Cracked MOI for PT Slabs

[dengebre]

ACI 318-05 [10.11.1] allows the use of the reduction factor 0.25Ig for estimating the cracked MOI for concrete flat plates. However, that is not for prestressed flat plates. Assuming a minimum precompression of 125 psi, would not the cracked MOI be something higher than 0.25Ig? If so, what would be a reasonable factor to use? Remember this section is referring to estimating the cracked MOI for the ultimate load case and not the Ig used in computing deflections for the service load case. Thank you in advance for your help.

 

[rapt]

Except for the effect of secondary moments on the total ultimate design moment, it will not change much at all, but will probably reduce due to the smaller steel area. The depth of neutral axis at ultimate is the same for the RC and the PT member for the same tension/compression force couple, so the concrete contribution to Icd does not change, but the steel contribution will reduce due to the reduced area of steel.

Using an estimate nlike this for the RC is very approximate as it will vary significantly with steel ratio!