In each element, three field displacements (in-plane components) and (transverse component) along and x-z directions are considered. Nodal DOF chosen are grouped into three vectors as

(18.4)

where,

(18.5)

(18.6)

(18.7)

and  etc. are the first order derivatives with respect to the node Similarly,   etc. are the second order derivatives;  denote the displacements along x and y direction at the centroid of the element. The in-plane displacement functions are assumed to be complete cubic polynomial such as

or,

(18.8)

where

(18.9)

and

(18.10)