Concrete T beam with flange in tension

[JVon1]

Good day,

I have a question about designing T-beams with the flange in tension using the ACI code,
The details of the problem are as follows:
f'c = 3 ksi, fy = 60 ksi
flange width = 24", flange thickness = 6"
web width = 12" and overall depth = 30"
Rebar detail: 6 #3 bars at 2" and 6 #3 @ 28" (from the compression face)

The problems I'm having are: the cracking moment, Mcr is greater than the nominal moment capacity, Mn...which does not seem to make sense (any advice?)
And, as a result of the depth of the neutral axis, all bars are in tension. Therefore, should I determine the centroid of the tensile rebar "d", using force equilibrium...in order to compute As_min (Eqn 9.6.1.2, ACI318-14)? or can I use "d" as 28"?

 

[KootK]

The conditions that you've encountered are possible with lightly reinforced beams. Double check your numbers and then:

1) If it's new construction, add some bars.
2) If it's existing construction, work out capacity based on strain compatibility.

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.

 

[rapt]

It does make sense as the flange is affecting the cracking moment but not the ultimate moment capacity where it is considered to be cracked to the neutral axis and therefore the tension face flange does not affect the results.

I would base my minimum reinforcement on the reinforcement at the tension face only and use d = 28". The reinforcement at the compression face would not be at yield, it would be at very low stress.

And following on from the other thread on concrete strength, 3000psi is low for a significant structural member!

 

[JVon1]

Thank you very much for your assistance, gentlemen.