Applicability of moment of inertia reduction on beam/slab on grade

[TTomaz]

Hi there!

ACI 318-19 states at Table 6.6.3.1.1(a) that a moment of inertia reduction is permitted for elastic analysis at factored load level of nonprestressed beams (0.35 x Ig) and slabs (0.25 x Ig), where Ig is the moment of inertia of gross concrete section.

The question is rather simple: can these reductions be applied to beam and slab on grade at structural model level? The code apparently doesn't mention, which makes me assume that these reductions can be applied, even though the behavior could be different from a typical beam or slab.

 

[JAE]

If the beam on grade is a part of the overall lateral system, and if it will receive bending from the adjoining frame columns, then I would think you would have to use a reduced moment of inertia to accentuate the second order effects in the frame to properly estimate the frame behavior.

Slabs on grade, however, to my way of thinking are NOT a part of the overall framing of the building but rather a separate on grade slab used as a floor, independent of the frame behavior.

If it is a slab-on-grade, but a STRUCTURAL slab participating in the overall frame behavior, then I would also model it in with the framing and reduce its moment as well.