The plywood will be 8 feet long. If your beams are to be 16 feet long, for each beam cut two pieces of plywood 11-1/2 inches wide by 8 feet long, and three pieces 11-1/2 inches wide by 5ft 4 inches long. That way the joints will overlap. You will need four 2x12 boards total and three sheets of plywood. I believe the wood would be less expensive than two new galvanized steel I-beams or T-beams, and easier to drill. Use plenty of bolts.
You could check local scrap yards for steel. Maybe someone recently sold an old lattice tower that might have nice galvanized steel.
Perpendicular stringers could be 2x8 treated wood on edge about two feet apart with braces between for the deck screws. Nails will pop out.
