how do I build a foot and atv bridge any suggestions?

[Keary]

ok add details span should be around 40 ft it will go over a 6 foot deep gully. The creek trickles in summer but when we get rain it runs wild and washes out trees and banks. My idea is to put concrete piers and maybe some angled steel into the banks 5 ft in the ground to support the gusset plates. I turn to the vast knowledge base we have here to request ideas or maybe even a program that could mock up a bridge for me