cracked sections for frame analysis

[genzo]

when analysing a reinforced concrete moment frame do i have to use the effective moment of inertia specified in ACI such as col:Ie=0.7Ig;beam:Ie=0.35 Ig and so on...when i used this and satisfy the drift requirement i always get results of bigger sections than analysing it with out modifying the moment of Inertia, structural system is SMRF.can somone help me with this one?thanks

 

[JAE]

Are you using the ACI rational method (ACI 318, Section 10.10) or the magnified moment method (ACI 318, Section 10.11, 12, etc)?

 

[genzo]

JAE,
yes i am using the Ie as provided for moment magnifications.
thanks for the reply guys.

 

[JAE]

Section 10.11.1 requires you to take into account axial loads, cracked regions, and duration of loads. You can do this manually, interating to see where and if various parts of your structure crack, and modeling these parts as cracked...or you can "alternatively" use the reduced values given in 10.11.1 (columns - 0.7 x Ig etc.)

It specifically states that this is for the calculation of Pu, M1, M2, and delta(o).