Open stirrups in the prestressed precast concrete girder

[KimWT]

Hi!

This 40ft long girder was designed to support 60ft long precast double tees at both sides in the parking garage.
Recently, I saw some outside engineers use open stirrups in the girder design;
personally, I have never seen this reinforcement design before.

I am curious what other engineers think about this reinforcement design.

Thanks!

 

[KootK]

1) I've never seen it before, except for the reinforcing in the nibs.

2) Gives the heebeejeebees on a gut feel level.

3) Technically, you would seem to have your shear and torsion capacities still.

4) I'd still like to see the stirrup legs hooked around some rebar.

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.

 

[jayrod12]

I've seen it before but flipped the other direction with the hooks bent around the top reinforcing. It was on an older drawing set from the 40's or 50's.

I've also seen ICF grade beams reinforced with masonry style single legged stirrups on each face. In a non-seismic zone.

 

[TehMightyEngineer]

I'd think that this would technically work and may even be allowed by the code but agree with KootK on the heebee-jeebees.

I'll check me PCI Handbook when I'm back in the office to see what guidance they have on inverted tee girders supporting double tees. I've never seen much reason from the typical PCI reinforcement details, I always figured they had it optimized so why reinvent the wheel?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[rapt]

Definitely not recommended. Open stirrups in the compression zone are sometimes allowed but not a good idea. In the tension zone, never.

 

[TehMightyEngineer]

Well, you probably get some confinement from the compression strut off of the double tees but relying on that gives me even more heebee-jeebees.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[KimWT]

I was told this reinforcement design is not rare among precast engineers.
Any opinion from precast specialty engineers?

 

[PMR06]

If the bent legs beyond the lower corners provide ldh to develop a standard hook, then could the upper core be confined? Seems like you'd be hitting the horizontal tie double time for its function as a corbel too though?

I can definitely see why the precaster would do this though. They can just drop the ties down over the tendons and mild reinforcing. It would be cumbersome to string those through closed ties otherwise.

 

[TehMightyEngineer]

Any opinion from precast specialty engineers?

Yo! Precaster here, but full disclosure is I've never had to design a inverted tee yet in practice.

Precasters for some reason hate hoops and stirrups, likely because it's more time consuming for the shop guys to bend and form up so straight bars are always being asked of me. I try to accommodate them where possible.

But for this ACI 318-14 25.7.1.2 and 25.7.1.3 is pretty clear. Per the commentary, straight deformed bar and wire anchorage is not permitted. The only exception is "joists". I highly doubt that you can call a inverted tee a "joist".

I did a pretty extensive google search and all the research and details I can find point to closed hoops or traditional stirrups.

The PCI Design Handbook is a little vague on this but does show similar details to yours in a number of places.

That said, I interpret the un-hooked bars they show as being intended as horizontal shear reinforcement for a composite topping. PCI also notes that the hanger reinforcement for the ledge is in addition to the shear reinforcement for the beam. Are they perhaps mixing closed stirrups or hoops in with those straight anchored bars in your girder?

The PCI handbook example and equations for calculating reinforcement for ledge attachment to the web isn't quite clear on this. There's a factor γ[sub]t[/sub] that states: "If closed stirrups are provided in the ledge, γ[sub]t[/sub] may conservatively be taken as 1.0. If closed stirrups are not provided in the ledge, γ[sub]t[/sub] may be taken as zero." However, they don't indicate if the closed stirrups they're referring to are the ledge reinforcement for torsion in the ledge, or the ledge attachment to the web (the bars your detail shows as not hooked around long. rebar).

Regardless, ACI controls and seems clear to me on what they require.

If anyone has a copy of these papers (I don't) it might shed some light on this:
[URL unfurl="true"]http://ascelibrary.org/doi/abs/10.1061/(ASCE)0733-9445(1983)109%3A8(1836)[/url]
[URL unfurl="true"]https://www.concrete.org/publications/internationalconcreteabstractsportal/m/details/id/10850[/url]

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[KimWT]

In the perspective of production efficiency, U shape rebars are not different from closed stirrups for experienced production crew.
When missing stirrups sometimes need to be added after everything is assembled, U shape rebars should be used.
But, this is exception...

I am asking if 'inverted' U-shaped rebars can be used instead of closed stirrups as main stirrups.
(In fact, this engineer used closed stirrups 3ft from both ends and open stirrups elsewhere.)

Thanks!

 

[TehMightyEngineer]

I am asking if 'inverted' U-shaped rebars can be used instead of closed stirrups as main stirrups.

I'm confused as that's what PMR06 and I were discussing. What rebar did you think I was referring to?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[TehMightyEngineer]

Oh, just found this. Really interesting stuff, gonna take me a bit to read through it all though: [URL unfurl="true"]https://www.lrrb.org/PDF/201436.pdf[/url]

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[MotorCity]

Depending on what SDC you're in, you may be required to use closed stirrups. Not sure how precasters would get around this.

 

[KootK]

Precast newb question here: are these things cast right side up or upside down?

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.

 

[TehMightyEngineer]

KootK: There would be advantages to casting them either way (and you could do it either way) but I believe most precasters have their forms setup to cast them right side up.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[jayrod12]

would that be to avoid having to spin it on its axis prior to erection?

I would just think from a concrete consolidation and rebar layout scheme it may be easier to cast it upside down.

 

[TehMightyEngineer]

Jayrod: Yeah, that and the bars extending from the piece. Hence there being advantages of pouring it either way.

Flipping wouldn't be that hard (we do it with much larger pieces all the time) but whenever we've broken a piece in the shop it's always during flipping so avoiding it does have benefits.

Given that precast is often poured with SCC, the consolidation under the shelf of the form wouldn't be that bad. The biggest issue pouring it upside down is now you have the horizontal shear rebar extending out the bottom of the form (I can hear our shop foreman complaining in my head already). I would suspect this is the biggest reason they don't cast them upside down.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[jayrod12]

Right,

I totally overlooked the rebar sticking out of the top of the stem. That makes perfect sense then.

 

[KimWT]

"Anchorage of Shear Reinforcement in Prestressed Concrete Bridge Girders-by MnDOT"
Though I read only Executive Summary, it gave me insight to some extent.
Thanks, TheMightyEngineer!

This detail is related with torsion, shear and flexural strength design,
while this paper is mainly interested in shear capacity if bridge I-girder due to the straight rebar with no hook.
This paper refers to ACI318 commentary:
"A longitudinal bar within a stirrup hook limits the width of any flexural cracks,
which acts to debond the stirrups from the concrete."
...
So, how will open stirrups affect this girder design?
Can I accept this transverse reinforcement design?

 

[TehMightyEngineer]

So, how will open stirrups affect this girder design?
Can I accept this transverse reinforcement design?

Torsional forces (which are definitely present in inverted tees) will likely need a closed stirrup. Plus, the note about seismic from MotorCity. I agree with your quote from ACI that flexural cracks will tend to debond the stirrups and you really need a longitudinal bar for it to anchor to and to restrain those cracks.

Overall I would not accept it; or I would at least ask them how they're getting around the requirement in ACI 318 regarding anchorage of transverse reinforcement. A prestressed bridge girder is not going to see anywhere near the torsional forces that a inverted tee will so while it's encouraging to see that MnDOT has proven it can work I don't think their research is 100% applicable to this application.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

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