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Torsional modulus constant for square and rectangular hollow sections
Torsional modulus constant
$$ C_{RHS} = {{( t_{RHS} ^ 3 \cdot {h_{RHS} \over 3} + 2 \cdot k_{RHS} \cdot A_{h_{RHS}} ) \over \left({ t_{RHS} + \left({ {k_{RHS} \over t_{RHS}} }\right) }\right)}}
\; \; , {mm ^ 3}
$$