Question

The chances of diagonal tension cracks in R.C.C. members reduce when

• A. Axial compression and shear force act simultaneously
• B. Axial tension and shear force act simultaneously
• C. Only shear force acts
• D. Flexural and shear force acts

Hint:

Axial compression improves the shear capacity of RCC members because it lowers the principal tensile stress responsible for diagonal tension cracks.

Answer 1

The Correct Option is A

Concept: Diagonal tension cracks in reinforced concrete members are mainly caused by principal tensile stresses developed due to shear. Axial compression reduces these principal tensile stresses, thereby improving the shear resistance of the member. The principal tensile stress is given by \[ \sigma_1=\frac{\sigma}{2}+\sqrt{\left(\frac{\sigma}{2}\right)^2+\tau^2}, \] where \[ \sigma=\text{Normal stress}, \qquad \tau=\text{Shear stress}. \] A compressive normal stress decreases the value of the principal tensile stress.

Step 1: Understand the effect of axial compression. When axial compression acts along with shear, \[ \text{Principal tensile stress decreases.} \] As a result, \[ \text{Possibility of diagonal tension cracks reduces.} \]

Step 2: Compare with other cases.
• Axial compression + Shear $\rightarrow$ Reduces cracking.
• Axial tension + Shear $\rightarrow$ Increases cracking.
• Shear alone $\rightarrow$ Higher principal tensile stress.
• Flexure + Shear $\rightarrow$ Does not reduce diagonal tension cracks as effectively as axial compression. Hence, \[ \boxed{\text{Axial compression acting simultaneously with shear reduces diagonal tension cracks.}} \] Therefore, the correct option is \[ \boxed{(A)\;\text{Axial compression and shear force act simultaneously}.} \]