etabs frame forces

huangyhg

etabs : frame forces
folks,
i have a question regarding the frame forces reported by etabs for beam design.
i have a frame element for the concrete beam and shell elements (meshed along the length of the beam) for the slab. the frame forces (positive moment) that is reported is much less than what will be computed from hand calcs.
i see that there is quite a bit of force being taken into the slab (which is obviously composite with the beam). i am wondering if it is unconservative to use these reported moments to design the reinforcing for the beam.
i am assuming that the program maybe internally doing something to figure out the n.a. of the section and computing the tension at the extreme fiber and back calculating a moment, but i am not certain.
i have emailed csi tech support to get their input, and i am sure that users here would have more valuable practical input in this matter.