According to ACI the stiffness modifier for moment of inertia for flat plate and flat slabs is 0.25 (ACI318/318R-02 10.11.1) to mitigate for the cracked section.

But in a prestressed slab (post-tensioned) the crack is controlled can the stiffness modifiers be taken higher than 0.25?

My professor suggests that it could be taken between 0.6-0.8, can anyone suggest any article or document or even a discussion about this topic to be sure about this problem ?


1 Answer 1


The Los Angeles Tall Buildings Structural Design Council (LATBSDC) 2017 guidelines suggest using the following (Table 3, LATBSDC 2017):

Service-Level Evaluation:

  • Axial : $0.8E_cA_g$
  • Flexural : $0.8E_cI_g$
  • Shear : $0.4E_cA_g$

MCE-Level Evaluation:

  • Axial : $0.5E_cA_g$
  • Flexural : $0.5E_cI_g$
  • Shear : $0.2E_cA_g$

For code-level evaluation you could use some value between those two, as you professor recommended. However, it is common practice to use the full section for PT slabs (i.e., modifier = 1.0).


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