[scook]

Like Smoke says, it's a bridge. Any bridge, or simpler to visualize, a beam relies on tension and compression. When you stand on (load) a beam, the downward force tries to bend the beam down. That makes the bottom want to stretch and the top want to compress (get shorter). Fiberglass is is both hard (resists being compressed) and the fibers are strong (resist stretching).

Like he says, when you glass the bottom, particularly the support stringers, the reinforcing in the glass will add LOTS of strength. You do also want to adhere the plywood well to the fiberglass deck above, so that when the load makes it want to compress, it can't buckle (to get shorter) instead of staying in place and resisting compression.