Deflections - Mild
Codes
ACI 318-14
ACI 318-19
Effective Moment of Inertia
For mildly reinforced members, deflections are computed using the effective member of inertia from Equation 24.2.3.5. Depending on the member being designed, up the three different effective moment of inertias can be used based on three different loading levels. These three loading levels are:
All loads
Computed based on the composite section properties (cracked and gross)
Used to compute the instantaneous deflection due to all loads applied to the composite section
All sustained loads
Computed based on the composite section properties (cracked and gross)
Used to compute the additional deflections due to long term creep and shrinkage effects.
The composite section properties were chosen since all long term effects will take place when the section is composite
All loads applied to the non-composite section
Computed based on the non-composite section properties (cracked and gross)
Used to compute the instantaneous deflection due to all loads applied to the non-composite section
If the section is not a composite section, this effective moment of inertia is not used
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 either at midspan or over the support. The program will check the supports if the cantilever exceeds more than 10% of the beams total length.
The additional deflection multiplier is used in combination with the deflections caused by all sustained loading using the corresponding effective moment of inertia. The final deflections are the sum of the instantaneous deflections and the additional deflections.