Explain under, over and balanced section w.r.t Limit state method of RC design.

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teamques10 ★ 68k

Balanced Section or Critical Section: When the ratio of steel to concrete in a section is such that the strain in steel and strain in concrete reach their maximum values simultaneously the section is referred to as a balanced or critical section and the percentage of steel in this section is known as critical steel percentage.
Under-reinforced section: A section having steel percentage less than the critical percentage is known as under-reinforced section. Since steel is insufficient to balance compression in concrete, the tensile strain in steel reaches yield value while the maximum compressive strain in concrete is less than its ultimate crushing value. The section undergoes large rotational deformations from the initial stage of yielding of steel to the final stage of crushing of concrete, giving sufficient warning of impending failure.
Over-reinforced Section: A section having percentage of tension steel greater than the critical percentage is known as over-reinforced section. Since the amount of steel is more than that in balanced section, the neutral axis moves below the balanced neutral axis to satisfy the equilibrium condition resulting in the concrete reaching its ultimate strain while the strain in steel remains less than its yield value. This type of failure is usually avoided in ultimate strength design.

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