I'm in the process of designing and installing a new deck. The deck shall be approximately 40ftx15ft with wood framing and composite/vinyl decking. To minimize the amount of wood columns, I plan to use 3 steel columns with an 8 inch I beam (girder/header). The 12 inch joists will span approximatley 15 feet from the ledger to the 8 inch steel girder supported by the 3 4inch or 6inch columns. The steel is more than enough to support the deck load, but not sure as to what the footings should be builit to? My town code requires footings to be lower than frost line (42 inches). Not sure what size footings and or reinforcement I would need?? Any advice would be valuable...
The AWC's Prescriptive Residential Deck Design Guide has a table for footings. Unfortunately, your 20 foot beam span is off the footing sizing charts. Max the table goes up to is 18 ft span. Not to say it can't be done, but thats what the PRDGG maxes out at.
Are you going to cantilever the joists at all? Cantilever the outside end of the beams a foot or two? Read the guide, and you'll get a better idea of what I'm talking about. Page 9. It's free on the internet.
Footing reinforcement calcs come from ACI's SP-17 design handbook. If doing or paying for the calcs isn't in the cards, try talking to the building code official. He might give you some direction and possibly tell you what he'd be comfortable with.