Increase moment of inertia vs. increasing the height of the cross section

[Anonymous]

Hi,

In my model I have modelled a concrete slab and a concrete beam below it. They will be cast together, with coherent reinforcement. To model the increased stiffness of the beam due to the offset below the slab, I have tried both (seperately) increasing the I_y with "reduction of moment of inertia" and increasing the cross section height. The two methods yield quite different results w.r.t. bending moments M_y. Anyone know why?

Thanks,

[Artur.Kosakowski]

To have the correct results you need to "place" the beam below the slab. As the offset will not give you the expected values of bending moment the proposed solution is to increase the IY instead. If you only increase beam's height but it is still defined with its center at the level of the center of the slab the bending moment in this beam will be too small.

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[Anonymous]

I experience that the bending moments from the modell with the increased heights become the largest. I model I slab of 250 mm and a beam of 600 mm (the slab included), i.e. 350 mm below the lower edge of the slab. I have calculated a equivalent height of 759 mm.