Is a col tie or beam stirrup smaller than a #3 bar possible

[samdamon]

Gentlemen-

Anyone ever used anything smaller than a number three bar to tie columns or provide stirrups for shear reinforcement of a beam, in cast-in-place concrete construction? Is this feasible?.

 

[JAE]

ACI section 7.10.5.1 states (for column ties):
"All nonprestressed bars shall be enclosed by lateral ties, at least No. 3 in size for longitudinal bars No. 10 or smaller, and at least No. 4 in size for No. 11, No. 14, No. 18, and bundled longitudinal bars. Deformed wire or welded wire fabric of equivalent area shall be permitted."

It appears that you can substitute WWF or wires - providing the same area but it would probably require a lot more labor if you needed cross ties throughout the column section.

 

[sven]

In Australia our codes allow the use of ligatures down to 6mm dia in columns, to restrain small bars (max 20mm I think).
For beams, I have used 6mm ligs in small, narrow and lightly loaded beams without problems. The main problem with small dia bars is the lack of section area to resist shear loads. Codes also restrict their use for restraining longituduial bars in compression, but I believe that this is more of an "accepted practice" requirement than a structural one.

 

[samdamon]

Gents- Thanks for your replies.

At the columns, I want to use the ties more for holding the vertical steel in place during concrete placement, than for retsraining the bars during service. The columns I am talking about are lightly loaded and have less than 1% area of steel, so from a code perspective, ties are not required. However, I am concerned that without some restraint the bars will move as the forms are filled. I have never specified a tie smaller than a number three before, but in this case there is very little clearance for 3/8" of tie on each side of the cage. Also don't know what type of wire would be appropriate to specify. JAE, agree with you that using wwf would be cumbersome.

At the beams, I AM interested in having the steel stirrups add to the shear strength of the section. The area of shear steel required is not high, and using a #3 stirrup will be overkill. Clearances are also a concern at these beams.

Sven, do you specify the yield strength of the ligatures? Is there an ASTM number or other code reference that you use to specify the wire?

 

[curvbridger]

If column is lightly loaded, why can't you reduce the size of your cage and use #3's?

Also, if all you want is vertical steel stability, just detail your #3 tie to meet the interior dimension of the main bars!

curvbridger

 

[Qshake]

What about the possibility of using sprial ties. This is very common in the US, especially in seismic zones. One of the benefits of spirals is the way that they are shipped, erected and set in place. Since the spiral comes to the job in a single length it is packaged for shipping much like a slinky and fits well over the main vertical bars via crane and pulled up from the bottom and tied as it is pulled. Really slick application and out-performs the standard ties with regard to confinement.

 

[sven]

SamDamon,
Here we use "hard drawn wire" ties, grade 500MPa steel. Spiral ties are a good solution, if you can get them in a suitably rectangular shape. To restrain your steel during concreting, I would recommend maximum spacing 300mm for 6mm dia ties.