Shear reinforcement - U-stirrups 1

[sdz]

Is there any problem with providing shear reinf by having double U stirrups lapping on the sides. I can't find anything specific in AS3600 but did find something in ACI 318M 12.13.5.

 

[apsix]

Cl 8.2.12.4(d); "fitment cogs shall not be used when the anchorage of the fitment is solely in the concrete cover of the beam."

This rules out double U's due to the location of the anchorage.

Presumably ACI 318M specifically rules out double U's?

 

[hokie66]

I don't read Cl 8.2.12.4 as prohibiting double U's. They are not commonly used because the required lap length can govern the beam depth.

I don't like double U's because the top U is in practice often not well tied to the other bars, meaning that it can easily be dislodged during placement, causing a cover problem.

I never have understood subparagragh (d), as anchorage can never be in the concrete cover, as concrete cover is outside the reinforcement. But whatever it means, this is for cogs, not laps.

 

[hokie66]

After posting, I looked at the Commentary, which says that lapping shear fitments "should be avoided". Don't know why they didn't just say they the practice is prohibited.

 

[apsix]

I agree it doesn't seem to be directly applicable to double U's. I had the advantage of learning it at a seminar introducing AS3600:2001.

I believe it is still OK to use open topped ligs where the ends are hooked around the top corner bars. The top can then be closed with a U-bar.

 

[csd72]

Check that 90 degree hooks are okay, I think this is the reason why they dont work.

I wouldnt use then anyhow as it is a poor detail.

I would avoid these if at all possible with an open top lig hooks as per apsix's comment (unless there is torsion/seismic, in which case you need fully closed ties).

csd

 

[apsix]

By hook I meant a 180 deg hook.
By Cl.13.1.2.5 a hook has at least a 135 deg bend. Any less and they are called cogs.

 

[sdz]

Thanks all, but especially hokie66. I checked the commentary and it says "Lapping fitments in the manner illustrated in Figure C8.2.12.4 should be avoided."

ACI 318M does allow lapping in this fashion but requires lap lengths be increased by 30%.