One Piece vs. Two Piece Stirrups

[abusementpark]

Structurally, is there any significant difference between one piece and two piece stirrups in a concrete beam?

I read recently that two piece stirrups are preferred because they are better for constructibility, which makes senses to me. What do you guys normally specify?

 

[Qshake]

abusementpark -

For me it depends on what sort of work is being performed and the work space provided. When doing new work I just go with single or double stirrups in one shape 13. When doing retrofit work it's often better to use the two piece shape 10 with overlapping legs.

I'm sure there are pros and cons to both when using new construction. For example, I can see a case made for fewer pieces on the jobsite and if the beam cages are assembled and dropped into the forms this is probably best. Otherwise there is case to be made for better fit with the two pieces and some may say better quality. But that also means someone has to make sure the legs are tied together and not a gap between them etc.

Just my $0.02.

Regards,
Qshake

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[JAE]

You might clarify a bit what you mean by two-piece stirrups.

There are two-piece stirrups that are composed of double "U" shapes such that they make four legs. Is that what you mean?

Or are you talking about a typical "U" shaped stirrup with a separate top "cap" over top bars that forms a complete rectangular stirrup leg all around?

 

[abusementpark]

Or are you talking about a typical "U" shaped stirrup with a separate top "cap" over top bars that forms a complete rectangular stirrup leg all around?

This.

 

[JLNJ]

If you need closed stirrups and you want to go easy on the guys in the field, then specifiy the two-piece type with the "cap". Be careful that the 90 degree hooked and is confined on the slab side.

 

[Lion06]

We show closed stirrups on our drawings. We've been seeing a lot of shops come in lately with the two-piece stirrups that you mentioned. There's no difference, so we just let them go.

 

[hokie66]

Two piece stirrups are commonly used for ease of construction. But if a continuous stirrup is required, e.g. for torsion, you have to be careful with development.

 

[JAE]

JLNJ and hokie66 point out the main design concern - that of torsion resistance. If you use a two piece, then the lower "U" stirrup should have 135 degree hooks at the top, not 90 degree hooks. The cap then would typically have two 135 degree hooks for a beam with no slab attached. If you have a slab attached on one side, then the hook in the cap at the slab side can be 90 degrees. If slab on both sides, then both ends of the cap can be 90 degrees.

 

[PUEngineer]

I'd like to point out that there are different requirements for frame beams in a special moment frame (ACI 318, Ch. 21) and possibly even an intermediate moment frame. I know that wasn't part of the original question, but thought I would add that. The basic difference is that the legs of the bottom 'U' have to have 135 degree hooks at the top. The top cap shall be the typical 'candy cane' bar but have the 135 degree hook alternated along the length of the frame beam where there is slab on both sides. Or as noted above by JAE if there is slab on only one side.

 

[doka1]

As a contractor I usually request the detailer to show 2 piece stirrup when the ends of the bottom and top bars are not the same. Usually that happens in a continuous beam over 3 supports. It becomes very hard to weave these bars in closed stirrups especially with hooked ends and with the column bars projecting up.
Another note is that it actually recommends it in the crsi manual

 

[abusementpark]

If you use a two piece, then the lower "U" stirrup should have 135 degree hooks at the top, not 90 degree hooks. The cap then would typically have two 135 degree hooks for a beam with no slab attached. If you have a slab attached on one side, then the hook in the cap at the slab side can be 90 degrees. If slab on both sides, then both ends of the cap can be 90 degrees.

This is in ACI 318?

 

[csd72]

As some have indicated, if this frame is in a seismic zone then the implications of the stirrups being required for confinement need to be considered.

In these cases if the cover concrete spalls then 90 degree hooks or laps can become innefective.

 

[JAE]

abusementpark, Not in ACI 318 but recommended by ACI in their detailing manual.

 

[doka1]

just to be clear, crsi illustrates that the cap piece must be a 90 degree on one side and 135 on the other. The "U", main or bottom piece (whatever you want to call it) has to have to have both ends with 135 degree bends

 

[spats]

I don't have an ACI Detailing Manual, but how would you install a cap piece with 180 degree hooks at both ends? Also, I think it's customary to alternate the 180 degree hooked ends on the cap.

 

[hokie66]

135 degrees was discussed. If done this way, you place the top bars somewhere inside the cage, then lift them into the hooks. In spandrel beams, you don't want a 90 degree hook on the outside corner.

 

[spats]

Oops... I meant 135 degrees!

 

[rscassar]

When the beam requires closed ties then I detail one piece stirrups such that the cage can be built and lifted into place. I try not to use two piece u stirrups unless 1.3 times the development length can be used to lap the bars along the vertical leg. This detail increases the total steel tonnage because of the increased number of laps.

 

[hokie66]

For spandrel beams, the ACI Commentary to paragraph 7.13 says that two U-shaped stirrups are not allowed. That is a better reason not to do it than saving steel. It does allow the cap piece as discussed above.