time. Friction under the slab is not great when the vapor retarder is placed directly under the slab as is often done in my area.

Using a rod continuous across the building and wrapping it around the anchors would be a tie beam when encased in concrete.