I believe you are correct when you say, "Well the problem is there seems to be a height limit."

At a certain distance from base terrain, no amount of wood, log, stone, or iron-reinforced wood will prevent a new structural member from collapsing due to its own weight. Judging from your screenshot, at the center of the span this limit has been reached. (Sorry, I don't recall the exact distance at the moment.)

What you should be able to do is use the hoe to raise the ground level on either side of the stream at the bottom of the ravine. Raising the ground level shortens the distance to the point where you want to place the finishing pieces of you span. If you raise the ground level enough you may be back inside the height limit.