References
ACI 318-14
Effective Moment of Inertia
Mildly reinforced members compute deflections using the effective member of inertia from equation 24.2.3.5.1 in ACI 318-14. The loading is split into the portion on the non-composite section and the portion on the composite section. Each of these loading stages calculate the effective moment of inertia independently. The difference between these two stages is the gross section properties as we always use the final applied load. This prevents rare cases where the cracked composite section is a stiffer section than the gross precast section.
Sustained Load Multiplier
Additional time-dependent deflections are accounted for by following section 24.2.4.1.1 of ACI 318-14. This provision provides a deflection multiplier to use on all sustained loads. The compression reinforcement ratio used in equation 24.2.4.1.1 is computed as the average ratio over the length of the member. This is typically a conservative assumption as it results in a higher deflection multiplier.