Open Stirrups in Pretensioned Girders 1

[JohnRwals]

Hi!

Sometimes I have seen open stirrups in the midspan of precast concrete girders.
They may be better for precaster's production efficiency.
But, I wonder this design is good for flexural strength.

Can anybody explain how open stirrups can be accepted?
Is this design method popular and accepted in the cast-in-place concrete beam design?
(Some details are omitted in my sketch.)

Thanks!

JRW

 

[dik]

less costly? Are they installed after the stressing strand is in place (before tensioning)... if so, then much easier to fabricate.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r13]

It looks like a typical practice for prestressed concrete girders. Note that shear reinforcement usually is not required in the midspan region, in which shear demand is low. You can check PCI design manual, see what it says. Also the upside down open stirrups is used when top face subjects to tension.

 

[jayrod12]

Is it possible they're only for compression reinforcing confinement instead of shear capacity?

 

[KootK]

Is this design method popular and accepted in the cast-in-place concrete beam design?

Open stirrups are popular with precasters for the reason that you clearly suspect: fabrication process efficiency. It's much easier to place the reinforcement with the top open. Sometimes precaster's even orient the hooks along the longitudinal axis of the beam which is sketchier still.

All that said, I never see the open stirrups oriented as you have them shown. The hooks are usually at the top. Some issues with the arrangement that you've shown:

1) In general, it's nice to have the hooks in the flexural compression zone whereas you'll likely have them in the tension zone where anchorage is weaker.

2) For shear stirrup development, you want the longitudinal bars placed inside the hook knuckle, not outside of it.

3) An important function of these bars in an IT beam will be to act as the hanger reinforcement for the ledge. Again, your arrangement is a poor choice for that function.

I'm lucky in that the precaster that I design for actually prefers closed stirrups everywhere for holding things in place robustly.

 

[r13]

Note depends on the design, the invert U stirrups can be seen as shear reinforcement, but the bottom hooks shall be anchored by longitudinal bars, and I prefer to see 135° hook leg.