Double Rebar Matt?

[BSVBD]

I do not do a lot of concrete design other than foundations, footings and occasional beams in foundation walls.

On past projects, whenever i've designed above grade concrete walls, regardless of whether the wall needed it or not, we always put a double rebar matt similar to (2) layers or matts of #4 bar at 12" oc E.W.

On elevators and bank vaults we've always done it this way, with an assumed understanding that this was code for such applications.

Other than elevator or bank vault applications, is there a reason for having a double matt of rebar in above grade concrete walls even when the stress for a single layer is well below 50%?

The majority of concrete, cantilevered retaining or basement walls only have ONE layer of rebar.

Why two?

Thank you!

 

[Lomarandil]

ACI 318 14.3.4 requires two layers for most walls thicker than 10"

I don't know if that applies in your case, or if there are other code requirements that also steer you toward two layers.

 

[JAE]

Well, if the wall is bending in both directions - (either face could be in tension) you'd want two mats.

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[hokie66]

Thin walls, less than 8" thick, typically have one layer. This is typical in tilt up construction.

 

[KootK]

On elevators and bank vaults we've always done it this way, with an assumed understanding that this was code for such applications.

I always go with one centered layer in walls < 10" unless loads demand a double layer. I also know of no such requirement for elevator walls.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

For greater crack control requirements 2 layers may be recommended/required to get the rebar closer to the surface. For example, AASHTO requires 2 layers of regularly distributed reinforcement for temp. & shrinkage in bridge structures unless 6 inches or less (essentially at the point that 2 layers becomes unfeasible). I believe ACI 350 has a similar requirement.

But for most typical buildings I would agree with the above. One layer (unless required for strength reasons) at t < 10", at t > 10" use 2 layers.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[rapt]

There were also lots of problems with walls with one layer in the Christchurch earthquake. Lots of spalling and resulting failures!