Reinforcement percentage 1

[bjoyt1]

Thanks for the reply. What I mean is what should be the safe range of percentage of reinforcement in a double reinforced rcc beam. and the disadvantage of over reinforcemtn & under reinforcement.

 

[KootK]

In a beam with only tension reinforcement, the percentage is simply the area of steel reinforcing divided by the area of concrete in the section. If a reinforced concrete beam is over-reinforced then the concrete compression zone may crush before the reinforcing steel yields. That is a sudden, brittle failure that does not provide much warning to structure occupants and is undesirable for that reason.

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.

 

[PicoStruc]

Same as Kootk, but i will add that in Canada for example, it is simply not allowed to have over-reinforced beam, by code !

 

[TXStructural]

IBC and ACI allow over-reinforced sections, but penalize the design by decreasing allowable loads to a level where one would not expect failure. This is done with a change in phi factor. But when it does fail, it can be sudden and catastrophic.
Doubly-reinforced sections are limited by practical issues of what will fit and what can be constructed. You could put 8% in both the top and bottom of a beam. Practically, this would be nearly impossible to build due to conflicts at supports and congestion issues. Even if the bar fits, getting concrete into excessively-reinforced members and consolidating it is problematic. We recommend not more than 4% in top or bottom. As an excessively simple generalization, most singly-reinforced sections use around 2% for optimal economy. That said, very few sections are ever built fully singly-reinforced due to practical construction considerations.