We just bought a house and are building a bridge across a creek. Presently there is what looks like a mobile home frame over the creek which the neighbors told us it once had lumber on it but apparently has rotted away. The frame is 40' foot long (from one footing to the other)and is made of 4 each 3/16" thick x 10" deep I-beams that are connected together with cross pieces of steel.
Question: As far as the frame itself is concerned - how much weight would this frame be able to support including the lumber used for decking?

I doubt if even a structural engineer can answer this. If its so old that the wood has rotted of it there must be plenty of rust which would weaking the steel. I would not bother with repairing it, sounds jury rigged to start with.

I would say that would be an acceptable frame for a foot bridge or to take a riding mower over, that is totally inadaquate for any type of vhiecle traffic. If you are in a somewhat rural area make a trip to your local farm co-op or elevator they can more than likely direct you to a supplier of prefab bridge components, as many farmers have a need to get heavy farming equipment across creeks to access farm fields.