calculating rho prime for long-term deflections

[Lion06]

This might sound like a silly question, but I have to ask. The question is which b to use when calc'ing rho prime in order to get the long term multiplier for long term concrete deflections.
Let's say you have a concrete T-beam with a flange width of 120" and a stem width of 36". You will get a significantly different rho prime depending which b you use for As/(bd).
I have always used the flange width, but have never had such a big disparity before and needed so much compression steel to cut down the long term deflections so this is the first time I've thought this much about it.

 

[WillisV]

I would go with flange width as well. b it typicaly defined as the width of the compression face, which will primarily be the flange of the T in this case.

 

[miecz]

I have to think that if you used 120" to compute short term deflections, then you need to use 120" for [ρ]'. However, if using b=36" for short and long term deflections results in a lower long term deflection, then I'd use 36" for both. Are you considering the longitudinal slab steel in your As'?

 

[JAE]

[ρ]' is based on bd where b is the compression flange and this is taken at the midspan (by definitions from ACI 318).

So you'd use the 120" and the area of compression steel at midspan.