I think you're wanting to determine when the box beam will collapse or begin yield (crush).
I haven't any idea however with the known yield of the base material a load design that is 1/2 of that one could half argue that you have a FOS 2. Which I know is mostly not correct.
With testing one could create an estimated typical yield crush load..
Engineers Edge does have the following calculator where one could set the foundation deflection to a very low # (.0001) and then determine the Moment, shear and deflection of the beam.
Beam with Distributed Loading on Elastic Foundation Calculator and Equations