mobile home I beams for foot bridge
The topic of building a footbridge over a creek comes up pretty regularly on this forum. Unfortunately there are no prescriptive standards that I am aware of for the design and construction of bridges, each one is custom designed based on the span, the load, the soil type, and the bridge geometry. Bridge design is generally more complex than building design because the loads are moving, and there are many configurations of possible loads which can be the critical load for the bridge. For this reason, it is not generally possible to go to a steel fabricator and ask them for the "right" size beam for the bridge.
An experienced engineer who has done bridge design can generally determine the maximum possible load on your bridge, and can then tell you if the specific I beams you are proposing to purchase will work. No building inspector I have ever met would stick their neck out and bless a particular I beam, all the ones I know would tell you to get a design from a professional engineer, else use a pre-engineered bridge designed for a particular load (bridges are generally rated for axle loads such as HS-20).
In addition to being structurally adequate, bridges need to be designed so they do not wash out in high water events. Determining the maximum height of a river or creek is a specialized job typically done by the engineer who designs the bridge, however your local building department may have flood maps for your area that will show hundred year flood elevations.