Consider that cattle grate useless from a structural standpoint. Also know that bending is related to distance by the exponent of 4. 33x33x33x33. She's going to sag significantly with a MOI of a railroad tie. Model would look like a spaghetti. 33' is a big boy span. What's the repercussions of failure? 2' drop into shallow water/muck or deadly heights? Understand no scenario is the bridge, as described, safe. The worst is if he welded ties together to span 33'. Welding between the two could be done with care not shown to be present in design. Wouldn't matter if the welds were performed by a roll cage fabricator, cattle guard is useless no mater how well it's attached to railroad ties. Do the concrete poles have rebar? Sounds like you should go the timber route, much more fogiving engineering and fabrication wise. If you're not a seasoned welder, you really shouldn't be welding on structures that support people.