Steel + Long Reinf (n=n) vs. Steel + Long Reinf (n=3n) section property

In my composite steel girder run I notice the MOI in the negative bending region where the steel member and the reinforcing steel are the only parts contributing to the property that the 3N moment of inertia is different than the N moment of inertia.  I think N should be 1 since E of the girder steel is the same as E of the reinforcing steel (N = 29000/29000).  So why would the properties for Steel+Long Reinf (n=n) and Steel + Long Reinf (n=3n) be different?

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  • Please follow FHWA (and AISC) design example which explains why would the properties for Steel+Long Reinf (n=n) and Steel + Long Reinf (n=3n) be different.

    Also there is explanation in the FHWA example which says :


    “Although not required by specification, for stress calculations involving the application of long-term loads to the composite section
    in regions of negative flexure in this example, the area of the longitudinal reinforcement is conservatively adjusted for the effects
    of concrete creep by dividing the area by 3 . The concrete is assumed to transfer the force from the longitudinal deck steel to the rest
     of the cross-section and concrete creep acts to reduce that force over time”

    Reference : FHWA design example

    Answer Verified By: MarcosBeier