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Hairpin & Floor Mesh Reinforcing

Hairpin & Floor Mesh Reinforcing

Hairpin & Floor Mesh Reinforcing

(OP)
I've been designing PEMB (Pre-Engineered Metal Building) foundations with hairpins for 20 years.

I've always specified either #4 bar at 12" o.c. or less or 6x6-#6 or #10 WWM.

The contractor informed me that the owner and sub-contractor want to use #4 at 24" o.c.

The numbers in my calcs work!

BUT... is there any concern for having the reinforcing at greater than 12" o.c.?

Thank you!

RE: Hairpin & Floor Mesh Reinforcing

For non structural typically use 5X slab thickness (used to be a code requirement) or for structural things 3X the slab thickness... How thick is your slab... I would not use #4@24..., #4@12 is quite reasonable.

A bit of a dilemma; if owner wants @24, I would send him a note that you would recommend against it and that your spacing is more appropriate. If you use your spacing make sure the sawcutting is done at the correct timing and that the contractor is aware that timing is critical. If you use your spacing and delay sawcutting, you will likely end up with unwanted cracks, the very thing you warned the client about...

Dik

RE: Hairpin & Floor Mesh Reinforcing

Quote:

The contractor informed me that the owner and sub-contractor want to use #4 at 24" o.c.

The numbers in my calcs work!

BUT... is there any concern for having the reinforcing at greater than 12" o.c.?

I would say: no. Hairpins are built around "engaging" a certain width of slab (with enough reinforcement to handle the force from the hairpin). If the slab is properly reinforced.....there should be no issue.

RE: Hairpin & Floor Mesh Reinforcing

You can make the hairpins longer to engage more slab reinforcing.

Dik

RE: Hairpin & Floor Mesh Reinforcing

The main concern is that, with less reinforcement, the cracks in the slab will be wider.

RE: Hairpin & Floor Mesh Reinforcing

Yup... that's why I wouldn't use it.

Dik

RE: Hairpin & Floor Mesh Reinforcing

I wouldn't sweat it if the numbers work. Around my locale, there's a school of thought that SOG rebar should be 16-18" spaced so that you can step through it kind of safely. Terrible things do not seem to be happening as a result. And who doesn't like rebar over WWM pretty much anytime?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Hairpin & Floor Mesh Reinforcing

You could point to ACI 318 where it limits the spacing of bars designed to take structural stress to 18” max. We use 18” typically in slabs for the same reason KootK mentioned. Just don’t show anyone where ACI 318 says that it doesn’t apply to slabs on grade

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