eh, well, I think I figured it out on my own. Much of my mass comes from the modeled infrastructure (the foundations) and the bottom floor slab. These elements do not participate in the vibration modes. This is why the accumulated percentage of mass will never come close to 100%.
Hello,
Usually, when one wants to apply a distributed load to a 2D plane stress model of a simply supported beam, for example, we have, in theory, to calculate the load contributions in every node of the finite elements (nodal forces). This is what I learned in my courses in college, anyway...
