Discounting other codes which may control in your area, AASHTO (roadway bridges) specs call for an 85#/sq. ft pedestrian loading and in some cases increases to 100#/sq. ft.
For the 85#/sq. ft. load and four foot loading and assuming your going to use two beams that support a 4" concrete deck (conservative, may use wood), this translates to a uniform load of 270#/lin. ft acting on each beam (328 including the beam). Assuming that you have simple supports and that whatever decking you will use (concrete or wood) will brace the beams properly the mid span moment is 6.6 kip-ft (8.0 including the beam).
If you assume that the beams are grade 36 steel then the allowable flexural stress is limited to 0.55Fy and will be 20ksi. This will require a section modulus of 5 cubic inches. A W8x58 provides a section modulus of 52 cubic inches. This, of course, assumes strong axix bending.
This solution appears to provide a FS of nearly 10. Which depending on the application may not be so bad.
One should check deflection as people are very sensitive to bridges that deflect considerably under normal loading.
Speaking of railings and party-goers in Chicago...I forgot to add my usual limitations of well intended advice.
Please have your design checked by a professional engineer with experience in structures and who completely understands the application. This will afford you many a full night asleep.
Incidentally, if this is for more than your backyard excursion to the otherside of the creek, please look into the following companies as they provide full service pedestrian bridge products and engineering.
