Balanced Section vs Under Reinforced Section vs Over Reinforced Section
In this article, we learn about what is balanced, under reinforced, and over the reinforced section with difference between under reinforced over reinforced and balanced section with real-time design application. Briefly write the terms ‘balanced’, ‘over reinforced’, and ‘under reinforced’ sections in bending.
Balanced Under and Over Reinforced Sections is a type of section and it depends on the percentage used for concrete and steel in that beam section
Balanced Section
A balanced section is that in which stress in steel and stress in concrete achieve their permissible limit value at the same time.
The stress diagram of the balanced section is shown in Fig. (b).
The percentage of steel related to the balanced section is known as balanced steel and the neutral axis is called a critical neutral axis (NC).
This design is not suitable for real life as an over reinforcement concrete beam section because of sudden failure.
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| balanced, under reinforced, and over reinforced section |
Under Reinforced Beam Section
Under Reinforced Beam Section is the section in which the percentage of steel provided is less than balanced section
Neutral axis transfer to upward as shown in Fig (c)
In this Under Reinforced Section steel is fully utilized and reaches its permissible limit but at the same time concrete is not fully utilized
Under reinforced section is preferred as a safe section as well as economical by designers because of ductile failure. “In ductile failure first steel fully utilized and then fail on other hand concrete has its own strength so sufficient warning sign is provided in this section before collapse or failure of the section”
This section is economical because steel is fully utilized and then concrete plays a role in the stability of the structure. It means that both section materials are used as a safe limit
Each and every singly reinforced beam section should be designed as per under reinforcement beam section because of safe design which provides a warning before failure
NOTE: failure of steel or fully utilized of steel in under reinforced beam section done not means that section is failed because concrete is not utilized so that we received the sign of failure like cracking and deflection so that we have sufficient time to escape before building failure
Over Reinforced Beam Section
Over Reinforced Beam Section is the section in which the percentage of steel provided is higher than the balanced section.
The neutral axis transfers downward as shown in Fig (d).
In this Over Reinforced Section concrete is fully utilized and reaches its permissible limit but at the same time steel is not fully utilized.
This section is uneconomical because concrete is fully utilized and concrete is a brittle material so it fails suddenly at the same time steel is not fully utilized but only steel is not sufficient to stand alone and make section stable so section collapse without steel utilized
Why Over Reinforced Sections are not preferred?
Over reinforced sections is uneconomical because concrete is fully utilised and concrete is a brittle material so it fails suddenly at the same time steel is not fully utilised but only steel is not sufficient to stand alone and make section stable so section collapse without steel utilised
When Doubly Reinforced Sections are adopted?
Doubly Reinforced Sections are adopted when: Over reinforced sections fail immediately because steel is not sufficient to carry design load alone and first of all, steel is not able to stand without concrete support, so we do not have time or not received sign before failure, so it is very dangerous in the real-time application of RCC beam
Doubly reinforced sections are adopted when: Under reinforced design always preferred by a designer due to the Ductile failure nature
You can use ORC whenever needed like the depth of the beam is restricted
It’s doesn’t mean that you never create ORC (over reinforced section) but for the safety proposed we highly used URC (under reinforced section)
Different Between Balanced Section vs Under Reinforced Section vs Over Reinforced Section
Difference between balanced under reinforced and over reinforced section are given below in detail by civil experience. Don't forget to join us on Whatsapp, Facebook, Linkedin… for the latest civil engineering updates.
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Balanced Section |
Under Reinforced Section |
Over Reinforced Section |
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Steel and Concrete both are fully stressed |
Steel is fully stressed |
Concrete is fully stressed |
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n = nc |
Neutral axis lies above the critical neutral axis n < nc |
Neutral axis lies below the critical neutral axis n > nc |
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Xu = Xu max Xu= (0.87*fy*Ast)/(0.36*fck*b) |
Xu < Xu max (this row only for if you familiar with Xu) |
Xu > Xu max |
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Percentage of steel axis lies on the natural axis |
Percentage of steel is less than balanced section |
Percentage of steel is more than balanced section |
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un economical |
Economical |
un economical |
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The moment is balanced at the natural axis |
The moment of resistance is less than balanced section |
The moment of resistance is more than a balanced section |
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Sudden failure |
Safe failure Ductile failure |
Sudden failure Brittle failure – crushing failure |
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Not Preferred by designers |
Preferred by designers |
Not Preferred by designers |
Civil experience article on balanced reinforcement is great and in depth.
Your way of explanation is more relate me to this information.
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