I tend to agree with the contractor, and think that ice lifting will be a non-issue.
Yellow Pine weighs somewhere around 37#-41# per cubic foot (roughly one 2x12x8). How many cubic feet of lumber will the bridge be made from?
You will have 10 sq. ft. of interface between the 4x6's and the ice if the ice is 18" thick.
The total square footage of the Bridge is 225.sqf. In this neck of the woods structural timber is usually what we call "Hem-Fir", which is most of the time Douglas Fir. Notably PT DF DOES weigh more than untreated DF. I was planning to use 5/4 cedar deck planks for the actual decking. I will also construct a 42" high hand rail on both sides of the bridge, also made of a combination of PT and cedar balistars.
Because both ends of the 12'long bridge ramp sections will be supported from the ground at both the shores, the four stanchions will support the full weight of the 9' center section and 1/2 the weight of the two ramp sections. ie effective bridge area with associated weight bearing load is 153.sqf. I have not completed a material list so I don't know what the board feet will be yet.
Shearing 10.sqf of ice sounds like a very difficult thing to do. I imagine the bridge weight would have to be in the thousands of lbs to break that level of adhesion between the ice and the 4 wood stanchions? On the other hand the ice is usually not that thick.
