Here, positive and negative reinforcement directly relates to positive and negative bending in the member. So wherever we anticipate postive bending or sagging in a member, we provide postive reinforcement and wherever we anticipate negative bending or hogging, we provide negative reinforcement. Most of the times these two may be the same bar.
What is positive and negative reinforcement in reinforced concrete? What is positive and negative reinforcement in reinforced concrete?
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Positive and negative reinforcement in reinforced concrete:
positive reinforcement: Whenever the reinforcement is provided in the reinforced concrete structural element to resist the positive bending moment then and that type of reinforcement is called positive reinforcement.
Negative reinforcement: When the reinforcement is provided to resist the negative bending moment in the RCC element then that reinforcement is called as negative reinforcement.
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A beam is a horizontal member and it takes load by bending itself only. So you know very well the beam will either sag(like a valley) or hog(summit forms ). In both the cases upper fibres of beam undergoes compression(in sagging) or tension(in hogging) and lower fibres goes the same in reverse order.
As we know that concrete is very good in compression but is a brittle material hence weak in tension. So to overcome this weakness steel is provided in it. Steel provides the beam flexibility it requires for bending.
Sagging is also called positive bending and hogging is called negative bending.
If sagging occurs- it means beam is undergoing positive bending moment so the reinforcement provided for this moment resistance will be termed as Positive Steel.
If hogging occurs- it means beam is undergoing negative bending moment so the reinforcement provided for this moment resistance will be termed as Negative Steel and this steel is always provided in tension zone. or part undergoing hogging moment.
There may be a case in which beam is undergoing both positive and negative moment, in that case there exist a point of contraflexure, to negate the effect of this point Negative Steel is provided. Like practically almost every concrete joint is either fixed or continous so definitely there will be a contraflexure point near the supports. Hence the type of reinforcement provided is termed as Negative Steel.
Main Reinforcement always provide to resist tension in the member.
The concept of +ve and -ve reinforcement is come when we talk about beam or beam like other structure.
In beam, part where moment is sagging(generally, at near mid point)reinforcement provided in lower side that is called positive reinforcement. If moment is hogging(generally, at near support), then reinforcement provided at upper side of beam, it is called as negative reinforcement.