I'm using the AIAA paper AIAA 94-1579 to calculate the D matrix of a stiffened panel, however I can't match the HyperSizer calculated stiffness results. Other than the papers on Thermoelastic Formulation of Stiffened Composite Panels, is there any documentation on how you calculate the D-matrix?

Related to the same problem, how does HyperSizer account for the panel width in excess of the stringer spacing? For example, an 11" panel with 3" stiffener spacing results in... 3 stiffeners or 4? If 3, what happens to the other 2" -- is that spread evenly into the D-matrix as an effective stiffener spacing of 3.66" or is the excess just ignored? If 4 stringers, what happens to the missing skin width?