The stiffness terms for stiffened panels are represented in NASTRAN as a PSHELL with three MAT2 cards representing the A, B, and D matrices respectively. By using a thickness of exactly 1.0, this allows us to enter the classical ABD terms directly into their MAT2 representations. If we used a thickness other than 1.0, we would have to factor this thickness into the ABD terms before we could put them into MAT2s.

The bending stiffness is inherently taken care of with the D terms spelled out by MID2 on the PSHELL card. These D terms directly relate bending moment of the panel to curvature.

If you were to use a thickness other than 1.0, then you would have to factor this thickness into the D terms before entering into a MAT2 card.

The actual thickness entered on the PSHELL card is unimportant because we are not representing the actual panel dimensions, rather we are representing a smeared stiffness representation of the panel in the FEA. The important thing is that the membrane, bending and membrane-bending coupling stiffnesses are all accurately captured, and this is accomplished very accurately by using the ABD representation.