RCC beam can be classified as balanced, under-reinforced, and over-reinforced sections based on the percentage of reinforcement it has. In this post, I have discussed the balanced, under-reinforced, and over-reinforced beam sections.
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Balanced Beam Section
Reinforced concrete beam (RCC) sections in which the tension steel also reaches yield strain at the same time as the concrete reaches the failure strain in bending, are called balanced beam sections.
Under-Reinforced Beam Section
Reinforced concrete (RCC) beam sections in which the steel reaches yield strain at loads lower than the load at which the concrete reaches failure strain are known as under-reinforced sections.
Every singly reinforced beam should be designed as under-reinforced sections because this section gives an adequate warning before failure occur. Yielding of steel in the under-reinforced beam section does not mean the structure has failed, as when steel yields, excessive deflection and cracking in the beam will appear before failure which provides enough time for building occupants to escape before the section fails.
The failure in the under-reinforced beam section is because o the concrete reaches its ultimate failure strain of 0.0035 before the steel reaches its failure strain which is much greater than 0.20 to 0.25.
Over-Reinforced Beam Sections
Reinforced concrete (RCC) beam sections in which the failure strain in concrete is reached earlier than the yield strain of steel is reached, are known as over-reinforced beam sections.
If the over-reinforced beam is designed and loaded to full capacity then the steel in the tension zone will not yield much before the concrete reaches its ultimate strain of 0.0035. This is because of the small yielding of steel the deflection and cracking of the beam do not occur and do not give enough warning before failure.
Failures in over-reinforced sections immediately. This kind of design is not recommended in real practice for the design of the RCC beam.