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josh68 (Civil/Environmental) (OP)
23 Jan 09 16:45
Hi, first off I'm kind of a noob at this whole estimating concrete business. Well im looking at this project and I need some help interpreting what a line in our specifications means, and also how to apply it. The line refers to the structure were going to build regardless of what the structure is the line states
"structure...will be constructed of 4,000 psi reinforced concrete with a maximum reinforcement ratio of .25 pb."
Thats all it gives me. There are no details of any rebar called out on the drawings, nor any other info about reinforcement. I would like to understand what the pb means and how to incorporate it into our project. If anyone can help that would be great!
 

Geaux Tigers!

JAE (Structural)
23 Jan 09 18:47
pb is really ρb which is a reinforcement ratio.

The actual reinforcement ratio ρ = As / (bd)
where As is the area of tension steel reinforcing
b is the beam width
d is the depth from compression side of the beam to the centroid of the reinforcing.

ρb is the balanced steel ratio.

If you have a reinforced beam with As steel in it and you start applying greater and greater bending moment to it, at some point one of two failures can occur.
1.  The compressed side of the beam will fail in concrete crushing.
2.  The tension side of the beam will fail by the steel yielding.

So if you have too much rebar on the tension side, the concrete will crush first.

If you have too little reinforcing on the tension side the rebar will yield first.

The rebar yielding is what is desired as it offers advance warnings of overload while the concrete crushing is sudden and catastrophic.

The balanced ratio ρb is when you have just the right amount of rebar so the concrete AND the steel fail at the same bending moment.

So the codes (used to) limit the maximum amount of reinforcing to 75% of the balanced ratio to ensure that there would never be that catastrophic failure.

0.25ρb doesn't make any sense to me other than to offer a suggested amount of rebar based upon a size of beam.  If you know the b and the d of a beam, then you still need to determine ρb and I can't do that for you here in this forum.

 
civilperson (Structural)
23 Jan 09 18:53
Yes as stated by JAE and please no duplicate posts.
kslee1000 (Civil/Environmental)
24 Jan 09 19:21
Balanced reinforcing ratio is a function of fc', fy and beta1 (I don't know how to use greek here). The equation is listed in Chaper 8 (8.4.3) of ACI 318. For fc'=4 ksi, fy=60 ksi, beta1=0.85, the balanced reinforcing ratio is 0.0285 (2.85%). For estimating purpose, assuming a concrete cover of 2", knowing b & h, you can get very good feel of reinforcing required. I think 25% of balanced steel covers the majority of medium to light duty structures' needs.

JAE: Excellent response.
dik (Structural)
26 Jan 09 9:40
It's an odd way to call it up...

Is there any compression reinforcing?  This can have an impact on your answer.

Dik
josh68 (Civil/Environmental) (OP)
30 Jan 09 9:39
thanks for your help guys!

Geaux Tigers!

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