I know in the old days galvanized could react with the PT chemicals (copper, Arsenic). I don't know if there is any issue with the new chemicals.

And 16 feet seems kind of long to span. It should not be too hard to add some support. It could sag after a while, which could affect drainage. Just make sure it never slopes toward the house.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra