# What is the stiffness modifiers for a prestressed slab?

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?

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).