Splice at Beam Stirrup? 2

[zlindauer]

I am working on a project with an elevated concrete beam. The beam is fully loaded from above and will not experience any torsion. The stirrups in the beam were fabricated a few inches too tall. The contractor has asked to cut the stirrups just below the top to turn them into U-shapes. He would like to pair these cut stirrups together to create a closed loop. This would cover a half of the beam's length, and he would order correctly sized stirrups for the other half of the beam. The length of the vertical leg is long enough to meet the requirements of a Class B splice. This project is in Texas.

Can anyone think of a reason that this should not be allowed?

Thanks.

 

[BAretired]

Could you place the stirrups on an angle of 45 degrees or less? They would be more effective than vertical stirrups. Make sure you slope them in the correct direction, i.e.the top should be closer to the nearest support.

BA

 

[zlindauer]

I'd rather not suggest that to the contractor because I don't trust them to slope the stirrups in the right direction, and I am not out there to see every rebar before the concrete is cast. There is a third party inspector doing the day to day inspections. My fear is that if I let them do something like that here, they might find themselves with a similar problem down the road and misapply this solution. If those stirrups get sloped the wrong way, bad things could happen.

 

[DST148]

Technically it is acceptable. Assuming symmetrically loaded beam, place 2 x U-shaped stirrups in the middle half of the span and correct size stirrups at the ends of the beam. Alternatively, the odd sized stirrups may be used in some other location.

 

[hokie66]

The use of U-bars as stirrups is poor practice due to the difficulty of obtaining adequate anchorage beyond any potential crack. Assuming you are using ACI 318-2008, refer to paragraph 11.4.4 and the commentary therto. That paragraph refers to 12.13, where further guidance is given, including conditions where use of U stirrups is acceptable. Personally, I wouldn't allow it if these stirrups are working hard.

 

[dhengr]

I agree with BA that those stirrups could be used quite effectively if they are sloped as he suggests. In fact that is really the most efficient way to install stirrups, more stirrups cross any potential shear crack (diagonal tension lines), except they are kinda cumbersome for the iron workers to install correctly. Show them a sketch of what you want and tell them to get it done, or else remake the stirrups. I agree with Hokie that U shaped stirrups have potential anchorage problems. And, if you let them start cutting these, however you would do that, and installing them to be able to use them, you will have a real mess on your hands with field cutting and bending, etc. Talk about putting bad ideas in their heads for the future... Ah, we’ll just cut em and then they’ll fit, remember the last job? And, there might be only a few inches of lap and/or no development if they cut them. And, fire the contractor and inspector if they can’t do this right. What the heck are you paying the inspector for?

 

[nac521]

See ACI 318-11 section 12.13.5. -Requirements for lapping of double U-stirrups to form closed stirrups.

 

[DST148]

I fully agree with BAretired's suggestion of sloping the over sized stirrups. However, the OPer does not want to suggest that alternative to the contractor.
I have ACI 318-05. Section 12.13.5 gives requirements for splicing of pairs of U-stirrups to form a close unit. The commentary R12.13.5 - These requirements for lapping of double U-stirrups to form closed stirrups control over the provisions of section 12.15 - splicing of deformed bars.
The OP clearly says that the overlaps for U-stirrups meet Class B splice requirements. Therefore, pairs of U-stirrups may be acceptable within the middle-half of the span, assuming they are not expected to work very hard. A sketch showing splicing requirements and limitations on the use of that sketch can be issued.

 

[zlindauer]

Thank you very much to everyone who gave input on this. I learned a couple of things here. The stirrups in this beam don't have to do much work, so I am comfortable allowing the paired u-stirrups.

 

[asixth]

Thanks BA for making the point regarding inclined stirrups. I am just reading thru the Australian concrete code and it increases the steel contribution to shear strength by 10-40% when inclined at 45 degrees.

Makes me think back to an argument I had on-site recently with a steel fixer when I picked him up for not having the ligs vertical.

 

[slickdeals]

Key is to having the links sloping in the right direction though, as pointed out by BA.

It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN

 

[BAretired]

Stirrups are most effective when they are parallel to diagonal tension stress (or perpendicular to diagonal tension cracking).

BA

 

[TXStructural]

(Sorry for missing this - was in the IBC hearings the last few days.)

I would never allow such an adjustment, based on the actual function of the stirrups. Except in large members, the overlap of the u-stirrups will not develop sufficient strength for shear demand. (ACI 318 12.3.5 describes when that can be used.) The biggest problem with it is that behind only concrete cover, failure will be brittle once the capacity of the cover to restrain the lap spliced stirrup is exceeded. Stirrups must remain ductile to provide the intended warning in the event of shear failure, and a detail which has unconfined, lapped U-stirrups is a dangerous practice. (I will post a link to high speed video of some cover tests that were just completed, once the university approves them to be posted.)

The supplier should be permitted to either provide correct stirrups, or modify the stirrups in the fabrication facility. It sounds like they could be cut and bent to conventional open stirrups with 90 or 135 hooks. If the fix has to be done in the field (seldom the case), make absolutely sure that any field bending conforms to CRSI/ACI bend diameters for stirrups.