How to fix the bottom flange face of a beam?

Hi All

I have a simple structure made of I-Beams. At the bottom I have a couple of beams that sit on the ground. I want to define a fixture on the bottom face of beam flanges, however SW doesn't allow you to select beam faces to define a fixture. I'm wondering what the best way is to define a constraint for a fully supported beam.

Thanks,

Reza

SolidworksSimulation