Find the maximum torque - I-section:
An I-section 150 × 80 mm has flanges 6 mm thick and web 4 mm thick. Discover the maximum torque that may be safely applied if the shear stress is limited to 30 N/mm^{2} and twist per metre length to 6^{o}. G = 80 GPa.
Solution
∑ h t ^{2} = 2 × 80 × 6^{2} + 150 × 4^{2} = 8160 mm^{3}
T = τ_{max} . (1/3) (∑ h t 2 ) = 30 × (1 /3) × 8160 = 81, 600 N-mm
∑ h t_{3} = 2 × 80 × 6^{3} + 150 × 4^{3} = 44,160 mm^{4}
θ= 3Tl / G (∑ h t^{3} )
⇒ 6 × (π /180) = 3T × 1000 / (80 × 10^{3} × 44160)
∴ T = 123318 N-mm
∴ T = 44,160 N-mm (least value).