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- You could create a sequence of section cuts to obtain the design forces - see related FAQ on Section cut FAQ, item "Can the program display force diagrams based on a sequence of section cuts?" for additional details. This may require significant effort if done manually, but it would be a reasonable approach if it is automated using Application Programming Interface (API). The discretization should be refined as needed in order to adequately define the section cuts.
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To model a condition when the slab does not contribute any stiffness, you could use property modifiers to reduce the flexural and axial stiffness of the slab in the direction parallel with the girders. I would recommend to use very small values for the property modifiers, say 1e-3, to avoid numerical problems that may occur when zero modifiers are used. Property modifiers for the direction perpendicular to the girders could be also modified to account for the stiffness of the forms.