100 psf seems like a good design load for this application. There isn't a category for "motorcycle stand," so you have to estimate what's going to take place. To compare to other design loads, residential floors are generally 40psf live and 10 psf dead, decks and balconies are 60 or 100psf depending on area, and 10psf dead load.

2.7psf is the actual dead weight of a 3/16" thick sheet of aluminum.

The 1.6 multiplier is a standard factor of safety for live load in LRFD design to account for impact effects and uncertainty in load magnitude... as is 1.2 for dead load (lower uncertainty).