Hairpins in the place of beam stirrups

[efFeb]

Hi,
A contractor has asked to replace beam stirrups at several locations with hairpins (135deg hook each end).
I don't love this idea, but can't seem to find any reason to say no.
I am wondering if there is any reason or code clause that would go against this change?
Thanks!

 

[KootK]

I'm cool with it so long as you don't need stirrup torsional capacity.

My only real concern is that it might make it tougher to get all of the stirrups plumb and accurately places. For that, however, I think it's appropriate to just trust the contractor and then verify a bit of it on site.

 

[dik]

I used these for concrete beams for a project outside of Ottawa and was queried... no reason not to. Reinforcing of the beams was a single bar top and bot.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[rapt]

Any beam where you ignore "compatibility" torsion still requires minimum torsion reinforcement, so at least the outside stirrups need to be continuous around the perimeter.

I would also have the outside stirrup continuous for construction reasons to keep the reinforcement in place during pouring.

Internal legs I have no problem.

 

[TLHS]

What are we talking about exactly by hairpins? Are we talking about open stirrups where you've got a u bar with hooks at the top? Are we talking single leg shear reinforcement / stirrups where you've just got a vertical leg of rebar with a hook at the top and the bottom? Or are you using a bobbypin style hairpin vee that has some sort of hook on it?

Also, where are you? Detailing requirements for shear reinforcement actually varies a reasonable amount based on the country and code. Not sure I remember all that much of it, but I did a pretty deep dive a couple of years back trying to retrofit something.

 

[Tomfh]

Any beam where you ignore "compatibility" torsion still requires minimum torsion reinforcement, so at least the outside stirrups need to be continuous around the perimeter.

I would also have the outside stirrup continuous for construction reasons to keep the reinforcement in place during pouring

We see slabs all the time without any u-bar closer on the perimeter beams, and very rarely a continuous tie - at least not without a fight. So many post tension designers seem perfectly happy to ignore the rule.

 

[hokie66]

Many post-tensioned structure designers ignore a lot of rules. Sort of like a lot of precast designers.

I agree with rapt. Spandrel beams need closed stirrups.

 

[rapt]

Tomfh,

That approach cost a builder of a multi level car park structure in Gosford in NSW as lot of money in the 1970's, and a very good very experienced PT designing Consulting Engineer ended his career because of it after agreeing to design it as was "common" practice in the industry. It was never our common practice, but for some PT companies it was (and still is) and he was dealing with one of those in that case.

Amazing what the combined effects of flexure, shear, torsion and temperature differential can do to a basically unreinforced (in shear and torsion) spandrel beam!

Every edge beam or band beam should have closed ties.

 

[Settingsun]

Whilst I use closed ties to account for compatibility torsion, one example 50 years ago sounds like either bad luck or the expected failure rate arising from the reliability index that we adopt.

 

[rapt]

Steveh49,

Just because I can only quote 1 case I know very well does not mean there are not more.

AS3600 specifically says you must provide minimum torsion reinforcement in cases where compatibility torsion has been ignored in design. That is basically in every edge beam and to a lesser extent in nearly every internal beam.

As torsion reinforcement requires closed ties, then the code requires them in all edge beams.

 

[dik]

The ones I did were all interior... no torsion. It was one of the easiest floors I ever designed... 3'x3' columns on an 18'x18' grid with 3'x4' deep beams on the column lines and 1'x4' beams @ 3' on centre... single bar T&B and single leg ties for the length. The floor framing was for an ultra stiff microprocessor lab. Loading was light... The department head wondered about the single bars and the AHJ also queried...no issue. I don't recall, but I think the columns were 18' high...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[slickdeals]

I have never not done a closed stirrup on the outside of any beam. In lieu of a single piece closed stirrup for a unconfined beam (no slabs), I have allowed for a two-piece stirrup consisting of a U bar with 135 degree hooks and a cap tie with 135 degree hooks on each side.

 

[dik]

I like to, to accommodate torsion, but the top dowel effectively provides a closed stirrup. I often see beams with closed stirrups used 'all the time'. It makes it difficult to place reinforcing.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik