Concrete Technology and Design of Concrete Structures
Critical section for shear in case of flat slabs is at a distance of

effective depth of slab from periphery of column/drop panel
at the periphery of column
at the drop panel of slab
d/2 from periphery of column/capital/ drop panel

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Concrete Technology and Design of Concrete Structures
In prestressed concrete

both forces of tension and compres-sion as well as lever arm remain unchanged
both forces of tension and compres-sion as well as lever arm change
forces of tension and compressions remain unchanged but lever arm changes with the moment
forces of tension and compression change but lever arm remains unchanged

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