Flexural Steel in Beam ACI 318-19(22) 9.6.1.3

[jStadts]

I've got a 'beam' section that is 22" wide by 118" deep and 114" long. Factored moment on the beam is 188.61 kip-ft so I'm getting from analysis .32 in2 of steel required. The minimum steel from ACI 318 comes to 8.45in2. Is this where section 9.6.1.3 comes into play? Saying that if As is 1/3 above As required by analysis then you don't need to follow 9.6.1.2 and 9.6.1.1?

Just seems wild to me that the code calls for over 8in2 of reinforcement but only need .44in2. I just wanted to make sure I'm understanding the code correctly.

 

[Celt83]

your beam is technically a deep beam, refer to Section 9.9 in ACI 318-19.

 

[jStadts]

your beam is technically a deep beam, refer to Section 9.9 in ACI 318-19.

9.9.3.2 still refers to 9.6.1 for min area of flexural tension reinforcement unless I'm mistaken.

 

[Celt83]

good call, believe you are correct. I'd probably double check against any requirements for the tie reinforcement but barring anything there then I do think you could take advantage of 9.6.1.3.

The intent of 9.6.1 is to have enough reinforcement in the section so that Mn exceeds the cracking moment (Mcr) for a more ductile failure mode.

 

[cliff234]

That's a deep beam. Follow the deep beam provisions in 318-22. You are not only going to have to provide much more flexural steel in the bottom, you are also going need to provide horizontal and vertical bars each face for the full depth of the beam and you're going have to make sure that your detail at each end of the span develops the bottom tension tie bars within the nodal zones. While design of deep beams is fairly simple, you need to spend time and make sure you've properly detail the reinforcing steel in the member.

 

[jStadts]

That's a deep beam. Follow the deep beam provisions in 318-22. You are not only going to have to provide much more flexural steel in the bottom, you are also going need to provide horizontal and vertical bars each face for the full depth of the beam and you're going have to make sure that your detail at each end of the span develops the bottom tension tie bars within the nodal zones. While design of deep beams is fairly simple, you need to spend time and make sure you've properly detail the reinforcing steel in the member.

I understand from the deep beam section in ACI to add reinforcing to the faces which I have already. My question was more about the flexural steel which leads to section 9.6.1.3 which was my original confusion. Rather than needing two layers of 6 #8 bars or something like that I can get away with way less since the steel required from the moment is way less than that.

 

[rapt]

It is a very illogical concession. If you do the calculations by strain compatibility and check the strain in the reinforcement against the breaking strain, you will find that reducing the minimum with this rule will often result in the actual steel strain being significantly higher than breaking strain.

Unfortunately codes for some reason do not require this check, but is is illogical not to.