It is a plane problem where the external loadings are applied in the vertical (y, z)
plane, and the cross section (perpendicular to z) is symmetrical respect to y.
This means that the deformed shape is lying in the (y, z) plane.
Small displacements and rotations are assumed.
• the eect of curvature is neglected
• the elongation for an elemental length is calculated neglecting the rotation.