$4-$WING RIBBED $($IN TWO

WAYS$)$

$4\mathrm{.}1$ Definition

$4\mathrm{.}2$ Characteristics

$4\mathrm{.}3$ When to use them

$4\mathrm{.}4$ Applications

$4\mathrm{.}5$ Thickness design

$4\mathrm{.}6$ Design standards