In this article, we try to explain the reason behind less economy of doubly reinforced concrete beam section as compared to singly RC beam which is mostly not suitable in construction.
So please read the article till the end and if you get any value from this article please share with your friends.
Major Disadvantages of Doubly Reinforced Concrete Beam
In design of RCC beam, when the moment on beam is greater than limiting moment of beam. In this case beam is designed as doubly RC beam.
Mu > Mulim (Mulim = limiting moment of resistant)
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While the extra moment on beam (after subtracting limiting moment from actual moment) will increase the bottom reinforcements quantity in section of the beam to resist extra moment.
Which is more than limiting area of bottom steel.
Actual bottom Ast > Limiting bottom Ast
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But in our practice, we try to make balanced section and provide ast as per the balanced section.
So that extra tension reinforcements (Actual Ast – Limiting Ast) increase the tension force in beam and make section unbalanced.
To make beam section balanced, we try to Increase area of steel in the compression zone of beam.
And this compression reinforcements become worthless. We only provide compression reinforcements in case of negative bending moment.
And negative bending moment mostly occurs at near the support. So in this case we provide extra top bars in compression zone.
But in Doubly RC beam, we provide compression reinforcements only for make balanced section.
Means we provide extra reinforcements over the span of beam which not take any type of positive moment.
Now let’s consider, the cost of extra reinforcements that is much higher as compared to concrete. And also make beam Costly as compared to singly RC beam.
And this is the reason behind less economical doubly reinforced concrete beam
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When we used Doubly reinforced concrete beam in construction?
In general practice of design, we always try to design beam as a single reinforced concrete beam.
But in the case of the moment is greater than the limiting moment and size of beams such as depth and width is given by architecture.
That means we are not able to increase the size of the beam. In this situation, you can design beam as Doubly reinforced concrete beam.
Otherwise provide extra bottom bars in beam to resist an extra moment.
So this is the simple concept behind the doubly reinforced concrete beam.
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