Balanced reinforced typically refers to a reinforced concrete section, such as a beam, that is specifically designed or behaves as a balanced section.
Understanding a Balanced Section
According to the provided reference, a balanced section is defined as:
"The section of a reinforced beam where most distant concrete fibre in compression and tension in steel attains permissible stresses simultaneously, is called balanced section."
This type of section is also known as a critical section.
In simpler terms, in a balanced reinforced section:
- The concrete at its furthest point from the neutral axis on the compression side reaches its maximum allowable stress (permissible stress) at the exact same time that
- The steel reinforcement on the tension side reaches its maximum allowable stress (permissible stress).
This simultaneous yielding means that both materials are utilized to their full permissible capacity under the design load.
Significance of a Balanced Section
Designing a reinforced concrete element to be a balanced section is significant in structural engineering for several reasons:
- Optimal Material Usage: It theoretically represents an ideal scenario where neither the concrete nor the steel is underutilized when the section reaches its permissible stress limit.
- Predictable Failure Mode: A balanced section is designed to fail in a specific, predictable manner, typically involving simultaneous crushing of concrete and yielding of steel.
- Comparison Baseline: Balanced sections serve as a critical baseline for classifying other types of reinforced concrete sections:
- Under-reinforced sections: The steel reaches its permissible stress before the concrete. This is generally preferred as it leads to a ductile failure with visible cracking and deflection, providing warning signs before collapse.
- Over-reinforced sections: The concrete reaches its permissible stress before the steel. This is generally avoided as it leads to a brittle failure where the concrete crushes suddenly without significant warning.
Practical Considerations
While a perfectly balanced section is an ideal theoretical condition, designing for slightly under-reinforced sections is often preferred in practice to ensure ductile failure. However, understanding the balanced condition is fundamental to determining the limits for under-reinforced and over-reinforced designs.
Knowing the properties of a balanced section allows engineers to calculate:
- The maximum amount of steel reinforcement that can be used before the section becomes over-reinforced.
- The minimum amount of steel reinforcement required to prevent brittle failure.
Therefore, "balanced reinforced" is not just about the presence of reinforcement, but about the proportion of steel reinforcement relative to the concrete section such that it achieves the "balanced section" criteria under load.
