What is a "hairpin"? 1

[SoiloftheMonth]

I have been seeing "hairpin reinforcement bar" and "hairpin foundation" in the area of metal building and their foundations. Appears to me to be a horizontal bar that ties a slab on grade to a concrete foundation, heard this is the "cheap" way to do foundations for metal building systems. Anyone share insight on what a hairpin is in reference to and insights on it?

 

[JAE]

You are correct that hairpin refers to a horizontal bar in a "V" shape, or "U" shape that wraps around the column anchor bolt assembly and extends inward into the building inside the slab-on-grade.

Some advantages:
1. Takes the horizontal outward thrust from the column reaction and delivers it into the interior slab.
2. Can lap with slab reinforcement to tie in to the slab's on-grade friction as well as the opposite columns opposing reaction.
3. Eliminates lateral pushover forces on the footing below - reducing the footing size.
4. Simple to construct

Some disadvantages
1. Relies on the slab-on-grade competence for structural stability.
2. If the slab is ever removed for any reason (plumbing, repair, etc.) the structure can become unstable.
3. May allow some initial movement or rotation of the column pedestal until the reinforcement is fully engaged.
4. Requires the slab to be placed prior to the structure erection - staging issues.




Check out Eng-Tips Forum's Policies here:
faq731-376

 

[Ron247]

Another disadvantage is you cannot use in a fiber reinforced slab unless you are tying it to some type of actual steel reinforcement.

They tend to have maximum magnitude of outward thrust they can resist. It is good for buildings probably 80' or less in clear span in low roof load areas. Bigger than 80' they are more difficult to get to work.

 

[CANPRO]

I have two disadvantages to add:

- If the slab settles relative to the foundation wall, it will pull on the hairpin and creates additional stress in the bar
- There is a gap between the edge of slab and foundation wall and the hairpin is prone to corrosion at this location

 

[JAE]

CANPRO - we typically provide a length of foam, or copper pipe, around the bar for the first 12" or so in the slab to allow a more gradual transition should there be settlement in the slab.
Granted, this will not necessarily eliminate the potential for slab cracking since the bar may eventually bear on the inside of the resulting concrete void, but it certainly minimizes it.

I agree with the corrosion concern.

Check out Eng-Tips Forum's Policies here:
faq731-376