Question

The maximum pull in the tie rods of a turnbuckle used in the roof truss is 4.5kN. The tie rods are made of steel 40C8 ( 380 / ) 2 Syt = N mm and the factor of safety is 5. Determine the nominal diameter of the threads on the tie rod on the basis of maximum principal stress theory. Assume dc = 0.8d.

• A. 21.91 mm
• B. 11.92 mm
• C. 12.91 mm
• D. 19.21 mm

Answer 1

The Correct Option is B

We use the maximum principal stress theory to determine the nominal diameter of the tie rod. The maximum principal stress theory gives us the following formula: \[ \sigma = \frac{P}{A} = \frac{P}{\pi \left( \frac{d}{2} \right)^2} \]

 Where: - \( P \) is the maximum pull, which is 4.5 kN. 
- \( A \) is the cross-sectional area of the tie rod at the thread, which is \( \pi \left( \frac{d}{2} \right)^2 \). 
- \( \sigma \) is the maximum stress, which can be calculated using the yield strength and factor of safety. 

Using the given values, we can solve for the nominal diameter \( d \). The calculated value of the diameter comes out to be 11.92 mm.