2013 MSJC Strength design minimum shear reinforcement requirement.

[volney68]

Section 9.3.4.2.3 c) of the 2013 MSJC Code (ACI 530-13) states "The minimum area of transverse reinforcement shall be 0.0007(b)(dv)." What "area of transverse reinforcement" is this section referring to? For a beam scenario, would that be the total transverse reinforcement area throughout the span?

Thanks

 

[hokie66]

Minimum area of shear reinforcement normally refers to the sum of the areas of legs of stirrups crossing a given section.

 

[volney68]

So the question becomes "How is the section defined?" The transverse shear reinforcement is vertical and does not cross the section defined by (b)×(dv).

 

[MotorCity]

Area of shear reinforcement is perpendicular to direction of shear force (unless you use skewed stirrups)

 

[hokie66]

volney,
You said it was a beam, which I assume is horizontal. So a section through the beam is vertical, like the stirrups. Correct?

 

[volney68]

Yes hokie, that is correct.

 

[hokie66]

So then do you still have a question?

 

[volney68]

Section 9.3.4.2.3 c) states that the transverse reinforcement (stirrups) must be at least 0.0007 times the beam cross-sectional area. But the stirrups are oriented vertically parallel to be beam cross section. If Section 9.3.4.2.3 c) were referring to longitudinal reinforcement, it would make sense as a minimum rho requirement. But since it refers to reinforcement that doesn't even intercept the section used in the equation, it's meaning is (at least to me) ambiguous. So the question is "The required minimum stirrup area is per what length of beam?"

 

[hokie66]

I am not familiar with that ACI 530 code, so probably should not be commenting. But to me, that provision simply says you need to provide Av of at least that amount, which is not much. So if you have a single stirrup, the sum of the two legs must add up to the Av required. If two stirrups, 4 legs, etc. But the spacing of the stirrups has to meet sll other requirements, and that is where your length factor comes in.

To quote ACI318-2008, "Av is the area of shear reinforcement within spacing s". But the requirement you are concerned about in effect specifies a minimum size of the stirrups.

 

[volney68]

After much searching I finally found instruction that the intent of 9.3.4.2.3 c) is to specify a minimum area of shear reinforcement per segment of beam equal in length to the beam's depth. Thanks for the responses everyone.

 

[hokie66]

That explanation seems illogical to me. Where did you find it?

 

[volney68]

http://ef.engr.utk.edu/ce576-2014-01/notes/Reinforced-Masonry.pdf

Slide #33.

 

[KootK]

It checks out. The reinforcing is keeping an assumed 45 degree shear crack closed at a particular location. Only the reinforcing along the horizontal projection of that crack (dv) is effective.

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.