Geometric mean radius of armour

The formula for the GMR was taken from the book Electric Power Generation, Transmission, and Distribution by Leonard L. Grigsby (2006).

Symbol
$GMR_{ar}$
Unit
mm
Formulae
$\left(0.7788\frac{t_{ar}}{2} n_{ar} \left(\frac{t_{ar}}{2}\right)^{n_{ar}-1}\right)^{\frac{1}{n_{ar}}}$