… in a closed thin-walled non-circular cross-section#

For closed thin-walled non-circular cross-sections the shear stress is defined by:
\[\tau = \frac{M_t}{2A_mt}, \quad I_t=\frac{4A_m^2}{\oint\frac{1}{t}dm}\]
This is treated in chapter 6.3.1 of the book Engineering Mechanics Volume 2 (Hartsuijker and Welleman, 2007).
Exercises#
Exercises 6.14, 6.16, 6.17, 6.20 of the book Engineering Mechanics Volume 2 (Hartsuijker and Welleman, 2007). Answers are available on this website.