Effective Moment of Inertia
For mildly reinforced members, deflections are computed using the effective member of inertia from Equation 24.2.3.5.1. The loading is split into the portion supported by the non-composite cross-section and the portion supported by the composite cross-section. For each of these loading stages, the effective moment of inertia is calculated independently.
Sustained Load Multiplier
Additional time-dependent effects are accounted for by following Section 24.2.4.1.1. 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 can be a conservative assumption as it typically results in a higher deflection multiplier.
Codes
ACI 318-14