等效方形截面尺寸

Equivalent round section dimension for timber design

d_{i} = \sqrt{\frac{π \cdot d^{2}}{4}} (CSA O 86)

a_{eq} = \sqrt{\frac{π \cdot d^{2}}{4}} (NDS)

d_i	Initial section depth
d	Diameter (section depth)
a_eq	Equivalent square section dimension

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