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When the depth of the actual neutral axis in a beam is greater than the depth of the critical neutral axis, then the beam is called:

When the depth of the actual neutral axis in a beam is greater than the depth of the critical neutral axis, then the beam is called: Correct Answer over-reinforced section

Explanation:

​Over-reinforced section:

  • The depth of the actual neutral axis (Xu) is greater than the depth of the critical neutral axis (Xulim).
  • The stress in concrete reaches its permissible value before steel.
  • Failure is also sudden in over-reinforced sections because concrete is brittle in nature and in over-reinforced sections concrete fails prior to steel.
  • In the over-reinforced section, the steels are not yielded fully, so the over-reinforced section is considered uneconomical.


Comparison Between under-reinforced section, balanced section, and over-reinforced section:

Under reinforced

Balanced section

Over reinforced

Xu < Xu(lim)

Xu = Xu(lim)

Xu > Xu(lim)

Ast < Ast(lim)

Ast = Ast(lim)

Ast > Ast(lim)

MOR < MORlim

MOR = MORlim

MOR > MORlim

Z > Zlim

Z = Zlim

Z < Zlim

Steel yields first then concrete reaches a maximum strain of 0.0035

Both steel yields and concrete reaches a maximum strain of 0.0035 simultaneously

Concrete reaches a maximum strain of 0.0035 before steel yields

The final failure is due to cracking of concrete (when strain is 0.0035)

Ductile failure (Gradual / Warning is issued)

 

Brittle failure (Sudden / No warning is issued)

Secondary compression failure

 

Primary compression failure

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