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Area enclosed by mid-contour used in calculating properties of square or rectangular hollow section
Area
$$ A_{h_{RHS}} = {\left({ b_{RHS} - t_{RHS} }\right) \cdot \left({ d_{RHS} - t_{RHS} }\right) - \left({ {( R_{o_{RHS}} + R_{i_{RHS}} ) \over 2 }}\right) ^ 2 \cdot \left({ 4 - \pi }\right)} \:, mm^2 $$