How do I model a slab having two different thicknesses such that their top faces are aligned?
This can be accomplished using the ELEMENT OFFSET specification. Consider an example in which a slab is divided into two parts: one half is 12 inches thick and the other half is 24 inches thick. To model this in STAAD, you would first assign 12 inch and 24 inch thicknesses to the appropriate plate elements. At this point the 3D rendering would show the variable thickness, but without the top surfaces being aligned:
To achieve the desired orientation, the 24 inch plate elements must be offset downward by a distance of 6 inches. To do this click on the Specifications menu as shown below:
You will see the Specifications - Whole Structure dialog box appear on the right-hand side of the screen. In this dialog box, click on "Plate". In the window that pops up, click on the "Offset" tab. Here you can specify the direction and distance by which the plates will be offset (-6 inches downward in this example):
Click "Add" and assign to the 24 inch thick plates to complete the process. Once this is done, the 3D rendering will show the slab correctly aligned: