Propped cantilever bending moment

[Tstruct]

A propped cantilever element (fixed at one end and pinned at the other) loaded with triangular load should produce the bending diagram as following:

Now I have an ETABS model of an underground septic tank. Since walls are connected to foundation at the bottom (fixed), with slab at the top (pinned) and loaded with triangular load of full height; it should behave like a propped cantilever. But the bending moment I am getting doesn't have any possitive moment.
What am I doing wrong here?

 

[JLNJ]

Looking at your top & bottom reactions at the different nodes will probably tell you how your model is acting. Maybe your lid is imposing counter-moment into the walls.

For these types of structures I first see if I can design the walls as three-sided members free at the top so that the contractor can backfill the chamber without having to shore the top or put the lid on prior to backfilling. Given your wall proportions, the panels might work quite nicely as fixed-fixed-fixed free.

 

[HDStructural]

Any chance Major (V2 and M3) may show the diagrams you are expecting to see?. I would expect it to be considered major axis bending, not minor.

 

[hardbutmild]

Given your wall proportions, the panels might work quite nicely as fixed-fixed-fixed free.

I'd consider the vertical edge to be fixed only between the two "rooms" on two walls in x-z plane if wall connections have no haunch.

 

[TRAK.Structural]

I think you should model the top slab instead of using node supports along the top edge. The stiffness of the top slab will affect the wall stresses. I think it's also important to understand that you will have horizontal bending in combination with vertical bending. PCA has a rectangular tank manual with lots of helpful tables that will get you reasonably close results for moments and shears in both directions. You could check that out to see if your ETABS model is in the right ballpark as far as results go.