MMM,
Provided you have a good footing and cross bracing, the house trailer frames I have seen would be VERY strong for a 10-15' span. A long trailer would almost yield edge-to-edge beams the entire 15' length of the bridge.
Why not test it? Using 4x4 or 4x6 posts, block up only 2 of the beams at 15' intervals, drive a heavily overloaded {whatever vehicle you intend to drive over it} and observe the flex. If it flexes no more than an inch or 2, you are good to go. If it flexes a lot, you can weld some 1/8" x2" strapping in a zig-zag pattern along the length. This will make the
I-beam act more like a box beam, and practally double the strength for a few bucks.
Be sure the decking is well attached, as light beams can twist torsionally.
Having said that, less than 15' is very forgiving in a bridge span. When you get over 15', every extra foot will require heavier beams. For example, a 20' span would need about twice the strength of a 15' span.
A great thing about solid steel beams is they rarely fail catastrophically unless used in sub-zero weather. They flex, bend, sag, creak and groan, and give you plenty of warning before they break. In fact, a short bridge made from homogeneous beams would probably never actually break.
Merry Christmas to all!