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I have worked with some engineers that like to assign high stiffness modifiers to rafts to get conservative flexural and shear design of foundation. Assigning modifiers would increase amount of rebar in your raft. I personally think that this is good practice as nothing is perfectly rigid and cracking in inevitable, which would result in loss of inertia and high flexural stresses. The general procedure is to create two models. One with no stiffness modifiers and one with modifiers for foundation/ raft. You should use the first model to calculate piles reactions and the second to do flexural and shear design. Thanks.

Minimum reinforcement in a mat footing, is to be provided as per Section 7.12.2 of ACI 318-08, as specified in ACI Section 15.10.4. The amount of reinforcement to be provided (e.g., 0.0018 b h for Grade 60 reinforcing steel), is for each face (top or bottom). This is because of the fact the mat reinforcement is provided to resist tensile stresses (rather than temperature or shrinkage) on the relevant (top or bottom) face of the mat, similar to minimum reinforcement provided in RC beams. Regards.