Curvature of zeroLength element

[BLJ]

Hello, can you tell me how is this curvature defined in a zeroLength element?
set Ky [expr $epsy/(0.7*$d)]

EXAMPLE 2 -- Moment-Curvature Analysis of a RC Section
epsy is steel yield strain, d is colDepth-$cover.

then why the curvature is the epsy/(0.7*$d),

Thank you.