This one shows the framing of the ramp, with the two six foot sections on one side bolted together. and 12' 2x4's going all the way across the flat.

Make sure you build the ramp in separate sections, so you can have the 2x4's line up as though they were one 12' 2x4.

Some people build the side as one section, with a single piece of plywood in the center, with notches cut in it to allow the joists to go through. they like it because you don't have to cut as many 2x4's and you can save a little bit of plywood. We don't recommend this though, you have to cut just as many notches, as you would 2x4's and the notches are harder to cut. It's easier, faster, better, to just build the ramp in separate 6' sections and bolt them together.