I am building a bridge across a small stream. I have 3 hemlock planks that are 24 feet long, and are 6 inches by 4 inches in dimension. The hemlock planks are positioned so that the 6 inch sides are vertical, and the 4 inch sides are horizontal.

The 3 hemlock beams are resting on the sides of the river bank. There are no supports in the middle for the planks. The beams overlap the banks of the stream by about 4 feet on either side, so there is 16 feet of unsupported beam crossing the stream.

I am putting 2x6 planking on top of the hemlock beams to complete the top-side of the bridge.

I want to calculate if I can drive my 1.5 ton ATV across the bridge. Will the hemlock beams support its weight, or will they crack when my ATV gets into the middle of the bridge?