What is balanced, under reinforced and over reinforced section?

We learn about what is balanced, under reinforced and over the reinforced section with differences between them with real-time design application

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 differences between them with real-time design application. Briefly write the terms ‘balanced’, ‘over reinforced’ and ‘under reinforced’ sections in bending.

Balanced, under reinforced, and over reinforced section 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 over reinforcement concrete beam section because of sudden failure

    balanced, under reinforced and over reinforced section
    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 

    Neutral axis transfer to 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

    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 prefer 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

    Balanced Section

    Under Reinforced Section

    Over Reinforced Section

    Steel and Concrete both are fully stressed

    Steel is fully stressed

    Concrete is fully stressed

     

    n = nc

    Neutral axis lies above the critical neutral axis

    n < nc

    Neutral axis lies below the critical neutral axis

    n > nc

    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

    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

    un economical

    Economical

    un economical

    The moment is balanced at the natural axis

    The moment of resistance is less than balanced section

    The moment of resistance is more than balanced section

    Sudden failure

     

    Safe failure

    Ductile failure

    Sudden failure

    Brittle failure - crushing failure

    Not Preferred by designers

    Preferred by designers

    Not Preferred by designers


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