Select the correct statement with respect to an over-reinforced cement concrete section under the working stress method.

Select the correct statement with respect to an over-reinforced cement concrete section under the working stress method.

Right Answer is:

The allowable limiting stress is reached in concrete before the steel stress reaches the limiting value.

SOLUTION

Balanced (WSM)’ section constants:- In the working stress method, the ‘balanced’ section is one in which both tensile steel stress fst and maximum compressive stress fc simultaneously reach their allowable limits σst, and σabc, respectively under service loads.

Under-Reinforced (WSM)’ Sections:- According to the traditional WSM terminology, a given section is said to be ‘under-reinforced (WSM)’ if its area of tension steel is less than that corresponding to balanced conditions so that the tensile stress in steel reaches the allowable limit before the maximum compressive stress in concrete reaches its allowable limit, and the allowable moment capacity is limited by the stress in steel, and not by the stress in concrete.

Over-Reinforced (WSM)’ Sections:– According to the traditional WSM terminology, a given section is said to be ‘over-reinforced (WSM)’ if its area of tension steel is more than 4, ;, so that allowable limiting stress is reached in the concrete before the steel stress reaches the limiting value, and the allowable moment capacity is limited by the stress in concrete, and not by the stress in steel.

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 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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