i am planing to build a w16xL40xh12 pole barn over my camper and slowly finish it in to a house over time then pull the camper out. im going to use 4x4 post 8' o/c. i think i can go with a 16 foot span useing 2x6x16 joist and 2x4purlins? i dont know if i should put the purlins and joist 2'o/c or...