Till now, staad pro has an option to assign plates of various thickness, but it should also have an option to model such plates in TRAPEZOIDAL PATTERN as shown in below images,which is frequently used to model raft foundation, also placement and bending of reinforcement at site for trapezoidal pattern plates is very convenient compared to earlier pattern (different thickness of plates in uniform pattern)..
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IMG_20240119_104407.jpg
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Thank you for posting this idea. STAAD has this capability already. As you have noted it is possible to defined plates with variable thicknesses. the larger thickness should be assigned to the deeper section and then smaller to the thinner section. Note that you sections are through the thickness of plates and as such the plane of the plate is the neutral axis (as is the case with beams). If you want to account for the variation of the neutral axes, then use offsets on the plates to account for this difference. Plate offsets were added several years back.