I thin your definition of moment of inertia is wrong but “(spine + compressed abdominal cavity take the compression, instead of spine alone?)” yes, basically.
This is way to complicated to figure out with some engineering formulas.
You can get an idea, but it’s still a guesstimate at best. One example, what role does the fascial system play and how do you account for that? And yes the body’s fascia will play a role in dissipating stress.
This is coming from a chiropractor buy the way who picks up heavy things. If you want to figure it out, have at it. If you want to lift heavy stuff, learn to lift heavy stuff.
I just wanted to have a basic idea, all the while doing a little mental gymnastics applying some engineering knowledge in order to understand (a little) how the body works (and since I know no professor of bio-mechanics…) 
And again, thank you very much Pvoosen. You answered my question.
While I know the concept of moment of inertia is not applied to trusses, I meant that practically the compressed area goes from just the vertebrae to vertebrae + pressurized abdominal cavity. (while the tensed area remains the same, muscle + conjunctive tissue)
Thank you,
Vlad
Has to be a troll.
Vlad,
In regard to the nucleus of the disc being pushed in all directions that is accurate, however the important thing to note is that as you go lower into the lumbar spine, the posterior longitudinal ligament, which is the main “belt” of the posterior and lateral aspects of the annulus and thus the disc becomes narrower covering less and less of the posterior portion and the opposite happens in the upper cervical spine, there is less and less of the anterior longitudinal ligament. This is why most lumbar herniations/protrusions (depending on your nomenclature) are of the postero-lateral variety.
I know you are making a model to figure the compressive loads of the spine on a “perfect” spine but there is no known optimal curvature of lumbar lordosis, thoracic kyphosis, or cervical lordosis. Other items of considereation when analyzing the forces on the spine are the facets which change direction throughout the levels of the spine to account for more movement in certain directions based on need. This will certainly change the amount of stress on discs at certain levels.
The IAP will absolutely act as a brace supporting the spine. In regard to whether it will decrease compressive loads there is variable evidence. People like McGill state that it does decrease the compressive loads and I very much respect his opinion but there are other recent studies that suggest the opposite that it will increase the compressive load on the disc and create a moment arm in the erectors. I hope some of this information is helpful.