I'm building a bridge across a ditch in my nephews front yard. Shown here is an elevation of one of the four joists that will support 2 x 6 redwood treads, width of 5.5'. To achieve the curve, I will need to assemble the joists from 2 pieces that will need to be scabbed together in the center. The unsupported part of the span measures 11.5 feet, the vertical thickness of the majority of the span is 10", it measures about 8.5" where it leaves the supporting pads at the ends. The wood to be used is pressure treated Doug Fir, structural grade. My intent is to scab these two pieces with a 4' long piece of 2 x 8 PT Fir, same as above. I'm a long time builder but I'm a bit nervous on this one. I've not, in memory, seen similar. Think this is sound? If so, how best to secure the scabs? Many screws? Carriage bolts? Use iron versus the wood scabs? (I'd rather avoid that.) Wood-scab both sides of the interior joists? I guess my biggest concern is stress, or wear from use, on holes going through the joist halves.
I could run the design flat with a single piece but that would require steps on the ends and would not look nearly as nice in its intended location.