Minimum Number of Stirrup Legs in Wide Beams 1

[MasterB]

REQUIRED: I am looking for a written publication or a consensus on this issue.

As an example take a 48" wide concrete beam. The shear in the beam requires the use of only 2 shear legs. Given a large number of top bars the top horizontal leg of the stirrup will want to sag due to the weight of the bars. How many bars vertical stirrup legs should be placed in this beam to prevent the sag?

Say for a beam width of 30" to 48" 3 legs would be required. Or for a beam width of 48" to 60" 4 legs would be required.

There is a graphic in CRSI 2002 (page 12-4)close to what I am looking for. However, this is not directed at my question.

Any and all help would be greatly appreciated.

 

[UcfSE]

That sounds like "means and methods" to me, not usually something we address as engineers unless you were hired or paid to do so. Either way, if you want more legs, put in more legs. Using a larger bar size for the stirrup would help with that also.

 

[jike]

I know of no written guide but, I would probably use a minimum of 2 legs for up to 30" width and 4 legs for beams up to 60".

 

[MichSt]

I agree with UcfSE, it’s up to the contractor, detailer, etc. to deal with this type of issue. In bridge applications, two legged stirrups are typically used for shear reinforcement in 36” and 48” box beams. I’ve designed quite a few 48” wide box beams with 2-legged stirrups and a decent amount of top reinforcement and I’ve never seen shop drawings come back for review showing extra stirrup legs.

I don’t have the CRSI pub you mention, and I’m not exactly sure I understand what you’re describing. But, I would think that if the top longitudinal bars were causing the top horizontal leg of the stirrups to sag, the top longitudinal bars could be supported off the formwork to prevent the stirrups from sagging.

 

[MasterB]

I agree that this is a "means and methods" issue and many engineers have the same opinion. However, I feel this will benefit everyone in the long run. If I put this on the fabricator/contractor it will most probably not be paid attention to. Then when I get in the field everything is sagging and we start the run around. Concrete cover is all messed up and I would like to be certain that the placement of the top bars, especially in the tension region, is were they are supposed to be. As a fellow colleague put it "To me, it is the engineer’s responsibility to make sure his design is constructible to his design assumptions."

Any other opinions?

 

[UcfSE]

You can try a tighter stirrup spacing as well.

Whiie I agree that our design should be buildable, I don't think we have to go to the extent you describe necessarily. It's good that you want to though. The easier you make it for them, the easier it will be for you during construction.

How are you modeling the stirrup to figure up it's strength?

 

[rapt]

MasterB

The Australian concrete code specifies and maximum transverse spacing of legs of the lesser of D or 600mm (24"). So minimum 3 legs for your 48" beam.

 

[JAE]

I've designed numerous concrete structures over the years and was taught by my earlier mentors (three guys who started their careers in 1927, 1945, and 1946) that any beam wider than 36" should have doubled stirrups - where you take two twin "U" shaped stirrups and overlap them to produce four, equally spaced vertical stirrup legs. I think the ACI Detailing Book has pictures of this.

This is not necessarily a totally means and method thing. For very wide beams, it think the research is somewhat lacking so the idea here is to follow the standard of practice in all concrete engineering...i.e. - always try to distribute reinforcing evenly in sections and slabs, unless the stress (following stiffness) demands an uneven distribution.

 

[AggieYank]

My office's design methodology is similar to JAE's. When a concrete beam is wider than 24", I usually specify double stirrups (4 legs). If it is lightly loaded, or a short beam, I may use 2 legs.

As you mentioned, only 2 are required, but it is good practice to use 4 legs for wide beams in my opinion.

In the site, maybe all it takes to hold up with middle bars in a wide beam is tying the middle top steel every so often to the slab steel running accross it above. I wouldn't worry about how the guys in the site deal with it, but there are several options for them, and all of them should fall in means and methods.

 

[Dinosaur]

I also consider this a means-and-methods question. If the main bars are not placed withing CRSI tolerances, then you can require the contractor to provide adequate support.

In my career, often when I try and go the extra mile for the contractor by detailing my structure to accommodate construction outside the normal standard of care, he will offer an alternative. His alternative will require less work, diminishes the quality somewhat or increases his compensation.

The most comical of these is in the bridge industry. Deck slabs are detailed in this market with main bars perpendicular to the traffic. Every other row of bars is a bar pair, same size top and bottom, to provide capacity against positive and negative bending. The remaining alternating rows have something called a truss-bar. The truss bar is bent into the top over beams and the bottom between where the moment peaks occur. Contractors are always complaining about the truss bar. They say we could do it cheaper if we did the whole bridge using the straight bars because bending the truss bar is so expensive. On the surface this all sounds reasonable except ... If we replace the truss bar with a bar pair the quantity of deck reinforcing steel (the basis of payment) goes up about 15-20%. Contractor says, "But I'll save more than that in labor." Instead we place a plan note saying the contractor can replace the truss bar with a bar pair at no additional cost to the owner. I have yet to see this happen. Bottom line, contractor is not telling the truth. He wants to be paid more (remember the total bar weight is the basis of payment) for doing less work, since he won't have to have the truss bars fabricated.

I'll get off my soap box now.