I am spanning a creek that is separated by an island. the driveway going across is only 10' wide made possible by the ten foot culvert pipes. it is filled and graveled level, but I wish to build a bridge structure for the weight and width of the concrete trucks. I have 25 feet of distance to cover, and already have about 70 pieces of 4x6 pressure treated that are 12 feet long, and these will run side to side on the bridge. I am wondering what I can attach these to, whether it can be a couple of utility poles stretching across or if I can get by with 6x6 pressure treated stock. the area is level and they will lay flat, but I wanted a little wider (hence the 12' 4x6's) and looking cool too. can this hold up to a concrete truck or dump truck?