Typically the load on a floor is measured in pounds per square feet (psf). Where did you get the "20,000 lbs" figure you quote? Do you have a specific load in mind? Is this for general storage? Residential living space? The use of the space would dictate how much load the floor should be designed for. Typical values would be from 40 psf to 100 psf or more, depending on the use, local code requirements, etc. An engineer or other qualified building professional can help you determine the appropriate design loads, and then you can decide how to support it.
If you have a single point load (e.g. a winch suspended from the ceiling for moving around equipment or something), that would be a totally different design requirement.
Side note: where are you planning on finding 4"x12" joists and 3"x12" floor boards?? Those are not standards sizes and would be extremely difficult & expensive to obtain unless you have a known source of reclaimed lumber or something.