8 12-1 Basic Hypothesis For small bending problem of sheet, we generally adopt these assumptions (1) fixity of the board thickness The normal strain E perpendicular to middle plane is very small, thus we can ignore it. Namely .0 From geometric equations, we have =0, thus we get w=w(x, y) Namely: any normal which is perpendicular to middle plane has the same bending (2) fixity of normal of the middle plane9 § 12－1 Basic Hypothesis For small bending problem of sheet, we generally adopt these assumptions: （1）fixity of the board thickness Namely: any normal which is perpendicular to middle plane has the same bending. （2）fixity of normal of the middle plane w= w(x, y) The normal strain perpendicular to middle plane is very small, thus we can ignore it. Namely . From geometric equations, we have ,thus we get: = 0 z  = 0   z w z 