Hi Dale and Welcome to the forum.
Without any calculations I'll tell you one piece standing alone won't handle much before it'll start twisting. Pair it up with another angle and now you have effected a channel or a T beam which will handle a good bit more. You don't really need to know what one piece will handle. You need to know what the entire structure will handle. The amount that two angles tied together will handle is a lot more than twice the load a single angle can carry. The geometry has a big effect.
Was at a big aution and watched a guy with some home-made ramps back off a good sized lawn mower out of his high 4x4 pick-up. As soon as a decent part of the weight got on the ramps they collapsed and the tractor and he flipped over backwards and to the side. Lucky he wasn't killed.
There are more things to consider in designing ramps than the load they'll carry, and most on this site will be reluctant to tell you what to make them out of for fear of the unknowns. Weight... application of that weight... angle of the dangle... potential for shock load or bouncing... welding or bolting and how good or how many... etc.
With more information could possibly help you out. Also there are calculators on this site that make doing the calculations quick and slick.
And sometimes it's best to just go out and buy things designed for the purpose?
Good luck and be careful,