Reply to "Maximum bridge length without center support"

To Allen Miller and AGHRMatt:  I do believe that 1 x 2 boards would be sufficient and I also like the "shape" of the parabolic support.  I can do a revision to my drawing and it would meld both ideas together.  I would build the supports from 1 x 4 boards.  The middle 18" would be the thickness of a 1 x 2.  This would give me the height I need to walk under without hitting my head.  From each end of the 18", it would curve out to the far left and right ends of the 1 x 4) at a height of 3.5".  The curve on each end would give the bridge a nicer look.