The most common place for a bridge to rust out is where the planks lay on the flange of the i-beam trapping water and at the abutments.

What can be put between the planks and i-beams to stop this? I'm thinking tar or a synthetic liquid rubber type product. Any advice?

Zip siding tape.

Paint?  I would think that a couple of coats of good paint would last as long as the oak planks, and when it comes time to re-deck you'll probably want to take the time to repaint anyway.

If the planks are just basically lying across the beams, a layer of silicone caulk between them wouldn't hurt.

Definitely prime the steel and coat w/ an appropriate "exterior performance grade steel coating".
Using a barrier between the two materials is a good idea.
The Zip tape is in line with this. Something like "rubber roof repair roll" might be more suitable.
Zinc fasteners and make sure the wood is properly dried, before use.

I put metal pipe posts in the ground 15 years ago.  I coated them with bed liner before I planted them. Pulled them last summer and they were in perfect shape.  I was really impressed.  I wouldn't hesitate to give that a try.

Butyl tape on the flange and a good coat of paint near the abutment (eta, and everywhere else too).  If its in good shape when laid it will outlast you.

When making snowmobile bridges with the clubs. We put down a layer of tar or asphalt roofing shingles then the planks. Bridges held up for years/decades. Usually the wood rotted out and the steel was just fine.

Duh, I don't know why I didn't think of butyl tape. Unsure of what type out of the many different options, I contacted a manufacturer to ask questions. This is actually for a non-profit kids summer camp. I volunteered to help them with the bridge replacement project bc the new director is in pretty deep with projects. Once they heard what it was for they said they will send us whatever butyl tape we need at no cost. That's pretty awesome!



The hope here is that the steel beams last through several wood deck replacements. I'm thinking about every 20 years the deck will get torn off, beams stripped and repainted, then new deck put on. Done right the bridge should last over 50 years. The age of the current bridge is unknown but it could be original to the camp (60s).

Sure is!  Post up a photo when you get the project complete!

Shingles