Are you proposing to hang the pipe from rods embedded in the deck? Typically water mains on bridges sit on steel supports, though on one project I had to mount a 12” pipe to brackets mounted to the parapet.
The loads are no mystery. Weight of pipe and water, impact, perhaps wind depending on how you mount the pipe
They do this all the time in Florida. I'm sure if you do a search (the drawings are mostly public domain), you'll find details.
It's best to overdo the supports, both in capacity and spacing as these are subject to vibration, environmental loads and abuse.
