Personally, I would want to model the actual plate for the floor, then apply the load to the floor. If the load is something that readily deforms, like sand, then I would apply a pressure, equal to the total load divided by the area of the plate. If the plate is thin, then you will get a lot of deformation between the beams. If the plate is thick, then there will be much less deformation between the beams and the plate will add significantly to the stiffness of the whole structure.
Another approach, but one I wouldn't recommend, would be to divide the load between the beams based on the total length of the beams. You would divide the load by the total length of the beams, then multipy that number times the length of each beam to get the appropriate load for each beam.